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Binaural microphone and inherent boost of high frequencies
Old 15th December 2012
  #1
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Binaural microphone and inherent boost of high frequencies

Recently, I bought this cheap but surprisingly decent binaural microphone called the Free Space from 3Dio. I've been using it for recording effects with great success, but when recording musical material, such as ambience in a live room, I noticed that there was a siginificant boost of high frequencies. So loud in fact, that you may pull your headphones off your head by pure reflex when the drummer goes for the cymbals. After some analysis, the high frequency boost seems to correspond to the natural resonance of the pinnae (outer ears): One wide peak centered at 4.5kHz, and another narrow peak at 15kHz. This microphone uses silicone pinnae to create the binaural illusion, so I guess this result is to be expected.

- Is this high frequency boost any different (worse) than what you get from the high-end binaural microphones such as the Neumann KU100?
- Do you think I should treat such a result in order to make it listenable, and if so: how should it be treated? The high frequency boost seems to vary depending on the type of high frequency material, and there seems to be a certain smearing in time in the boosted frequencies.

I've included a sample of music loops from Garage Band + brown/pink/white noise, recorded 3m away from my studio monitors playing at just below 75dBA. In the source file, I boosted/dampened all frequencies above 1kHz with 6dB. Watch your ears when listening! Screen shots are the frequency response from the white noise in the end of the clip. Also, I switched the Free Space for a pair of Behringer ECM8000 with a Jecklin disk, so you can compare. To my ears, although the frontal imaging is less detailed with the Jecklin disk setup, it's staggering how much more natural it sounds compared to the binaural. And finally, I've included the same recording after being processed with Logic Pro's Linear Phase EQ, with a funky-looking curve that I'm a bit frightened to use in any serious work.

Any ideas are welcome, as I would really like to be able to use this Free Space microphone for musical material, and not just special effects.
Old 15th December 2012
  #2
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Attachments below
Attached Images
File Type: png Binaural3Dio-FreqResponse.png (61.4 KB, 762 views) File Type: png JecklinECM8000-FreqResponse.png (61.8 KB, 703 views) File Type: png Binaural3Dio-FreqResponsePostEQ.png (61.0 KB, 573 views) File Type: png LinearPhaseEQCompensation.png (27.4 KB, 852 views)
Attached Files

1-Binaural3Dio-Music&Noise.mp3 (2.94 MB, 4317 views)

2-JecklinECM8000-Music&Noise.mp3 (2.94 MB, 3944 views)

3-Binaural3Dio-PostLinPhaseEQ.mp3 (2.94 MB, 3873 views)

Old 16th December 2012
  #3
Schallfeldwebel
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Save your time.

This is no binaural or dummy head stereo. What you see in your picture, the raise between 2-8kHz, is caused by the fact the ears are placed on a circular object, a sort of dish instead of a dummy head. From presence boost's center frequency I estimate the diameter of this saucer/dish about 13cm?

This is a total faulty constructed microphone, people who made this microphone have not taken the correct microphone physics into the equation.

Buy a decent dummyhead instead. Compensating with a phase linear filter may also not be correct. Some of the lift creates minimum phase shift, and if you compensate that with a linear phase filter you end up with a flatter frequency response, but the phase response still shows a bumpy ride.
Old 16th December 2012
  #4
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Thanks for listening and sharing your thoughts.

I haven't measured, but maybe the discs are slightly less than 13cm, yes. Hard to say if they really are the problem here, but I think it's a mixture of several things going on.

The Free Space uses Panasonic WM-61A capsules, but the guy who is making these has applied something he calls the Linkwitz Mod, which according to the website is done "to brighten the sound". I've e-mailed him asking if he would offer versions without this high-frequency boost, and he answered no, but said that he was considering making a pro version with DPA 406x capsules. Hopefully without performing any aftermarket diffuse-field compensation…

I have a plan B instead of trying to flatten the frequency response using EQ: Tilting the microphone about 45 degress backward loses some horizontal imaging, but the high-frequency boost is less severe (4-6dB instead of 10-12dB). And then maybe apply just a nice broad EQ to beef up the recording (maybe simply a flat bass boost below 1Khz) without adding any abrupt phase shift issues to the treble, where most of the directionality information resides. Of course, then the listener would have to accept that the result sounds colored and not transparent.

As much as I would like to follow your "buy a decent dummyhead" advice, there is quite a step up from $300 to $8000 which is what a brand new Neumann KU100 goes for. And what if such an expensive microphone still had similar issues? Too risky for me, and I'm not planning to do recordings like this often enough to reach break-even.

I wouldn't go as far as you do dismissing the Free Space as a binaural microphone, but it appears that it is best suited for special effects (nature ambience, virtual haircuts, scaring the crap out of people on Skype, you name it), I'll keep using it for those tasks, and instead use a Jecklin disk or other well-tested stereo recording techniques whenever I will record musical material.

Do the rest of you who listened to the clips share the same view as Schallfeldwebel, that I shouldn't use this microphone for such types of recording, and that there is no point trying to fix it with EQ afterwards?

(EDIT: Now look at that. Attach a PNG file = download only. Attach a JPG file = display and thumbnail. You live and learn, I guess.)
Attached Thumbnails
Binaural microphone and inherent boost of high frequencies-freespace.jpg  
Old 17th December 2012
  #5
Schallfeldwebel
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Ears

Take the silicon ears off from the unit and make your own dummy head and place them on there, and use e.g. DPA 4000 miniature series microphones.
Old 17th December 2012
  #6
TSM
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Firstly, I'm no expert on Binaural recording, but I did build my own head using the Ears shown just to see what it would sound like. The results I got initially were similar...way too bright. I'm guessing the system you bought has "ear canals"? I tried this method as described on his website. Even with very radical EQ curves, it never sounded that natural. I found by taking the canals out and placing the mic just inside the ear of my dummy head, the sound was way more natural. I have since played around with Panasonic mics stuck in my own ears via inear headphones (ripped out the speaker and put in the mic caps) and on my head, the sound is amazing.Have you tried this method? I made a set for less than £15.
Old 19th December 2012
  #7
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You propose a very sensible solution, TSM. However, it might not be so practical for me. The reason why I bought this mic compared to for instance the Soundman OKM (in-ear), was because it's difficult to have a person sit in the same spot/angle without breathing during prolonged tracking sessions.

Perhaps I could try some simple modifications using what I have available. A friend of mine has a couple of lapel mics (Countryman B3, IIRC) that I can probably borrow and experiment with, placing them at varying distances from the ear canal and test how it sounds. And then try mounting the silicon ears onto a styrofoam head and see if it improves the boundary effect that Schallfeldwebel mentioned.
Old 20th December 2012
  #8
Schallfeldwebel
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Sennheiser

On Ebay sometimes people offer a Sennheiser dummy head from the 70s, which has no microphones build in. In this Sennheiser dummy head you can mount any in ear microphone. The ears are not so precise as in the unit above, but it might be a good solution if you do not want to spend too much money.
Old 21st December 2012
  #9
nkf
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Quote:
Originally Posted by Schallfeldwebel View Post
On Ebay sometimes people offer a Sennheiser dummy head from the 70s, which has no microphones build in. In this Sennheiser dummy head you can mount any in ear microphone. The ears are not so precise as in the unit above, but it might be a good solution if you do not want to spend too much money.
If you mean this blue one you can mount on a mic stand? I have one and sometimes mount a pair of DPA 4060 in it. They're fitting quite well even with foam.
Old 21st December 2012
  #10
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that right,On Ebay sometimes people offer a Sennheiser dummy head from the 70s, which has no microphones build in
Old 21st December 2012
  #11
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Something no one has mentioned about Binaural recording in this thread is the importance of what is called Diffuse Field Equalization. Diffuse Field Equalization is electronic equalization of the signal recorded from the dummy head to compensate for numerous factors, namely artificial high frequency build up and optimization of Binaural signals for loudspeaker playback in addition to headphones.

It is critically important to apply this Diffuse Field EQ curve to the signal to both "flatten" frequency response and make the recording playable over speakers.
The exact curve to apply is unique to the physical and electronic characteristics of the dummy head itself and can only be determined through sophisticated acoustic measurements in a lab. Absent lab data you might as well just EQ to taste by ear.

The Neumann KU 100 dummy heads have built in electronic equalization to render the output usable without any post processing. By the way the Neumann is a great mic, it works really well.
Old 23rd December 2012
  #12
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Yes, that's probably why professional binaural microphones cost as much as they do, due to all the research that lies behind the final product. I can guarantee that I have not enlisted the help of any german scientists in my attempt at post-EQing, as you might have heard from the results

But it's good to know that at least the professionals are able to get it right. They probably know better than anyone what attributes to compensate for, and what attributes should be left untouched. Not just adding a flattening EQ curve and hoping for the best. There are likely many, many papers on this subject, but it's kind of difficult to know where to start.
Old 23rd December 2012
  #13
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KU 100 is fantastic and offers a very natural sound. Even at its high price, it is worth acquiring.

