# What causes cabin gain?



## stochastic (Jan 24, 2012)

I've seen the graph of multiple car's interiors at low frequencies that floats around this forum, and I've heard lots of people talk about working around cabin gain, but can anyone expain the phenomenon?


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## chad (Jun 30, 2005)

It's when the closed space that a driver is playing in is smaller than the wavelength of the reproduced signal. At this point the wave propagation goes from modal to pressure... In other words the transfer function begins at the point where you are merely pressurizing the cabin and no longer allowing wavefront propagation.

http://www.mcsquared.com/wavelength.htm

by knowing approximate dimensions of the internal space you can begin to get an idea where this point begins.

Since all cabin dimensions are different for different models of vehicles the amount and beginning of this point is dependent on vehicle.


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## Swanson's Performance (Mar 27, 2011)

So how do you calculate peak gain frequency and gain amount?

Sent from my A101IT using Tapatalk


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## chad (Jun 30, 2005)

Take a test enclosure, measure it outside, close to the cone, hanging from a tree, on a calm day, use pink noise. That is your reference.

Put same thing in car, measure again. That's going to give you frequency, or rather the entire picture of the gain... As for amount... time to take that to the SPL gurus 

It can be done mathematically I'm sure, but imagine calculating the cubic volume of your car accurately then the amount of possible gain/attenuation you will get at each frequency from SUPPORTED wavelengths piling up.


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## thehatedguy (May 4, 2007)

Cabin gain is a 12db/oct rise.


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## chad (Jun 30, 2005)

thehatedguy said:


> Cabin gain is a 12db/oct rise.



It's a bumpy ride.


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## Gary S (Dec 11, 2007)

A simple way to think of it - it's easier to pressurize a small space than a larger one. Free extra bass.


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## The Baron Groog (Mar 15, 2010)

Patricks thread http://www.diymobileaudio.com/forum...120190-does-anyone-understand-cabin-gain.html will be a useful read, he believes and argues well that standing waves have a large part to play. That's why the graph you've seen doesn just raise at 12dB per octave as thehatedguy states, it swings up and down at nodes and peaks in the standing waves within your vehicle.

I'm currently using cabin gain to great effect-my enclosure has and FB below the F3 of the enclosure and is crossed over below the F3, on paper should do squat and in reality sounds great I was very worried that it wouldn't work and that it would be incredibly peaky (mine is the mini on said graph) and surprisingly it worked and is flat without EQ from 30-60Hz, with EQ flat to 20Hz.


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## chad (Jun 30, 2005)

chad said:


> but imagine calculating the cubic volume of your car accurately then the amount of possible gain/attenuation you will get at each frequency from SUPPORTED wavelengths piling up.





The Baron Groog said:


> he believes and argues well that standing waves have a large part to play. That's why the graph you've seen doesn just raise at 12dB per octave as thehatedguy states, it swings up and down at nodes and peaks in the standing waves within your vehicle.


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## 14642 (May 19, 2008)

Yeah, the rise begins at a higher frequency than is easily determined in the graph. In most cars, the rise is "interrupted" by a cancellation somewhere between 50 and 120Hz. Below 50Hz, the graphs are very similar. 

An old version of LEAP included a simple calculator that approximated cabin gain and i found it to be pretty close below 50Hz, but not close above, for this reason. The newer LEAP includes more domain parameters that can be adjusted, so it's a bit better. 

In all of the measurements I've made over the years, the frequency response of the vehicle (cabin gain) continues to increase below 20Hz, so the idea that there's a peak seems to be moot. It's just a rise at low frequencies for the audio band. The peak that you see in the in-car measurement of the woofer in its enclosure is NOT the resonance frequency of the car, as so many have proposed. It's the point at which the the combination of the woofer/enclosure and the car combine for maximum output and the attenuation below that is the result of the low frequency rolloff of the speaker/enclosure exceeding the magnitude of the car's boost. This is why sealed boxes measure close to flat (12dB/octave rolloff which compliments the car's gain) and with vented boxes, you see a big peak (the region of flat response is extended will below the point at which cabin gain begins and the rolloff below Fb is steeper than the gain).


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## avanti1960 (Sep 24, 2011)

by calculation my car's cabin gain should begin at 50Hz. Unfortunately it's more like a cabin peak at 50Hz that falls like a rock into oblivion and -26db at 20Hz. It may not even be cabin gain but possibly my subwoofer's F3. 
I am attempting to correct this by installing a new sub that will have an F3 around 37 to 39 Hz (the best I could achieve with my current box).

