# Questions about correlating RTA plots with what being heard.



## aztec1 (Jun 13, 2008)

I recently sprung for the Behringer ECM8000 and the 1/24 version of TruRTA  It's a great tool, but raises some questions for me about how my system is behaving.

Here's the setup, the Behringer plugged into the MicMate stuck in the headrest.









Here's the plot of the right mid only, flat EQ, no TA. The background plot is after smoothing.









Here's the plot of the left mid only, flat EQ, no TA. The background plot is after smoothing.









Overlayed to see the difference in response, which really isn't _that_ terrible I guess...









Here's the summed L+R plot, still flat EQ with no TA, background is smoothed.









The huge dip at 200Hz cannot be fixed with the EQ in the 880 because the I think the Q is too wide...it just pulls the whole curve up from 100-300Hz or so. I can't adjust the peaks on either side because those frequencies aren't present in the EQ. TA doesn't do anything to the plots, neither does changing the volume. I've read through tyroneshoes' excellent thread and tried some things from that thread to no avail. I have some similar issues ie, pink noise sounds very different from the left to right speakers, summed or not. It's interesting to me that both speakers played alone have similar curves, considering where they're mounted, yet sound so very different.

Is there a guideline like "plus or minus x-dB over x-octaves is audible"? I've done some reading on the equal loudness curve, but I still don't know if the peaks and dips are too narrow (or wide, or deep) to even be heard. I can say the tonality of pink noise of the left is night and day from the right. Frequency sweeps "change sides" multiple times. How does one correlate what the plots show with what's being heard? Sorry if this is a stupid question, but I just can't get my head wrapped around this at all.


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## pwnt by pat (Mar 13, 2006)

The 200hz dip is probably to blame on location - something you cannot fix with the drivers in their current location. Since the flaw in apparent in both doors when played independently, I would look at door skin resonances or reflections off of the lower dash as a culprit.



aztec1 said:


> Next are the baffles and ID OEM mids. They are sealed with wood glue and bolted to the doorskin with 1.25" bolts, nuts, and washers. For some reason I can't find the pics of these things mounted, but they're VERY solid.


Also try taking a left measurement by your left ear when you are sitting in the truck, and likewise with the right side. Measuring FR without taking diffraction caused by your body in to account is silly.


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## candaddy (May 21, 2008)

Just FYI, I've found it about 1000 x easier to make comparisons by clicking off the bar graph button at the top so you get a trace line instead. That way dips on the background traces are viewable. 

10db is perceived as half or 2x as loud, so anything that major should be really noticeable. When I EQ for left and right, a 2db change is audible, a 4db difference is pretty big and a 6 or 8db difference left/right is major. I know your question was more focused to the over-all response curve, but that might serve as some reference to you.

To actually calibrate your program, you'll have to expose the microphone to a known db level and use the db calibration in the program, or it will more than likely be off. I haven't done this with mine and I just take what I get with a major grain of salt. 

If you can't adjust a certain frequency accurately, it is probably because there is a cancellation and I think you will "hear" that if it's real. I put "hear" in quotes because you can't hear what isn't making noise, but that was also the point, the loss will be there and you won't hear it. I've always found that when you have a hole in your response, you'll go round and round with eq, tones, level setting, etc trying to get that hole back, without even knowing what is really wrong. 

I am not totally convinced the holes that show up in the graph are always present however, but I can't currently prove that hunch right or wrong. I was setting subwoofer levels one night about a year ago with my old setup and when I upped the sub level, the graph went backwards (showed less bass) which was clearly not the case by my ears. I can't say I wasn't listening to other portions of the bass increasing, but I'm pretty sure it was a bogus result. I have had better results since then however with totally predictable and responsive measurements, but that time I just frustrate myself to no end. 

Have you tried with your tweeters hooked up yet?


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## smellygas (Feb 21, 2008)

aztec1 said:


> I recently sprung for the Behringer ECM8000 and the 1/24 version of TruRTA  It's a great tool, but raises some questions for me about how my system is behaving.
> 
> The huge dip at 200Hz cannot be fixed with the EQ in the 880 because the I think the Q is too wide...it just pulls the whole curve up from 100-300Hz or so. I can't adjust the peaks on either side because those frequencies aren't present in the EQ. TA doesn't do anything to the plots, neither does changing the volume. I've read through tyroneshoes' excellent thread and tried some things from that thread to no avail. I have some similar issues ie, pink noise sounds very different from the left to right speakers, summed or not. It's interesting to me that both speakers played alone have similar curves, considering where they're mounted, yet sound so very different.
> 
> Is there a guideline like "plus or minus x-dB over x-octaves is audible"? I've done some reading on the equal loudness curve, but I still don't know if the peaks and dips are too narrow (or wide, or deep) to even be heard. I can say the tonality of pink noise of the left is night and day from the right. Frequency sweeps "change sides" multiple times. How does one correlate what the plots show with what's being heard? Sorry if this is a stupid question, but I just can't get my head wrapped around this at all.


This isn't a stupid question. In fact, I've had the same question for at least 5 years, and there's a lot of misinformation out there, and I learned a lot of things the hard way. I ended up reading about 10 key articles published in JAES and everything kind of fell into place. A lot of the following will sound like voodoo, and I'm sure the people who repeat the same misinformation will chime in soon, but here goes:

a) *you absolutely must take several measurements in slightly different locations and average them together to get a valid measurement. This is called spatial averaging.* Geddes in an older JAES article found that taking one random time-averaged (not spatial) RTA measurement gave you a measurement accuracy window of about +/- 8dB or so...which basically is almost useless. You'll find that just moving your mic a few inches changes your RTA curve completely. He found that you required a minimum of 6 measurements in 6 strategic locations to yield a precision of +/- 0.5dB. 
b) *A flat RTA with a pink noise signal is NOT desirable. It will sound overly bright. * Ignore the IASCA metric. Everybody who has done RTA work will say "flat sounds terrible." The reason, very briefly, is that you're measuring the power response, which includes direct and reflected sound. A flat power response sounds too bright. What you want is a flat ANECHOIC response. You can't measure anechoic response without sophisticated software. TrueRTA doesn't do it. So the next best thing is to model the power response curve (what you measure) after what a flat anechoic response speaker would sound like. This has been studied. It is a downsloping line from about 100-400Hz to 10-20kHz at a rate of 1-1.7db/octave. That's your target. Some people have also demonstrated that a power reponse that starts at around 2kHz or so and downslopes about 1-2dB/oct towards 20kHz is also desirable, but the study that supported it was not as authoritative. 
c) some peaks/valleys in your response are caused by reflections and other non-time-coherent processes, and are what is called *"non-minimum-phase." What that means is that no matter how much you equalize, it will make little to no difference in the actual measured response.* If this is the case, there's nothing you can do other than move your listening position or the speakers.
d) if you're equalizing with a 1/3-octave-band equalizer or some other fixed-frequency equalizer, you might be doing more harm then good. You can actually generate high Q high value peaks/valleys that measure well to a 1/3-octave RTA, but are only seen with higher resolutions (like 1/6 or 1/12 octave). The problem is that in order to eq out a peak or valley, you need to have the correct frequency and Q. *A parametric equalizer would be a better tool.*
e) to answer your question, low Q (wide) peaks/valleys are more audible than high Q (narrow) aberrations, given the same amplitude. this has been studied. i don't have specific numbers.
f) In Olive's study, a large double-blind study with 70 loudspeakers covering $100 to tens of thousands, showed that *listeners definitely preferred speakers with a flat, smooth response, and extended low frequency cutoff.* Smooth meant very little deviation from flat when examined at 1/20th octave resolution. 
g) based on Olive's paper, and a few others, 1/3 octave resolution is NOT the ideal resolution to examine loudspeakers, because it doesn't reveal low-Q high amplitude variations that "average" out to a smooth 1/3 octave response.
h) Keep in mind that the default ECM8000 calibration file that comes with trueRTA is not very accurate. the ECM8000 has a lot of sample variation, so each one should ideally be calibrated. 

Hope this is a good starting place. I'm sure this posted will be followed up by someone saying "yo, RTA is a good starting place, but i just trust my ears to tune," which will be followed by "yo, I use the RTA, but flat sounds terrible," which will be followed with "yo, RTA is crap." Good luck.

SG


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## candaddy (May 21, 2008)

smellygas said:


> This isn't a stupid question. In fact, I've had the same question for at least 5 years, and there's a lot of misinformation out there, and I learned a lot of things the hard way. I ended up reading about 10 key articles published in JAES and everything kind of fell into place. A lot of the following will sound like voodoo, and I'm sure the people who repeat the same misinformation will chime in soon, but here goes:
> 
> a) *you absolutely must take several measurements in slightly different locations and average them together to get a valid measurement. This is called spatial averaging.* Toole in an older JAES article found that taking one random time-averaged (not spatial) RTA measurement gave you a measurement accuracy window of about +/- 8dB or so...which basically is almost useless. You'll find that just moving your mic a few inches changes your RTA curve completely. He found that you required a minimum of 6 measurements in 6 strategic locations to yield a precision of +/- 0.5dB.
> b) *A flat RTA with a pink noise signal is NOT desirable. It will sound overly bright. * Ignore the IASCA metric. Everybody who has done RTA work will say "flat sounds terrible." The reason, very briefly, is that you're measuring the power response, which includes direct and reflected sound. A flat power response sounds too bright. What you want is a flat ANECHOIC response. You can't measure anechoic response without sophisticated software. TrueRTA doesn't do it. So the next best thing is to model the power response curve (what you measure) after what a flat anechoic response speaker would sound like. This has been studied. It is a downsloping line from about 100-400Hz to 10-20kHz at a rate of 1db/octave. That's your target. Some people have also demonstrated that a power reponse that starts at around 2kHz or so and downslopes about 1-2dB/oct towards 20kHz is also desirable, but the study that supported it was not authoritative.
> ...


Awesome post! I agree completely.

I think Navone said to hold the mic in the listening position and rotate it for about 30 seconds while gathering averages to equal out the response. 

I think in theory, flat should be ideal, but in reality you are picking up reflections and other problems that your ears may be tuning out. Sometimes highly compressed music forms (like crappy recordings and MP3's) sound better when you do some lame eq settings like the classic "smiley face" or whatever, it's only because you're trying to doctor up something that sounds god-awful to begin with though.. IMO.

I use the RTA to find problems I'm having trouble hearing. It's a good visual for me to compare to my aural results, but it's far from something as bone head as setting resistance with a volt meter and variable resistor though (which is how some people try to use them). It would be nice if it were.


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## npdang (Jul 29, 2005)

Very good response from SmellyGas. Should probably be stickied somewhere. In short, RTA is a tool of limited usefulness for the purpose you're describing. 

In addition I'd like to add the following:

1. Equal loudness curves - It doesn't mean what you think it does. You DO NOT TUNE so that every single frequency is "equally" loud to your ear!! That's not natural. Do not confuse your ear's sensitivity to the sensitivity of a speaker which is REPRODUCING a recording, and not evaluating (listening) to one.

2. Consider not only using spatial averaging, but also frequency adaptive windowing. In short, the measurement window length changes with frequency. So at higher frequencies the window would be relatively short and progressively longer as you go down. The result of this is a more anechoic response which would window out more reflections instead of averaging them in. Playing with the window length yields dramatically different responses, and it does take some play time to get good results. (for example you don't want to window out high amplitude early reflections which are known to affect tonality)

3. I'm not certain how effective equalisation is beyond 1/3rd octave. IIRC the ear doesn't differentiate between sounds that are less than 1/3rd octave in distance. Can't cite it, and could be wrong in how that applies to tuning however but in my experience I haven't found any significant qualitative differences using convolvers with very fine frequency correction and with 1/3rd octave correction.