Its frequency balance is very even with nary a hint of treble boost.

The best binaural mics also have a torso attached. The best result with these pro binaural dummies is when you clothe the torso in a sweater as well.

Leaving that approach to real freaks, the Neumann KU 100 is the best followed by the German in ear mics from SoundMan.

In my signature I refer listeners to my Soundcloud page where you can hear some offerings I did with the Neumann dummy head.

Merry Christmas to ALL.
Old 18th February 2013
  #14
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Soundman OKM II Classic/Studio

Update: I've now tried the Soundman OKM II Classic/Studio. The time-domain definition is much more precise, and the capsules are EQ-ed in a more gentle fashion.

Looking at the frequency response, it's actually EQ-ed down just about where there was a heavy boost caused by the ears in my other clips. I have the option of either putting them in my own ears, or in the fake ears of the 3Dio FreeSpace, and the difference in imaging between the two methods is obvious. Their difference in frequency response is actually not that different, mainly a smoother and more solid response around 1Khz-2kHz in my own ears vs. the fake ones.

One thing that bothers me with the OKM is the resonance-like self-noise around 400-500Hz – I can't filter it out without destroying the sound. So that means that I can only use this micing technique with loud sound sources, which is a bummer.

I've attached a re-take of the original clip from the top of this thread, using the Soundman OKM in my own ears. It sounds better (more comfortable), apart from the self-noise.

In a rush of inspiration, I tried holding the old Behringer omnis just outside my own ear canals (maybe 1cm distance). Fairly good, actually. The trouble with that method is keeping the mics in the exact same position for the duration of the take, when my arms start to shake and get tired quickly! A method for rare situations only.

So in the end, I'll use the Soundman OKM pair when the material will be blended into other tracks, but due to the self-noise I will not use it for quiet solo material. For those instances I will likely upgrade from the Behringers to some Earthworks QTC or DPA omnis, and stick to using the safe Jecklin disk configuration.
Attached Thumbnails
Binaural microphone and inherent boost of high frequencies-whitenoise-okm-ear.jpg  
Attached Files

OKM-in-Human-Ears.mp3 (2.94 MB, 4106 views)

ECM-Outside-of-Human-Ears.mp3 (2.93 MB, 3687 views)

Old 10th June 2013
  #15
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Hjelmevold, I'd like to go back to your original question if I may.

"- Do you think I should treat such a result in order to make it listenable, and if so: how should it be treated? The high frequency boost seems to vary depending on the type of high frequency material, and there seems to be a certain smearing in time in the boosted frequencies."

Yes, with binaural, you will always run into situations where you need to apply post EQ to balance things out. But nobody has talked about the real important issues here. I'd really like to know the details of the room you were in. How big was it? How close were you to the drummer? Was the drummer to the side of the mic, or directly in front?

Without hearing the live drum tracks I am not able to say exactly what you experienced. The samples you posted sounded good to me, but just needed some lower energy boost. Did you detect any actual distortion? Can you post it?

You get the best binaural result by treating the room first, if possible. If that's not possible or not enough, you can also re-orient your mic to make sure that the drums are not directly to the side of the mic and are more centered, possibly even behind the mic, so that the pinnae dampen the higher frequencies.

For live band binaural with a drummer, I would definitely expect to apply 6-8 db of EQ to treat for the cymbals and the overall increased high frequency energy. A gentle limiter can also help you as well, especially if you use a multi-band compressor tool, and target the harsher frequencies.

Jeff
Old 12th June 2013
  #16
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Hey Jeff,

I've been able to experiment quite a lot during the last couple of months, and now I think I've come up with a setup that works well without any immediate need for EQ.

In the example when recording drum kit ambience, I now typically place the mic stand about 2 meters away from the drum kit (that's closer than I usually would place a single omni for ambience). The live room is about 5*7*3m in size. The trick is that I position the mic more than 2m up high, effectively creating a slope at about 30 degrees from the cymbals up to the mic. At this angle, the frequency response is smoother with the ears of the Free Space than at uniform height. You would think that the end result wouldn't sound as natural, but the binaural imaging is in fact relatively unaffected because of all the reflections.

The second part of the trick is that I've switched to using a different set of capsules. Thanks to the great advice from Plush a couple of posts above, I'm now shoving the SoundMan OKM capsules into the ears of the 3Dio Free Space. The result is the best of both worlds: Higher-quality (Sennheiser?) capsules that are diffuse-field equalized, and at the same time more practical than having a guy with in-ear mics to stand perfectly still in front of the drum kit.

I've attached some takes that I did for a project a while back, at a time when I was still using both the original Free Space and the SoundMan capsules.

I see there's a Free Space Pro out now, and that might be better-suited for these situations than the original model. Nevertheless, I'm very happy with the setup that I've come up with, placing the SoundMan capsules into the Free Space ears. Granted, it's no Neumann KU 100, but the setup costs only a fraction of that and is good enough for my purposes.
Attached Files

Amb_3DioFreeSpace.mp3 (1.18 MB, 3707 views)

Amb_SoundManOKM.mp3 (1.05 MB, 3732 views)

Old 11th September 2013
  #17
Binaural high frequency 'boost'

Quote:
Originally Posted by Hjelmevold View Post
....

- Is this high frequency boost any different (worse) than what you get from the high-end binaural microphones such as the Neumann KU100?
- Do you think I should treat such a result in order to make it listenable, and if so: how should it be treated? The high frequency boost seems to vary depending on the type of high frequency material, and there seems to be a certain smearing in time in the boosted frequencies...
Firstly, I'm with Plush in that I am a strong advocate of the KU 100; having worked with mannequins from Neumann, B&K, Head Acoustics, and others over the past 20+ years, I feel the KU 100 is pretty neutral...and while arguably expensive, does a really nice job in the vast majority of settings. Still, I'll touch on the EQ issues in a bit.

The silicone ear thing that you show (the guy who did the 'diggadag' build your own binaural mannequin way back when) is, I believe, the guy behind that design. Personally, and based on my years working in sound quality, noise control, and recording, the ear-only approach is interesting, but I don't think it's really faithful to the concept (and math) required to refer to something as HRTF based.

Having said that, one important aspect of all binaural mannequins is that they are equalized for specific conditions. For example, the KU 100 is equalized to have flat nth octave response when in a diffuse field. Several mannequin heads allow the user to alter the EQ based on boundary conditions (such as free field, diffuse field, or ear-canal-based only).

When a mannequin can be EQ'd to 'free field" EQ, there is a huge lift to the treble. This is because the 'free field' conditions assume that the source is directly in front of the mannequin (perhaps 3m away), and the space is truly free-field (absolutely no reflections). This EQ is really used more in the Acoustics world when one wants to compare the results one would get using a standard free field mic with the data obtained from the mannequin. Now, because the source is directly in front, the HF response is peaked (by design) to compensate for the fact that the ears of the mannequin are not facing the source. This should sort of make sense - think of the source as a light, and the mannequin as a simple sphere, and you can envision the ears being in shadow. So, on the mannequin heads that allow EQ settings, recordings made using this setting will seem really, really bright.

By contrast, the diffuse field EQ assumes that sound is striking the ears (and thus the mics inside of those ears) from all directions - like what one would experience in a reverberation chamber. As an example, if you recorded something with a mannequin head using the 'free' setting and then the 'diffuse' setting (using the exact same set up etc), the recording made with the 'free' setting would seem very, very bright as compared to the recording made with the 'diffuse' setting.

The KU 100 uses diffuse field equalization, but I know not how the 'ears on a tube' thing that you have is EQ'd, but if you have access to a reverb chamber, you can find out, and where there are peaks and troughs in the response, you could build an FIR filter to flatten that out as much as possible. Mind you, this is beyond the scope of a post, but you can feel free to write me if you want to know the methodology.

Additionally, correcting the EQ to a diffuse field condition will likely improve the timbre (of your ears on a tube thing), but not necessarily the imaging - much of the HRTF comes from the shape and dimensions of the head between the ears, which reflect and refract various frequencies. That's conspicuously absent from the 'ears only' approach. Again, I'm not denigrating the ears only approach per se - if you like how it works for achieving an aesthetic, then that's all that matters, but I wouldn't be so generous to refer to that as a binaural mannequin or approximating an HRTF approach.

The main issue, as I see it (and assuming you have MOST of the important elements of the HRTF intact), is what you deem to be most pleasing, EQ wise.

