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

[Marc Lavallée]

Hi, and thanks for the news!

The new designs are just beautiful. I like the "flower power" one; it's 
so Californian... ;)

I still hope that one day I'll be able to build and calibrate some sort 
of ambisonics microphone, so the SpHEAR project is on my radar, along 
with the AAMA project (https://iaem.at/kurse/projekte/iemkit/aama)

I would appreciate opinions on this:

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

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)?

Marc


Le 2018-06-23 à 04:43 PM, Fernando Lopez-Lezcano a écrit :
> Hi all,
>
> I have been meaning to send an update about the project for a loooong 
> time... Many many changes (not all of them in current git yet). I'll 
> try to summarize...
>
> I have a new design for the Zapnspark phantom power printed circuit 
> board. I decided to make a double sided board as I was going to 
> manufacture a few (double sided would be much harder with our small 
> mill), and then went overboard and tried to make it as small as 
> possible without using surface mounted components. I found some 
> smaller form factor capacitors, which helped shrink it.
>
> Using those, the four capsule microphone is much smaller, and the 
> Octathingy benefits as well (smaller but not quite as much). There is 
> a control variable that lets you switch between the two PCB sizes, and 
> the assemblies mostly scale (but work is not finished yet - for 
> example I need new windshield and shock mount 3d models).
>
> Speaking of the Octathingy. I have been working on the calibration 
> side a lot. Looking at many graphs made it more apparent that the 
> initial design, which is a straight scale up from the four capsule 
> design, was "clever" but not the best possible due to the resonance 
> from the cavity formed by the capsules (but then Rode, SPS, Core Sound 
> and others of course already knew about that - hmmm, but not 
> Senheisser? :-).
>
> So I went ahead with a newer "flower power" or "trumpets of doom" 
> design with individual conical capsule holders and the smallest 
> practical spherical core they could plug into. A measurement of a test 
> 3D print (all dummy capsules except for one) showed better performance 
> at high frequencies. Just yesterday I finished building the first full 
> prototype, now I have to measure it (see pictures, all three 
> prototypes and the comparison of the array between the last two).
>
> Measuring... oh so not easy. I have to work on compensating the low 
> end of the measurement frequency range as that is currently (I think) 
> the weakest link in the calibration data. Juan Sierra here at CCRMA 
> helped me understand Eric's paper on the matter, and it looks like I 
> was doing the inverse filter and first reflection trimming in the 
> wrong order (to begin with!). I have to see which type of window is 
> best (right now using a full blackman centered on the impulse)...
>
> The current Octathingy calibration includes equalizing the 8 
> individual capsules, deriving an 8x8 matrix of A to B filters for low 
> and mid frequencies, and finally creating 8 individual filters for the 
> B format components at high frequencies, all collapsable into a single 
> 8x8 matrix of filters.
>
> More about measuring...
>
> I (well, CCRMA) bought a small robotic arm. And customized it to be a 
> better measurement rig. The inverse kinematics are now working 
> correctly and the arm is now pointing the microphone in the right 
> directions. The arm is a bit short (longer would have been much more 
> expensive), but with the current dimensions, and depending on how it 
> orients the microphone, I can get vertical coverage up to around +/-40 
> degrees of elevation (and complete horizontal coverage of course), or 
> +60-20 or so. We'll see what's best, or if both can be combined.
>
> The arm is controlled from a SuperCollider program which does the 
> inverse kinematics and will also will play back the sine sweeps and 
> record the output of the capsules. Hopefully easier and more 
> repeatable at the end of the road (which is still long). I really need 
> better data...
>
> Comments welcome...
> -- Fernando
>
>
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