# RTA: Binaural Microphones and their application



## ErinH (Feb 14, 2007)

After doing quite a bit of research on the topic and finding many sparse threads here and on other websites, I decided to write down what I’ve found. Hopefully you guys will benefit from this and save yourself the headaches of tracking down information on this subject.

So, here’s my attempt at showing you what Binaural Microphones and Spatial Averaging are about, and how they can help us to better tune our cars with the use of a Real Time Analyzer (RTA).

*Preface:*
The following thread assumes you are familiar with what an RTA is, and more specifically, how to use programs such as TrueRTA.
I also assume you know how to use this program, but I will try my best to detail specific steps throughout the tutorial, providing links along the way to appropriate threads/sites for further information. This is not specifically a tutorial on TrueRTA itself, but rather a tutorial on how to use TrueRTA with binaural microphones and how to use the results in a meaningful way to help you get a more pleasant stereo system in your car. I’m not a degreed acoustic engineer or anything of the sort. Those of you who are may be able to articulate some of the below with more ease and precision than I can, and if you find something wrong, please jump in. Ge0 wrote a very good tutorial a couple years back on this subject. I really suggest you guys take a look at his thread, too. 
http://www.diymobileaudio.com/forum...680-case-study-system-frequency-response.html

This thread will not get into phase and time alignment features because, frankly, that opens up an ENTIRE other can of worms and I don’t want to bog this thread down anymore than it’s going to be. However, I will touch on how to tell if drivers are in phase with each other in the last portion of this tutorial. I just can’t get into the time alignment aspect due to it being very complicated (and in my experience easier to set by ear anyway).

This is obviously not geared to be an ‘ultimate tuning thread’. The RTA is a _tool_ and that’s it. It will not fix everything for you. The more you use it, however, the better you will understand what it’s telling you and with some experimentation in crossover slopes, EQ, time alignment, install tweaker, etc, you’ll find that it is a very powerful tool. It is not a replacement for your ears. Always take the time out of an RTA tune to listen to things and verify that your efforts are not being wasted. I’ve changed my methods about 3 times, but the underlying factor has always been the same: experimentation to find what works best for me.​Having said all of that, let’s start the show… 

*What is a Binaural Microphone?*
Simply put, a binaural mic is 2 microphones used instead of a single microphone. These are used to make binaural recordings or to tune via spatial averaging. 
They can go in your ear, or over your ear. There are various types of use and many versions of these. The price can range from $50 up to $250+. The kind you choose depends on the application you employ them in. Luckily, in our case, the cheaper options are sufficient.​

*What benefit does it have over a mono mic, such as my behringer ecm8000?*

Interestingly enough, the mics I will recommend you purchase use the same mics found in first gen models of the behringer ecm8000: Panasonic WM-60 electrets. 

The main benefits to us are:

*L+R over single mono*
Rather than recording from a single microphone source, you get the benefit of recording in true stereo format, to give you a more accurate representation of what you hear. 

*Fits in ear/more true results*
Simply put, everyone’s ear is different. Your ear is a huge factor of how you hear. That’s why one person may sit in the same location and experience a different response. Another reason why audio is such a tough thing to get right and explains why there is not ‘one size fits all’ system response. The in-ear binaural mics are also influenced by your own ear. In this case, you’re not measuring response with a mic on the end of a stick. While that method may be fine for measuring FR of a speaker or system, it’s not ideal for measuring response with the intended purpose of shaping the FR to your liking. 


*Allows for spatial averaging*
The problem with taking a single mic measurement is that it’s not entirely indicative of what’s happening in your car. We all know this. 
Reflections in the car and even your headrest wreak havoc on a measurement and can often give you very erroneous results in regards to what you see on the RTA graph and what you hear in the car. Similarly, the shape of your body, and most importantly your head and ears, will shape your response. This is why you see some using dummy heads. We all understand the limitations to the single mic, and we live with it. As long as you are aware of these limitations you can still get good helpful information, but it’s not bulletproof by any means.
Enter ‘spatial averaging’. This is something that has been touched on here by guys such as Andy Wehmeyer, NPDang, Ge0 and many others in numerous threads. Just do a search and look at the results. 

So, what is spatial averaging, exactly? I could be long winded on the subject, but the simple answer is this: an average of response at multiple locations within a seating area.
Spatial averaging can also be more than a single area of seating. It can be multiple seats to achieve a better response for multiple seating areas in the listening environment. This is what the MS-8 and Alpine’s Imprint account for. This isn’t just a car audio thing. Many home audio systems such as Denon’s receivers, have these kind of abilities built in. 

