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?
_-_-bear
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?
_-_-bear
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.
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.
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.
Bear, might be worth talking to this guy
http://people.rit.edu/andpph/text-schlieren-focus.html
http://people.rit.edu/andpph/
http://people.rit.edu/andpph/text-schlieren-focus.html
http://people.rit.edu/andpph/
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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.
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.
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.
Hi John ,
- 3 decades ago I found a book in the library with photos similar to what you've shown .
- The book was called "Seeing Sound" .
- The book I read so long ago may be the same as this one at Amazon .
- It was a very enlightening book with many interesting photos .
- I'm quite sure the process used to photograph soundwaves was documented within the first portion of the book .
>< cheers
Thanks Earl for bringing up "Seeing Sound". I came across the book in the Tektronix library many years ago. If I remember correctly, the method of photography used a microphone and neon bulb mounted on a pivoted boom. The microphone feed an amplifier that drove the neon bulb. Bulb brightness was proportional to sound level at the microphone. The boom was mounted on a track and was swung vertically in front of the speaker under test. The camera was positioned to the side of the speaker and boom. The boom was moved away from the speaker slowly while swinging vertically while the shutter of the camera was open. This created photos that showed the sound intensity with respect to vertical angle and distance from the speaker.
They had lots of neat photos of different driver combos including some photos of drivers with different variations of acoustic lenses set in front of the drivers.
Too bad the current corporate environment led to the demise of the library. There were many books in there that were real gems.
Gary
They had lots of neat photos of different driver combos including some photos of drivers with different variations of acoustic lenses set in front of the drivers.
Too bad the current corporate environment led to the demise of the library. There were many books in there that were real gems.
Gary
Yes, I can see that working if you have the resources of an automotive company (years ago that is - not these days, even they couldn't afford it now. Maybe Chrysler has a setup that could be bought at bargin prices.) Although measureing static flow and measuring dynamic oscillations at >> 1 kHz are not really the same thing.
gedlee said:
Bet again. Note the caption, "Sound field....". These particular photos are from a book on compressible gas dynamics and presented as an example of visualization techniques: shadowgraph, schlieren and inferometry. All are based on the continuous refraction of light by gases. When I said I wasn't sure of the method I meant which of the three was used for each photo.
john k... said:
Shadowgraph is a ripple tank as far as I recall which is what I was refering to. You are not going to get enough refraction from air desnity changes to do any thing with without a seriuosly complex setup. I'm sure that there are ways to measure what Bear is asking about, but only the one that I mentioned is within the realm of doable outside of a mjor laboratory setup. And again, most of the techniques mentioned work fine for static flow or low frequencies, but we are talking about a 1" aperature in the kilo-hertz region and I have serious doubts that many, if any, of them would work on that problem. And let's not forget that the aperature has to be loaded with a plane wave tube or the like because the aperature velocities change completely otherwise. This imposes some serious restrictions on what can be done.
Ok, I'll bite, please explain. I've only ever seen a ripple tank used with a film exposed underneith from a light source above. The pictures look just like what you showed. There are so many questions that come to mind, but I'll just wait for your explaination.
Edit: Googled it, understood now. But I don't see how this could actually be done for a circular source since this type of photography can only do a single plane. You might be able to catch a pulse from a Compression Driver and use that as the impulse response, but the tube which was on it would be difficult to account for in the photograph unless it was optically benign, which is doubtful. Seems like it would be a research project to actually pull off.
I think I'll stick with the method of multiple mics along a tube.
Edit: Googled it, understood now. But I don't see how this could actually be done for a circular source since this type of photography can only do a single plane. You might be able to catch a pulse from a Compression Driver and use that as the impulse response, but the tube which was on it would be difficult to account for in the photograph unless it was optically benign, which is doubtful. Seems like it would be a research project to actually pull off.
I think I'll stick with the method of multiple mics along a tube.
Ok,
How about this?
One inserts a suitably small sensor/microphone into the area proximate to the exit of the compression driver, with repeatable precision, but moved in an X/Y location per sample. Each sample is made with an impulse sensed by the tiny microphone. The difference in time for the impulse will then show the shape of the wavefront exiting the driver. We sample across the face of the exit.
If it was a planar wave, all the samples would be at the same time. If it was spherical, the centermost sample would be ahead of the one at the edge in time. The shape would be shown by the time.
Of course this might preclude the detection of oddly shaped wavefronts, and harmonic interference... so it is like a low bitrate audio sample to some extent... but perhaps useable and useful albiet crude?
_-_-bear
How about this?
One inserts a suitably small sensor/microphone into the area proximate to the exit of the compression driver, with repeatable precision, but moved in an X/Y location per sample. Each sample is made with an impulse sensed by the tiny microphone. The difference in time for the impulse will then show the shape of the wavefront exiting the driver. We sample across the face of the exit.
If it was a planar wave, all the samples would be at the same time. If it was spherical, the centermost sample would be ahead of the one at the edge in time. The shape would be shown by the time.
Of course this might preclude the detection of oddly shaped wavefronts, and harmonic interference... so it is like a low bitrate audio sample to some extent... but perhaps useable and useful albiet crude?
_-_-bear
bear said:
Sure this would work, IF the mic were small enough to not influence the sound field - good luck! (And remember a plane wave tube has to be attached throughout this whole neasurement!), and you have a bank account sufficient to fund such a project as this approach would be very expensive.
I have studied this problem extensively and I gave you what is the only viable way that I found to do it. Take it or leave it.
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