Anyone know if I am headed in the right direction?


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## chad (Jun 30, 2005)

Move the mic... or work on absolute phase.


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## Brian Steele (Jun 1, 2007)

chad said:


> Take a test enclosure, measure it outside, close to the cone, hanging from a tree, on a calm day, use pink noise. That is your reference.


No need to do all that. Just do a close-miked measurement of the test box in the car where you expect the sub to be eventually mounted, and open all doors and trunk of the car. Doors and trunk open = no cabin gain .


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## chad (Jun 30, 2005)

Brian Steele said:


> No need to do all that. Just do a close-miked measurement of the test box in the car where you expect the sub to be eventually mounted, and open all doors and trunk of the car. Doors and trunk open = no cabin gain .


Tried that, does not work, especially in a garage.

OHHHH trust me, I tried  I'm all about the easy way 

That's what brought the accelerometers into play on body panels.


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## Brian Steele (Jun 1, 2007)

chad said:


> Tried that, does not work, especially in a garage.
> 
> OHHHH trust me, I tried  I'm all about the easy way
> 
> That's what brought the accelerometers into play on body panels.


Curious. I've done it once or twice and I haven't run into that issue.

The supposed advantage of using close-miked measurement (i.e. mike placed as close as possible to the speaker's cone) is that it supposedly minimizes the effect of the environment on the frequency response - the measurement is basically swamped by the output from the driver. In theory, there should only be deviation from predicted frequency response if the driver itself is loaded significantly by that environment, which in a typical car audio setup with all doors and trunk open it isn't (unless of course the driver is in a bandpass box or horn, LOL), and you can check this by doing an impedance response measurement as well. In reality the driver's cone size does affect the measurement a bit if the driver's a big one, but that's a whole other discusion...

Have a look at The Subwoofer DIY Page: Measuring a car's transfer function, which includes a nearfield measurement done with the test driver mounted in the door for example.


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## chad (Jun 30, 2005)

There IS a way to do it with gating, but then again some car audio enclosures will not capture the vent tuning in the gating. I tried to capture the gating with the accelerometers. No matter what, it still "lights the car up"

You still have a car there, doors open or not, now for a door speaker, bandwidth limited, yes, that. But a sub anywhere near a car, nope. 

Put it on the picka-nik table in the yard, on a quiet day, preferably snowy, pink it, put it in the car, pink it again... That's where it is at.

Next, step, put the enclosure OUTSIDE the car, about 3' away (1M) Hammer it with pink noise with the mic in the car.....

Extrapolate data.


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## tnbubba (Mar 1, 2008)

elementaryphysics


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## Brian Steele (Jun 1, 2007)

chad said:


> There IS a way to do it with gating, but then again some car audio enclosures will not capture the vent tuning in the gating. I tried to capture the gating with the accelerometers. No matter what, it still "lights the car up"
> 
> You still have a car there, doors open or not, now for a door speaker, bandwidth limited, yes, that. But a sub anywhere near a car, nope.


The door speaker's FR curve extended down to 60 Hz, which is within the frequencies affected by cabin gain. In fact, the writer of that article referred to a peak in the cabin gain at that frequency. That peak does not appear in the measured close-miked response of the door speaker. 




chad said:


> Put it on the picka-nik table in the yard, on a quiet day, preferably snowy, pink it, put it in the car, pink it again... That's where it is at.
> 
> Next, step, put the enclosure OUTSIDE the car, about 3' away (1M) Hammer it with pink noise with the mic in the car.....
> 
> Extrapolate data.


I'm not sure exactly what that's supposed to prove. The supposed advantage of closed-miked measurement, which is a technique specifically recommended for measuring low frequency performance, is that, as the mic is mounted very close to the driver (e.g. 0.25" from the cone), the effect of the environment on the measured response is supposed to be swamped out by the driver's output. You appear to be suggesting that it isn't. I'm just curious as to the reason why.

Finally, using vented systems to measure the car transfer function is not something I'd recommend, unless the vent can be plugged.


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## chad (Jun 30, 2005)

Brian Steele said:


> The supposed advantage of closed-miked measurement, which is a technique specifically recommended for measuring low frequency performance, is that, as the mic is mounted very close to the driver (e.g. 0.25" from the cone), the effect of the environment on the measured response is supposed to be swamped out by the driver's output. You appear to be suggesting that it isn't. I'm just curious as to the reason why.