Yes, I use the KU 100 exclusively for all of my live work, but it's not true to say that its EQ will be 'accurate' for every recording situation...because...only in a diffuse field will its nth octave response be flat. Most live performances do not take place in wholly diffuse or free field conditions (for obvious reasons). Then again, many mics are chosen for specific recording tasks because they are NOT flat, but instead, suit a specific aesthetic.

The thing is...I think it's fair to say that most of the time, the conditions in which one records live music will be more diffuse than they will be free-field. So, while it's fair to say that the KU 100's EQ isn't 'exact' (flat nth octave response in diffuse field), its EQ is biased more towards the conditions in which one would most likely record music; you'd never really record a live show in an anechoic chamber. So, the 'deviation' from the ideal EQ is comparatively minor most of the time (when using the KU 100) and why a free field equalization on a mannequin head would seem overly bright (very much so) in most instances. I think it's also reasonable to say that the HRTF of the KU 100 is reasonably close the the 'average' HRTF of most people if measured in such a space, whereas the free equalization would be a much bigger deviation. Thus, because the EQ is reasonably close to what would be observed in-situ, its timbre is pretty neutral, but again, I'm open to any EQ adjustment that I think is necessary to achieve the desired aesthetic.

So, from that angle, I see no reason why you should not EQ any binaural mic's signals to improve the timbre based on your desired aesthetic - even the kind-of-sort-of-but-not-really-binaural-microphone that you mention. Mind you, for perceptual research work (rather than recording), I don't think one can be so cavalier with the shape of the EQ as I see perceptual research recordings (more appropriately 'acquired data') as disconnected from the 'pleasant aesthetic' of music recording. No, they do not have to be mutually exclusive, but many times (for live recording), a reasonable approximation is all that's required. In research, the focus is on accuracy, but many times in recording, the focus is on the aesthetic; sometimes... 'truth' and 'accuracy' in recordings are at odds with the desired aesthetic - in fact, they can be the enemy.

After all, for any other mics you may use, you're open to EQ'ng them as necessary to get the required aesthetic (the timbre), right? I mean, if you think a mic needs EQ, you use it...right?

Now, the last EQ that I mentioned is one that only considers the ear canal and meatus of the ear, and neglects the effects of the pinna, head, shoulders (if present), torso (if present). This is done in an attempt to make the mannequin head response suitable for most conditions rather than explicitly diffuse or explicitly free (because the ear canal resonance is independent of boundary conditions). However, and as far as I know, only one manufacturer has this provision and cost roughly 2x or 3x that of a KU 100.

So, while it seems to me that this approach is the most 'technically accurate', for aesthetic recording purposes, I don't think its cost is wholly justified - for research, perhaps, but not so much for recording music. I happen to believe, right or wrong, that working with the EQ from a proper binaural mic is really all that's required to achieve the proper desired timbre (setting aside proper placement, etc., etc., etc). Imaging is another matter, and again, I think the issue with the ears on a tube approach is that there is nothing in-between to shadow one ear from the other, or to reflect / retract energy as a function of angle.

Like Plush, you can find some of my binaural stuff on soundcloud. Just query immersifi, and you will find various binaural recordings of mine there, made in a wide variety of conditions...and all of them made using a KU 100. Some are 'as is', wherein literally nothing was done to the KU 100's signals, but in other tracks I have posted there, differing EQ schemes have been used.

I hope this helps...

Mark

Last edited by Mark A. Jay; 11th September 2013 at 07:48 PM.. Reason: added a small section
Old 12th September 2013
  #18
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Thank you for your extensive and insightful post, Mark. I have no further questions.
Old 3rd September 2014
  #19
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Wondering if anyone has recorded dolby atmos array with binaural in ear mircophones
Old 3rd September 2014
  #20
Gear interested
 

Wondering if anyone has recorded dolby atmos array with binaural in ear mircophones
Old 3rd September 2014
  #21
Gear interested
 

Wondering if anyone has recorded dolby atmos array with binaural in ear mircophones
Old 9th April 2017
  #22
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Hello,

I've been working with binaural microphones over the past few years and the frequency boost that you experienced (and measured) is to be totally expected.
What happens is that the sound wave is recorded in a spot (the capsule spot) that, due to the boundary shape of the pinna, adds to the original frequency content an extra frequency shaping, not unlike an EQ filter. This added shaping is the frequency response of the pinna at the capsule's location.
As somebody pointed out, I wouldn't go as far as calling that an HRTF, since this specific microphone is lacking the ear canals and the head.

This frequency shaping may instinctively make you steer away from binaural, if one is interested in making natural sounding music recordings. But the binaural technique not only CAN produce natural recordings, but it is REQUIRED if one is really serious about the degree of naturalness they want in their recordings.

What's required with binaural recordings is a reverse EQ filter applied to the recorded sound in the mixing phase, to undo the fact that when you listen to the recording, there is another EQ filter applied, which is your own ears.
So, why go through the trouble to apply the frequency boost to the recorded material physically, using a physical boundary, if you are then required to undo it with an additional filter before being able to listen to it?

There two main reasons:

1. If the head replica is done right, you get encoded in your L and R channels all the spatial cues that make it possible to hear a 3D sound stage when played back (provided that the reverse filter above is done right too).
Spaced pair and Jecklin disc try to approximate these cues and are successful to an extent, but they are not able to encode in their L and R channels all the time shift, amplitude, frequency response and phase differences that your left and right eardrums experience when you actually are there listening to the sound that you are trying to faithfully record.

2. The second reason is more of an indirect benefit. The S/N is increased by about 15-20 dB A-weighted!
This is something that is most of the time overlooked, but very important. By increasing the power of the sound at the frequency most sensitive to humans, let's say 15 dB at 4 kHz, when the reverse filter is applied, the power of the noise at that frequency is lowered by 15 dB.
This is because the power of the sound is increased by the physical boundary, so that there is an effective 15 dB S/N increase at that frequency in the signal recorded by the microphone.
This is huge, as it allows to use smaller microphone capsules which are not that quiet if used in traditional ways. The smaller capsules are required if someone is to make proper binaural recordings, as they are comparable to the size of our eardrums.
Using 10+ mm capsules requires the boundary geometry to get distorted too much, and jeopardize the recreation of accurate HRTF functions at the capsule's location (try to fit 10 mm capsules at the eardrum position, and your ear canal needs to necessarily stretch as a consequence, changing its shape and consequently the HRTF).
Smaller capsules also have higher SPL handling, which is nice when you consider that the SPL they receive is boosted 15-20 dB for the reasons above mentioned. For example, Panasonic WM-61A already saturate with a 6 ft away piano recording when used in a binaural microphone.

Neumann KU 100 probably has the above mentioned reverse EQ embedded in its circuitry, and that's why it can sound natural with musical content too.
The good news is that one doesn't need to spend close to $ 10,000 to get the same or better results. A matching EQ filter can do the trick (and with less added noise than passive circuitry).

Speaking of accurate HRTFs, as I was saying required for a natural reproduction of the 3D sound stage, a true to life boundary geometry is necessary.
For this reason I have (proudly, I must say) created the first anatomically accurate pinna + ear canal 3D printable models available out there.
These can be used with any mannequin head of reasonable dimensions (most of them are too small, so some time sorting out the right one is highly recommended).

https://www.shapeways.com/shops/bina...icrophone-ears

With my ear models one no longer has to settle for a cheap makeshift DIY microphone, but rather they can build a professional quality one for pennies on the dollar.

Can DIY recordings really sound that good? I actually had one of my binaural recordings used in an audiophile speakers listening session and, as difficult as it is to please those audiophile people, I was pleasantly surprised of how it stacked up against much more 'boutique' labels.
This article mentions that recording.

Flying Blind: Another Visit to the Magnepan Skunkworks - The Audio Beat - www.TheAudioBeat.com
Old 10th April 2017
  #23
Some Thoughts

My [comments] appear in the body of the email. I hope they help.

Quote:
Originally Posted by sax512 View Post
Hello,

I've been working with binaural microphones over the past few years and the frequency boost that you experienced (and measured) is to be totally expected.
What happens is that the sound wave is recorded in a spot (the capsule spot) that, due to the boundary shape of the pinna, adds to the original frequency content an extra frequency shaping, not unlike an EQ filter. This added shaping is the frequency response of the pinna at the capsule's location.
As somebody pointed out, I wouldn't go as far as calling that an HRTF, since this specific microphone is lacking the ear canals and the head.