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## michaelsil1 (May 24, 2007)

smellygas said:


> b) *A flat RTA with a pink noise signal is NOT desirable. It will sound overly bright. * Ignore the IASCA metric. Everybody who has done RTA work will say "flat sounds terrible." The reason, very briefly, is that you're measuring the power response, which includes direct and reflected sound. A flat power response sounds too bright. What you want is a flat ANECHOIC response. You can't measure anechoic response without sophisticated software.



I was wondering why I was getting an overly bright system after tuning.


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## vecc205 (Nov 18, 2007)

This is from Linkwitz's website about an equalizer for a headphone. I too have made binaural recordings and when I play them back on headphones that are equalized to sound flat to my ear they sound the most realistic. This is why I am confused when I am told that you shouldn't tune flat to your ear. I currently tune so that if I were to play a sine sweep it would "sound" constant in volume to my ear, no dips where it sounds like it fades out for a little and no peaks where it starts to get louder. Am I missing something here? 

The equalizer is formed by inserting a passive network between the earphone amplifier output and the connector to the ER-4S. The headphone amplifier's open circuit output voltage is Vs and its output impedance is R1. The earphone impedance is 100 ohm and symbolized by R2 in the schematic below. 



The insertion loss of the network will be 1.6 dB when a value of 20 ohm is assumed for R1. The inductor L must have sufficiently low dc resistance R3 compared to R1 to achieve the necessary notch depth. The inductance value determines the width of the notch. Larger values make it narrower. The notch frequency is then adjusted by C. *The optimum values for L, C, and R3 are determined empirically by adjusting them for a constant amplitude sound while a sinewave generator's frequency is changed.*


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## smellygas (Feb 21, 2008)

npdang said:


> 2. Consider not only using spatial averaging, but also frequency adaptive windowing. In short, the measurement window length changes with frequency. So at higher frequencies the window would be relatively short and progressively longer as you go down. The result of this is a more anechoic response which would window out more reflections instead of averaging them in. Playing with the window length yields dramatically different responses, and it does take some play time to get good results. (for example you don't want to window out high amplitude early reflections which are known to affect tonality)


This is a highly desirable technique when used in listening rooms, where you have early reflections and late reflections/reverberation. Only the first 30ms or so is "registered" as the primary sound, based on the Haas effect. I believe MLSSA uses this technique. However, in one JAES paper, they showed that in the CAR (not a room), 90% of the sound is actually received within the first 10ms, because the sound that reaches the listener (in the driver's seat) is direct + reflected only a short distance. (remember, sound traves approx 1ft in a millisecond). The volume of the reflected sound is often just as loud as that of the direct sound, in fact. I have not been able to find a method to use time gating to approximate an anechoic response in the car.



> 3. I'm not certain how effective equalisation is beyond 1/3rd octave. IIRC the ear doesn't differentiate between sounds that are less than 1/3rd octave in distance. Can't cite it, and could be wrong in how that applies to tuning however but in my experience I haven't found any significant qualitative differences using convolvers with very fine frequency correction and with 1/3rd octave correction.


It was widely accepted for a long time that 1/3 octave is the desired resolution to achieve smoothness and the best SQ. However, this was specifically studied in Olive's JAES paper and it was disproven. Yes, 1/3 octave smoothness correlates with listener preferences, however, it does NOT correlate nearly as well as smoothness at 1/20th octave resolution. They found that loudspeakers with better smoothness at the higher resolution (1/20th vs. 1/3rd) actually sounded better to a large group of listeners over a large, diverse sample of loudspeakers. The range that correlated best was around 100Hz or so up to 10-16kHz, depending on the metric. 

Do you remember the Consumer Reports loudspeaker ratings that were based on 1/3 octave power-response (=RTA)?...the one that ranked Bose #1?? Well Olive took 13 loudspeakers tested by CR and did double-blind listening tests with a group of listeners. There was ZERO correlation between CR's ratings and listener preferences. Astonishing. Basically, a flat 1/3-octave power response did not mean people thought the speaker sounded good. How come?
Two reasons, and this was studied in an elegant design in one of Olive's JAES papers.
#1 - 1/3 octave response was not accurate enough. when the same loudspeakers were evaluated for smoothness at 1/20th octave, there was a different rank order! which means 1/3 octave resolution failed to reveal problems.
#2 - a flat POWER response, which is what the RTA measures, is again, NOT desirable. Speaker that are preferred in blinded tests have a downsloping power-response, not flat/horizontal. Speakers that are preferred have a flat ANECHOIC response, but that isn't what CR was testing!

My own personal take on it is that smoothness at 1/3 octave is still very important, and that a downsloping power/RTA response is desirable. However, if you have extra processing power, it may be desirable to correct peaks/valleys at higher than 1/3-octave resolution, PROVIDED that you have measured them accurately and they actually exist (i.e. good spatial averaging). Things get tricky in a car at higher frequencies because the spacing of nodes and antinodes are very close, sometimes within a few inches, so moving your head (or measurement mic) slightly may cause or eliminate peaks/dips. So it's very had to equalize them out reliably. Plus, if you're using spatial averaging, a peak to your left ear and a dip to your right ear may actuall measure FLAT. This may be why I think there's only so much you can do with EQ, even if you're using a convolver. (I'm using DRC+convolver with good results).


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## smellygas (Feb 21, 2008)

vecc205 said:


> This is from Linkwitz's website about an equalizer for a headphone. I too have made binaural recordings and when I play them back on headphones that are equalized to sound flat to my ear they sound the most realistic. This is why I am confused when I am told that you shouldn't tune flat to your ear. I currently tune so that if I were to play a sine sweep it would "sound" constant in volume to my ear, no dips where it sounds like it fades out for a little and no peaks where it starts to get louder. Am I missing something here?


Honestly, if it sounds good to you, just do it. Tuning by ear for equal loudness, assuming you do this accurately, will yield a measured RTA response that follows an equal-loudness curve (an example would be the Fletcher-Munson...there are many of them). This is similar to what Whitledge did in his Magic Bus, although he didn't do this by ear. There was a long thread over on DIYaudio, and nobody agreed that an EQ curve that followed an equal-loudness curve was desirable. It also is at odds with the findings of Toole and Olive in published JAES articles. I skimmed the relevant article on Linkwitz's site, and I'm not convinced he advises people to eq to flat by ear, he just says that you can hear large peaks and dips easily and you can EQ them out. The electronic LCR circuit he describes does just that - it can equalize a single peak/dip.



> The equalizer is formed by inserting a passive network between the earphone amplifier output and the connector to the ER-4S. The headphone amplifier's open circuit output voltage is Vs and its output impedance is R1. The earphone impedance is 100 ohm and symbolized by R2 in the schematic below.


I actually used to own a pair of ER-4S. However, I had heard some better and more comfortable 'phones for the same price or less, so I sent them back. At any rate, headphones are a completely different ball game. The actual eardrum-heard "frequency response" curve is way different from the measured microphone freq resp because of manipulation from your external ear (auricle), the ear canal, and vibration through your head. Your actual ear-drum heard response is also different if you have a free-field source vs. a diffuse-field source, which tends to radiate sound from all directions. An external (non-canal) headphone radiates sound onto your auricle and canal similar to how a distant diffuse-field would, and so most good headphones use a diffuse-field equalization. Using canal phones is a different story. Basically, you would want to the frequency response in the ear canal that is generated by a flat distant source, and reproduce it with the ER4's (canal phones). Because people have different ear canal anatomies, there may be differences among individuals, and this would only reflect an "average" equalization that might be universally appealing. 

The short version of the above is that headphone equalization and listening is a completely different entity from listening to car loudspeakers. A lot of it isn't very generalizable back and forth. The only thing I think is worth looking at more ist hat some people argue that the vehicle listening environment replicates a "diffuse-field" environment, which implies that a free-field to diffuse-field equalization should be applied. I've tried this and it sounds okay (improvement) but my convolver made everything sound even better, and I'm too lazy to experiment more. There is an ISO curve that equalizes free-field to diffuse field. This may be the true target curve (applied on top of the downsloping curve that starts at 100-400Hz or so) in a car. I bet there are some car companies or car audio manufacturers that have discovered this in their labs, but aren't sharing it...it hasn't been published in any journal that I've found and it's my own theory, so take it for what it is. 

SG


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## aztec1 (Jun 13, 2008)

Thank you for all of these great replies. Each of them is worth probably months of research and experimentation. It's going to take me quite awhile to digest it all. Admittedly, it seems I'm in a bit over my head at the moment and much more learning is required.


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## aztec1 (Jun 13, 2008)

pwnt by pat said:


> The 200hz dip is probably to blame on location - something you cannot fix with the drivers in their current location. Since the flaw in apparent in both doors when played independently, I would look at door skin resonances or reflections off of the lower dash as a culprit.
> 
> Also try taking a left measurement by your left ear when you are sitting in the truck, and likewise with the right side. Measuring FR without taking diffraction caused by your body in to account is silly.


I will definitely do this. At the minimum, now I can compare what the response looks like with me sitting in the cabin versus the graphs I already have. Do you think it's worth trying a nearfield measurement with the door open? Maybe that will help rule out some interior reflections?


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## aztec1 (Jun 13, 2008)

candaddy said:


> Just FYI, I've found it about 1000 x easier to make comparisons by clicking off the bar graph button at the top so you get a trace line instead. That way dips on the background traces are viewable.
> 
> 10db is perceived as half or 2x as loud, so anything that major should be really noticeable. When I EQ for left and right, a 2db change is audible, a 4db difference is pretty big and a 6 or 8db difference left/right is major. I know your question was more focused to the over-all response curve, but that might serve as some reference to you.
> 
> ...


This is good info, I'm going to try tackling the calibration within the next few days. I was under the impression that the USB setup didn't require it, but it can't hurt to try. I don't know of a way to cal the mic at this time. It would be great if that's what's causing all of this though 

Regarding what's been bolded in your response above, I have to admit that I don't have a very trained ear. I've been listening to music my whole life and like it to sound as good as possible, but this is the first time I've used a tool to evaluate anything. I feel like I'm chasing my tail because of the learning curve associated with this, and I think my struggle is finding the comparison between what I like and what it's supposed to _sound _like.


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## aztec1 (Jun 13, 2008)

smellygas said:


> This isn't a stupid question. In fact, I've had the same question for at least 5 years, and there's a lot of misinformation out there, and I learned a lot of things the hard way. I ended up reading about 10 key articles published in JAES and everything kind of fell into place. A lot of the following will sound like voodoo, and I'm sure the people who repeat the same misinformation will chime in soon, but here goes:
> 
> a) *you absolutely must take several measurements in slightly different locations and average them together to get a valid measurement. This is called spatial averaging.* Toole in an older JAES article found that taking one random time-averaged (not spatial) RTA measurement gave you a measurement accuracy window of about +/- 8dB or so...which basically is almost useless. You'll find that just moving your mic a few inches changes your RTA curve completely. He found that you required a minimum of 6 measurements in 6 strategic locations to yield a precision of +/- 0.5dB.
> b) *A flat RTA with a pink noise signal is NOT desirable. It will sound overly bright. * Ignore the IASCA metric. Everybody who has done RTA work will say "flat sounds terrible." The reason, very briefly, is that you're measuring the power response, which includes direct and reflected sound. A flat power response sounds too bright. What you want is a flat ANECHOIC response. You can't measure anechoic response without sophisticated software. TrueRTA doesn't do it. So the next best thing is to model the power response curve (what you measure) after what a flat anechoic response speaker would sound like. This has been studied. It is a downsloping line from about 100-400Hz to 10-20kHz at a rate of 1db/octave. That's your target. Some people have also demonstrated that a power reponse that starts at around 2kHz or so and downslopes about 1-2dB/oct towards 20kHz is also desirable, but the study that supported it was not authoritative.
> ...


I need to go out to the truck with a printout of this. Tons of info, I'll need to read it a couple more hundred times to let it sink in.

