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

[Fernando Lopez-Lezcano]

On 06/26/2018 05:32 AM, Umashankar Mantravadi wrote:
> Dear Fernando

Hi!,

> I am enclosing photograph of my 16 position rotator, based on an Arduino
> card and stepper motor.

Nice! Thanks for the picture...

>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.

I'm doing something similar with an emm-6 calibrated omni used as a 
reference...

>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.

Horizontal only measurements? Weird. At the beginning of my tests of the 
Octathingy I did just that, as I did not have a measuring rig that could 
do elevation. With 16 horizontal plane measurements I calibrated 
W/X/Y/U/V quite well. But for calibrating Z/S/T you would need 
measurements above and below the horizontal plane (or maybe - just maybe 
- above only if you can exploit the vertical axis symmetry). Those 
components have a null in the horizontal plane so horizontal plane only 
measurements give you no information about them (for R there is no hope 
as it aliases to W with that configuration of capsules).

The only way to do that that I can imagine is if you know or previously 
measured (in full 3D) a similar microphone, and use the 16 measurements 
to derive the capsule gains and horizontal components calibration, and 
then assume that the behavior for the other spherical harmonics is 
similar to the ones you measured before (and more or less replicate the 
same filters).

> 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.

Yes, much better to not have reflections at all. We need to levitate the 
microphone!

> Will send you the rotor design when I finish it.

It'd be nice to see what you come up with!
Best,
-- Fernando