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GGBoCa 30th June 2020 08:08 AM

Stereo microphones charts (once again)

First post, first question. I’m a reader for years, but I don’t find what i’m looking for now. Time to create an account.

It’s about a very original subject : stereo recording. When I have to record something, use for years Michel Williams diagram from the well known “Stereophonic Zoom”.

Before continue, I said now this is more about curiosity and understanding of technical concepts, more than artistical point of vu. I already know what I like, or what I don’t.

Let’s back to the subject. There is other charts available. For exemple :

They all look pro, and at least 2 of them come from serious microphone manufacturers. But, if I understand well (and maybe I don’t), they do not have the same conclusion. At all.

If we take the simplest sitation, the AB omni, because we don’t care about the directivity (theorically speaking, let’s say that) there is such a huge difference on the angle of recording, I just ask myself, how it’s possible ?

If I’m right, the distance between 2 mics to have a 180° recording angle is :

38cm for Williams (and Rycote)
51cm for Tian Hejia
51cm for Neumann
28cm for Schoeps

The distance to the source doesn’t really matter I guess. Is there a frequency impact on this ? I mean, one of them take the signal between 20Hz and 20kHz, and the second one is focus on 1kHz ? The the Williams results are based on listening tests, where other ones are calculate.

Two years ago I made some tests only for omni AB with double blind test with some collegues and I found (modestly) that Williams say the truth. But it was only for AB. I took lot of time to make it right and I only change distance between mics.

Thanks for reading !


jimjazzdad 3rd July 2020 11:34 AM

I can't answer the question but, like you, I have always relied on Williams' Stereophonic Zoom paper or Sengpiel (seems to agree with Williams). I suspect that most of the research was done in anechoic chambers or other laboratory settings, so the calculations reflect the ideal rather than real world...

deedeeyeah 3rd July 2020 12:22 PM

the discrepancy between what various prediction tools, the literature or your experience tell you stems from how measuring data gets smoothed, weighted and interpreted: when you get to see an fft, you might be surprised how spot on or how far off things measure regarding specific frequency areas!

on the mixing side (i know, you're more interested in the recording side or actually the setup before recording), using allpass filters can both reveal and mitigate (some) issues stemming from different spacings between mics.

on the setup side and in practical terms, imo one doesn't need to deal with more than two or three setups unless one is recording in a room with a severe issue or happens to put mics exactly in a strong room mode: to find out about the former and to avoid the latter, you need additional tools though as none of the 'mic apps' teach you anything about this.

one can however learn to interpret measurement data and then decide on positioning and spacing of mics based on measurements which is what do IF i have all the necessary gear and enough time available...

tourtelot 3rd July 2020 07:53 PM

And then, and not to be sarcastic, there is always "does this sound good?"


GGBoCa 4th July 2020 01:54 PM

@ tourtelot : and you are :lol:

It's exactly the part of the subject I don't want to talk about. Even if, obviously, you're right.

In this story, what's completely crazy in my point of view, is stereo recording is something I learned at highschool (15 years ago now). It's like the beginning of everything my job. The base. Specially in my domain, classical music.

And of course it's not as simple it looks. We can improve our knowledge about it recording after recording. And it's normal to take years of practice to have pro results. But until now, I modestly met quite a lot of great sound engineers during this last years. And no one is really able to explain this common fact. Even if, once again, not precisely understand what technically happened don't prevent to make great recordings and have good empiric knowledge.

Maybe I didn't find the good person. Maybe I will on GS. I hope !

Thanks anyway for your answers !
Keep digging !


David Rick 4th July 2020 08:16 PM

Stereo Array Design as an Imperfect Art
1 Attachment(s)
The various tools available for designing stereo microphone arrays do indeed produce differing results. That's because the underlying psychoacoustic data are scattered and inconsistent. The results depend on the type of program material used (clicks, tone bursts, instruments, etc.) and the conditions under which those data were acquired. I've excerpted a couple a graphs from Wittek & Theile's 2002 convention paper that illustrate this clearly. The complete paper is well worth reading; you can find it here. Not only are the experimental curves different from one another, but they are all non-linear. The authors point out that the worst-case deviations occur at the outside edges of the stereo image, but that there is much more consistency if only the middle 75% of the image width is considered because the localization curves are much more linear there.

There is also confusion between the ~1 ms ITD found by many researchers to produce full left (right) imaging and the ~ 1.5 ms ICTD assumed in the Neumann and Sengpiel apps. To further confuse matters, charts for omni arrays based on the 1 ms assumption appear elsewhere on the Sengpiel website! I believe the ICTD number to be what's required for playback over typically-arranged loudspeakers, while the ITD limit was derived in headphone testing.

In actual field practice, I find that the Neumann Recording Tools app produces more reliable results for me than the Williams curves. In the years that I used the latter, a second, post audition tweak was almost always required to give good results. The fact that the Neumann app runs on my phone is a big advantage.

I use the Schoeps Image Assistant app when I need to design LCR arrays. Its prediction of the central "flat spot" in the localization curve of an OCT array matches well with my experience. I use it less often in the field, because it requires web access.

The MARRS app, written at the University of Huddersfield, is also available. It is noteworthy in that it allows you to have the mic array at a different height than the ensemble, and to accommodate wide playback baselines. The underlying theory is explained here.

David L. Rick
Seventh String Recording

tourtelot 4th July 2020 10:50 PM

I don't think it has been mentioned here but the Mic Tools app is also one to check out and my go-to when I need to look at an app. :)


GGBoCa 5th July 2020 12:05 PM


Thanks to both of you for your messages.

The Mic Tools app is one iPhone, and I do not have one, but thanks anyway.

David, thanks for your very complete answer. It start to be more clear in my small brain. When I did my tests for AB, I used a bunch of keys, so my personal results can be useful for the high frequencies and maybe less for the others.

The Huddersfield's app looks great. I'll will compare it to the rest and read carefully the paper. I'll will certainly come back to talk about it soon.


GGBoCa 5th July 2020 08:47 PM


I finally read Lee's paper. And thanks again, it's quite exactly what I'm looking for. It explains why there is differences on the same models a talked before. And even better, it seems to increase the level of correctness.

Now I just have to try. But maybe, if it sounds good, Hyunkook Lee will be my new reference. After more than 30 years of Williams domination (ok i'm 36...). I'm quite enthusiastic at the idea it's possible to increase our level of knowledge on a subject (stereo recording) I though, and obviously I was wrong, totally understand and measured.


studer58 6th July 2020 06:13 AM


Originally Posted by GGBoCa (Post 14839812)
The Mic Tools app is one iPhone, and I do not have one, but thanks anyway.

It could even be worth buying a very old used iPhone, for little money, just to have this app in your pocket, and use the phone for nothing else !