I think I'll start with your point c) in bold, it seems reasonable to determine if this is the case before pulling what little hair I have left out trying to tune, when I need to move the speakers instead.


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## smellygas (Feb 21, 2008)

aztec1 said:


> This is good info, I'm going to try tackling the calibration within the next few days. I was under the impression that the USB setup didn't require it, but it can't hurt to try. I don't know of a way to cal the mic at this time. It would be great if that's what's causing all of this though


There are two things you can calibrate: the sound card (or USB interface for you) and the microphone. You generally calibrate the sound card by using a loopback device (lineout to line in). There is very little benefit though, since most sound cards are already very flat. The more important thing to calibrate is your microphone. The ECM8000 is very accurate, perhaps +/- 1 dB from 20Hz all the way up to about 1-2kHz or so, but it can vary by as much as 6dB or so once you go higher than that, and even worse when you measure off-axis. Calibrating the mic involves sending it in to a professional, and having them measure the mic, and send you a file with the actual response of the mic. This file can be plugged into TrueRTA. It generally costs upwards of $50 for this service.



> Regarding what's been bolded in your response above, I have to admit that I don't have a very trained ear. I've been listening to music my whole life and like it to sound as good as possible, but this is the first time I've used a tool to evaluate anything. I feel like I'm chasing my tail because of the learning curve associated with this, and I think my struggle is finding the comparison between what I like and what it's supposed to _sound _like.


Here's an easy way to use the RTA to improve your system. It will be far from perfect, but it should at least help.
1) Use the ECM8000 mic calibration file that is included with TrueRTA. Load it.
2) Take at the very minimum 4 averaged readings at your headrest location, spaced out over the width of about a foot or so. Save each one to a memory location in TrueRTA. Then use the "average" function and average the 4 memory tracings.
3) Do this for each channel (left/right) individually, with the other one muted, provided you can make EQ adjustments to each channel separately.
3) Make EQ adjustments off the averaged responses
4) Adjust the LEVELSS of your individual speakers so that you have a roughly downsloping RTA response. If you have a passive crossover setup, then you can increase the tweeter attenuation if you have this option.
5)Since you have a 1/3-octave equalizer, you're better off looking at the 1/3-octave smoothed RTA plot. Pick an arbitrary target straight line from ~300Hz or so down to 10kHz at 1db/octave or so. 
6) Decide how much deviation in dB there is between the actual response and the target line for each band on your EQ
7) Set your eq accordingly to "counteract" each deviation.
8) Do the average of 4 measurements again, and compare to the pre-EQ plot. Make sure you seen an improvement. 

This is a good way to start, and it doesn't really require more than a basic understanding of RTA's.

SG


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## smellygas (Feb 21, 2008)

aztec1 said:


> I think I'll start with your point c) in bold, it seems reasonable to determine if this is the case before pulling what little hair I have left out trying to tune, when I need to move the speakers instead.


Not all peaks/valleys are non-minimum-phase, but it's dangerous to try to correct the non-minimum-phase valleys because this can overdrive your speaker without actually improving frequency response. Also, in a car, it isn't really practical to move the speaker locations, especially since the new location might also have issues as well. Good luck.

SG


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## aztec1 (Jun 13, 2008)

npdang said:


> Very good response from SmellyGas. Should probably be stickied somewhere. In short, RTA is a tool of limited usefulness for the purpose you're describing.
> 
> In addition I'd like to add the following:
> 
> ...


This clarifies a lot of things for me, especially the equal loudness curves. You're absolutely right that I was thinking in term of "to my ear". 

I'm not too sure if I fully understand what frequency adaptive windowing is, but could I just manually change the frequency scale in TrueRTA and take the response data in say 500hz chunks, then use something else to graph the exported data? Or would that skew the curve too much?

It sounds like a big problem of mine is confusing speakers and ears when it comes to tuning. I can only adjust the speakers, but I don't have an RTA for my ears for a comparison


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## michaelsil1 (May 24, 2007)

smellygas said:


> Calibrating the mic involves sending it in to a professional, and having them measure the mic, and send you a file with the actual response of the mic. This file can be plugged into TrueRTA. It generally costs upwards of $50 for this service.



Do you know anyone in Los Angeles that can calibrate my Mic?


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## aztec1 (Jun 13, 2008)

smellygas said:


> There are two things you can calibrate: the sound card (or USB interface for you) and the microphone. You generally calibrate the sound card by using a loopback device (lineout to line in). There is very little benefit though, since most sound cards are already very flat. The more important thing to calibrate is your microphone. The ECM8000 is very accurate, perhaps +/- 1 dB from 20Hz all the way up to about 1-2kHz or so, but it can vary by as much as 6dB or so once you go higher than that, and even worse when you measure off-axis. Calibrating the mic involves sending it in to a professional, and having them measure the mic, and send you a file with the actual response of the mic. This file can be plugged into TrueRTA. It generally costs upwards of $50 for this service.
> 
> Here's an easy way to use the RTA to improve your system. It will be far from perfect, but it should at least help.
> 1) Use the ECM8000 mic calibration file that is included with TrueRTA. Load it.
> ...





smellygas said:


> Not all peaks/valleys are non-minimum-phase, but it's dangerous to try to correct the non-minimum-phase valleys because this can overdrive your speaker without actually improving frequency response. Also, in a car, it isn't really practical to move the speaker locations, especially since the new location might also have issues as well. Good luck.
> 
> SG


Thanks a lot for writing the step by step, I will do it sometime tonight. I didn't think of swapping a current set of problems for a potential set of completely different ones by changing speaker locations! I'm glad I haven't torn my door apart yet


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## michaelsil1 (May 24, 2007)

I'm wondering if getting an overly bright system after measuring is because the Mic compensation values were not entered?


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## dkm201 (Nov 29, 2007)

Thanks to SG for such an in-depth, thoughtful and useful explanation...


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## smellygas (Feb 21, 2008)

michaelsil1 said:


> Do you know anyone in Los Angeles that can calibrate my Mic?


Most of the services I've seen are mail-in. The two most commonly mentioned services are provided by Herb Singleton and Kim Girardin. You can google them. I personally went through Herb and I highly recommend him. He is also the official "ECM8000 calibrator" over at hometheatershack.com too. 



michaelsil1 said:


> I'm wondering if getting an overly bright system after measuring is because the Mic compensation values were not entered?


Possibly. You can't tell because there is so much variability in ECM8000's that you don't really know if there was a big peak or a dip or both in the last 1-2 octaves. This is why I highly recommend getting the mic calibrated if you're serious about using the RTA to fine tune your system. Without a calibrated mic, you could still use it to EQ out any peaks and valleys you see to get a "smooth" response, which we know is an independent predictor of perceived sound quality, based on Olive's big study. You can then adjust your tweeter level by ear to get the most pleasing balance, which will get you closer to your target downsloping curve without knowing exactly the response in the last 1-2 octaves, where the ECM8000 has the most variability between units.

SG


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## michaelsil1 (May 24, 2007)

smellygas said:


> Possibly. You can't tell because there is so much variability in ECM8000's that you don't really know if there was a big peak or a dip or both in the last 1-2 octaves. This is why I highly recommend getting the mic calibrated if you're serious about using the RTA to fine tune your system.
> 
> SG


I think getting my Behringer ECM8000 Calibrated is a great idea since our ears are most sensitive in the 1KHz-4KHz range.


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## M-Dub (Nov 29, 2006)

After reading this I think I'm going to tune my new system with sine waves and my ears, to get equal loudness to my ears. I have the Behringer ECM8000 and the 1/24 version of TruRTA combo, and pink noise on my last car, and got some strange results. First of all "flat" sounds too bright. 
Second, I know I had a huge dip between 50 Hz and 80Hz (cause I had crossovers set that way) but TruRTA showed flat. 
I'll still prolly check out the TruRTA curves just for funsies, but I'm just gona trust my ears on this one.


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## michaelsil1 (May 24, 2007)

Megalo said:


> After reading this I think I'm going to tune my new system with sine waves and my ears, to get equal loudness to my ears. I have the Behringer ECM8000 and the 1/24 version of TruRTA combo, and pink noise on my last car, and got some strange results. First of all "flat" sounds too bright.
> Second, I know I had a huge dip between 50 Hz and 80Hz (cause I had crossovers set that way) but TruRTA showed flat.
> I'll still prolly check out the TruRTA curves just for funsies, but I'm just gona trust my ears on this one.


Measuring the lower frequencies requires a higher sensitivity setting. Try going from 20db to 5db for the first line.


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## smellygas (Feb 21, 2008)

Megalo said:


> After reading this I think I'm going to tune my new system with sine waves and my ears, to get equal loudness to my ears. I have the Behringer ECM8000 and the 1/24 version of TruRTA combo, and pink noise on my last car, and got some strange results. First of all "flat" sounds too bright.


As I predicted 19 hours ago, *"I'm sure this posted will be followed up by someone saying "yo, RTA is a good starting place, but i just trust my ears to tune," which will be followed by "yo, I use the RTA, but flat sounds terrible," which will be followed with "yo, RTA is crap." *

If you're still wondering why flat sounds too bright and why equal-loudness to sine waves is the wrong target curve, you obviously didn't read/understand anything I wrote. Good luck with your sine wave tuning!!!

SG


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## M-Dub (Nov 29, 2006)

michaelsil1 said:


> Measuring the lower frequencies requires a higher sensitivity setting. Try going from 20db to 5db for the first line.


I tried that, but I think maybe cabin gain "filled in" the 50Hz - 80Hz. But ya know I would just crank the bass settings on the fly while driving anyways (based on the song). I guess I just can't get rid of some of my old spl ways  I guess I'm not really concerned about flat & I'm not competing. This car audio thing is just fun and just for me.


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## M-Dub (Nov 29, 2006)

smellygas said:


> As I predicted 19 hours ago, *"I'm sure this posted will be followed up by someone saying "yo, RTA is a good starting place, but i just trust my ears to tune," which will be followed by "yo, I use the RTA, but flat sounds terrible," which will be followed with "yo, RTA is crap." *
> 
> If you're still wondering why flat sounds too bright and why equal-loudness to sine waves is the wrong target curve, you obviously didn't read/understand anything I wrote. Good luck with your sine wave tuning!!!
> 
> SG


I totaly read your why "flat sounds too bright" that's why I mentioned that. I thank you for your insite on why "flat sounds too bright". That is exactly why I am inspired to tune by "sound" (to my ears). By the way... I love the tool for helping flattening out 80 Hz - 4 kHz. I'll tune my own tweeters & subs thank you.


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## vecc205 (Nov 18, 2007)

smellygas said:


> Honestly, if it sounds good to you, just do it. Tuning by ear for equal loudness, assuming you do this accurately, will yield a measured RTA response that follows an equal-loudness curve (an example would be the Fletcher-Munson...there are many of them). This is similar to what Whitledge did in his Magic Bus, although he didn't do this by ear. There was a long thread over on DIYaudio, and nobody agreed that an EQ curve that followed an equal-loudness curve was desirable. It also is at odds with the findings of Toole and Olive in published JAES articles. I skimmed the relevant article on Linkwitz's site, and I'm not convinced he advises people to eq to flat by ear, he just says that you can hear large peaks and dips easily and you can EQ them out. The electronic LCR circuit he describes does just that - it can equalize a single peak/dip.
> 
> 
> 
> ...


Do you believe diffuse-field equalized headphones are accurate in the frequency response?


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## 14642 (May 19, 2008)

Smellygas,
Thanks. Your answers are absolutely correct--because your research was top-notch. Sean's (Olive) test are designed to help correlate objective measurements with preference--and after years and years he does have a frequency response curve and a directivity index that people do, in fact, prefer. Listening to speakers in a room and in a car are two completely different things and Sean's group at Harman is now begining to conduct similar tests in cars. Look for some upcoming papers from AES.

BTW, it appears thus far, that listeners prefer the second of the high-frequency contours you described--gradual attenuation above about 2kHz, rather than the longer slope from a lower frequency.