[The HRTF is a rather involved set of filter 'arguments' that are a function of X, Y, and Z (at a constant distance) as well as another arbitrary distance. That is, the HRTF varies spatially as described, but the far field HRTF is significantly different than when the HRTF is measured in the near field. Because of this, much of the HRTF measurements that are now undertaken are done in accordance with the SOFA criteria as outlined by the AES. This means two main methods: the first is simply a fixed elevation but varying azimuth (working your way around the mannequin head) and comprises 360 measurement points - one per degree of rotation. The second employs what is known as a Lebedev surface and rather than being measured in a fixed plane is measured in X, Y, and Z; a total of 2702 (as I recall) measurements are made. This is a painstaking process that requires an anechoic chamber, a source with known characteristics, and robotic control of the mannequin head. These measurements are then made SOFA-compliant so that plug-ins (such as those from Noisemaker et al) can read the HRTF as a function of angle - this is how the VR stuff is done (in part anyway). In short not all of the HRTF stuff is intuitively obvious, nor is it consistent as a function of distance to the mannequin. However, this should make sense as the near field and far field are two very different things indeed (for any acoustical situation, not just binaural), and when one thinks about the size of the 'melon' (the mannequin head) it is appreciable as compared to a typical microphone. Thus, the HRTF is affected by the pinnae (and where they are placed), the head geometry, whether the shoulders are present or absent, the angle at which sound is presented, the frequency, and the distance. In short, a lot more than what normally goes on in a gradient (cardioid) pattern or a velocity (figure-8) pattern.]

This frequency shaping may instinctively make you steer away from binaural, if one is interested in making natural sounding music recordings. But the binaural technique not only CAN produce natural recordings, but it is REQUIRED if one is really serious about the degree of naturalness they want in their recordings. [Readers wanting to know about the treble lift described elsewhere should refer to my post about the differences in available equalizations - these are particularly important for binaural as they affect the timbre of the recorded signal. The KU 100 is equalized to have flat diffuse-field response. This is where many DIY mannequin head builders fall down. Without the proper EQ the timbre just won't be right. This can be EQ'd out by trial and error, but Neumann chose to make the response flat in a diffuse field, which seems a reasonable compromise. Keep in mind that the directivity of the microphone is not affected by the EQ - those things are affected by the physical and mechanical properties of the mannequin head - the timbre comes from the overall EQ. Again, refer to my post on EQ and why each is done for more detail.]

This frequency shaping may instinctively make you steer away from binaural, if one is interested in making natural sounding music recordings. But the binaural technique not only CAN produce natural recordings, but it is REQUIRED if one is really serious about the degree of naturalness they want in their recordings.

What's required with binaural recordings is a reverse EQ filter applied to the recorded sound in the mixing phase, to undo the fact that when you listen to the recording, there is another EQ filter applied, which is your own ears.
So, why go through the trouble to apply the frequency boost to the recorded material physically, using a physical boundary, if you are then required to undo it with an additional filter before being able to listen to it?

There two main reasons:

1. If the head replica is done right, you get encoded in your L and R channels all the spatial cues that make it possible to hear a 3D sound stage when played back (provided that the reverse filter above is done right too).
Spaced pair and Jecklin disc try to approximate these cues and are successful to an extent, but they are not able to encode in their L and R channels all the time shift, amplitude, frequency response and phase differences that your left and right eardrums experience when you actually are there listening to the sound that you are trying to faithfully record.

[In essence, yes. The idea behind the Jecklin is to try and emulate the shadowing of binaural (this is the purpose of the absorptive boundary between the mics) and the somewhat forward-looking pattern of one's ears (this is why a cardioid is often used). However, an object such as a mannequin head (an ovoid shape, for instance) will reflect and refract sound in different ways than will a disc. A lot of people have modeled this in mannequin heads using BEM (boundary element methodology) in an attempt to better understand the near-field HRTFS and how those vary with respect to the far-field HRTFs). Anyway, the fact that the playback is meant to be delivered by headphones has its roots in the fact that crosstalk (and watch out...some headphones have crosstalk built in) must not be allowed to exist if binaural is to be rendered properly. This is why when using binaural with speakers, one has to sit very close to the speakers (and in a very well-damped space in proximity to the speakers) or construct a plane to prevent crosstalk (consult the works of Don D.B." Keele, AES Member). If you think about it, sitting in the near field (very close to the speakers) in a room that has been properly treated (LE/DE with speakers in the DE) approximates the headphone experience because most of the damaging early reflections that would otherwise cause crosstalk are mitigated by the absorptive materials that line the walls of the DE area. In short, you are sort of sitting inside a giant pair of headphones at that point, facilitated by the small distance between you and the speakers, and the minimal amounts of crosstalk that result due to the absorptive treatments on the walls.]

2. The second reason is more of an indirect benefit. The S/N is increased by about 15-20 dB A-weighted!
This is something that is most of the time overlooked, but very important. By increasing the power of the sound at the frequency most sensitive to humans, let's say 15 dB at 4 kHz, when the reverse filter is applied, the power of the noise at that frequency is lowered by 15 dB.
This is because the power of the sound is increased by the physical boundary, so that there is an effective 15 dB S/N increase at that frequency in the signal recorded by the microphone.
This is huge, as it allows to use smaller microphone capsules which are not that quiet if used in traditional ways. The smaller capsules are required if someone is to make proper binaural recordings, as they are comparable to the size of our eardrums.
Using 10+ mm capsules requires the boundary geometry to get distorted too much, and jeopardize the recreation of accurate HRTF functions at the capsule's location (try to fit 10 mm capsules at the eardrum position, and your ear canal needs to necessarily stretch as a consequence, changing its shape and consequently the HRTF).
Smaller capsules also have higher SPL handling, which is nice when you consider that the SPL they receive is boosted 15-20 dB for the reasons above mentioned. For example, Panasonic WM-61A already saturate with a 6 ft away piano recording when used in a binaural microphone.

[Again, a review of the various EQ schemes used / available in commercially-avaailable mannequin heads should be reviewed here. There is an enormous difference between the free-field response (for example) and diffuse field response. Please remember, in this context these mean 'equalized for flat response in a free field and equalized for flat response in a diffuse field'.]

Neumann KU 100 probably has the above mentioned reverse EQ embedded in its circuitry, and that's why it can sound natural with musical content too.
The good news is that one doesn't need to spend close to $ 10,000 to get the same or better results. A matching EQ filter can do the trick (and with less added noise than passive circuitry).

[Again, the Neumann is equalized for flat diffuse field response. That is, the FRF is computed between a reference microphone (an omni) and the ears for a variety of locations as tested in a reverberation chamber. In the end a bunch of FRFs are averaged so that the inverse of that FRF's shape can be constructed in a hardware filter, which is an analog filter within the KU 100 itself. Other mannequin heads from other vendors achieve similar results either though an analog filter (Bruel & Kjaer use a specific outboard mic supply that toggles the response from free field to diffuse field, in hardware, Head Acoustics does some of the equalizations in software and some in hardware (and also allows for 'user EQ').]

Speaking of accurate HRTFs, as I was saying required for a natural reproduction of the 3D sound stage, a true to life boundary geometry is necessary.
For this reason I have (proudly, I must say) created the first anatomically accurate pinna + ear canal 3D printable models available out there.
These can be used with any mannequin head of reasonable dimensions (most of them are too small, so some time sorting out the right one is highly recommended).

[I am not certain about that. Years ago, ISO had a working group on the geometry of the mannequin ears. The issue was - as I recall - that many on the committee worked for providers of mannequin head microphones. Thus, each had a sort of vested interest as to what 'proposed geometry' was the 'right' geometry (lots of players' technical reputations on the table and at risk...politics...), trying to take into account a median anthropomorphology of the pinnae, as defined by various (medical) databases kept by universities and such. The idea was to use the median as everyone's pinnae are different (and of course, this remains a reason why some would argue that your own ears should be used for your own binaural recordings. This of course solves the issue for you, but continues the same issue of the mannequin's hears being different from those of the listener. I cannot recall where ISO ended up with this, but suffice to say that the pinnae are but one part of the overall HRTF. That is, assuming one had the 'correct' pinnae (based on what criteria ?). Keep in mind that some of the most expensive and most respected mannequin head mics (those made by Head Acoustics) do not even have a pinna that is close to that of a person - they are essentially in the shape of apple wedges. I would argue that virtually no humans on the planet have pinnae shaped that way, but again, the pinnae are (in my opinion) but one element in a rather involved series of elements.]

https://www.shapeways.com/shops/bina...icrophone-ears

With my ear models one no longer has to settle for a cheap makeshift DIY microphone, but rather they can build a professional quality one for pennies on the dollar.

