Simple, spooky sound semicircles.

July 3, 2007 6:01 PM

What is the name of the simple dish-shaped device used to project and receive sound across a large room?

I've seen them at a bunch of science museums. They're usually big white dishes, positioned on opposite sides of a large exhibit floor, with a small metal loop that one may speak into, in order to have their voice transmitted from one dish to the other. What are these called? Points for any information on how to build one.

posted by UnnotciedTypo to technology (10 comments total)

Parabolic reflector.
posted by paulsc at 6:06 PM on July 3, 2007

A parabolic dish. Edmund Scientific sells them.

Attach a microphone at the focal point, where the converging soundwaves meet and you've got a parabolic microphone

On preview, what paulsc said.
posted by acro at 6:16 PM on July 3, 2007

Beautiful. Thanks guys! Another question - How much does material matter when using these for acoustic purposes? I've found a few reflectors bigger than those sold by Edmund (and more likely to survive the elements, which is important for me), but they're designed for lighting. I'm certain they'd work, but which materials would be best for sound transmission? Can I add soundproofing material to make them more efficient? I'm going for something as close to museum quality as possible (and I'm assuming the ones in museums are custom produced, and not available for purchase somewhere).
posted by UnnotciedTypo at 6:28 PM on July 3, 2007

Telinga makes their soft dishes out of latex/some plastic, and they seem to be one of the only commercial outlets for birdwatching/audio recording parabolic microphones.
posted by acro at 6:55 PM on July 3, 2007

Every 'professional' parabolic microphone I've seen is made of either hard, clear acrylic or some similar material. Soft materials tend to dampen the sound reflections. Concrete would be best for sound concentration/reflection if weight isn't an issue.
posted by IronLizard at 8:01 PM on July 3, 2007

There's a pair at the Exploratorium in San Francisco that are made from what appears to be concrete. I think that you can be pretty flexible in the materials you use.
posted by lekvar at 8:02 PM on July 3, 2007

You're basically looking for hard, rigid and smooth. So something like glass or acrylic would be ideal, and the polar opposite, something like foam or stetched fabric would be the worst.
posted by doctor_negative at 9:25 PM on July 3, 2007

"... How much does material matter when using these for acoustic purposes? ..."

If your mission is vital you could take a lesson from the WWI era Brits.
posted by paulsc at 9:56 PM on July 3, 2007

Actually, the very best shape for mirrors in the situation you describe, where you want transmission between two points a fixed distance apart, would be an ellipse with foci at the two points, provided the ellipse is 'complete' enough.

I believe the ellipse has an advantage over the parabola no matter how small the mirrors are, since it will focus all sound emitted from the source and reflected first from either of the two mirrors, and the parabola will always fail to focus most sound from the source which hits the far mirror first, but this advantage is very tiny when the diameter of the mirrors is small compared to the distance between them.

As far as materials are concerned, a parachutist friend of mine told me sounds from the ground are preternaturally loud from remarkably high up, and I attribute that to reflection from the canopy, for whatever that's worth.

It would be fun to have inflatable sound mirrors at the beach, I bet.
posted by jamjam at 11:43 AM on July 4, 2007

a case of this phenomenon is located at NYC's Grand Central Station:

http://localoaf.org/2006/12/30/grand-central-stations-hidden-acoustic-wonder/
posted by k7lim at 11:21 AM on July 6, 2007 [1 favorite has favorites]