I have been using a stepper motor (of the kind used in 3d printer ) driven
by a low cost Arduino and motor control board. I 3d print a snug fitting
fixture for the microphone with the motor shaft aligned to the array
centre. It is low cost so I design a fitting for each mic I test, including
the Brahma-in-Zoom. A small Arduino script rotates the stepper 25 steps
each time I press a button (for 16 positions) and 50 steps (for 8
positions). I was worried about the stepper skipping with the weight of the
microphone, but that is not happening, even with a five volt supply. I was
ready with a thrust bearing between the motor housing and the microphone
housing but it was not necessary. I plan to get rid of the switch and use a
pulse on the right channel instead, though I generally do not like to
automate things too much.
umashankar
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________________________________
From: Sursound <sursound-boun...@music.vt.edu> on behalf of Fernando
Lopez-Lezcano <na...@ccrma.stanford.edu>
Sent: Tuesday, April 24, 2018 1:40:56 AM
To: Surround Sound discussion group
Subject: Re: [Sursound] RIR measuring, how to capture a higher order
Ambisonic room responce?
On 04/23/2018 12:42 PM, Stefan Schreiber wrote:
I can do the 4 measurements with 45 degrees rotation of my tetramic,
that
is not so difficult, the next step to create a second order ambisonic
RIR
that is where I will fail :-).
You would need to "calibrate" the created 8 capsule array. That is,
record impulse responses all around it in a big space or anechoic room
(enough to accurately sample the spherical harmonics you want), and then
derive an A to B converter from that. I have some preliminary code in my
SpHEAR project that tries to do that, but it is not a "push a button and
you are done" thing at all...
For Fons's code, and to do this the "right way"...
On 03/27/2018 01:18 PM, Fons Adriaensen wrote:
... you'll have to sell your soul :-)
:-P
I believe you might need a quite high precision to be successful even at
the first step...
(A SF mike has narrowly spaced capsules, and needs calibration....The
mechanical precision you need to measure 2nd order with a FOA mike is
IMHO high.)
Based on my experience with the Octathingy's I have built I would agree,
you would need to be very precise (and repeatable).
In my case to get good calibration data I need to rotate the microphone
with no wobble and at different orientations (or if it is not _exactly_
perfect, try to get away with calibrating out the average delays to all
capsules).
BTW, I cannot move the speaker around which would probably be a better
solution because of space constraints... I can barely get 4.5mSecs of IR
data in the spaces I can use.
So the mathematical methods (based on FOA but improving the RIR
resolution, as suggested by Archontis) should be a better way to go
on... Especially since you could receive even higher resolutions/orders,
and in practice.
So the presented ideas to capture 2nd order RIRs via a 1st order mike
are brilliant, but are they practical?
Probably not practical IMHO.
And even if somebody could succeed in a very careful process: this does
not look to be a robust measurement method. ..
We always talk about the 1st reflections, in this case. Not reverb,
which is kind of statistical.
Of course you can try, but how much precision is really needed? (Should
be clarified before...)
I would have to go to my data to get some numbers... I definitely can
see effects at high frequencies when the data capture is not precise
(I'm in the process of trying to build a better measuring rig).
-- Fernando
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