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Old 17th April 2009, 02:08 AM   #1
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Question Find Wavefront Shape @ Compression driver exit??

Ok, here's a question about how to determine wavefront shape via an empirical measurement method.

I am wondering what the method would be to find what the wavefront looks like as it exits a compression driver? Or for that matter, any exit or orifice where sound is emanating?

I'm not talking about modeling and using a mathematical or simulation method here, I want to know if anyone knows exactly what proceedure(s) can be used to "look" at what the shape of the wavefront is as it exits (something).

The basic idea is to know if for example the wavefront is planar (flat) or "spherical" or some distortion beyond spherical, and if spherical, what is the radius of the apparent sphere?

Anyone got the inside info on this one?

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Old 17th April 2009, 02:29 AM   #2
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There's probably a laser technique that could be adapted. Imaging suspended particles?
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Old 17th April 2009, 02:31 AM   #3
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Scott,

great idea... have you become William S.Burroughs now??



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Old 17th April 2009, 02:42 AM   #4
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I blow with the wind.

Might be something along the lines of Schlieren photography too.
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Old 17th April 2009, 06:58 PM   #5
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Bear

Sound cannot be seen, so any visual method won't work. Dr. Behler at Auchen used a microflown (a hot wire microphone that measures velocity) and used this to scan the aperature. It required a very sophisticated measurement technique that only a university could afford.

I did some work for B&C where I showed how microphone measurements along the length of a plane wave tube could reconstruct the motion right at the throat aperature (or anywhere else for that matter). The technique was very mathematically intensive however, although the equipment requirements were minimal.

I'd love to interest someone in this technique.

It is theroretically possible to reconstruct the motion in the aperature from near field or far field pressure measurements at several polar angles etc. I have done this before. The problem is that the equations are very unstable (nearly singular) and its hard to find a reversal technique that always works.
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Old 18th April 2009, 03:55 AM   #6
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Dr. G

Isn't there any application of laser doppler velocimetry that could apply here? I know there are some researchers trying to use this as an absolute SPL calibration. It seems intuitively like "seeing" sound to me, but if I'm off base no biggie.
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Old 18th April 2009, 04:34 AM   #7
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Unless your talking about a new technique that I have never heard of, no. Sound is transparent to a laser so there is no way to measure its velocity without a reflection. Lasers work great on solid objects that reflect the laser, this is quite a well developed technique, but sound waves!? Never heard of that.
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Old 18th April 2009, 07:01 AM   #8
FrankWW is offline FrankWW  Canada
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Bear, might be worth talking to this guy

http://people.rit.edu/andpph/text-schlieren-focus.html

http://people.rit.edu/andpph/
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Old 18th April 2009, 10:40 AM   #9
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It's been done with a number of techniques, including laser interferometry. I posted some pictures of such a measurement in the old Enable thread but I don't recall the source material or author. But here is a photo taken form my old compressible flow text (circa '69) of the radiation pattern and and wave pattern. The distortion you see in the axis wave fronts is due to the limited pixel resolution in my copy.

Click the image to open in full size.

I don't know the technique used but it was not lasers here. All the techniques are based on the very slight but continuous variation of the index of refraction of light by gases due to changes in density.
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Old 18th April 2009, 04:06 PM   #10
gedlee is offline gedlee  United States
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John

I'm betting that those photos were done in a ripple tank as I've seen this done before. The change in the index of refraction of air would be very small and would require an incredibly high resolution system to resolve these small changes. I've never heard of someone doing this for sound waves.
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