[I think the ears that you have shown here look great. Well done. However, I would argue (as would others) that the shape of the pinna is not the be all and end all to binaural. Is it important ? Of course. Should one strive to have an ear that is anatomially correct ? Sure. However, we're right back to the issue of the mannequins' ears being different than those of the listener. Personally (based on having worked with binaural for around 25 years now) I think the ears are important, but again, there are so many other things that have to be addressed, especially if the near-field HRTFS are to be accurate (again, the far field HRTFS are much more stable as a function of angle, frequency, and so on when compared to the near-field results). When you get right down to it only a portion of the real cost in a mannequin head is in the bits (parts). What you are paying for are things like approved measurement facilities (reverb and anechoic chambers can easily cost $1,000,000 US) let alone the cost of the data acquisition systems, transducers, and other associated bits of hardware, not to mention as well as the salaries of the scientists and technicians who research these issues, many of whom regularly write peer-reviewed articles for journals associated with the A.E.S., A.S.A., I.N.C.E. et al. In other words, there's a reason - a business reason - why such microphones are as expensive as they are. Can you find 'great value' mics ? Of course. Can you make inexpensive ears ? Certainly. However, there are many other factors that one must consider and remedy / address, but I do believe that for a mannequin head to sound 'natural' for music, a frequency response that is close to flat diffuse field will probably sound more natural than one that has not had this EQ adjustment made. Is diffuse field 'correct' ? I wrestle with this all of the time, at least intellectually. I mean, when I use my KU 100 for live recording, it's never in fully diffuse field conditions - what would be the point of playing music in a reverb chamber ? Conversely, the conditions are almost never free field, so that doesn't seem like the right EQ either (and trust me, it's not...the FF equalization has a very specific function in the test and measurement world). Having said that I treat the EQ in the KU-100 not as a sacred cow, but like any mic. That is, I EQ until I arrive at the desired timbre from the mic. In the end, what matters most is how it sounds, pure and simple. Also, as has been pointed out, the 'right' EQ often has the benefit of improving the SNR as often times the mids / highs end up being shelved or notched (again, not a true correction...just an adjustment), and thus, the noise as well as the signal (in the EQ range) are attenuated.]

Can DIY recordings really sound that good? I actually had one of my binaural recordings used in an audiophile speakers listening session and, as difficult as it is to please those audiophile people, I was pleasantly surprised of how it stacked up against much more 'boutique' labels.
This article mentions that recording.

[They can sound really good...again, in my opinion, as long as one starts with relatively flat, quiet mics, and one is willing to EQ to arrive at a pleasing timbre. Good luck with your ears. I am glad that there appears to be a source for 'reasonable' ears so that experimenters have some options. They do look similar to those made by the guy selling mannequin heads out of Poland (though truth be told those look a LOT like the ears from the KU 100 - kind of makes me wonder if he laser-scanned the ears on a KU 100 and then used that process to generate CAD data for an injection mold process... Anyway, the ears that you make look nice.]

Flying Blind: Another Visit to the Magnepan Skunkworks - The Audio Beat - www.TheAudioBeat.com
Old 10th April 2017
  #24
Gear Head
 

Wow! Lot's of info and considerations, Mark.
Thank you very much.
I don't know where to start, so I guess I'll do that from the top..

[The HRTF is a rather involved set of filter 'arguments' that are a function of X, Y, and Z (at a constant distance) as well as another arbitrary distance. That is, the HRTF varies spatially as described, but the far field HRTF is significantly different than when the HRTF is measured in the near field. Because of this, much of the HRTF measurements that are now undertaken are done in accordance with the SOFA criteria as outlined by the AES. This means two main methods: the first is simply a fixed elevation but varying azimuth (working your way around the mannequin head) and comprises 360 measurement points - one per degree of rotation. The second employs what is known as a Lebedev surface and rather than being measured in a fixed plane is measured in X, Y, and Z; a total of 2702 (as I recall) measurements are made. This is a painstaking process that requires an anechoic chamber, a source with known characteristics, and robotic control of the mannequin head. These measurements are then made SOFA-compliant so that plug-ins (such as those from Noisemaker et al) can read the HRTF as a function of angle - this is how the VR stuff is done (in part anyway). In short not all of the HRTF stuff is intuitively obvious, nor is it consistent as a function of distance to the mannequin. However, this should make sense as the near field and far field are two very different things indeed (for any acoustical situation, not just binaural), and when one thinks about the size of the 'melon' (the mannequin head) it is appreciable as compared to a typical microphone. Thus, the HRTF is affected by the pinnae (and where they are placed), the head geometry, whether the shoulders are present or absent, the angle at which sound is presented, the frequency, and the distance. In short, a lot more than what normally goes on in a gradient (cardioid) pattern or a velocity (figure-8) pattern.]
This is great information. I was trying to keep it simple and explain why the experienced frequency boost in an intuitive way, but you went all the way and left nothing uncovered here. Very much appreciated!


[Readers wanting to know about the treble lift described elsewhere should refer to my post about the differences in available equalizations - these are particularly important for binaural as they affect the timbre of the recorded signal. The KU 100 is equalized to have flat diffuse-field response. This is where many DIY mannequin head builders fall down. Without the proper EQ the timbre just won't be right. This can be EQ'd out by trial and error, but Neumann chose to make the response flat in a diffuse field, which seems a reasonable compromise. Keep in mind that the directivity of the microphone is not affected by the EQ - those things are affected by the physical and mechanical properties of the mannequin head - the timbre comes from the overall EQ. Again, refer to my post on EQ and why each is done for more detail.]

I follow a different procedure, which has produced very good results for me so far. See below.

[In essence, yes. The idea behind the Jecklin is to try and emulate the shadowing of binaural (this is the purpose of the absorptive boundary between the mics) and the somewhat forward-looking pattern of one's ears (this is why a cardioid is often used). However, an object such as a mannequin head (an ovoid shape, for instance) will reflect and refract sound in different ways than will a disc. A lot of people have modeled this in mannequin heads using BEM (boundary element methodology) in an attempt to better understand the near-field HRTFS and how those vary with respect to the far-field HRTFs). Anyway, the fact that the playback is meant to be delivered by headphones has its roots in the fact that crosstalk (and watch out...some headphones have crosstalk built in) must not be allowed to exist if binaural is to be rendered properly. This is why when using binaural with speakers, one has to sit very close to the speakers (and in a very well-damped space in proximity to the speakers) or construct a plane to prevent crosstalk (consult the works of Don D.B." Keele, AES Member). If you think about it, sitting in the near field (very close to the speakers) in a room that has been properly treated (LE/DE with speakers in the DE) approximates the headphone experience because most of the damaging early reflections that would otherwise cause crosstalk are mitigated by the absorptive materials that line the walls of the DE area. In short, you are sort of sitting inside a giant pair of headphones at that point, facilitated by the small distance between you and the speakers, and the minimal amounts of crosstalk that result due to the absorptive treatments on the walls.]
Absolutely correct! This is why I listen in very near field conditions (all recordings, binaural and not). This brings about another issue, which is the speaker's own frequency response and polar pattern, but that's another story(I am actually in the process of designing a full range single driver speaker to overcome this).

[Again, the Neumann is equalized for flat diffuse field response. That is, the FRF is computed between a reference microphone (an omni) and the ears for a variety of locations as tested in a reverberation chamber. In the end a bunch of FRFs are averaged so that the inverse of that FRF's shape can be constructed in a hardware filter, which is an analog filter within the KU 100 itself. Other mannequin heads from other vendors achieve similar results either though an analog filter (Bruel & Kjaer use a specific outboard mic supply that toggles the response from free field to diffuse field, in hardware, Head Acoustics does some of the equalizations in software and some in hardware (and also allows for 'user EQ').]
My equalization starts with putting the head at the listener's location in a normal stereo loudspeaker setup. I then play white noise from one channel and record the frequency response at the respective ear.
Then I use a software inverse filter plug-in (Logic MatchEQ) and apply it in the mixing phase.
The reasoning for this is the following.
Assuming that the microphone is a good enough replica of my own head, I want the sound wave recorded by the microphone capsule to be exactly what hits my eardrum (that's another reason why the capsule's location has to be at the end of the ear canal).
To make sure this happens, I have to undo the frequency shaping caused by the fact that the microphone signal is played through the speaker and passed through my head a second time (I say a second time under the hypothesis that the microphone is, for all practical purposes, reproducing my head effect when recording).
The best one can do to satisfy the hypothesis above is to use a model of an averaged ear, to my knowledge.