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## benthe8track (Nov 23, 2008)

smellygas said:


> Most of the services I've seen are mail-in. The two most commonly mentioned services are provided by Herb Singleton and Kim Girardin. You can google them. I personally went through Herb and I highly recommend him. He is also the official "ECM8000 calibrator" over at hometheatershack.com too.
> 
> SG


Do you know if either of those guys provide the phase calibration as well? I calibrated my ECM8000 off of one of our B&K calibrated mics in the Anechoic chamber at school but I can't find a reasonable way to work out the phase adjustments.


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## smellygas (Feb 21, 2008)

vecc205 said:


> Do you believe diffuse-field equalized headphones are accurate in the frequency response?


This is kind of off topic, but the answer is yes, and it's been studied. Listeners tend to favor headphones that have diffuse-field equalization over pressure-field eq over flat-at-the-canal. It makes sense. A perfectly flat loudspeaker will have various frequencies amplified or attenuated by the interaction with the external ear, with resonances within the ear canal, and vibration of the head/bone/etc before it hits the eardrum, which determines the vast majority of what is heard/perceived. So the frequency vs. amplitude curve at the eardrum won't be flat like when measured by a microphone sitting right next to you. Over-the-ear headphones emit sound that hits the external ear from all sorts of incident angles. If you didn't have headphones, the rough equivalent of that would be sitting in a diffuse field, in which sound is coming from all directions at the same time. So, the equalization that would apply if you were sitting in a diffuse field should also apply with over-the-ear headphones. This has been published in JAES, and I makes sense to me. The only criticism is that the correction curve for pressure to diffuse field eq is derived empirically - i.e. from testing a large group of people and average the curve...which means some people may not like the way a pair of headphones are eq'd by the manufacturer. This is because people have different external ear shapes and canal lengths/diameters. 

SG


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## vecc205 (Nov 18, 2007)

Andy Wehmeyer said:


> Diffuse field equalization is designed to sound the same (or similar) for many different listeners, as I understand it. That means that the final "curve" can be right or wrong, but wither way, it'll sound the same for everyone.
> 
> Smellygas,
> Thanks. Your answers are absolutely correct--because your research was top-notch. Sean's (Olive) test are designed to help correlate objective measurements with preference--and after years and years he does have a frequency response curve and a directivity index that people do, in fact, prefer. Listening to speakers in a room and in a car are two completely different things and Sean's group at Harman is now begining to conduct similar tests in cars. Look for some upcoming papers from AES.
> ...


Andy, I mention diffuse-field equalized headphones because I own a pair and they give a response that is flat to my ear. But then I see some saying don't tune flat to the ear it's not natural. But if this is the case then all diffuse field equalized headphones are not natural? I'm confused.


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## candaddy (May 21, 2008)

aztec1 said:


> This is good info, I'm going to try tackling the calibration within the next few days. I was under the impression that the USB setup didn't require it, but it can't hurt to try. I don't know of a way to cal the mic at this time. It would be great if that's what's causing all of this though
> 
> Regarding what's been bolded in your response above, I have to admit that I don't have a very trained ear. I've been listening to music my whole life and like it to sound as good as possible, but this is the first time I've used a tool to evaluate anything. I feel like I'm chasing my tail because of the learning curve associated with this, and I think my struggle is finding the comparison between what I like and what it's supposed to _sound _like.


I don't know how it is possible to calibrate the line-in/out with the USB pre-amp. I don't use the generator in the program, I play pink noise on a CD and hope that the USB thing is accurate (which it might not be?). I'd like to find if someone has or could calibrate the USB preamp and I'm definitely going to get my mic calibrated now after reading through this thread.

The calibration I'm talking about is the db calibration. Go to the Audio I/O button at the top and scroll down to the "SPL calibration" and click it. Once you've done that it will have some directions on how to calibrate the actual SPL level on the screen. You must have a known db level sound playing at the tip of the mic to do this. I don't have that, so I haven't done the calibration. I was just suggesting earlier that what you see on the graph might be wrong because of the lack of having calibrated this function of the program.

My ears are not terribly trained either, but I know what sounds like crap and with the help of some reference CD's (like the Autosound set) I have trained myself to recognize some problems pretty readily. Actually, just from screwing up things in the past I've learned quite a bit! LOL

I'm loving this thread! Tons of good info.


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## npdang (Jul 29, 2005)

smellygas said:


> This is a highly desirable technique when used in listening rooms, where you have early reflections and late reflections/reverberation. Only the first 30ms or so is "registered" as the primary sound, based on the Haas effect. I believe MLSSA uses this technique. However, in one JAES paper, they showed that in the CAR (not a room), 90% of the sound is actually received within the first 10ms, because the sound that reaches the listener (in the driver's seat) is direct + reflected only a short distance. (remember, sound traves approx 1ft in a millisecond). The volume of the reflected sound is often just as loud as that of the direct sound, in fact. I have not been able to find a method to use time gating to approximate an anechoic response in the car.
> 
> 
> 
> ...


That's very true and something I didn't consider when typing my response. I've noticed as well when taking in car IR measurements that there are many significant peaks in the response all occurring well within 10ms of each other. I'm assuming then, that tuning toward a flat anechoic profile is simply impossible given measurement limitations and that utilizing an ideal power response target curve is the only way to go?

WRT to 1/20th octave measurement resolution is there really a point though in car audio? You couldn't correct for it anyways with traditional 1/3rd octave eq's or limited band parametric ones that are commonly available. You also begin seeing some rather significant variations. Also, do the results from "Olive" test also apply to car audio? There maybe masking effects or other variables that would interfere.


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## sqshoestring (Jun 19, 2007)

smellygas said:


> The only criticism is that the correction curve for pressure to diffuse field eq is derived empirically - i.e. from testing a large group of people and average the curve...which means some people may not like the way a pair of headphones are eq'd by the manufacturer. This is because people have different external ear shapes and canal lengths/diameters.
> 
> SG


Am I off base here or would these people who are not happy be the ones who just realized they have been hearing things incorrectly from birth because their ear is shaped the wrong way?


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## vecc205 (Nov 18, 2007)

sqshoestring said:


> Am I off base here or would these people who are not happy be the ones who just realized they have been hearing things incorrectly from birth because their ear is shaped the wrong way?


What if their ear is shaped differently then the engineer who mixed the tracks? Or even better, what if the engineer had weird shaped ears? :laugh:


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## ErinH (Feb 14, 2007)

Now THIS is a nice thread!

I'll have to re-read SG and NP's posts again to fully get it, but I really like the suggestions here. I've used the RTA multiple times but never do spatial averaging, though I've seen it discussed quite a bit. I need to get my but into gear.

When you guys do the longer windows, how are you doing this? MLS based at a higher resolution or pink noise or ???

Thanks, fellas.


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## sqshoestring (Jun 19, 2007)

vecc205 said:


> What if their ear is shaped differently then the engineer who mixed the tracks? Or even better, what if the engineer had weird shaped ears? :laugh:


My point is how would they know since they have always heard everything in their lives the same way. I mean ten people might see the color red differently, but if they were all taught that was red then it is red. Sure an engineer might suck, but (IMO) it is their job to make the song sound good to everyone not what they think is good. On the other hand elected officials are supposed to serve the people and they don't do that either.


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## aztec1 (Jun 13, 2008)

I still don't fully understand this. I don't even know if I'm asking the right question here :blush:

I believe the Quicksweep in TrueRTA is 1.5ms from 20hz-20khz, is that fast enough to limit high frequency reflections in the curve? Do I need to make longer frequency-limited sweeps for the lower octaves ie 20hz-500hz over a full second to allow for reflections?


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## smellygas (Feb 21, 2008)

npdang said:


> WRT to 1/20th octave measurement resolution is there really a point though in car audio? You couldn't correct for it anyways with traditional 1/3rd octave eq's or limited band parametric ones that are commonly available.


Exactly. And it would also be very tedious to correct response variations at higher resolutions. Digital room correction with a DSP and a sophisticated algorithm might be an option.



> Also, do the results from "Olive" test also apply to car audio? There maybe masking effects or other variables that would interfere.


This is another excellent point. Olive's study (and Toole's work as well) only demonstrates that listeners prefer loudspeakers that have smooth responses overall and at high resolution. It does NOT necessarily imply that CORRECTING the roughness in a response actually IMPROVES the perceived sound quality, although honestly I think that most people would assume that it would. These studies were also done in a listening room with distant loudspeakers vs. the diffuse-field and highly reflective environment in the car. The problem is, there is no other well-conducted study on objective measurements and perceived speaker quality that I am aware of that can be more generalizable to the car. So you have to decide how different the car environment is and how the results may be skewed, if at all.

SG


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## aztec1 (Jun 13, 2008)

smellygas said:


> This is another excellent point. Olive's study (and Toole's work as well) only demonstrates that listeners prefer loudspeakers that have smooth responses overall and at high resolution. It does NOT necessarily imply that CORRECTING the roughness in a response actually IMPROVES the perceived sound quality, although honestly I think that most people would assume that it would. These studies were also done in a listening room with distant loudspeakers vs. the diffuse-field and highly reflective environment in the car. The problem is, there is no other well-conducted study on objective measurements and perceived speaker quality that I am aware of that can be more generalizable to the car. So you have to decide how different the car environment is and how the results may be skewed, if at all.
> 
> SG


Very interesting! Can I take this to mean that I should get the 1/3 or 1/6 curve to look and sound right before even touching the higher resolutions (as they may be unnecessary in a car)?


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## smellygas (Feb 21, 2008)

Autiophile said:


> x2 on a great thread.
> 
> For higher frequencies where I can get anechoic data (or something close) by sufficiently limiting the measurement window to eliminate extraneous reflections (which are plainly visible in the impulse response) I take several independent measurements with the mic in different positions then average the curves. I use fuzzmeasure's sine sweep for these, my window is often in the 7-10 ms range.


That's pretty interesting! I might have to try that when my new mic preamp arrives. Are the results with 10ms windows significantly different from longer windows?

SG


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## smellygas (Feb 21, 2008)

bikinpunk said:


> Now THIS is a nice thread!
> I've used the RTA multiple times but never do spatial averaging, though I've seen it discussed quite a bit. I need to get my but into gear.


According to the older article by Geddes, they actually needed 18 measurements placed in various positions of the region in 3-d space where the driver's would be like 90% of the time - and this was go achieve the +/- 0.dB accuracy. However, if 6 measurements were taken in an exact configuration of 3 pairs of 2, separated by at least the width of a head, and moving up a ladder (it's hard to describe), you could you get the same precision in your measurements. But the variability in response curves was so much when taking a single measurement, I think its almost useless for fine eq work. [/QUOTE]



sqshoestring said:


> My point is how would they know since they have always heard everything in their lives the same way. I mean ten people might see the color red differently, but if they were all taught that was red then it is red.
> 
> 
> > This is exactly right. The actual frequency response at the eardrum that occurs when a person is exposed to a flat response sound source will vary from individual to individual....but each individual will still "perceive" the sound as flat.
> ...


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## ErinH (Feb 14, 2007)

smellygas said:


> According to the older article by Geddes, they actually needed 18 measurements placed in various positions of the region in 3-d space where the driver's would be like 90% of the time - and this was go achieve the +/- 0.dB accuracy. However, if 6 measurements were taken in an exact configuration of 3 pairs of 2, separated by at least the width of a head, and moving up a ladder (it's hard to describe), you could you get the same precision in your measurements. But the variability in response curves was so much when taking a single measurement, I think its almost useless for fine eq work.
> 
> SG


So, 6 measurements is considered ‘okay’?

In 3 pairs of 2.

Are these all centered at the listening position in the X-axis and then move along the Y-axis (vertically), the width of the head in each step? 

Do you have something that describes this or illustrates it?