To do this with a single mic you simply place the mic at multiple locations and record the response in up to 4 or more different spots all within the region of your head. Then you average all of those responses in the software and you end up with a single, averaged response. You can also manually turn your microphone while you’re recording the response. This is a bit easier. You can do this with a dummy head, with a tennis ball, with a mic on a stick, whatever you want. The problem is still what I’ve listed above. It’s not perfect, but it’s acceptable up until recently. 

With the advent of binaural mics, you are able to sit in the car, and turn your head from side to side slowly while averaging the results in real time (via TrueRTA’s ‘average’ feature and other settings). This provides a more realistic result of your system’s response. As noted above, this benefits you because you then have a nice averaged response of your listening spot, but you also have accounted for your body, your head and your ears. A very great tool to have.



*Calibration Note:*
Like all recording aspects, though, you really need to make sure that the source is as free from error as possible. So, we calibrate the microphones. This isn’t terribly hard to do, given you have the gear to do it and the environment that allows it.

The real problem, however, is the fact that binaurals use two mics. Well, TrueRTA only allows for one mic calibration file to be loaded per measurement. This is fine with a single mic, but with two mics you’ve got a problem. 
As mentioned above, the mics used in most of the binaural sets sold by the above company, and specifically in the binaurals I’ve suggested, are the Panasonic WM-60. If you do a search on these mics you’ll find quite a bit of info. From the research I’ve done, they seem to have a pretty accurate response curve. The problem areas seem to be 5khz and it’s harmonics. From what I’ve seen, the response typically dips a couple dB at 5khz and rises around 10khz. But, overall, it’s pretty nominal. That’s good. Here’s more technical information on the wm-60’s:
http://www.melaudia.net/zdoc/em07_wm60_a_dne.PDF

Here’s the mfg FR curve:










Now, if both your mics are pretty much the same in regards to their FR curve, then you’d be just fine to use a single calibration file. In my short (VERY short) time with my own set of binaurals, I’ve found the response to be pretty close to what I have with my calibrated behringer mic. 
However, I’ve also obtained a calibration file to correct the variances shown in the graph above. I’ve attached it to this thread. It is titled ‘WM-60 Calibration’. Again, this is a generic cal file. Use only if you’ve been able to verify that it helps you. In some cases, using a calibration file can hurt you by adding or taking away from things that are not an issue with your particular measuring device.​


*Where do I buy it? Can I make my own? What else is needed?*

This is where things get _very _hairy. 

Let’s break this down into 3 requirements you need to have this setup:

*Binaural Mics*. 
The ones I bought are these. They are in-ear binaural microphones. The same company provides many options which can be found here, but I found this to be the best compromise and the reviews were very good. I bought those. It's my suggestion that you do the same. 

*‘Plug in power’ *
This provides you with anywhere from 4.5v to as much as 10v, depending on what you use. In a nutshell, the higher the voltage, the more sensitive the mics are and the lower volumes will be recorded more accurately. In our case, I don’t see the benefit so I don’t feel it’s as important to achieve a very high voltage.

*A stereo microphone input * 
Now THIS is the tough part. Most all standard audio cards do not have this. The mobile pre does, so if you own this already you’re set. Don’t be confused by measuring your laptop’s sound card output and getting 5v. That’s bias voltage for a mono mic; it’s not plug-in power which powers both microphones needed to achieve a binaural recording. I’ve searched for days trying to find other options and there’s just no way around this. You will all but have to buy a new soundcard if you do not have a soundcard that has stereo microphone input. 


You have to have all three of the above. If you do, by chance, have a product that has stereo 1/8” input for microphone recording, make sure that it also provides power. The mobile pre DOES NOT. Absolutely does not. Therefore, you’ll have to buy or build a battery module.
Alright, so what do I suggest for the last 2 items? I suggest the Andrea USB-SA USB Sound Card. You can find it on the same site:
USB Microphone, Stereo Microphone, Preamplifier, Digital Recorder, Custom Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!

This gives you both stereo mic input and plug-in power of about 5v. It’s a simple, and easy solution without a huge price tag. Plug it in, and that’s it. 
*So to recap, I suggest you buy the two following items, and these are the only two things you need to buy if you don't want to go the DIY route:*
In-Ear Binaural Mics
USB sound card​
*Total Cost = ~$140 shipped.*



*For the DIY route, consider this:*
I really don’t want to get into how to do all of the above. Rather, I’ve found a few links to help those who may want to DIY their own setup. I’ll tell you right now, that I did some rough estimations and found that you would only save about $50 if you built your own binaurals+battery module, BUT, you’ll still HAVE to have a recording source that allows for stereo microphone input. If you do not, you’ll have to add at least $50 for a sound card that allows for this (cheapest being the one I’ve suggested to buy which also provides 5v plug-in power). DIY’ing may be worth it to you, but it wasn’t to me. 
Here’s a link, though, if you’re interested in building your own headset and battery module setup:
Binaural Microphone Construction​





*What software is needed?*
This thread only covers use of binaurals with TrueRTA, as it is the only software I know of which allows for L+R summing. I’m sure there are others, but I can’t recall any off the top of my head. Without summing, you’ll only get one channel’s recording, and this is typically the left channel. 