[I am not certain about that. Years ago, ISO had a working group on the geometry of the mannequin ears. The issue was - as I recall - that many on the committee worked for providers of mannequin head microphones. Thus, each had a sort of vested interest as to what 'proposed geometry' was the 'right' geometry (lots of players' technical reputations on the table and at risk...politics...), trying to take into account a median anthropomorphology of the pinnae, as defined by various (medical) databases kept by universities and such. The idea was to use the median as everyone's pinnae are different (and of course, this remains a reason why some would argue that your own ears should be used for your own binaural recordings. This of course solves the issue for you, but continues the same issue of the mannequin's hears being different from those of the listener. I cannot recall where ISO ended up with this, but suffice to say that the pinnae are but one part of the overall HRTF. That is, assuming one had the 'correct' pinnae (based on what criteria ?). Keep in mind that some of the most expensive and most respected mannequin head mics (those made by Head Acoustics) do not even have a pinna that is close to that of a person - they are essentially in the shape of apple wedges. I would argue that virtually no humans on the planet have pinnae shaped that way, but again, the pinnae are (in my opinion) but one element in a rather involved series of elements.]
I would think that approximate realistic dimensions of the head are necessary to encode the correct time shifts at the eardum signals. Shorter distance = less time shift between L and R than there should be.
I agree that the pinna is but one of the pieces of the puzzle. The main thing about my models is actually the anatomically accurate ear canal.
This piece is missing in any of the DIY incarnations I see posted on line and also in any of the professional ones too.
I believe that is really what sets my models apart

[I think the ears that you have shown here look great. Well done. However, I would argue (as would others) that the shape of the pinna is not the be all and end all to binaural. Is it important ? Of course. Should one strive to have an ear that is anatomially correct ? Sure. However, we're right back to the issue of the mannequins' ears being different than those of the listener. Personally (based on having worked with binaural for around 25 years now) I think the ears are important, but again, there are so many other things that have to be addressed, especially if the near-field HRTFS are to be accurate (again, the far field HRTFS are much more stable as a function of angle, frequency, and so on when compared to the near-field results). When you get right down to it only a portion of the real cost in a mannequin head is in the bits (parts). What you are paying for are things like approved measurement facilities (reverb and anechoic chambers can easily cost $1,000,000 US) let alone the cost of the data acquisition systems, transducers, and other associated bits of hardware, not to mention as well as the salaries of the scientists and technicians who research these issues, many of whom regularly write peer-reviewed articles for journals associated with the A.E.S., A.S.A., I.N.C.E. et al. In other words, there's a reason - a business reason - why such microphones are as expensive as they are. Can you find 'great value' mics ? Of course. Can you make inexpensive ears ? Certainly. However, there are many other factors that one must consider and remedy / address, but I do believe that for a mannequin head to sound 'natural' for music, a frequency response that is close to flat diffuse field will probably sound more natural than one that has not had this EQ adjustment made. Is diffuse field 'correct' ? I wrestle with this all of the time, at least intellectually. I mean, when I use my KU 100 for live recording, it's never in fully diffuse field conditions - what would be the point of playing music in a reverb chamber ? Conversely, the conditions are almost never free field, so that doesn't seem like the right EQ either (and trust me, it's not...the FF equalization has a very specific function in the test and measurement world). Having said that I treat the EQ in the KU-100 not as a sacred cow, but like any mic. That is, I EQ until I arrive at the desired timbre from the mic. In the end, what matters most is how it sounds, pure and simple. Also, as has been pointed out, the 'right' EQ often has the benefit of improving the SNR as often times the mids / highs end up being shelved or notched (again, not a true correction...just an adjustment), and thus, the noise as well as the signal (in the EQ range) are attenuated.]
I'm glad you like my models. I want to say that I absolutely don't think that Neumann is trying to rip people off by selling any of their microphones. Simply, I created my models because I couldn't afford one, and in the process I tried to go even a step further with the modelization of an anatomically accurate ear canal.
My models are far from being a 'plug in and hit record' thing. They are simply a tool to perfect DIY binaural microphone builds.
If I were to provide fully operational DIY microphones I would probably put a price tag of at least $2000 on them, so I don't think that a big company as Neumann, with all its overhead, is asking the moon with their price. But... it is a fact that not everybody can afford the price tag. Myself for one.

[They can sound really good...again, in my opinion, as long as one starts with relatively flat, quiet mics, and one is willing to EQ to arrive at a pleasing timbre. Good luck with your ears. I am glad that there appears to be a source for 'reasonable' ears so that experimenters have some options. They do look similar to those made by the guy selling mannequin heads out of Poland (though truth be told those look a LOT like the ears from the KU 100 - kind of makes me wonder if he laser-scanned the ears on a KU 100 and then used that process to generate CAD data for an injection mold process... Anyway, the ears that you make look nice.]
Primo capsules do the trick for me. They are flat and enough quiet to make professional sounding recordings.
I don't know who the guy from Poland is. There are a lot of people selling silicone ears on line. I bought a pair from the 3-Dio guy when I first started (from the digdagga era).
There are some sold for much less on Amazon used for acupuncture, but they are rather sticky to the touch.
None of them, anyway, have an ear canal, let alone an anatomically accurate one.
Old 10th April 2017
  #25
Some links you may find to be of use - 'hope these are useful

[OK.some more comments and links...I hope these are useful.]

Quote:
Originally Posted by sax512 View Post
Wow! Lot's of info and considerations, Mark.
Thank you very much.
I don't know where to start, so I guess I'll do that from the top..

[The HRTF is a rather involved set of filter 'arguments' that are a function of X, Y, and Z (at a constant distance) as well as another arbitrary distance. That is, the HRTF varies spatially as described, but the far field HRTF is significantly different than when the HRTF is measured in the near field. Because of this, much of the HRTF measurements that are now undertaken are done in accordance with the SOFA criteria as outlined by the AES. This means two main methods: the first is simply a fixed elevation but varying azimuth (working your way around the mannequin head) and comprises 360 measurement points - one per degree of rotation. The second employs what is known as a Lebedev surface and rather than being measured in a fixed plane is measured in X, Y, and Z; a total of 2702 (as I recall) measurements are made. This is a painstaking process that requires an anechoic chamber, a source with known characteristics, and robotic control of the mannequin head. These measurements are then made SOFA-compliant so that plug-ins (such as those from Noisemaker et al) can read the HRTF as a function of angle - this is how the VR stuff is done (in part anyway). In short not all of the HRTF stuff is intuitively obvious, nor is it consistent as a function of distance to the mannequin. However, this should make sense as the near field and far field are two very different things indeed (for any acoustical situation, not just binaural), and when one thinks about the size of the 'melon' (the mannequin head) it is appreciable as compared to a typical microphone. Thus, the HRTF is affected by the pinnae (and where they are placed), the head geometry, whether the shoulders are present or absent, the angle at which sound is presented, the frequency, and the distance. In short, a lot more than what normally goes on in a gradient (cardioid) pattern or a velocity (figure-8) pattern.]
This is great information. I was trying to keep it simple and explain why the experienced frequency boost in an intuitive way, but you went all the way and left nothing uncovered here. Very much appreciated!


[Readers wanting to know about the treble lift described elsewhere should refer to my post about the differences in available equalizations - these are particularly important for binaural as they affect the timbre of the recorded signal. The KU 100 is equalized to have flat diffuse-field response. This is where many DIY mannequin head builders fall down. Without the proper EQ the timbre just won't be right. This can be EQ'd out by trial and error, but Neumann chose to make the response flat in a diffuse field, which seems a reasonable compromise. Keep in mind that the directivity of the microphone is not affected by the EQ - those things are affected by the physical and mechanical properties of the mannequin head - the timbre comes from the overall EQ. Again, refer to my post on EQ and why each is done for more detail.]

I follow a different procedure, which has produced very good results for me so far. See below.

[In essence, yes. The idea behind the Jecklin is to try and emulate the shadowing of binaural (this is the purpose of the absorptive boundary between the mics) and the somewhat forward-looking pattern of one's ears (this is why a cardioid is often used). However, an object such as a mannequin head (an ovoid shape, for instance) will reflect and refract sound in different ways than will a disc. A lot of people have modeled this in mannequin heads using BEM (boundary element methodology) in an attempt to better understand the near-field HRTFS and how those vary with respect to the far-field HRTFs). Anyway, the fact that the playback is meant to be delivered by headphones has its roots in the fact that crosstalk (and watch out...some headphones have crosstalk built in) must not be allowed to exist if binaural is to be rendered properly. This is why when using binaural with speakers, one has to sit very close to the speakers (and in a very well-damped space in proximity to the speakers) or construct a plane to prevent crosstalk (consult the works of Don D.B." Keele, AES Member). If you think about it, sitting in the near field (very close to the speakers) in a room that has been properly treated (LE/DE with speakers in the DE) approximates the headphone experience because most of the damaging early reflections that would otherwise cause crosstalk are mitigated by the absorptive materials that line the walls of the DE area. In short, you are sort of sitting inside a giant pair of headphones at that point, facilitated by the small distance between you and the speakers, and the minimal amounts of crosstalk that result due to the absorptive treatments on the walls.]
Absolutely correct! This is why I listen in very near field conditions (all recordings, binaural and not). This brings about another issue, which is the speaker's own frequency response and polar pattern, but that's another story(I am actually in the process of designing a full range single driver speaker to overcome this).