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## smellygas (Feb 21, 2008)

Andy Wehmeyer said:


> Smellygas,
> Thanks. Your answers are absolutely correct--because your research was top-notch. Sean's (Olive) test are designed to help correlate objective measurements with preference--and after years and years he does have a frequency response curve and a directivity index that people do, in fact, prefer. Listening to speakers in a room and in a car are two completely different things and Sean's group at Harman is now begining to conduct similar tests in cars. Look for some upcoming papers from AES.
> 
> BTW, it appears thus far, that listeners prefer the second of the high-frequency contours you described--gradual attenuation above about 2kHz, rather than the longer slope from a lower frequency.


OH, hallelujah. I was beginning to think audio engineers didn't care about car audio anymore! So 2k is interesting. So does the preferred curve follow the Dolby house curve? Surround Professional. And is Olive going to reveal which loudspeaker models ranked highest in the blinded listening tests? 

SG


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## smellygas (Feb 21, 2008)

bikinpunk said:


> So, 6 measurements is considered ‘okay’?


It depends on how precise and repeatable you want your overall measurement to be. 6 in those strategic positions would give you +/- 0.5dB. For comparison, if you took 4 samples in any old place around where the head usually is, the 95% confidence interval is within +3dB and -4dB...terrible. Which means you may end up with some deviations within that range if you compared different average-of-4 samples that were done at random places.



> Are these all centered at the listening position in the X-axis and then move along the Y-axis (vertically), the width of the head in each step?
> 
> Do you have something that describes this or illustrates it?


Geddes didn't precisely describe the locations, but a pair was 2 mics 15cm apart, and the pair was then together moved forward then vertical (from the perspective of soemone sitting in the driver's seat facing ahead), and another measurement taken, then moved forward then vertical and the last set taken. I don't know the distance it is moved, but it looks like it's about 15cm horizontal and vertical approximately. If you want to look at the article, it's Geddes E and Blind H, "The Localized Sound Power Method," presented at 
AES 76th, 1984 Oct 8-11. 

SG


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## SSSnake (Mar 8, 2007)

MODS!

Can someone sticky this thread. This is perhaps THE most beneficial thread I have seen on this forum for the past 6 months.

Special thanks to smellygas, autiophile, NP, and Andy.


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## aztec1 (Jun 13, 2008)

Autiophile said:


> Plots in blue are of my left front loudspeaker at 1m. Orange are at 4 meters. Each color represents only one measurement. The difference between plots of the same color is only in the window duration.


I can't believe how different these plots are. What causes the additional peaks and valleys with a longer duration? Is it something like sampling rate in a ADC that improves the resolution?


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## blackwolf (Feb 21, 2008)

SSSnake said:


> MODS!
> 
> Can someone sticky this thread. This is perhaps THE most beneficial thread I have seen on this forum for the past 6 months.
> 
> Special thanks to smellygas, autiophile, NP, and Andy.



I second SSSnake's opinion. All of it. Thank's guys.


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## BMWturbo (Apr 11, 2008)

+1 on great thread Gents.

I'm getting motivation back to drag out the CBRTA rig again tonight to do some more testing, this time with a little bit more direction then before 

My aim is to develop a methodology that will work 'for me' and be able to be relliably replicate results when instigated.

Currently frustration often set in after hours spent testing and tuning only to decide that I'm not at all happy with the resultant tune.

I've printed off some more pages from this thread to add to my ever-increasing reference material.


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## aztec1 (Jun 13, 2008)

Autiophile said:


> Part of what you are seeing is the lack of resolution below a given frequency based on the window size. On the right hand side of the plots it shows the window duration, and the frequency above which the measurement is valid.
> 
> You also get into room modes and constructive and destructive reflections as the window get's larger and you allow the reflections hitting the mic to be included in the measurement.


Thank you for clarifying what these show.

I recall something in the TrueRTA manual that describes how the gating is wide enough on the Quicksweep to include reflections with the response. (If this is the equivalent of window duration in your program) I'm assuming the gate is wide enough to allow low frequency reflections to occur and doesn't change scale as frequencies rise. I can't find any information that says at what frequencies the responses become invalid...but if I'm interpreting this correctly, TrueRTA sounds much more suited to a home stereo where it's possible to move speakers to obtain a desired response coupled with _room_ reflections. 

If the duration is long enough, and a sweep is run in such a confined space as a truck cabin, is it conceivable that the response would show multiple reflections and totally muddy anything I'm trying to see, especially with higher frequencies?


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## npdang (Jul 29, 2005)

I'm wondering if taking binaural measurements with a certain kind of target response would be more effective as well. 

There's actually a wide availability of affordable earbud mounted mics nowadays.


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## npdang (Jul 29, 2005)

I'm wondering if taking binaural measurements with a certain kind of target response would be more effective as well. 

There's actually a wide availability of affordable earbud mounted mics nowadays.


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## smellygas (Feb 21, 2008)

npdang said:


> I'm wondering if taking binaural measurements with a certain kind of target response would be more effective as well.
> 
> There's actually a wide availability of affordable earbud mounted mics nowadays.


I have seen at least two JAES articles that specifically used binaural mics in dummy heads to take recordings in a car. The biggest argument is that the left and right ears receive different responses because of the presence of the head in between. That is, the right speakers may project sound to the right ear, but also reflect off the driver-side window glass and yield sounds that are just as loud to the left ear. Ordinarily, without that stupid window glass, the sound to the left ear would be attenuated as there is no direct path from the right speaker to the left ear. In some cases, the right speaker can result in a narrow band frequency spike at the left ear, and a dip to the right ear. The spike can be very audible and annoying and would only be picked up with the binaural headphones...but NOT with a single mic placed without a dummy head. The reason is that the spike and dip would cancel each other out on the single mic reading! 

However, there are a few problems with the binaural mics. 
1) You'll need a way to calibrate them. This is easier said than done, because of below:
2) A huge problem is what constitutes a valid frequency response? It is well known that the head and shoulder have a large influence over the frequency response measured in the binaural mic. This has been established using various dummy head models. This will clearly cause peaks/dips in the FR measured by your binaural mic. Thus a perfectly flat pink noies source would measure flat using a conventional mic, but NOT using a binaural mic because of the head/shoulder intereference. You would basically need to calibrate the mic using YOUR head/shoulder by generating a sound signal with known response and compensating the measured response of the binaural mics for the peaks/dips generated by the head/shoulders. Remember, your ears already perceive a sound generated by a flat source in a normal room as flat, if that makes sense. However, the actual FR next toy our head is not flat, but it still corresponds to your perception of flat. So if you eq to a flat response next to your head, it won't be perceived as flat anymore!
3) You still need to do spatial averaging because of the variation in FR among single measurements taken in a car (per Geddes' work).
4) This technique is at odds with how loudspeaker measurements at home were done in the best studies that correlated measurements with listener preferences. The characteristics of the FR curve that correlated with blinded listener preferences were done taking measurements with a microphone in the ABSENCE of a human body. Thus, a flat/smooth response with a low bass extension and downsloping power response correlates with the spatially averaged mic technique without a dummy head/body (or person). We honestly don't know what specific measurement characteristics sound best when we use an alternate measurement technique, like binaural mics with an actual or dummy head/body.

My take on it is that I try to replicate as much as possible what was done in the Olive study, because it is the best work to date (that i've seen) that relates cold measurements to subjective sound quality. On the other hand, I realize adjustments need to be made for the car environment, and the additional and different information derived from the binaural mics have been proven to be beneficial as well. My approach would be to use the spatially averaged single mic measurements to tune for the Olive characteristics, which is flat/smooth response, low bass ext, downsloping power response, and use the binaural measurements to identify big spikes that don't show up with the single-mic technique and try to eq them out. This approach, in my opinion, is most consistent with the literature. 

Sg


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## aztec1 (Jun 13, 2008)

Autiophile said:


> I'm going to quote Geddes on the subject of LF steady state conditions in a small room:
> diyAudio Forums - Setting up the Nathan 10 - Page 1
> 
> That's from a thread where someone was setting up some Nathan 10s (the same speakers I have) along with multiple subwoofers and Geddes sort of walked him through some measurement tactics. It's an interesting read, but get's terse as it goes on, as most do.
> ...


There's some great info in that thread too, I had no idea such things made a drastic difference in sound. Admittedly I'll have to read this thread as well as that one many more times before it fully sinks in, but I think I'm beginning to have a flicker of understanding. Now if I can just apply it...


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## aztec1 (Jun 13, 2008)

smellygas said:


> ...
> 
> 
> You should definitely try to get a flat 1/3 octave curve (along with an overall response slope that matches your target slope) before bothering with higher resolutions. However, you should double-check your new response in 1/6 and higher resolution to make sure you haven't inadvertendly introduced some big spikes or dips in the process of eq'ing with your coarse 1/3-octave adjustments. It happens.
> ...


I played with this a little bit last night, and found that at 1/3 octave, there was actually a *peak* at 200hz as opposed to a huge dip there at higher resolutions. I didn't spatially average any curves, just put the mic up against my left then right ear with only the right mid playing...there was quite a difference between the curves. With luck, I hope to find the sweet spot in TrueRTA that will allow for a nice spatial average at the given sample rate.


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## aztec1 (Jun 13, 2008)

I'd like to say thank you to everyone that's posted in this thread. I feel like learned more over the past few days than in all my years of installing systems. I appreciate you helping this noob out 

Cheers! :beerchug:


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## vecc205 (Nov 18, 2007)

npdang said:


> I'm wondering if taking binaural measurements with a certain kind of target response would be more effective as well.
> 
> There's actually a wide availability of affordable earbud mounted mics nowadays.


Stereo microphones, USB Microphones, Preamplifiers, Digital Recorders, Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!

I use these for dual measurements. I like it because I can sit in the seat and the mics are placed directly in the ears. I use Winmls and I do a spatial average from the drivers (left speakers) and passengers seat (right speakers) for matching the sides. Then I do a spatial average from the drivers side for the sum. I look at the left mirror and take a dual measurement look straight ahead and take a dual measurement then I look to the right to the passengers side mirror and I take a measurement.

In total this would be 6 measurements (dual left,straight and right). I take these measurements and average them. I used binaural recordings I made to help position the speakers. When I did this I got the same curve everytime repeatedly. I'm sure there are ways of improving this method and I'm hoping that this will help. The curve I got resembles John Whitledge's from his website.


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## vecc205 (Nov 18, 2007)

I will also go through a sine sweep and make sure there are no harsh points (resonances). The sweep should sound pretty consistent in amplitude no points where the sound fades out or starts getting loud or harsh. For me I seem to have good results doing this.


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## vecc205 (Nov 18, 2007)

I encourage people to make binaural recordings and use these as references. You won't need to guess what's accurate or what the artist intended. Your ears or what your hearing becomes the mic. Experiment people


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## aztec1 (Jun 13, 2008)

I've spent the last few hours taking more measurements. I spent a few hours before that re-reading this thread over and over. This is the result of trying to apply what's been learned so far. Assuming I've done this correctly, the results are a real eye opener for me. All of these plots are flat EQ, 1/3 octave. Gotta crawl before walking! 

Orange is the left mid, blue is the right mid. Each plot is average from 6 mic positions, 3 around my head and 3 about 4" in front of it, smoothed 1/3. 









This shows the smoothed average summed response, smoothed 1/3.









For me, this graph really puts the importance of spatial averaging into perspective. The difference in response between measurement positions is ginormous! Of course this shows left and right plots but still!









This process took alot longer than I thought it would, but if I did it correctly I think the results are well worth it. Hopefully I didn't miss something or make a gross error in the procedure. If that's the case, then let the equalizing begin!


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## Ge0 (Jul 23, 2007)

npdang said:


> I'm wondering if taking binaural measurements with a certain kind of target response would be more effective as well.
> 
> There's actually a wide availability of affordable earbud mounted mics nowadays.


Just my humble opinion...

Look at my posts last year under the search term "target curve".