In that situation, what gating is recommended? Even in a SMALL sealed enclosure I had under 40ms to capture, all open. Most people here have NO IDEA what gating means.

Why NOT use a vented system to measure? (although I normally use a simple cheapass driver in a sealed enclosure, predictable) Let's make the kiddies think.

You may as well get all the data you can from the intended device, and learn from it

They did not send men to the moon using a water cannon, albeit water cannons are fun.


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## Hanatsu (Nov 9, 2010)

Sealed box response is pretty easy to predict. Most sealed enclosures I've built measures up very close to predicted response. Lazy man's way to find out cabin gain. (if you don't wanna move the box I mean)

I agree that box response should be done outside where nothing affects the measurement. In a garage with doors/trunk open measurements will probably be off, more or less.


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## Brian Steele (Jun 1, 2007)

chad said:


> In that situation, what gating is recommended?


I'll answer that question with a question - what is the purpose of gating in FR measurement? .


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## Hanatsu (Nov 9, 2010)

Brian Steele said:


> I'll answer that question with a question - what is the purpose of gating in FR measurement? .


If you gate with a small time window, you could measure speaker response before cabin gain kicks in and presumably get a true box response inside the car. Has to be done in nearfield though.


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## Brian Steele (Jun 1, 2007)

Hanatsu said:


> If you gate with a small time window, you could measure speaker response before cabin gain kicks in and presumably get a true box response inside the car. Has to be done in nearfield though.


With all doors and trunk open, there should be no "cabin gain", because you've effectively unsealed the cabin.

I guess one could make the argument that if you've got a sedan with a small trunk, then maybe there might be *some* cabin again left within the passband of the subwoofer even with the trunk open and that *might* affect the FR measurement, but I'd likely opt for measuring that and confirming it does happen instead of just assuming that it exists.

Remember, we're talking about the close-miked measurement here, where output from the driver supposedly swamps everything else that can contribute to the measurement.


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## stochastic (Jan 24, 2012)

Hanatsu said:


> If you gate with a small time window, you could measure speaker response before cabin gain kicks in and presumably get a true box response inside the car. Has to be done in nearfield though.


LOL, the pressurization of the space because of low frequency boundary conditions will be very fast to take effect on the frequency response. Let's see, 340m/s with the average car's largest dimension at ~2m gives a window of maybe 5.8ms if you're really lucky. Realistically your boundaries will start to show up in FR at the smallest boundary condition, so closer to 2.5ms. Now how is it that you're measuring this frequency response? FFT you say? Hmm, what's your window size? no, not the windows on your car.

If you sample only 2.5ms of sound, then the lowest possible quarter wavelength that you'll ever see in that window - regardless of algorithm - will be in the ballpark of 100Hz (and a quarter wavelength is hard to identify). Sound is periodic so we generally need to see a few iterations of the period before an algorithm can identify it's period length.

Here's a quick graphical explanation of window size in fourier tranform algorithms: Short-time Fourier transform - Wikipedia, the free encyclopedia You'll notice that the smallest window size they use is 25ms, so you can imagine how blurred the graphing will be at 2.5ms right?


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## ErinH (Feb 14, 2007)

Not sure of te significance of quarter wavelength here but you are correct. There is a limit of usefulness. 

Moreso, the window size affects resolution of the data. If you choose a 2.5ms window, your resolution is only in 400hz increments. That means 400,800,1200,1600. So the 500, 630 and 1khz (1/3 octave) points needed to provide a decent eq basis are not valid here. While this specific example may not matter, resolution accuracy is certainly a point worth noting because in some cases it does impact how you view and use results. It can be misleading if this isn't considered. 

In a car, to achieve that kind of resolution you need at least 20hz accuracy. To get that, your window needs to be about 50ms gating. 

Of course, we fail to mention the various windows available (ie: rectangular, Blackman Harris, etc) that all shape the final result in some way. 

This is why I really find gating in a car useless. There are some merits I'm sure but for the average to more experienced car fi enthusiast I think it's trivial. And I think we all understand the use and limits of gating a response. It's just combining those two to give you something practical isn't exactly practical itself. 

As Chad said, the only real way to approximate cabin gain is an out of car ground plane measurement vs in car. Even then its tough to get right. Then you'll have to give a nod to modes causing the result to be anything but linear (not quite in the literal sense). But, really, what would that get you? What would you do with this information for your own personal use? I've toyed around with some ideas of simply letting cabin gain do its thing while only worrying about correlation causing modes. This meant measuring sub only and cabin gain only and IME its not an easy task if you're aiming for absolutes. And I really have had the most luck not only from a sound perspective but from an efficiency perspective where efficiency is measured in time spent trying to make something sound good.


