Acoustics: Directing Sound Waves

[Aero51]

Hey, I'm just curious if anyone has designed a device that can direct and concentrate sound waves like a parabolic mirror? Since both sound and light are waves, I assume there must be a shape that allows one to "target sound" much like using a magnifying glass to focus light.

On a related note: I remember seeing on TV many years ago (military weapons documentary I believe) that an engineer developed a device which can direct sound waves in a laser-like fashion. I've tried doing some Google searches with no luck. Anybody know where I can find some information as to how this device works?

Thanks.

 

[Curl]

I've tried it with an FEA model. I put a point source on the focal point of a parabola and ran the simulation, and it works. If you have COMSOL I can send you the file.

 

[Aero51]

Unfortunately I don't.

 

[Bobbywhy]

There are at least three ways to concentrate sound waves:
1. Parabolic concentrator (mentioned above by curl)
2. Parametric beamforming
3. Acoustic lenses

Number 1. Above is a straightforward method to concentrate sound, either to gather sound energy from a distant source and concentrate it at the acoustic focal point, or the reverse.

As for 2., here is an example of one technique: “Acoustic beamforming of a parametric speaker comprising ultrasonic transducers Abstract: A directional audible sound can be generated in air by means of the nonlinear interaction between intense amplitude modulated (AM) ultrasonic waves, which attracts much attention in the audio industry. In the case of sound reproduction by a parametric speaker comprising an array of ultrasonic transducers, a novel algorithm with Chebyshev window has been proposed to control the sidelobe level of the beam pattern by utilizing the acoustic nonlinearity and array signal processing technique.”

And here is a very good overview:
http://en.wikipedia.org/wiki/Parametric_array

Just as a note of interest, certain kinds of sonar use this “beamforming effect” to receive acoustic energy underwater to determine the relative bearing of the source.

As for 3., here is an article with one example: http://blogs.scientificamerican.com...ets-of-sound-that-may-lead-to-sonic-scalpels/

 

[sophiecentaur]

In order to 'focus' waves of any type, you need to have a setup that makes the path lengths for a lot of waves from the source to the detector (at the focus) to be the same so they all add up constructively. In a parabolic reflector, the total length of all sets of parallel lines is the same, as the diagram here shows:
Parabolic_reflector
So the waves all add up in phase at that point.

The same effect can be obtained with a lens (again for any wave) that uses a material in which the wave travels slower than in air. This link shows how the waves take longer to travel through the thick bits than those through the thin bits so they all are in phase at the focus.

There is a ray-based alternative explanation for both the above. The parabola can be though of as operating because the 'rays' all bounce off the surface symmetrically about the normal (laws of reflection) and the rays going across the surface of a lens obey Snell's Law.

 

[rorix_bw]

Hey, I'm just curious if anyone has designed a device that can direct and concentrate sound waves like a parabolic mirror? ... I remember seeing on TV many years ago (military weapons documentary I believe) that an engineer developed a device which can direct sound waves in a laser-like fashion

http://en.wikipedia.org/wiki/Sonic_weapon

These are currently in use. They have been tried in the Horn of Africa (somalia pirate zone). I don't know where their military deployments were.

It proved ineffective against Somali pirates. The first (few?) times it worked. It caused great pain and the pirates ran away. Subsequently they realized it was a "non-lethal" device, took it, and carried on. The private security (i.e. mercenary) crew operating the weapon abandoned ship for fear of what the pirates would do to them, and were picked up someone's navy that arrived too late.

I suppose you could increase the power, but then it would be a maiming or lethal weapon using sonics, and you would have all sorts of legal problems with it. And it wouldn't really be better than a gun.

 

[Aero51]

Thanks guys, this is all very interesting material. The Scientific American link does not work with chrome as I tried loading the page and received an error message. I am also interested in a molecular theory of wave propagation. My knowledge of this topic only consists of phonons in solids. Again, I have not been able to find many papers/websites dealing with this subject.

Also, if anyone is interested here is a link to the full paper mentioned by Bobbywhy "Acoustic beamforming of a parametric speaker comprising
ultrasonic transducers"
http://eeeweba.ntu.edu.sg/DSPLab/Publication/2005_J10a.pdf

 

[rorix_bw]

does it have to be a parabolic emitter?

Military sonar does not use a large parabolic emitter. I don't understand what it is, but they have a large number of smaller emitters that I believe can emit in various controlled directions without mechanical moving parts (perhaps electrical stimulation? maybe many wires going in, and emits in the direction of the current?). I am really not sure how it works but there is no large mechanically steered parabolic dish.

 

[nasu]

I don't understand what it is, but they have a large number of smaller emitters that I believe can emit in various controlled directions without mechanical moving parts (perhaps electrical stimulation? maybe many wires going in, and emits in the direction of the current?). I am really not sure how it works but there is no large mechanically steered parabolic dish.

This is an example of active beamforming, mentioned above. The device may be a "phased array".
Same thing is used in medical ultrasound imaging. Is not just focusing the beam but also scanning the field of interest (changing the focal point in a specific pattern).

 

[rorix_bw]

I have heard the term "phased array" regarding non-steerable emitters for radars. I wasn't aware of other uses - thanks!