Then, dump the Behringer mic and visit this link:
Stereo microphones, USB Microphones, Preamplifiers, Digital Recorders, Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!
or others like it.

This has worked like a charm for me. Mind you, I have some decent processing power that allows me to be VERY flexible in tuning, but, can re-tune my vehicle dead nuts on in about 2 hours using this setup.

Ge0


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## Ge0 (Jul 23, 2007)

Shame on me for not reading the post through entirely. I just don't have that amount of time on my hands these days.

RTA software can be a God send. How do you pinpoint a problem if you can't see what is really going on? RTA of course. But, many of us mis-use the tool and do not get the most out of it.

1.) Set up your measurement system properly.

2.) Use 1/3rd octave resolution to analyze things and make your rough corrections (left, then right, then both)

3.) Use 1/12th octave resolution or higher to see why you can't get your 1/3rd octave perfectly on target.

4.) Try acoustical treatments / loudspeaker positioning to correct this first. Use your RTA and EQ to compensate only after you have exhausted other means.

i know this is vague, but, all the information you need is already here that is necessary to fill in the blanks.

Good luck.

Ge0


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## michaelsil1 (May 24, 2007)

I noticed that one of the settings is Polynomial Degree in the Microphone Compensation, does anyone know what this is and what it should be set at?


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## Random456 (Sep 1, 2007)

Ge0 said:


> Just my humble opinion...
> Then, dump the Behringer mic and visit this link:
> Stereo microphones, USB Microphones, Preamplifiers, Digital Recorders, Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!
> or others like it. Ge0


With all due respect, your advice to "dump" the ECM8000 is really in conflict with the solid information available in the audio engineering literature. There is a preponderance of evidence that sound quality correlates when there is a flat/smooth response, downsloping power response, and low bass extension using *measurements taken with a conventional single microphone * in a spatialy averaged manner. The Behringer closely replicates this, especially when calibrated. Binaural headphones do NOT. 

On that note, I have a few concerns regarding the binaural headphones linked above.

1) They fit inside your ear. The frequency response picked up by this mic is therefore MODIFIED by the your auricle (your outer ear). Everybody's auricle is shaped differently. Therefore, BY DEFINITION, the measurement taken by this binaural mic will be DIFFERENT depending on who's ear it's in, even when you're measuring the SAME sound! THIS IS PROBLEMATIC TO ME, and it should be to you.

2) The amazingly flat "response curve" published on the linked site should raise skepticism. First of all, there is no indication of polarity. It is commonly known that the frequency response varies depending on the direction the sound strikes the ear. This raises more than one eyebrow. Is this for sound coming from the front of you? The side? All around (diffuse field)?? ...or MAYBE...the marketing guys just made it up

3) No microphone on the market selling for $40/each ($80/pair) has a response curve that FLAT with the quality control to keep them consistently within a close tolerance. Either these guys have found a magical mic element that costs them $5 to make, or... well, you decide.

*What *I* think is happening is that these are diffuse-field equalized when they give flat measurements. That is, when you equalize your sound such that these binaural mics MEASURE flat, then you have actually added diffuse-field equalization. Diffuse-field equalization is required to make everything "sound" subjectively flat (different from equal-loudness curve) when you your sound comes from a diffuse field. Headphone manufacturers commonly claim to have a flat response from 20Hz-20kHz, even though this is after diffuse-field equalization. It "sounds flat," even though it really isn't if you measure it. The fact that at least one person here has done this with good results in a CAR, suggests that diffuse-field equalization may be something desirable. I have seen more than one paper in JAES that claims that the sound field inside the car is a diffuse-field. It would be interesting to compare measurements taken with these binaural mics and with a calibrated conventional mic. *

SG


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## Ge0 (Jul 23, 2007)

Random456 said:


> With all due respect, your advice to "dump" the ECM8000 is really in conflict with the solid information available in the audio engineering literature. There is a preponderance of evidence that sound quality correlates when there is a flat/smooth response, downsloping power response, and low bass extension using *measurements taken with a conventional single microphone * in a spatialy averaged manner. The Behringer closely replicates this, especially when calibrated. Binaural headphones do NOT.
> 
> On that note, I have a few concerns regarding the binaural headphones linked above.
> 
> ...


Sounds like you have read a lot about acoustical theory. You are free to have your own opinion. But, have you ever tuned a car? Have you ever obtained pleasing/life like results fiddling around with a point source mic in a car?

I muddled around with the Behringer mic for two years trying to obtain consistent and accurate results. I could not for the life of me make my system sound natural and well balanced.

Then, I met this guy named David Clark of DLC Design. He showed me how a binaural microphone set summed and averaged over a few locations around your head (i.e. spatial averaging) could help me achieve sonic bliss. Read up on his JAES publications regarding the PTF. Damn near everyone who tunes vehciles as a profession knows of this system or has used it. I couldn't afford the steep price tag of a PTF system so made my own limited functionality version. 

I couldn't find the exact microphones I purchased last night so linked to something similar. But, here are the ones I purchased:
Stereo microphones, USB Microphones, Preamplifiers, Digital Recorders, Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!
Don't rag on them too hard. Sure, the published freq response plot is bogus. It's a plot of what the mic capsule is capable of, not necessarily the end product. They use the same series of Panasonic mic capsule the Behringer does. They were +/-2db from 40 to about 4KHz right out of the box. Approx 4db down to 20Hz and about 6db down at 20KHz. Yes, I did bother to have them calibrated and have an accurate compensation table loaded into my RTA programs. 

I found that actually tuning while wearing the microphones, using long averages, and in the process moving my head about as I would normally would seems to work great. Perhaps this is because I am actually accounting for the affect of my body and how this affects sound at MY ears??? 

Tell me I'm full of **** or wrong. I really don't care. Try it for yourself and then shoot holes in my theory.

Ge0


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## M-Dub (Nov 29, 2006)

Ge0 said:


> Sounds like you have read a lot about acoustical theory. You are free to have your own opinion. But, have you ever tuned a car? Have you ever obtained pleasing/life like results fiddling around with a point source mic in a car?
> 
> I muddled around with the Behringer mic for two years trying to obtain consistent and accurate results. I could not for the life of me make my system sound natural and well balanced.
> 
> ...


This is inpiring and refreshing. I also could not for the life of me make my system sound natural and well balanced with the Behringer. I would use it set a reference point and then adjust by ear. Those in-ear mics look great.


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## Random456 (Sep 1, 2007)

Ge0 said:


> But, have you ever tuned a car? Have you ever obtained pleasing/life like results fiddling around with a point source mic in a car?


No, never. I have a calibrated ECM8000, a computer-based DRC which generates digital filters that correct FR, reflections, and phase in realtime, DSP-based active crossovers, and the flagship Scanspeak drivers...but I don't actually do any tuning with it...it's mostly there for decoration. 



> I muddled around with the Behringer mic for two years trying to obtain consistent and accurate results. I could not for the life of me make my system sound natural and well balanced.


Did you follow the guidelines I wrote on page 1 of this thread?



> Then, I met this guy named David Clark of DLC Design. He showed me how a binaural microphone set summed and averaged over a few locations around your head (i.e. spatial averaging) could help me achieve sonic bliss. Read up on his JAES publications regarding the PTF. Damn near everyone who tunes vehciles as a profession knows of this system or has used it. I couldn't afford the steep price tag of a PTF system so made my own limited functionality version.


I'll take a look at his papers. Thank you. 



> Sure, the published freq response plot is bogus. It's a plot of what the mic capsule is capable of, not necessarily the end product. They use the same series of Panasonic mic capsule the Behringer does. They were +/-2db from 40 to about 4KHz right out of the box. Approx 4db down to 20Hz and about 6db down at 20KHz. Yes, I did bother to have them calibrated and have an accurate compensation table loaded into my RTA programs.


This is helpful information.



> I found that actually tuning while wearing the microphones, using long averages, and in the process moving my head about as I would normally would seems to work great. Perhaps this is because I am actually accounting for the affect of my body and how this affects sound at MY ears???


This reasoning does not make sense, and I have already explained why in a previous post.



> Tell me I'm full of **** or wrong. I really don't care. Try it for yourself and then shoot holes in my theory.
> Ge0


Hey, if it works for you, more power to you. It sounds like in order to make your method completely reproducible, the mics would need to be calibrated. I think people would also be interested how the binaural vs. ECM8000 techniques differed in terms of the RTA responses. If you have an alternate method that produces great results, I honestly think you should describe it! ...but just saying hey, go buy these mics and dump the ECM8000...well that's just not very helpful.

SG


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## Ge0 (Jul 23, 2007)

Random456 said:


> No, never. I have a calibrated ECM8000, a computer-based DRC which generates digital filters that correct FR, reflections, and phase in realtime, DSP-based active crossovers, and the flagship Scanspeak drivers...but I don't actually do any tuning with it...it's mostly there for decoration.


Hmm, I still think my dick is bigger than yours. Zapco based DSP, Computer based DSP, flagship Scan drivers, etc...

Mine are mostly hidden so decoration is not really a consideration .



> Did you follow the guidelines I wrote on page 1 of this thread?


With the exception of limiting my readings to 4 averages, yes. This has been common practice for some of us around here for a while now. Please explain why you limit the amount of averages to such a miniscule amount. At that rate one reading will look totally different than another taken at the same location, over and over again.



> I'll take a look at his papers. Thank you.


You should. I think you'll get a lot out of it. I've known Dave for about 9 years. He is brilliant and definitely a contributor to what we know about car audio today.

When I get some time on my hands next month I'll try out your short average method. I'll use my Behringer MIC then the Binaurals... Then I can compare the results to the corrections Audiolense puts in place. 



> This reasoning does not make sense, and I have already explained why in a previous post.


Are you saying having your body in the way of a direct path from your ears to a loudspeaker will have no affect? Sound is not reflected/absorbed by your body and create its own response anomalies? Recording the sound field close to where your noggin actually intercepts and processes data makes no sense? Hmm...



> Hey, if it works for you, more power to you. It sounds like in order to make your method completely reproducible, the mics would need to be calibrated.


Not true. A non-calibrated mic will give you consistent results. It's response does not change assuming you did not damage it. Its just the resulting response will be wrong unless you compensate for the mics own anomalies.



> I think people would also be interested how the binaural vs. ECM8000 techniques differed in terms of the RTA responses. If you have an alternate method that produces great results, I honestly think you should describe it! ...but just saying hey, go buy these mics and dump the ECM8000...well that's just not very helpful.


I've documented my prefered method throughout quite a few threads on this forum. Just do a search under my user name for more detail. I guess I was under the assumption that many people reading this thread were already familiar with my rantings. Well, my fault, I made an assumption... 

Ge0


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## Ge0 (Jul 23, 2007)

Autiophile said:


> Setting aside for a moment the issues with binaural mics due to head and shoulder interference, there is still a unique issue in many car installs where the listener has a portion of his/her body actually in the path between the driver and the listener. In the case of a low door or kickpanel mounted driver, legs are going to be a major obstruction.
> 
> So, perhaps it would be interesting, to take measurements with a normal mic, but with the lower extremities in place. My car is a two seater or I would go out and measure it with the seat fully reclined and the mic tripod on my chest while laying down so the mic was in the space my head would normally occupy. That would keep my legs in the offending position but still get a response without any issues from shoulders/head.
> 
> .....brainstorming


Now we are getting somewhere...

Ge0


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## Ge0 (Jul 23, 2007)

smellygas said:


> I have seen at least two JAES articles that specifically used binaural mics in dummy heads to take recordings in a car. The biggest argument is that the left and right ears receive different responses because of the presence of the head in between. That is, the right speakers may project sound to the right ear, but also reflect off the driver-side window glass and yield sounds that are just as loud to the left ear. Ordinarily, without that stupid window glass, the sound to the left ear would be attenuated as there is no direct path from the right speaker to the left ear. In some cases, the right speaker can result in a narrow band frequency spike at the left ear, and a dip to the right ear. The spike can be very audible and annoying and would only be picked up with the binaural headphones...but NOT with a single mic placed without a dummy head. The reason is that the spike and dip would cancel each other out on the single mic reading!
> 
> However, there are a few problems with the binaural mics.
> 1) You'll need a way to calibrate them. This is easier said than done, because of below:
> ...