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## Brian Steele (Jun 1, 2007)

bikinpunk said:


> This is why I really find gating in a car useless.


I agree, but for reasons that have to do with psychoacoustics, rather than various measurement technicalities. 

In any case, as I've mentioned before, the use of the close-miked measurement technique supposedly minimizes the effect of the environment on the results (by ensuring that the measured response from the driver swamps anything else that might contribute to the measurement) and therefore supposed to produce a result that is very close to a half-plane measurement of the same speaker at low frequencies, which is precisely the purpose for which this measurement technique is typically used. In other words, if you're trying to restrict your measurement "window" to reduce the impact of reflections for example, that should basically be a non-issue with the close-miked measurement technique. 

What I'm hearing here is that this isn't the case if the DUT happens to be a car with cabin gain significantly reduced if not eliminated, so my concern is what is it about a car environment that makes it so special that the close-miked measurement technique is rendered invalid.


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## minbari (Mar 3, 2011)

Brian Steele said:


> What I'm hearing here is that this isn't the case if the DUT happens to be a car with cabin gain significantly reduced if not eliminated, so my concern is what is it about a car environment that makes it so special that the close-miked measurement technique is rendered invalid.


because you are measuring the nearfield AND any cabin gain/interactions/standing waves/whatever. 

Things happen at the speed of sound. its not like you can measure the sub and then 5 seconds later you get a reflection that you ignore by only measuring for 50mS. cabin gain is as close to instantaneous as it gets. your mic dont know the difference, even close to the sub.

when you measure outside with no possible reflections and/or cabin gain, then you measure the subs true output.

my $.02 on the whole thing is, who cares. measure it in the car, cause that is where it is gonna go  then EQ it till it sounds good.


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## Brian Steele (Jun 1, 2007)

minbari said:


> because you are measuring the nearfield AND any cabin gain/interactions/standing waves/whatever.
> 
> Things happen at the speed of sound. its not like you can measure the sub and then 5 seconds later you get a reflection that you ignore by only measuring for 50mS.


With a close-miked measurement, that "reflected sound" should be signficantly lower in level than what was measured from the driver. Its impact on the results should therefore be minimal. That's basically the assumption behind the use of the close-miked measurement technique.


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## chad (Jun 30, 2005)

It takes around 35-40 ms for my car to "light up"


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## stochastic (Jan 24, 2012)

chad said:


> It takes around 35-40 ms for my car to "light up"



What's the process you use to measure this? Are you measuring total latency or just acoustic latency?


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## ErinH (Feb 14, 2007)

I imagine he's talking acoustic. But the system really shouldn't have a whole lot of latency in it. Mine uses about 40 feet of cabling between the mic and aux cables and I'm only off by about 9ms or so.


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## chad (Jun 30, 2005)

LOL cabling. Acoustic, measured via mic and with contact transducers on body panels.


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## Brian Steele (Jun 1, 2007)

I did some quick response measurements this afternoon using the close-miked method. 

Method: TrueRTA, sent to 20 Hz (slow), 1/24dB, 10 Hz-1kHz, average over 100 samples. RS SPL meter set at 110dB, C-weighting, feeding into external USB sound card. Pink noise at 0dB, input into the deck's AUX1 input from the same USB sound card. Deck adjusted until the meter read just under 109dB max. Test subwoofer (a small isobaric bandpass alignment) connected to the subwoofer amplifier with a 20 ft cable (to allow me to move it around and in and out of the car), active iltering at 125 Hz 12dB/octave (deck), in series with 200 Hz 24dB/octave (amp). The "car" in question is my Hyundai Tucson SUV. Pink noise left running at the same level through all the speakers for about 5 minutes before measurements were taken. The results are basically screen captures showing the raw (unsmoothed) response measurements, and for the comparisons I used a photo editing tool to superimpose the graphs and highlight the differences.


Here are the results:

1. Response with the subwoofer sitting on the garage floor, car doors closed. SPL meter placed in very close proximity to the subwoofer's vent. 

2. Comparison between (1) and measurement taken with car doors open (and all speakers playing pink noise). The response curve suggests that the effect of these additional sources of sound starts showing up above 200 Hz, where the response of the test subwoofer has dropped significantly.