Use of TrueRTA is documented in this forum, but the best information is the manual itself. You can find it here:
http://www.trueaudio.com/downloads/TrueRTA_Help_v3.pdf

You will need to calibrate your soundcard, which in this case was a bit aggravating to me with this setup, but I realized one thing: volume. You’ll want to look at this thread, which shows you how to calibrate your sound card if you plan to use your sound card to both send _and_ record data. Ie: You use TrueRTA’s pink noise generator to send pink noise through your stereo via an AUX input and use it to also record the stereo’s output. You’ll want to calibrate your sound card in this case. If you only plan to record with the sound card you do not need to calibrate it. Remember this! It’ll save you headaches. 

One great feature about TrueRTA is the ability to load a ‘house curve’. I’ll dive into this a bit further later on in the tutorial. Make sure you keep an eye out for it as it’s a nice feature.​


That’s it for now… measurements and how to read data in Part 2 which will come in the next day or two. 

-	Erin


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## ErinH (Feb 14, 2007)

PART 2
NOT FINISHED… PLACEHOLDER…..

*Measuring Time!*
Now that y you’ve read the above and gotten familiar with the equipment used, and the basis for using it, let’s put it to use.

First off, the gear. Again, I went the simple route and purchased the Anderson USB Sound car and in-ear Binaurals from thesoundprofessionals. The links provided in the previous sections will help you build your own setup if you choose to do so. IMO, it’s more cumbersome and not worth the $20-30 savings (estimated). 

The setup for me is easy: put in USB card, plug in headphones, get in car, play pink noise, measure, tune. Simple enough, right? Well, for those who want to know more, here we go…


Everything hooked up:

*Setting up the software:*
2 things to note you have selected here:

‘MIC CAL’ should be turned on (button highlighted) if you are using the cal file supplied above. However, do not turn this on until you have calibrated the sound card. If you load the cal file first, you’ll basically be trying to make the sound card flat with the cal file’s influence on the curve (ie: this is wrong).
‘L+R’ should be selected as your input. You are measuring both a left and right microphone and you’ll want the response as such because you can’t shut off just one of your ears. You may find this contradictory to what I’ve said in the past when measuring your system with a single mic, but think about it; back then you were only measuring from one mic source. Now you’ve got two mics to imitate _both_ your ears.


*Measuring:*
Now it’s time to get in the car with your mics in your ears and start recording. 

Splitting up your left channel and right channel response of your sound system is a very useful way to make sure that you do a good job of ‘level matching’ both sides. 

*What to do with the results*
Key here is a smooth response. 

House Curve. E-mail me for it.


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## ErinH (Feb 14, 2007)

PART 3, If needed...


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## thehatedguy (May 4, 2007)

http://www.linkwitzlab.com/images/graphics/microph2.gif


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## ErinH (Feb 14, 2007)

Thanks for the link.

If you check my build log you'll find I have about 5 links regarding this topic, and one of them has that picture. But, thanks for the reminder. Linkwitz had a really good page on this (too lazy to dig up the link). There were some who had issues with his mod, though:
Note for those doing the Linkwitz mod with the Panasonic Mic elements... - Head-Fi.org Community


Overall, IMO, the DIY route for this is not worth it. Most people just starting out do not have a mobile pre, and they are 99% likely to _not_ have a sound card that has stereo mic input capability. Most all sound cards are mono. So, by the time you do the mod to build your own plug-in power source you'll still have to buy a soundcard to give you stereo mic input. The Andrea USB card gives you plug-in power and stereo mic input for only $50. IMO, after days of research, you can't beat it if you don't already have something like a mobile pre (which I do).


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## oca123 (Aug 16, 2010)

I have two JBL MS-8 microphones.... can I use one of them with TrueRTA?


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## ErinH (Feb 14, 2007)

what ms-8 microphones?... you mean, something like these:
USB Microphone, Stereo Microphone, Preamplifier, Digital Recorder, Custom Cables and more at Rock Bottom Prices from The Sound Professionals - Great deals on Microphone, Preamplifier, Digital Recorder, Cable and more!




Take everything I said above, sub in your microphones for mine, and get to work.


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## usacimember (Dec 24, 2009)

Great Info!


