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

[Umashankar Mantravadi]

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 1st 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 for Windows 10

From: Fernando Lopez-Lezcano
Sent: Monday, June 25, 2018 4:36 AM
To: Marc Lavallée; 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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