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Old 14th February 2020, 04:20 PM   #1031
gedlee is offline gedlee  United States
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When I saw this paper at AES I swear they were using a wire microphone which measures velocity not pressure. In theory a two plane idea should work, but I would be concerned with measurements unless the mic was extremely small.

Really there are many ways to solve this problem and which works best remains to be seen, but none have actually come to be widely used and as such there is no data on performance.
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Old 14th February 2020, 04:43 PM   #1032
mabat is offline mabat  Czech Republic
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They mention B&K 4138 (~3.2 mm) or Sennheiser KE-4 (~5 mm). So if a 5 mm electret capsule should suffice (?), I'm quite tempted to try at least, even if I should move the driver manually on some primitive frame - that would cost next to nothing. Your technique looks most interesting and I would prefer that, I'm only scared of the math involved - not something I could cope with in a case of any trouble. This is the complete opposite - can't be any simpler.
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Old 14th February 2020, 05:08 PM   #1033
gedlee is offline gedlee  United States
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I could help with the math, let me find my notes. The biggest issue would be the matrix inversion as these can be problematic. One would likely need to use SVD, but algorithms for that are widely available these days. If you are comfortable with matrix inversion and SVD then I don't think that you would have a problem.

The Makarski technique uses a free field driver, correct? That would bother me as this is not the boundary condition that the driver would see in practice.
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Old 14th February 2020, 05:12 PM   #1034
mabat is offline mabat  Czech Republic
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We already touched that, if that's what you mean: #394, #395

OK, why not to try (both). I already looked for some aluminum tubes around, I can get 25 and 35 mm of inner diameter, up to 6 m long. I'd start a new thread when I have it ready (one mic only for the start).

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The Makarski technique uses a free field driver, correct? That would bother me as this is not the boundary condition that the driver would see in practice.
Yeah, that puzzles me as well. But neither the tube is...(?)
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Old 14th February 2020, 08:34 PM   #1035
gedlee is offline gedlee  United States
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Yea, that's the basic math. There is an excellent AES paper by my friend at B&C (Magalotti) on making PWT. You should look it up.
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Old 14th February 2020, 08:53 PM   #1036
mabat is offline mabat  Czech Republic
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I found this one, which also references the Magalotti paper. For $33 I could buy most if not all of the material to actually build the PWT, I guess

AES Information Document for Acoustics - Plane-Wave Tubes - Design and Practice
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Old 14th February 2020, 11:53 PM   #1037
gedlee is offline gedlee  United States
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Well that paper certainly works for background! Just need to space the mic ports along the tube, probably in some random, certainly not equal, spacing. How many is up to you and your computational capacity.
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Old 15th February 2020, 12:00 AM   #1038
Patrick Bateman is offline Patrick Bateman  United States
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Quote:
Originally Posted by mabat View Post
I found this one, which also references the Magalotti paper. For $33 I could buy most if not all of the material to actually build the PWT, I guess

AES Information Document for Acoustics - Plane-Wave Tubes - Design and Practice
I'll buy you an AES membership if you like, I've certainly got $125 worth of use from your software
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Old 15th February 2020, 04:09 AM   #1039
Dave Zan is offline Dave Zan  Australia
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Quote:
Originally Posted by mabat View Post
They mention B&K 4138 (~3.2 mm) or Sennheiser KE-4 (~5 mm). So if a 5 mm electret capsule should suffice (?), I'm quite tempted to try at least, even if I should move the driver manually on some primitive frame
The latest MEMS microphones have tiny ports, pretty decent specifications and cost only a few dollars each so well worth consideration.

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Old 15th February 2020, 06:45 AM   #1040
mabat is offline mabat  Czech Republic
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If I make the tube long enough to obtain a sufficiently long reflection-free time, I can make gated IR measurements and don't have to bother with damping at all, correct? How long is enough - 5 ms? That would be about 1.5 m of tube, which is perfectly fine. Is there any reason why not to do it this way?
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Last edited by mabat; 15th February 2020 at 06:49 AM.
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