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<p>Hi, and thanks for the news!<br>
</p>
<p>The new designs are just beautiful. I like the "flower power"
one; it's so Californian... ;)</p>
<p>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
(<a class="moz-txt-link-freetext" href="https://iaem.at/kurse/projekte/iemkit/aama">https://iaem.at/kurse/projekte/iemkit/aama</a>)</p>
<p>I would appreciate opinions on this:<br>
</p>
<p>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: <br>
</p>
<p><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>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)?</p>
<p>Marc<br>
</p>
<br>
<div class="moz-cite-prefix">Le 2018-06-23 à 04:43 PM, Fernando
Lopez-Lezcano a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:0f242ca0-56a5-464c-cc25-6ee714070650@ccrma.stanford.edu">Hi
all,
<br>
<br>
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...
<br>
<br>
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.
<br>
<br>
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).
<br>
<br>
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? :-).
<br>
<br>
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).
<br>
<br>
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)...
<br>
<br>
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.
<br>
<br>
More about measuring...
<br>
<br>
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.
<br>
<br>
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...
<br>
<br>
Comments welcome...
<br>
-- Fernando
<br>
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