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<p>Hi Umashankar (and all).</p>
<p>I did not forget the Matlab scripts I converted to Octave... I
just postponed my ambisonics related projects, partly because of
the difficulties to properly measure and calibrate a microphone. I
must admit that I never try to calibrate a microphone, and I yet
have to understand the Matlab scripts (converting a script does
not imply mastering it)... But it's never too late! I just found
the precious scripts in a folder named "brahmatlab"... ;-)
Publishing the scripts (properly edited and tested) as free
software would be great (if Angelo would allow it). Also, there's
probably other methods to calibrate a microphone, but I'm not
holding my breath; as Fons recently wrote on sursound: "So unless
you want to spend a few months full time to develop and test all
of this, I'd strongly advise to not even try it."<br>
</p>
<p> Because building an ambisonics microphone is a lot of work (even
the ones from the SpHEAR project), I got a cheap Twirling720 Lite;
it looks more a toy than a proper microphone, but I like its form
factor, its built-in USB interface, and maybe its a good enough
device to hack in order to learn the subtle art of measurement and
calibration. I just hope there's no processing between the
capsules and the USB interface (something to discover).<br>
</p>
<p>Fernando, I was asking about the article I found because I like
the idea of using a reference microphone to calibrate another one
(with a transfer function). So i wondered if a set of individually
calibrated capsules (using the same reference microphone) could be
used without further measurements to make a calibration profile
based on a theoretical ambisonics microphone, knowing the distance
between the capsules and the center of the array; I guess it would
fail because the capsules holder (and other capsules) are not
acoustically transparent. The devil is in the details (n'est-ce
pas?). Please excuse my naïve questions (this is the best way for
me to learn).<br>
</p>
Marc<br>
<br>
<br>
<div class="moz-cite-prefix">Le 2018-06-26 à 08:32 AM, Umashankar
Mantravadi a écrit :<br>
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<p class="MsoNormal">Dear Fernando</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am enclosing photograph of my 16 position
rotator, based on an Arduino card and stepper motor. It works
reliably and most importantly has almost no reflections when
recording test sweeps. (it was quite a problem in earlier
designs.) I measure an earthworks Measurement microphone
before and after the test sweeps. I use angelo’s log sine
sweep method, and for an eight position 1<sup>st</sup> order
measurement, the matlab scripts needed to create the
filtermatrix. One of the first steps the matlab scripts do is
use <b>invert kirkeby</b> to match the measurements to the
earthworks microphone. Angelo thinks that with 16 horizontal
positional measurements it should be possible to create a 8 x
8 filtermatrix for a second order microphone in the octathingy
style.</p>
<p class="MsoNormal">Nevertheless I am working on a two axis
rotator, using three steppers, With rigid carbon fiber rods
and minimal structures at about 30 cm from the array under
test, I should be able to control reflections. Angelo’s method
is to cover all the surfaces with acoustic foam, but I think
it would be better to minimize the reflections. Marc has seen
the matlab scripts (he had in fact converted them to Octave,
but he seems to have forgotten.</p>
<p class="MsoNormal">Will send you the rotor design when I
finish it.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">umashankar</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Sent from <a
href="https://go.microsoft.com/fwlink/?LinkId=550986"
moz-do-not-send="true">Mail</a> for Windows 10</p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="border:none;padding:0in"><b>From:
</b><a href="mailto:nando@ccrma.Stanford.EDU"
moz-do-not-send="true">Fernando Lopez-Lezcano</a><br>
<b>Sent: </b>Monday, June 25, 2018 4:36 AM<br>
<b>To: </b><a href="mailto:marc@hacklava.net"
moz-do-not-send="true">Marc Lavallée</a>; <a
href="mailto:sphear-devel@ccrma.Stanford.EDU"
moz-do-not-send="true">sphear-devel@ccrma.Stanford.EDU</a><br>
<b>Subject: </b>Re: [SpHEAR-devel] latest musings (PCB, new
Octathingy, calibrationand more)</p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">On 06/23/2018 02:20 PM, Marc Lavallée
wrote:</p>
<p class="MsoNormal">> Hi, and thanks for the news!</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi Marc,</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> I started a related project: building
a binaural microphone, for</p>
<p class="MsoNormal">> measurements and recording, using very
small Knowles capsules. One thing</p>
<p class="MsoNormal">> I have to learn is how to calibrate
microphone capsules, and I found an</p>
<p class="MsoNormal">> interesting article about a
calibration technique that doesn't require</p>
<p class="MsoNormal">> an anechoic environment, and I
wondered if a similar technique could be</p>
<p class="MsoNormal">> used for the measurement and
calibration of ambisonics microphones:</p>
<p class="MsoNormal">><o:p> </o:p></p>
<p class="MsoNormal">>
<a class="moz-txt-link-freetext"
href="https://www.scribd.com/document/321928725/Microphone-Calibration-by-Transfer-Function-Comparison-Method">https://www.scribd.com/document/321928725/Microphone-Calibration-by-Transfer-Function-Comparison-Method</a></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">They use SMART which is a (expensive)
professional software to run a </p>
<p class="MsoNormal">realtime transfer function measurement
between the two microphones.</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> Of course it'd be impossible to place
a reference microphone at the</p>
<p class="MsoNormal">> center of an ambisonics microphone,
but I guess it'd be fine to first</p>
<p class="MsoNormal">> measure the reference microphone (a
few times for averaging) then the</p>
<p class="MsoNormal">> ambisonics microphone by making sure
its center is at the same spot than</p>
<p class="MsoNormal">> the previously measured reference
microphone. Does it make sense (or am</p>
<p class="MsoNormal">> I too optimistic)?</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">It does make sense and that is what I am
doing (and presumably everybody </p>
<p class="MsoNormal">else), but not in realtime as described in
the article. I have a emm-6 </p>
<p class="MsoNormal">calibrated microphone that I use to have a
reference measurement by </p>
<p class="MsoNormal">placing it at the exact same position as
the microphone being </p>
<p class="MsoNormal">calibrated[*] - that measurement is used to
create an inverse filter </p>
<p class="MsoNormal">that "equalizes" the speaker, and that
filter is then used to calibrate </p>
<p class="MsoNormal">all measurements of the Ambisonics
microphone...</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">After that the fun begins - making sense of
the data :-)</p>
<p class="MsoNormal">-- Fernando</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">[*] well, not exactly the same position but
as close as I can get it</p>
<p class="MsoNormal">_______________________________________________</p>
<p class="MsoNormal">SpHEAR-devel mailing list</p>
<p class="MsoNormal"><a class="moz-txt-link-abbreviated"
href="mailto:SpHEAR-devel@ccrma.stanford.edu">SpHEAR-devel@ccrma.stanford.edu</a></p>
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href="https://cm-mail.stanford.edu/mailman/listinfo/sphear-devel">https://cm-mail.stanford.edu/mailman/listinfo/sphear-devel</a></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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