[SpHEAR-devel] latest musings (PCB, new Octathingy, calibrationand more)

[Marc Lavallée]

Hi Umashankar (and all).

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."

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).

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).

Marc


Le 2018-06-26 à 08:32 AM, Umashankar Mantravadi a écrit :
>
> Dear Fernando
>
> 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^st order measurement, the 
> matlab scripts needed to create the filtermatrix. One of the first 
> steps the matlab scripts do is use *invert kirkeby* 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.
>
> 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.
>
> Will send you the rotor design when I finish it.
>
> umashankar
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for 
> Windows 10
>
> *From: *Fernando Lopez-Lezcano <mailto:nando at ccrma.Stanford.EDU>
> *Sent: *Monday, June 25, 2018 4:36 AM
> *To: *Marc Lavallée <mailto:marc at hacklava.net>; 
> sphear-devel at ccrma.Stanford.EDU <mailto:sphear-devel at ccrma.Stanford.EDU>
> *Subject: *Re: [SpHEAR-devel] latest musings (PCB, new Octathingy, 
> calibrationand more)
>
> On 06/23/2018 02:20 PM, Marc Lavallée wrote:
>
> > Hi, and thanks for the news!
>
> Hi Marc,
>
> > I started a related project: building a binaural microphone, for
>
> > measurements and recording, using very small Knowles capsules. One thing
>
> > I have to learn is how to calibrate microphone capsules, and I found an
>
> > interesting article about a calibration technique that doesn't require
>
> > an anechoic environment, and I wondered if a similar technique could be
>
> > used for the measurement and calibration of ambisonics microphones:
>
> >
>
> > 
> https://www.scribd.com/document/321928725/Microphone-Calibration-by-Transfer-Function-Comparison-Method
>
> They use SMART which is a (expensive) professional software to run a
>
> realtime transfer function measurement between the two microphones.
>
> > Of course it'd be impossible to place a reference microphone at the
>
> > center of an ambisonics microphone, but I guess it'd be fine to first
>
> > measure the reference microphone (a few times for averaging) then the
>
> > ambisonics microphone by making sure its center is at the same spot than
>
> > the previously measured reference microphone. Does it make sense (or am
>
> > I too optimistic)?
>
> It does make sense and that is what I am doing (and presumably everybody
>
> else), but not in realtime as described in the article. I have a emm-6
>
> calibrated microphone that I use to have a reference measurement by
>
> placing it at the exact same position as the microphone being
>
> calibrated[*] - that measurement is used to create an inverse filter
>
> that "equalizes" the speaker, and that filter is then used to calibrate
>
> all measurements of the Ambisonics microphone...
>
> After that the fun begins - making sense of the data :-)
>
> -- Fernando
>
> [*] well, not exactly the same position but as close as I can get it
>
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>
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>
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