3. Comparison between (1) and measurement taken with subwoofer relocated in the car's trunk, trunk open, SPL meter again placed in close proximity to the subwoofer's vent. Again, the response curve suggests that the effect of these additional sources of sound starts showing up above 200 Hz, where the response of the test subwoofer has dropped significantly.

4. Comparison between (1) and measurement (3) repeated, this time with the trunk and doors closed. The outcome of this test was pretty interesting, because it suggests that, even with cabin gain fully in effect, the measured response at the subwoofer is completely swamping any effect the cabin has on the low frequency performance. Again, major response changes only show up above 200 Hz, where the subwoofer's output has dropped significantly and the measured response is now being significantly impacted by the pink noise playing through the door speakers.

5. Measurement taken with SPL meter relocated to the dashboard, everything else remaining the same. Here, even though I have not done any level matching, the effect of cabin gain on the rolloff at low frequencies is pretty clear to see. 

6. Comparison between measurement (2) and measurement (3). I'll get to why I did this below 

Some other conclusions I can draw from the measurements:

1. I suspect that most of the the minor changes observed below 200 Hz IMO for measurements 1 through 4 can all be put down to minor variations within the testing process. In fact, comparison 6 demonstrates this aptly, as the subwoofer and SPL meter were physically relocated and the other conditions of the test were changed significantly, yet, when the level of the graph is shifted to match, the actual variations in the response curve turn out to be LESS than when the subwoofer and mic were left in situ and the car doors were open and closed. One would have suspected that the exact opposite would have been observed.

2. If you're going to do this kind of measurement, you should consider using a test sub that is as flat as possible within the desired passband. The more the response falls off, the more the possibility of inaccuracies being introduced into the measurement due to the effect of other sources of sound. This effect can be seen at the high and low ends of the graphs. If you plan to move the subwoofer around, physically fastening the mic to the subwoofer at the correct distance for close-miked measurement should also increase measurement accuracy.


This weekend, if I have an hour or two, I'll try to repeat the measurements with a bit more accuracy, perhaps HolmImpulse and my ECM mike and USB interface, which are a bit more accurate at low frequencies than the RS SPL meter (though that is less of an issue if you're using it to compare responses, rather than measure absolute responses). I'm also going to find a way to fasten the mic to the subwoofer, as it appears that slight changes in the location of the mic with respect to the bandpass subwoofer's vent can change the recorded SPL level and therefore affect the accuracy of the comparisons.


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## ErinH (Feb 14, 2007)

chad said:


> LOL cabling. Acoustic, measured via mic and with contact transducers on body panels.


using what for the power? mobilepre?

just curious. I've used my acc but out of a phantom power port off the klippel.


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## Hanatsu (Nov 9, 2010)

Think I finally understand the point of why gating is practically useless in this regard. With the risk of straying offtopic, is there a benefit to use FFT over RTA at all if gating is pointless? Always thought that FFT was capable of having higher resolution and had less interference from other frequenicies causing resonance issues and so on while measuring compared to PN/RTA. Not that it has anything to do with this topic, but I thought I land the question anyways.


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## stochastic (Jan 24, 2012)

chad said:


> LOL cabling. Acoustic, measured via mic and with contact transducers on body panels.


Can you explain the total signal path from generation to your arrival at a conclusion of 30-40ms? What digital latency are you facing during the recording stage - all musicians know that their soundcard has latency when they're recording tracks. Have you run a loopback latency test?

I don't think cabling has that much effect on latency unless you have 500ft of cable you're working with.


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## chad (Jun 30, 2005)

stochastic said:


> Can you explain the total signal path from generation to your arrival at a conclusion of 30-40ms?


Yes.



stochastic said:


> Have you run a loopback latency test?


Whole different league of software I'm using than most here use. In other words the test procedure includes a hard-wire loopback during testing 100% of the time 

http://www.rationalacoustics.com/pages/Smaart_Landing_Page

In the past 7 years I have explained it at least 50 times.



stochastic said:


> I don't think cabling has that much effect on latency unless you have 500ft of cable you're working with.


500ft of cable has very little latency at that... Very little.

I was poking fun at Erin depicting his cable length that has electrons flowing at the speed of flippin light


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## minbari (Mar 3, 2011)

chad said:


> 500ft of cable has very little latency at that... Very little.
> 
> I was poking fun at Erin depicting his cable length that has electrons flowing at the speed of flippin light


is that daytime light or nighttime light? pretty sure dark light travels slower 


I dont think latency would be a problem until you got into miles of cable and at those kind of lengths, latency would be the least of your problems.