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## ErinH (Feb 14, 2007)

I realized I never finished up the 2nd post. Of course, only one person replied, so I didn't bother to waste my time. I might try to pick this back up soon, though if there's enough interest. 
Basically, this is a research paper... and I hate research papers.


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## DT053 (Mar 23, 2007)

I agree. Using a Binaural Microphone setup can sometimes help solve problems that are very hard to solve using other approach's. 

Example... While tuning a car about 6 or 8 years ago with a 7mic array, a guy with freakish hearing listened to the car and found it painful to sit in, even though the curve looked really good. Until we attached Binaural Mics to his head near his ear canals, and looked at the curve again, did we find a peak around 2K that that we hadn't noticed before.

dT


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## usacimember (Dec 24, 2009)

bikinpunk said:


> I realized I never finished up the 2nd post. Of course, only one person replied, so I didn't bother to waste my time. I might try to pick this back up soon, though if there's enough interest.
> Basically, this is a research paper... and I hate research papers.


LOL. Yes! Please finish!


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## oca123 (Aug 16, 2010)

Just bumping for maybe a follow-up.

Some research also digged up this thread: http://www.diymobileaudio.com/forum...our-ideal-frequency-response-curve-car-2.html which describes a method for tuning with binaural mics.

I have an MS8 in my car - but I can't get a good FR with it. I will post a curve later.


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## CraigMBA (Nov 19, 2010)

I'd like to see more on the subject...I bought a MobilePre to get started, but it was by luck more than anything else. The guys from REW perfer it over the MicMate, so that's what I got...


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## cvjoint (Mar 10, 2006)

Is L+R summing different than if I used a mono mic to measure at the left ear, then right and average them together?


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## cvjoint (Mar 10, 2006)

Check this out:
Etymotic Research, Inc. - How we measure response accuracy

I own those earphones. The fascinating thing is that they have a target response that boosts 2,700hz by 10db with a super wide Q! The company makes the best headphones out there imo, and they are also a professional hearing aid company.


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## funkalicious (Oct 8, 2007)

Any recommendations for a USB sound card that will work with an iPad for this application? I'll be using Studio Six Audio Tools.


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## leogun (Jan 18, 2009)

BINAURAL EAR MOUNTED ELECTRET CONDENSER MICROPHONE | eBay
this mic looks interesting, using panasonic condenser and matched pair within 1db
any comment?


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## leogun (Jan 18, 2009)

double post


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## ReticulatingPigeonElf (Sep 22, 2010)

leogun said:


> BINAURAL EAR MOUNTED ELECTRET CONDENSER MICROPHONE | eBay
> this mic looks interesting, using panasonic condenser and matched pair within 1db
> any comment?


yeah, really - i've also been looking for ear mounted mics.


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## ErinH (Feb 14, 2007)

ReticulatingPigeonElf said:


> yeah, really - i've also been looking for ear mounted mics.


Did you read the OP?... 



bikinpunk said:


> *Binaural Mics*.
> The ones I bought are these. They are in-ear binaural microphones. The same company provides many options which can be found here, but I found this to be the best compromise and the reviews were very good. I bought those. It's my suggestion that you do the same.
> 
> -	Erin


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## ReticulatingPigeonElf (Sep 22, 2010)

bikinpunk said:


> Did you read the OP?...


oh, haha, yeah saw that - I meant it would be cool to see comments on the performance of a variety of other models, as the previous poster suggested. But yeah, I'm leaning towards the OP's choice.

btw, I just realized your screen name says _bikinpunk _










- but for some reason up until now throughout the forum, I kept reading _bikin*i*punk_










sorry bout that :laugh:


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## leogun (Jan 18, 2009)

I like Bikini-Punk better hehehe


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## VP Electricity (Apr 11, 2009)

Erin, I see that these guys will sell the mikes with XLR for $90 more. Since I have the Mobile Pre and it supports phantom power, it seems that this is the way to go for simplicity (I have an Andrea sound card floating around somewhere, but it seems awfully simple to plug in the XLR and go...) Any comments?


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## mojozoom (Feb 11, 2012)

Two years...


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## VP Electricity (Apr 11, 2009)

mojozoom said:


> Two years...


I'm aware. Question still stands though.


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## ErinH (Feb 14, 2007)

makes sense to me to go the XLR route, then. only potential issue is that it is a thicker set of cabling. I don't think there's a clear better choice other than the one that is cheaper (ie: like you, someone already has the mobilePRE).


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## ErinH (Feb 14, 2007)

makes sense to me to go the XLR route, then. only potential issue is that it is a thicker set of cabling. I don't think there's a clear better choice other than the one that is cheaper (ie: like you, someone already has the mobilePRE).


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