Perhaps I am just not capable of absorbing what you described above... Microphones modeling how I actually hear sound and all, geesh. The audacity.

You know what the really ****ty thing is about listening to my vehicles sound system? I need to do so with my pesky head and shoulders in the way . Why not make measurements taking this into account?

I'm not trying to bust anyones balls here. Just trying to uncover something useful whether it contradicts my beliefs and opinions or supports them.

All is good.

Ge0


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## Random456 (Sep 1, 2007)

> With the exception of limiting my readings to 4 averages, yes. This has been common practice for some of us around here for a while now. Please explain why you limit the amount of averages to such a miniscule amount. At that rate one reading will look totally different than another taken at the same location, over and over again.


HUH? I specifically said *6* measurements at specific locations was the MINIMUM required to achieve +/- 0.5dB, based on Geddes' work, and I even provided the JAES citation.



> When I get some time on my hands next month I'll try out your short average method. I'll use my Behringer MIC then the Binaurals... Then I can compare the results to the corrections Audiolense puts in place.


I think people here would be interested in the results, especially the curves before Audiolense processing. Are you suggesting that using Audiolense with the binaurals instead of the Behringer provided more pleasing results? 



> Please either state this again or point me to where in this thread you explained this. I appologize but I don't have time to scour the whole thing.


Honestly, if you don't have time to scroll up, I don't have time to engage in a discussion with you. I have already provided several verbose criticisms of the binaural technique, and you have only addressed 1 or 2. 



> Not true. A non-calibrated mic will give you consistent results. It's response does not change assuming you did not damage it. Its just the resulting response will be wrong unless you compensate for the mics own anomalies.


No. When I say "reproducible" results, I am referring to the ability of other people to copy your methods and obtain similar quality measurements and sound tuning. This requires people to have their binaural mics calibrated, just like you did. 



> I've documented my prefered method throughout quite a few threads on this forum. Just do a search under my user name for more detail. I guess I was under the assumption that many people reading this thread were already familiar with my rantings. Well, my fault, I made an assumption...
> Ge0


Listen, I don't know you, and I'm honestly not very impressed. Basically, all I'm hearing from you is you got better results using binaural mics in your car and your setup, therefore everyone else should throw away their ECM8000's. You quote work by Clark, yet you admit that you don't use the same caliber of equipment that he used. You don't have time to scroll up and address the specific objections I have to your binaural mics, you post an extremely vague methodology in a previous post here, and you assume that you are some very well known person and everyone else here has read your posts and should be familiar with it. Are you for real?

SG


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## michaelsil1 (May 24, 2007)

I'm not going to ditch my Behringer ECM8000, but I'm in the process of getting it Calibrated.

I believe a lot of the tuning mistakes I've made is because of operator error. There isn't a lot of information on how to set up and read the results of a PC Based Measurement System. There's a lot to be learned and I'm glad that we're starting to get some useful tuning information.


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## vecc205 (Nov 18, 2007)

I'm going to have to agree with ge0 here. Using the binaural mic setup I'm able to get great results in short time. I don't bother using an rta mic either. I never felt happy with the results. I've had consistent results using the binaural setup. I consistently get a balanced non fatiguing sound after the tune. I do go in and tweak by ear after this but before I even start the tonality should be great. I want others to experiment with a binaural setup. I know at least for me I get better results then with using an rta mic. Just my humble opinion of course.


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## Random456 (Sep 1, 2007)

vecc205 said:


> I'm going to have to agree with ge0 here. Using the binaural mic setup I'm able to get great results in short time. I don't bother using an rta mic either. I never felt happy with the results. I've had consistent results using the binaural setup. I consistently get a balanced non fatiguing sound after the tune. I do go in and tweak by ear after this but before I even start the tonality should be great. I want others to experiment with a binaural setup. I know at least for me I get better results then with using an rta mic. Just my humble opinion of course.


If you don't mind me asking, what is your tuning method? (i.e. 1/3-octave equalizer, parametric equalizer, etc.)

SG


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## Ge0 (Jul 23, 2007)

I had some big and drawn out response drafted but then the computer timed out and I lost it all. Oh well... This would not help myself or others reading this anyway.



> No. When I say "reproducible" results, I am referring to the ability of other people to copy your methods and obtain similar quality measurements and sound tuning. This requires people to have their binaural mics calibrated, just like you did.


Stating the obvious here.



> Listen, I don't know you, and I'm honestly not very impressed. Basically, all I'm hearing from you is you got better results using binaural mics in your car and your setup, therefore everyone else should throw away their ECM8000's. You quote work by Clark, yet you admit that you don't use the same caliber of equipment that he used. You don't have time to scroll up and address the specific objections I have to your binaural mics, you post an extremely vague methodology in a previous post here, and you assume that you are some very well known person and everyone else here has read your posts and should be familiar with it. Are you for real?


You bet your ass I am for real sizzle chest. You can make all the personal attacks you want. Feel like a big man with a superior intellect over others on the internet. If it gives you a boner good for you. I could give a flying fug. This isn't doing this thread any good. Why don't you re-direct your efforts to something useful. Rather than spout off a bunch of information you read (within context or not), comment on your own personal experience. I tried to do this but you have ingored it in favor of discrediting me. 

Ge0


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## Ge0 (Jul 23, 2007)

michaelsil1 said:


> I believe a lot of the tuning mistakes I've made is because of operator error. There isn't a lot of information on how to set up and read the results of a PC Based Measurement System. There's a lot to be learned and I'm glad that we're starting to get some useful tuning information.


The sad thing is that even the "experts" vaguely understand the topic. It is extremely complex. So, don't discredit yourself for "user error". Keep trying. You'll figure it out to your satisfaction before to long. I did...

Ge0


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## Random456 (Sep 1, 2007)

Ge0 said:


> You bet your ass I am for real sizzle chest. You can make all the personal attacks you want. Feel like a big man with a superior intellect over others on the internet. If it gives you a boner good for you. I could give a flying fug. This isn't doing this thread any good. Why don't you re-direct your efforts to something useful. Rather than spout off a bunch of information you read (within context or not), comment on your own personal experience. I tried to do this but you have ingored it in favor of discrediting me.
> 
> Ge0


Regardless of the knowledge and experience you clearly have, I must say that you are an absolute displeasure to interact with. I mean who talks like this? 

SG


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## dkm201 (Nov 29, 2007)

Maybe we could have a separate thread about binaural mics? It would be a bummer for the first good thread in a year turn into something personal...


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## smellygas (Feb 21, 2008)

Irrespective of the individual who suggested the article, it looks like Clark has published some work on in-car measurements. It's kind of interesting. In his first paper on the perceptual transfer function measurement in a car (AES 108th) he basically describes an alternate method to measure the frequency response:

- two microphones embedded within a foam block, 15cm apart (acknowledging Geddes' work) each aimed at the side windows
- 4 pairs of measurements (8 total) = spatial averaging
- no shoulders or body or legs present, just the foam block to mimic a head
- ** interestingly, the microphones are equalized to measure flat when a free-field sound arrives dead ahead (+/- 20 degs) from the windshield direction.
- the foam that surrounds the mics is cut such that the polar frequency response is similar to that where they measured off a dummy head. 
- ** thus, the mics are equalized to have a flat-response for free-field sound that originates in front of the foam block and will have an appropriately modified response for sound that comes in at different angles, including that from reflected sound = "directional equzliation"
- no time windowing, as expected (reflected/reverbed sound beyond 50 ms is too far attenuated in the car, sound before 50ms is still perceived)
- signal is pink noise, measurement is 1/12th octave via FFT's

Thus, in order to reproduce this methodology with binaural headphones, the binaural mics would need to be calibrated to have a flat-response WHILE you are wearing them for sound coming at 0 degrees. Your head should then naturally modify the response for sound coming at differing angles. I think this is one clear advantage over the single mic. In a way, if you think about it, this is almost like diffuse-field equalization, but not quite because the sound isn't necessarily equal from all directions in 360 degrees, it's just widely spread out, mostly from the front/sides, as a result of direct and reflected sound. The next best thing may be to have someone calibrate the binaural mics while they are mounted on a dummy head. You would still need to do spatial averaging, as with the single mic. 

I don't think you could obtain similar results just by slapping on some uncalibrated binaural mics because of the need for frontal-sound field calibration/equalization...however, if they just so happend to be factory equalized to follow such a EQ curve, that might be fortuitous. 

Clark published a followup paper where he correlated measurements using his PTF system (described above) with listening tests. It is very rudimentary compared to the work by Toole and Olive (on home loudspeakers). 

At any rate, they published a TARGET curve that went along with this binaural measurement system. They do not cite where it came from, but I have seen Andy (from Harman) describe something similar in at least 2 threads here. It's basically +20dB at 16Hz, +8dB at 62Hz, 0dB at 315Hz, 0dB at 2kHz, then downsloping linearly to -10dB at 20kHz, which is about 3+dB/octave. However, this was validated only for his PTF system. Anyway, interesting.

SG


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## Ge0 (Jul 23, 2007)

dkm201 said:


> Maybe we could have a separate thread about binaural mics? It would be a bummer for the first good thread in a year turn into something personal...



Agreed...

Ge0


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## Ge0 (Jul 23, 2007)

smellygas said:


> Irrespective of the individual who suggested the article, it looks like Clark has published some work on in-car measurements. It's kind of interesting. In his first paper on the perceptual transfer function measurement in a car (AES 108th) he basically describes an alternate method to measure the frequency response:
> 
> - two microphones embedded within a foam block, 15cm apart (acknowledging Geddes' work) each aimed at the side windows
> - 4 pairs of measurements (8 total) = spatial averaging


No surprize here. Dave and Earl (Geddes) have known each other for years. They are both founders of the local audio society I belong to. I would not be surprized if Dave had input into Earls article and Earl had input into the development of the PTF. However, that is pure specualtion. 



> - no shoulders or body or legs present, just the foam block to mimic a head


We still need to answer the fundimental question. Does the presence of the body affect frequency response in vehicle?

Wouldn't it be BETTER to account for this when making corrections? After all, you can tune a vehicle (given your prefered method) without this major obstuction in the way, then sit in it, and change the response substantially. 




> Thus, in order to reproduce this methodology with binaural headphones, the binaural mics would need to be calibrated to have a flat-response WHILE you are wearing them for sound coming at 0 degrees. Your head should then naturally modify the response for sound coming at differing angles. I think this is one clear advantage over the single mic. In a way, if you think about it, this is almost like diffuse-field equalization, but not quite because the sound isn't necessarily equal from all directions in 360 degrees, it's just widely spread out, mostly from the front/sides, as a result of direct and reflected sound. The next best thing may be to have someone calibrate the binaural mics while they are mounted on a dummy head. You would still need to do spatial averaging, as with the single mic.
> 
> I don't think you could obtain similar results just by slapping on some uncalibrated binaural mics because of the need for frontal-sound field calibration/equalization...however, if they just so happend to be factory equalized to follow such a EQ curve, that might be fortuitous.


Unfortunately my only source to get a microphone calibrated is to send it off to have this done. I can't be wearing them at the time!

So, say you had the mics calibrated not plugged into your noggin. Now you know the response variation of the hardware itself and have a compensation table. 

What you say makes sense. Let me expand by postulating the act of plugging them into your ears makes your head become part of the microphone assembly. This will alter its response accordingly. By how much, I don't know. It will more than likely alter response for wavelengths smaller than the diameter of my head. This may explain a null in the mics response around 5KHz. I tune this range by ear to compensate.

