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finkelitis's avatar

What material reflects sound best, and is also inexpensive?

Asked by finkelitis (1905 points ) May 12th, 2009

I’m helping a student with a science project, and we’re trying to get sound to reflect off a shape we designed. We’ve tried making it out of a moderately hard plastic, but we’re not getting great containment of the sound. Would papier mache work better? Clay? What are the best options?

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12 Answers

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hearkat's avatar

“Moderately” hard plastic? Generally, the more rigid and smooth the surface is, the cleaner and stronger the sound reflection is. Consider which rooms have the most echo—tiled bathrooms and kitchens. Something funnel-shaped will gather the sound, and something flat will reflect it.

I’d love to hear more details about the project.

DarkScribe's avatar

Hmm. They removed my apparently less than appreciated comment re mothers in law.
I thought that it was amusing.

Sound will reflect more from hard substances than soft, so if you are using hard plastic it is not a matter of the material, it is a problem with the shape. A speaker for instance, is a cone shape, designed that way in order to “direct” the sound in one direction. The only way you are like to improve sound response it to either change the shape or add reflectors of some kind. To switch to a substance like papier mache would act as a damper, a sound absorber, and reduce the amount of sound reflected. If you were to place your object in a room corner you would probably have a better and more directional response, as you would by placing something like an umbrella behind it.

Knotmyday's avatar

Ok, imagine two surfaces- a soft, foamy surface like a mattress, and a hard, solid surface like a smooth slab of concrete. You have a super-ball in your hand, and you want to bounce it as high as you can. Which surface will bounce higher?

The same applies to aural reflectivity. Hard, smooth, and dense is the way to go.

On the other hand, maybe the shape of your design is affecting your results. What shape is it?

bpeoples's avatar

As mentioned above, sound reflectivity is affected by two things:

- Shape of the reflector
– Mass of the reflector (pounds per cubic inch and total mass)

As long as you’re looking at something “hard” (e.g., that’s not going to be easily moved by the sound pressure), you should be okay with making the shape what you want it to be. Paper, by itself, is hard enough and dense enough to reflect most sound above, maybe 1000Hz. (I don’t have the exact data here, so I’m sorry if my numbers are off)

Marble, on the other hand, is dense enough to reflect just about anything.

Sound proof doors and walls are generally made in layers—a thin outer layer (usually steel plate, 1/2” thick or so) to reflect higher frequencies, and a thicker inner layer (often lead wool) to absorb the lower frequencies.

I’d look at making the plastic what you need it to be, and then coating the outside of it in concrete or plaster. This will build up the total mass, and give you better reflectivity off the face of the plastic.

(I’m not an acoustical engineer, but I’ve had dinner with several)

hearkat's avatar

@bpeoples: Concrete and plaster may not have a smooth enough surface to get the reflectance they desire, unless they were to put some sort of glaze over it like ceramic tile has.

@finkelitis: It really would be beneficial to have more details about this project and your specific goals. BTW, I’m not an acoustical engineer,either; but my Uncle is… and I am an Audiologist.

bpeoples's avatar

@hearkat True! I was expecting that the plastic surface would be the acoustically reflective surface, with the plaster/concrete simply being a “backer”.

hearkat's avatar

@bpeoples: I’m a bit slow today… when you said “coating the outside” I pictured putting the concrete/plaster around the whole thing, not just the back. I understand what you meant now. I was born blonde… HaHaHa

finkelitis's avatar

The shape is elliptical.

Basically, the goal is to direct the sound emitted from a speaker so it goes towards the listener and away from others. Surrounding the speaker with half an ellipsoid (or elliptical trough) and placing the speaker at the focal point, and sitting the listener at the other focal point should work. But I need a good material to make it out of.

This is a kid’s project, and we don’t have much more time for a big production. Clay with a good glaze might work, but I don’t think we have that kind of access. Some kind of tile might work. Or what about a metal siding?

Knotmyday's avatar

You could always just use a plastic mixing bowl, and smear putty around the back.

hearkat's avatar

The ellipsis is concave? That might distort the sound rather than providing a clean reflection.

What are the dimensions of the display? What grade level?

bpeoples's avatar

Ellipse should work just fine (think US capitol building)—and you should be able to make it out of paper (not mache, just paper) if you make the shape right. If you’re just working in speech frequencies, you don’t need to get fancy, as long as your geometry is tight.

Getting the shape precise is going to be the hard part. My recommendation would be to draw it out large on the floor, using the two foci & string loop method. If money were no object, I’d get 4×8 sheets of 1/8” plexiglas. But a product called “lauan” will do just fine, it’s sold in 1/4” and 1/8” thicknesses from any hardware store. Should be under $20 a sheet. You should be able to have two people bend it to a pretty reasonable approximation of the ellipse, but you need the guide on the floor to make sure it’s close.

The OTHER option would be to make it out of a series of flat planes. Because of the way the ellipse works, you don’t need to have it be a continous curve (although it works better), if you break it up into small bits, you can just focus them individually.

Here’s what you do—take pieces of plywood, plastic sheet, or something like that that’s very smooth, and glue space blanket Use something like rubber cement or even elmer’s glue so you have a lot of time to get it smooth. The plates should be something like 6“x12” (doesn’t really matter, but pretty small), mount them to microphone stands (or whatever you have), at seated head height.

Arrange these on your ellipse shape on one side, and put something like a rolling chalkboard perpendicular to the line between the two foci. Put a light bulb at the foci at the seated head height. Now, focus the reflections so they make an even horizontal line across the chalkboard.

Set up the other half of the ellipse around the other foci, and have someone sit at the foci, and focus all of the reflected beams of light on their face until they can see all of them.

Now, if you swap out the light source at the other side with a person (they sit at the foci, back to back), they should be able to have a private, quiet, conversation without anyone in the middle being able to hear them.

Depending on the flatness of the boards and the care with which you arrange them, (as well as the total square footage of them) you could pretty easily beam sound from one corner of a gymansium to the other, with a basketball game going on in the middle.

I think the main trick here is to use a light source to focus your ellipse—it’s how the acoustics in opera houses (and whatnot) are tuned. Well, that and very expensive test gear.

Feel free to PM me if you have more questions, or if I wasn’t clear. It’s pretty late here, so I’m not sure if I’m making sense.

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