# Re: [Sursound] oktava 1st order mic

```Dear aaron
```
```

I do both. Make matched sets and calibrate the array, for Brahma microphones.
Having an open enough array is an issue I think I have solved (the array shape
has changed over time, without making it significantly larger. I have tried
making tangential arrays to avoid this, but I have not taken it further.

umashankar

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________________________________
From: Sursound <sursound-boun...@music.vt.edu> on behalf of Aaron Heller
<ajhel...@gmail.com>
Sent: Monday, March 12, 2018 3:17:41 PM
To: Surround Sound discussion group
Subject: Re: [Sursound] oktava 1st order mic

The radius of the tetrahedral array determines the frequency at which the
B-format polar patterns start to breakdown. The formula given by Gerzon is
c/(pi*r), where c is the speed of sound and r is the radius of the
array[1]. Depending of the design, the acoustic radius is about 10% larger
than the physical radius, because the sound has to diffract around the
structures. So, in round numbers 10/r kHz, with r in cm. In a Soundfield
mic, the physical radius 1.47cm, so around 6.8 kHz. The Octava is over 4
cm, so less than 2.5kHz.  Note that very small capsules tend to be noisy,
so there is a tradeoff between noise and integrity of the patterns at high
frequencies.

In many of the 3D printed designs, the array is not open enough and the
interior space behind the capsules becomes a resonant chamber. This causes
peaks, dips and phase shifts in the response of the individual capsules
that are difficult to correct and affect the resulting patterns. There is
also the general geometry of the microphone body that tells you how much
care went into the design in terms of acoustic shadowing, reflections, and
diffraction. The large flat surface on the top of the preamp enclosure in
the Octava does not look good to me.

Part of the magic of a tetrahedral microphone is that the free- and
diffuse-field responses track each other. To achieve this, it is important
that the directivities of the four capsules are well matched [2].
Calibration can compensate for this to some degree, but the better the
capsules match, the better the result will be.  The only way to do this is
have a large collection of capsules, measure them individually, pick sets
of four, and then calibrate the entire array.  I know that Core Sound does
this (and Calrec did this). I don't know about other companies. In general,
I am suspicious of any tetrahedral mic that uses generic A-to-B conversion,
with no individual calibration.

[1] M. A. Gerzon, "The Design of Precisely Coincident Microphone Arrays for
Stereo and Surround Sound," 50th AES Convention Preprints, London, no. 20,
1975.

[2] A. J. Heller and E. M. Benjamin, "Calibration of Soundfield Microphones
using the Diffuse-Field Response," 133rd AES Convention Preprints, San
Francisco, no. 7811, 2012.

On Sun, Mar 11, 2018 at 3:13 PM, Peter P. <peterpar...@fastmail.com> wrote:
>
> * Len Moskowitz <lenmoskow...@optonline.net> [2018-03-11 18:48]:
> > Gerard Lardner wrote:
> >
> > > Fons Adriaensen in Italy calibrated my Oktava. I believe Richard Lee
in
> > > Australia might still offer a calibration service, though he appears
to
> > > be less active on the internet these days, and I think Core Sound in
the
> > > USA also will do it - they used to say it on their website, but I
> > > haven't checked lately.
> >
> > We could, but in general we can confidently state that Oktava doesn't
> > understand how to build a first-order ambisonic microphone, and the
cost and
> > effort to calibrate it is not worthwhile.
>
> Thank you for your opinion Len. I am tempted to ask 'why' but let me
> ask instead what are the most difficult things to get right when
> building a first-order microphone.
>
> best, P
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