[Again, the Neumann is equalized for flat diffuse field response. That is, the FRF is computed between a reference microphone (an omni) and the ears for a variety of locations as tested in a reverberation chamber. In the end a bunch of FRFs are averaged so that the inverse of that FRF's shape can be constructed in a hardware filter, which is an analog filter within the KU 100 itself. Other mannequin heads from other vendors achieve similar results either though an analog filter (Bruel & Kjaer use a specific outboard mic supply that toggles the response from free field to diffuse field, in hardware, Head Acoustics does some of the equalizations in software and some in hardware (and also allows for 'user EQ').]
My equalization starts with putting the head at the listener's location in a normal stereo loudspeaker setup. I then play white noise from one channel and record the frequency response at the respective ear.
Then I use a software inverse filter plug-in (Logic MatchEQ) and apply it in the mixing phase.
The reasoning for this is the following.
Assuming that the microphone is a good enough replica of my own head, I want the sound wave recorded by the microphone capsule to be exactly what hits my eardrum (that's another reason why the capsule's location has to be at the end of the ear canal).
To make sure this happens, I have to undo the frequency shaping caused by the fact that the microphone signal is played through the speaker and passed through my head a second time (I say a second time under the hypothesis that the microphone is, for all practical purposes, reproducing my head effect when recording).
The best one can do to satisfy the hypothesis above is to use a model of an averaged ear, to my knowledge.

[Not disagreeing with this...but you should really look at this: http://audiogroup.web.th-koeln.de/PU...nd_TMT2016.pdf as it covers a lot of the salient information. I glossed-over one important part of all of the measurement stuff, namely, the Lebedev data are pretty much required by VR types who intend to convolve a three-axis orthogonal velocity signal (basically, B-format ambisonics) to binaural, but to so as as the person using the VR headset turns his or her head (the headsests are equipped with head tracking). You need the Lebedev data in the SOFA format because the other approach works only for a fixed elevation. What you describe is actually two nested functions - there's the ear canal resonance (which is part of the HRTF) but also the frequency and phase characteristics of the speaker...and in-situ, that is, the room boundary conditions comprise a third layer, each of which has to be un-done. This is why there are certain 'phones that are suggested (as based on the work of Nicole, Moeller, Hammershoi (Google those names - you will see) if binaural rendering is to be as 'accurate' as possible. Moeller et al speak about FEC (free-air equivalent) headphones, basically stating that such a phone has an acoustical impedance very close to that of air (in terms of its real and imaginary components). In the end, boundary conditions always matter, and one must always consider them. On that note, another really good primer that you should read is the AES Monograph "Binaural Technology" by Mme. Rozenn Nicole ( AES News » AES Publishes Monograph on Binaural Technology ). You should also purchase a copy of Durand Begault's book - https://www.amazon.com/3D-Sound-Virt.../dp/0120847353 ]

[I am not certain about that. Years ago, ISO had a working group on the geometry of the mannequin ears. The issue was - as I recall - that many on the committee worked for providers of mannequin head microphones. Thus, each had a sort of vested interest as to what 'proposed geometry' was the 'right' geometry (lots of players' technical reputations on the table and at risk...politics...), trying to take into account a median anthropomorphology of the pinnae, as defined by various (medical) databases kept by universities and such. The idea was to use the median as everyone's pinnae are different (and of course, this remains a reason why some would argue that your own ears should be used for your own binaural recordings. This of course solves the issue for you, but continues the same issue of the mannequin's hears being different from those of the listener. I cannot recall where ISO ended up with this, but suffice to say that the pinnae are but one part of the overall HRTF. That is, assuming one had the 'correct' pinnae (based on what criteria ?). Keep in mind that some of the most expensive and most respected mannequin head mics (those made by Head Acoustics) do not even have a pinna that is close to that of a person - they are essentially in the shape of apple wedges. I would argue that virtually no humans on the planet have pinnae shaped that way, but again, the pinnae are (in my opinion) but one element in a rather involved series of elements.]
I would think that approximate realistic dimensions of the head are necessary to encode the correct time shifts at the eardum signals. Shorter distance = less time shift between L and R than there should be.
I agree that the pinna is but one of the pieces of the puzzle. The main thing about my models is actually the anatomically accurate ear canal.
This piece is missing in any of the DIY incarnations I see posted on line and also in any of the professional ones too.
I believe that is really what sets my models apart

[Here is another, apart from the earlier reference, that you should look at: https://home.deib.polimi.it/antonacc...CASSP_16_2.pdf as well as the works of Farina. Believe it or not, Farina has been incredibly generous and has made these presentations and lectures available to the public - this is a treasure trove of information. So much good stuff here that it's crazy. Farina's a very bright cat, and a very nice person as well. Here is the link: http://pcfarina.eng.unipr.it/Public/.../Pres_list.htm

[I think the ears that you have shown here look great. Well done. However, I would argue (as would others) that the shape of the pinna is not the be all and end all to binaural. Is it important ? Of course. Should one strive to have an ear that is anatomially correct ? Sure. However, we're right back to the issue of the mannequins' ears being different than those of the listener. Personally (based on having worked with binaural for around 25 years now) I think the ears are important, but again, there are so many other things that have to be addressed, especially if the near-field HRTFS are to be accurate (again, the far field HRTFS are much more stable as a function of angle, frequency, and so on when compared to the near-field results). When you get right down to it only a portion of the real cost in a mannequin head is in the bits (parts). What you are paying for are things like approved measurement facilities (reverb and anechoic chambers can easily cost $1,000,000 US) let alone the cost of the data acquisition systems, transducers, and other associated bits of hardware, not to mention as well as the salaries of the scientists and technicians who research these issues, many of whom regularly write peer-reviewed articles for journals associated with the A.E.S., A.S.A., I.N.C.E. et al. In other words, there's a reason - a business reason - why such microphones are as expensive as they are. Can you find 'great value' mics ? Of course. Can you make inexpensive ears ? Certainly. However, there are many other factors that one must consider and remedy / address, but I do believe that for a mannequin head to sound 'natural' for music, a frequency response that is close to flat diffuse field will probably sound more natural than one that has not had this EQ adjustment made. Is diffuse field 'correct' ? I wrestle with this all of the time, at least intellectually. I mean, when I use my KU 100 for live recording, it's never in fully diffuse field conditions - what would be the point of playing music in a reverb chamber ? Conversely, the conditions are almost never free field, so that doesn't seem like the right EQ either (and trust me, it's not...the FF equalization has a very specific function in the test and measurement world). Having said that I treat the EQ in the KU-100 not as a sacred cow, but like any mic. That is, I EQ until I arrive at the desired timbre from the mic. In the end, what matters most is how it sounds, pure and simple. Also, as has been pointed out, the 'right' EQ often has the benefit of improving the SNR as often times the mids / highs end up being shelved or notched (again, not a true correction...just an adjustment), and thus, the noise as well as the signal (in the EQ range) are attenuated.]
I'm glad you like my models. I want to say that I absolutely don't think that Neumann is trying to rip people off by selling any of their microphones. Simply, I created my models because I couldn't afford one, and in the process I tried to go even a step further with the modelization of an anatomically accurate ear canal.
My models are far from being a 'plug in and hit record' thing. They are simply a tool to perfect DIY binaural microphone builds.
If I were to provide fully operational DIY microphones I would probably put a price tag of at least $2000 on them, so I don't think that a big company as Neumann, with all its overhead, is asking the moon with their price. But... it is a fact that not everybody can afford the price tag. Myself for one.

[They can sound really good...again, in my opinion, as long as one starts with relatively flat, quiet mics, and one is willing to EQ to arrive at a pleasing timbre. Good luck with your ears. I am glad that there appears to be a source for 'reasonable' ears so that experimenters have some options. They do look similar to those made by the guy selling mannequin heads out of Poland (though truth be told those look a LOT like the ears from the KU 100 - kind of makes me wonder if he laser-scanned the ears on a KU 100 and then used that process to generate CAD data for an injection mold process... Anyway, the ears that you make look nice.]
Primo capsules do the trick for me. They are flat and enough quiet to make professional sounding recordings.
I don't know who the guy from Poland is. There are a lot of people selling silicone ears on line. I bought a pair from the 3-Dio guy when I first started (from the digdagga era).
There are some sold for much less on Amazon used for acupuncture, but they are rather sticky to the touch.
None of them, anyway, have an ear canal, let alone an anatomically accurate one.