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## chad (Jun 30, 2005)

minbari said:


> is that daytime light or nighttime light?


Martian light... their sun is smaller than our sun you know?


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## minbari (Mar 3, 2011)

chad said:


> Martian light... their sun is smaller than our sun you know?


lol, but if thier sun is smaller than ours, that means it has less mass and the light should be able to achieve escape velocity quicker and thus martian light would go faster, right?


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## ErinH (Feb 14, 2007)

minbari said:


> is that daytime light or nighttime light? pretty sure dark light travels slower
> 
> 
> I dont think latency would be a problem until you got into miles of cable and at those kind of lengths, latency would be the least of your problems.


That was kinda my point. System delay is 9ms even with 40ft of cabling.


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## Thunderplains (Sep 6, 2009)

Driving at high altitude.. sorry no coffee yet.


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## donnieL72 (Jun 20, 2012)

minbari said:


> is that daytime light or nighttime light? pretty sure dark light travels slower
> QUOTE]
> 
> Since nighttime light makes ugly girls pretty, wouldn't it make music sound better? Or is that the beer???
> ...


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## stochastic (Jan 24, 2012)

donnieL72 said:


> Back on topic....
> Interesting comment about the car "lighting up" after a short time. Would this have to do with sound waves bouncing off of the reflective surfaces and somehow loading up? I know that the upper frequiencies are bad about reflections, but wouldn't any sound wave do the same no matter the length of the actual wave or am I thinking too much?


First, I'm glad to see that the latency was actually accounted for in the 30-40ms figure. I would make a blind guess that during that time frame the secondary air enclosures throughout the cabin (air pockets behind door card, above headliner, trunk walls, etc..) are being loaded up with acoustic energy and begin to resonate. Once they've been loaded with the acoustic energy, then their resonance feeds back into the car - at which point you would notice the 'lighting up' effect that has been described.

I'd still like to hear from chad what constitutes 'lighting up' and how it was actually determined when that occurs, even if he's already explained it to others 50+ times. Also, have you ever measured the R60 decay time, or 'dimming down' time of the same vehicle? Any correlation between the two numbers?


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## chad (Jun 30, 2005)

stochastic said:


> I would make a blind guess that during that time frame the secondary air enclosures throughout the cabin (air pockets behind door card, above headliner, trunk walls, etc..) are being loaded up with acoustic energy and begin to resonate. Once they've been loaded with the acoustic energy, then their resonance feeds back into the car - at which point you would notice the 'lighting up' effect that has been described.












It was evident in acoustic testing, when I used transducers on the roof, door, rear quarters, etc it was amazing how close they all were to being influenced at the same time.

I'm more inclined to believe that it is ALL the air in the cabin as opposed to the secondary pockets because of the ratios between the two.


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## thehatedguy (May 4, 2007)

Mine doesn't move because I put dynamat on the outside of the car


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## chad (Jun 30, 2005)

thehatedguy said:


> Mine doesn't move because I put dynamat on the outside of the car


Just takes longer to move and moves slower  :laugh:


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## ErinH (Feb 14, 2007)

Lethargic 'lectricity.


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## chad (Jun 30, 2005)

bikinpunk said:


> Lethargic 'lectricity.


Global warming.


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## minbari (Mar 3, 2011)

dolphins in tuna nets! :O


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## stochastic (Jan 24, 2012)

This is a common diagram to show acoustic space's interaction with an excitation: 








The figure of 50-80ms is for a room-sized environment, I would assume this is the 30-40ms period chad is seeing in his car too. Chances are that the reverb energy is quite high in the enironment and this is what is giving the 'light up' effect rather than the pictured/expected decay pattern - either that or there's some echo energy that is coming out of the previously mentioned reverberant air pockets to offset this expected decay pattern.

Chad, did you notice the 30-40ms figure across the frequency spectrum or in a certain range (maybe 200-400Hz, maybe below 100Hz, etc..)?


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## chad (Jun 30, 2005)

Low frequency only. 

Sorry for the brevity.... Tapatalk man.


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## ErinH (Feb 14, 2007)

If you're talking modal issues then it really comes down to two things: resolution (Q) and frequency where the response is pressure driven. My car transitions to pressure dominated at about 600hz. Below this, the response is pretty much the same from any position in the drivers seat. The lower in frequency it goes, the less variance in measurements from measurement location to measurement location. 

So, you need to determine a time that allows you to have the resolution needed at a frequency you're concerned with, depending on your goals.


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