Now let me know if this warrants any merit or not. Say I sit in the anocheic chamber at work and take a response plot with them inserted in my ears. Couldn't I then create another compensation table taking the effects of my head into account?

I realize not everyone has access to this sort of facility. But, couldn't one do something similar in a somewhat dead room, or, outdoors?



> Clark published a followup paper where he correlated measurements using his PTF system (described above) with listening tests. It is very rudimentary compared to the work by Toole and Olive (on home loudspeakers).
> 
> At any rate, they published a TARGET curve that went along with this binaural measurement system. They do not cite where it came from, but I have seen Andy (from Harman) describe something similar in at least 2 threads here. It's basically +20dB at 16Hz, +8dB at 62Hz, 0dB at 315Hz, 0dB at 2kHz, then downsloping linearly to -10dB at 20kHz, which is about 3+dB/octave. However, this was validated only for his PTF system. Anyway, interesting.
> 
> SG


I'll tell you where the target curve came from. Dave conducted a study using some 120 odd individuals as test subjects. He played back music with various equalization curves. The test subjects found that music played back using the target curve described above sounded the most pleasing/natural. This was not some make shift test. Knowing Dave, years of research went into this.

Part of the reason Harman may have adopted this is because their larger end customers (automotive OEM) have adopted it. But, this is pure speculation...

Ge0


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## candaddy (May 21, 2008)

Ge0 said:


> We still need to answer the fundimental question. Does the presence of the body affect frequency response in vehicle?


There is no doubt when I can shift leg positions in my car and hear the sound stage change. I doubt that's as big a problem in cars with high-door and sail panel type install locations, but still your body is a large object that sound waves will interact with regardless.


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## Ge0 (Jul 23, 2007)

aztec1 said:


> I recently sprung for the Behringer ECM8000 and the 1/24 version of TruRTA  It's a great tool, but raises some questions for me about how my system is behaving.
> 
> Here's the setup, the Behringer plugged into the MicMate stuck in the headrest.
> 
> ...


I've muddled up your thread enough with discussion regarding my chosen measurement input technique. To say this has drifted off topic is an understatement. So, I'll help you if I can.

1.) Don't trust that single reading you got from your headrest mounted mic. It's not the entire story. At the very least, with the equipment you possess, try the averaging method others have suggested.

2.) You got off to a good start. Independantly analysing left vs. right is a good part of the battle.

3.) Sometimes no amount of EQ will solve a problem. Sound reflects and bounces all over the place in a vehicle. Sometimes constructive interference happens, sometimes destructive. For some reason unknown to me you can easier tame a peak than fill in a null. Just call this good for now.

For instance. The null you see at 200Hz happens to just about anyone with door mounted drivers. Its a cancellation caused by sound reflecting off the opposite side of your vehicle (roughly 5.5 to 6ft away), inverting, then interfering with the original signal. Nothing you can do about it beyond moving your midbass or aiming it at some hard to accomplish angle. Forget about it unless you want to do serious mods to your vehicle and move on.

4.) How does what you see on the RTA affect what you hear? Well, I've been taught by some pretty credible sources that your brain buckets sonic data in 1/3rd octave increments. Thus the reasoning behind the 1/3rd octave EQ. A 6db difference in amplitude amounts to roughly a doubling in volume. Consider this when looking at you plots.

You'll want to make sure that your 1/3rd octave readings in TrueRTA are as flat as possible to your chosen curve. If you see anything above 3dB do what you can. Take the heavy hitters (greater differences first). This will help solidify your stage and keep it from wandering.

Read previous posts by NPDANG, CMUSIC, FOXPRO, and humbly myself to fill in some blanks.

One you have mastered this and have questions, or, are yearning for more, just ask. You can improve a few percent more.

Good luck. I wish you the best...

Ge0


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## 14642 (May 19, 2008)

Ge0 said:


> No surprize here. Dave and Earl (Geddes) have known each other for years. They are both founders of the local audio society I belong to. I would not be surprized if Dave had input into Earls article and Earl had input into the development of the PTF. However, that is pure specualtion.
> 
> 
> 
> ...


I don't think Harman has officially adopted anything--Olive and Company are condusting the research now. It takes a long time. 

In any case, there are a couple of important points to be made about the whole headphone mic thing--From my experience the only part of the curve tht's dramatically different from a spatially-averages measurement using a mic array that doesn't include a dumy head is the rate of attenuation above 2.5kHz. Everything else is the same. 

As far as the calibration thing goes, It's only important to calibrate in order to share the information easily. If I decide on a target curve using a microphone setup that I always use and if I tune every car to that same target, then it works fine. However, in order to sharae the benefir of the work, I have to suggest a partiular setup in order to achieve the same results. If it's calibrated, then the information is easier to understand and to use for people who can't or don't want to duplicate my setup. That goes for ANY measurement gathering technique. 

For the headphone mics (binaural) i use we calibrated them using a similar method--anechioc chamber, known speaker response measured on axis in the chamber with a simgle calibrated mic. Difference between the measurements is the difference between the mic responses. 

And to whomever suggested that it wasn't possible to slap a pair of headphone mics on and get a good response--Don't be so sure. Before calibrating the mics, I did--it just took longer.

Great sound is great sound no matter how you get there. All of these tools are designed to make getting there simpler and more efficient.


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## Ge0 (Jul 23, 2007)

candaddy said:


> There is no doubt when I can shift leg positions in my car and hear the sound stage change. I doubt that's as big a problem in cars with high-door and sail panel type install locations, but still your body is a large object that sound waves will interact with regardless.


Now you're talking. A phenomenon that is not accounted for when measuring frequency response using a mic mounted near the headrest. Multiple averages or not.

Here is my take on things. I have not dived into the physics behind what Earl Geddes has published in his paper. To be honest, I don't have the time of day to study it all and absorb it. Life is full of compromises. This is just one. For now, lets just take his word for it.

He suggests taking a number of readings in the general vicinity of your head and using the average to derive your frequency response. 

In my opinion, that is exactly what I am doing with a binaural headset. I take two mic readings simultaneously. Data from the two channels is averaged to obtain a plot. I have two trains of thought regarding what to do next.

a.) Move my head to a slightly different position (one that I would naturally do anyway) and take another set of readings that are averaged together. Do this for a few more head positions. Now, have the RTA program take the average of all the averaged readings you took to obtain your final response curve. Sounds kind of average huh? (for those of you dozing off that was a joke. I do that some times)

b.) Set the RTA program up to display a long average (perhaps 1000 readings). Move my head about as I would normally while I am sampling. The resulting plot should be an average of all the positions I moved my head while collecting data. I guess this is just a faster way to do method a).

Lately I have been using method b). Right or wrong? That is all a matter of opinion. An arguement can be waged for one side or the other. What I do know is it works well for me. I strongly encourage others to at least try it.

As I have stated before, one thing this does for you is take into account the affect of your head, torso, and legs while taking averaged readings over multiple locations.

Sounds right to me.

BTW: This only works for RTA software that accepts stereo mic input. If you are doing impulse response it gets kind of tricky. It can be done, but, you'll need to create the average between the two channels in hardware vs. software (a preamp that electrically sums the channels). My favorite RTA program ARTA does not support stereo mic input so i had to make my own preamp.

Also, if capturing impulse response to set up time alignment it's best to break out your single channel mic as well. 

Hope this helps...

Ge0


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## aztec1 (Jun 13, 2008)

Heh, I spilled beer on my computer and it whitesmoked...only to log back in and find even more helpful information here! Thanks guys, it's much appreciated. I'm about to get out there and get some more plots. Hopefully I understand your posts well enough to post something useful myself.

On a light note, I have an Xtant P500a on the way that should help with the massive peaks and dips in my response. With luck I can post some nice before/after curves.

:beerchug: Yes, I'm buzzing, some nice Grottenbier Pierre Celis Signature does the trick. Now if I can keep from spilling it in my computer,,,,,:blush:


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## vecc205 (Nov 18, 2007)

Random456 said:


> If you don't mind me asking, what is your tuning method? (i.e. 1/3-octave equalizer, parametric equalizer, etc.)
> 
> SG


I have an Audiocontrol DQT. So 1/3 octave stereo graphic eq, and two band summed parametric eq. I also have a 5 band parametric eq in my Alpine CDA9887, though you can only have center frequencies so close from band to band.


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## benthe8track (Nov 23, 2008)

Interesting I thought I had heard Toole mentioned before. I just picked up a copy of "Loudspeaker Measurements and Their Relationship to Listener Preferences" for a report I'm working on. Very interesting stuff!


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## smellygas (Feb 21, 2008)

Andy Wehmeyer said:


> For the headphone mics (binaural) i use we calibrated them using a similar method--anechioc chamber, known speaker response measured on axis in the chamber with a simgle calibrated mic. Difference between the measurements is the difference between the mic responses.


What type of binaural mics do you guys use? 

SG


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## BMWturbo (Apr 11, 2008)

I thought this information re some testing I did over the weekend would be relevant here.

http://www.diymobileaudio.com/forum/693824-post51.html

I did some back to back testing with and without windows up, which I hadn't previousyl done and noticed a response variation.


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## michaelsil1 (May 24, 2007)

I sent my Behringer ECM8000 to Herb Singleton to be calibrated, this is what he sent me:

Congratulations, you have by far the flattest ECM8000 I've ever 
measured.

Herb


He also sent the calibration file.


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## jsun_g (Jun 12, 2008)

Wish I'd found this thread earlier, thanks for all the insightful information! I am in the process of tweaking my car and up until now have relied on tuning by ear. I plan to eventually try the earbud mic approach while sitting in the driver's seat. It only makes sense to me to take measurements in the same environment you listen to music in. Also, for competition, with a judge in the driver's seat and you in the passenger's seat, it might be an option to tweak with a passenger in your car.

I had a few other random thoughts when reading thru this thread. On tweaking the high frequencies, say 5+kHz, I notice that moving my head slightly can make a BIG difference in how loud I perceive the sound to be (for e.g. the difference between perception of a smooth response to a peak that is very fatiguing on at least one of my ears). If I EQ out the peak, then when I move my head even the slightest, I am faced with a trough in the response. I have placed/angled my tweeters as best as I could (they are on the dash; I did not get satisfactory results from kickpanel mounts) and I am much happier with my current set of tweeters than others. Are there any effective methods of cutting down on the response variation over such a small change in position?

I notice a response variation in the midbass region as well when moving my head, but compared to the high frequency response variation, the midbass variation requires a greater distance of movement, and I can fix it relatively easily by experimentation with sub vs. midbass phase and time delay on the midbass. I would really like to get some control over the high frequency response variation next.

Finally what do you guys think of this...
when I listen to a 6.5" midrange playing music in its intended frequency range, in a large space, the closer my ear is to the speaker, the "worse" it sounds. When my ear is within a few inches of the speaker, I wonder to myself how it sounds great at a distance given that up-close it sounds horrible. In general, is distance your friend when it comes to frequency response smoothness? Any studies/papers done on this area?


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## digitalhifi (Mar 26, 2009)

Well, a company called Head Acoustics manufactures binaural dummy heads just for this reason. You can pick one up for a measly $10k. Luckily we have one in our acoustics lab...I'll definitely be using it to tune my new install soon.  I will report back with results!


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## jsun_g (Jun 12, 2008)

That'll be interesting...please post your findings.
I'd sooner pay $80 for earbud mics and play the role of a dummy for a savings of $9920. Besides, I have legs and arms and a torso which may affect the sounds coming from my doors and kickpods.


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## digitalhifi (Mar 26, 2009)

Its so expensive because not only does it include ridiculously nice microphones, but also pre amps and proprietary equalization curves for binaural aurilization or recording. Once calibrated you can set them to output a flat response from the microphones.


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## michaelsil1 (May 24, 2007)

It seems to me that a seamstress's mannequin would be a lot cheaper.


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