Last edited by Mark A. Jay; 10th April 2017 at 06:48 PM.. Reason: Added link to Farina's works
Old 10th April 2017
  #26
Gear Head
 

Thank you, Mark
Tons of info... I remember reading some of Farina's work. It might be a good time to refresh my memory.
The material about VR is interesting, but I have to say I don't have the funds for R&D into it.
I also appreciate the importance you put on near field HRTFs vs far field.
I can see how a modelized head with pinnae and canals, no matter how well thought out, could yield varying results when the source is right next to the microphone.
Luckily, most music is recorded in far field conditions with binaural.
Something that might be interesting for you to consider.. I was recording this soprano and she was moving around a lot. At some point she came less than 2 ft from the microphone, right during one of the most powerfully sung pieces (the only time when Primo capsules showed signs of SPL saturation. That's pretty impressive of them!). Her voice didn't change in tonal character even with such a close source location. I like to think this is a sign that I did things right with my models (and my equalization procedure).
Old 10th April 2017
  #27
Near and Far

Quote:
Originally Posted by sax512 View Post
Thank you, Mark
Tons of info... I remember reading some of Farina's work. It might be a good time to refresh my memory.
The material about VR is interesting, but I have to say I don't have the funds for R&D into it.
I also appreciate the importance you put on near field HRTFs vs far field.
I can see how a modelized head with pinnae and canals, no matter how well thought out, could yield varying results when the source is right next to the microphone.
Luckily, most music is recorded in far field conditions with binaural.
Something that might be interesting for you to consider.. I was recording this soprano and she was moving around a lot. At some point she came less than 2 ft from the microphone, right during one of the most powerfully sung pieces (the only time when Primo capsules showed signs of SPL saturation. That's pretty impressive of them!). Her voice didn't change in tonal character even with such a close source location. I like to think this is a sign that I did things right with my models (and my equalization procedure).
It's funny, but in some ways, a lot of folks know about near-field phenomena, if only as a consequence. For example, we all know about what happens to the LF in a gradient or velocity mic when a) the source has appreciable LF energy, and b) the source draws near the mic.

In a way...the principle of the near field versus far field HRTF are really the same as near and far field concerns with 'normal' mics (save for pressure transducers), but it's the sheer size of the mannequin that makes the near field issue somewhat more relevant than with traditional mics, which are (dimensionally) much smaller than the mannequin head. The shadowing issue is even more important when the source draws near (I like to remind people of the tennis ball-and-flashlight analogy.

Yes, almost everything recorded live (save for spot mics etc) is in the far field. How far this is typically located relative to the critical distance is a matter of taste and necessity. I will say this though...my preference is to try and get the mannequin head as close as I can, unless the client has a very specific sonic vision. I say that because of the strong lateral nature of binaural (and I would argue the same for Jecklin et al) means that as one passes the critical distance, one now starts to get a very 'open' sound, but one that cannot be un-done. Let's face it, one can always add judicious amounts of reverb (and if you have the IR or BRIR of the room in which you are recording, even better as you can use that in a convolution reverb program) but taking it out without adversely affecting the recording is basically impossible. Yes, echo cancellation is done all the time with mobile phones, but that's a special case.

You should also read the works of the people from Knowles and (I think) MIT. It seems they did a bunch of the original research on how the HRTFs morph as the distance changes from far to near field - it's pretty informative. Again, most engineers intrinsically know about near field effects vis a vis proximity effect and such, but binaural is at once 'simple' and yet very complicated (when you get down to what is really going on in the near field).

Like all things, it's a balancing act. I try to get the aesthetic that I want, but at the same time, mindful of the source-to-mannequin distance. Moreover, if I find that I need to supplement / tweak the signals in post to serve an aesthetic, then I have no issue doing so - under such circumstances, I refer to those mixes as 'hybrid' as they are no longer just the left and right ear signals from the mannequin head ears, or have content from some other source - often times, a board feed from a soloists mic can serve as a really nice anchor and complement the binaural spatial aspects - you just need to be careful about levels and EQ, but when done right, it can really help anchor the recording while surrendering little of the spatial aspects. At least, this has been my experience.

Yeah...near versus far field...in Acoustics and noise control measurements, these things can really, really matter, and in the recording world, play a big role in the aesthetic that we achieve in the end.
Old 10th April 2017
  #28
Gear Head
 

I Agree. I use IR convolution filters when the acoustics are not so great. Not all the time we want an exact replica of the real thing.
Unfortunately, great sounding spaces are hard to come by and expensive.

However, the 3D sound stage reproduction capabilities of well done binaural recordings are unsurpassed by any technique.

My experience is also that the timber of the instruments is better reproduced with binaural. Mono compatibility is pretty much a non issue, in all the recordings I have made. I believe this is because the cross talk is less destructive, given the high correlation between L and R. In my opinion, this fact also explains why, for same dBFS levels between a binaural and a, let's say, spaced pair, the binaural tends to sound always a little louder.

There are actually quite a few indirect benefits to a binaural recording, come to think of it..
I think the S/N boost has to be one of the most valued.
Old 11th April 2017
  #29
We're on the same page

Quote:
Originally Posted by sax512 View Post
I Agree. I use IR convolution filters when the acoustics are not so great. Not all the time we want an exact replica of the real thing.
Unfortunately, great sounding spaces are hard to come by and expensive.

[Yep. If you are capturing BRIRs then, of course, you want those to be accurate but as you have stated, sometimes 'truth' can be ugly. One of the things that I like about IR 'verb is that it is a philosophical bridge for me - adding reverb is never 'truth' per se, but if it serves the aesthetic, all the better. Moreover, if you are using IR from the space, then it feels more to me like a 'white lie' rather than an overt falsehood (from a sonic perspective). Mind you, I have indeed used the mannequin head to show someone (via playback) issues with a venue - much easier than explaining decay plots and so on, but hearing is as they say believing.]

However, the 3D sound stage reproduction capabilities of well done binaural recordings are unsurpassed by any technique.

[I tend to agree of course. You can find a lot of my binaural stuff on the immersifi soundcloud page, and other links posted on headfi.org (where I post as immersifi). The soundcloud page also features some hybrid stuff that I have done, as well as a few traditional stereo pair recordings. One of the things that I enjoy most about using headphones is the fact that I can negate room effects. This goes for stereo as well as binaural...but binaural is sort of the 'icing on the cake' (or the 'cherry on the cake' depending upon which side of the pond one finds oneself in terms of euphemisms) when it comes to headphones, of course. Might I make a suggestion ? Farina and I found out via email that we had both been using the same supplemental approach with our KU-100's, namely, and ORTF pair perched atop the KU-100. It can be pretty handy. Moreover, another all-time favorite of mine is to set anotehr mic in the proximal field in an M-S configuration (I use an Avantone CK-40 mic dedicated to this with one capsule set to velocity and the other to gradient. There are some real handy aspects of this approach as it allows you have a solid mono signal, but also (if you look up the paper by Wes Dooley on the subject of M-S you will see) the M-S in effect allows you to synthesize a virtual X-Y pair, whose included angle is a ratio of the M-S signals (you probably are well acquainted with this but for those not so, it is extremely powerful). These are always in my 'go-to' bag of tricks. That is, I find M-S can (when tweaked properly) generate some great stereo content, and when properly mixed and EQ'd with binaural, can actually solve some problems that may exist in a pure binaural rendering. Again...the aesthetic versus the truth.]

My experience is also that the timber of the instruments is better reproduced with binaural. Mono compatibility is pretty much a non issue, in all the recordings I have made. I believe this is because the cross talk is less destructive, given the high correlation between L and R. In my opinion, this fact also explains why, for same dBFS levels between a binaural and a, let's say, spaced pair, the binaural tends to sound always a little louder.

[Maybe. Capsule sensitivity and gain are important of course. Timbre comprises many things, but to your point, we usually experience live music binaurally (with our ears and brains) so I get where you are coming from. Again though, EQ is a huge factor...and it's why I keep going back to why (in my opinion) the best compromise (as a starting point) is to have something that is close to flat diffuse field response. If you ever have occasion, try listening to a recording made on a Bruel & Kajer type 4100 first with its power supply set to diffuse field and then free field (using the same stimulus of course). You will find that the diffuse field EQ is MUCH more pleasing than the free-field, and for good reason (as discussed above in the lengthy post that I wrote about the 'standard' EQs available in most commercial mannequin heads. Anyway, as we have discussed, since we are focused on the aesthetic, I have no issues whatsoever in using any EQ that 'sounds right'. Were we speaking of generating data, then I would not be so cavalier about how the EQ is done.]

There are actually quite a few indirect benefits to a binaural recording, come to think of it..
I think the S/N boost has to be one of the most valued.
[Right on. You should look me up on Linked In or FB sometime...seems our thought processes and artistic / aesthetic visions are similar.]
Old 11th April 2017
  #30
Gear Head
 

Absolutely! If you lived closer to me I would take you out for a couple beers. We could clear the place out with our nerdy talks
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