Acoustic Refraction

I read about this demonstration where all this guy did was put a balloon to his ear and somehow he could hear what he couldn’t before. Why is this? I was told it had something to do with the effect of air pressure on accoustic refracrion.
- Anonymous

I didn’t see this demonstration, but have seen something rather similar at a science museum, the Exploratorium in San Francisco (there may be others). The idea is that a gigantic rubber ball with thin walls -- a big balloon, essentially, is filled with carbon dioxide. Because sound waves travel more slowly in this gas than in air (carbon dioxide is more dense than nitrogen), this ball has the same effect as as lens has for light, for essentially the same reason.

Here is  of how light travels in different situations.

Because the balloon is round and the speed of sound is slower than that in air, it focuses sound waves like a convex lens. In the demonstration, it is important that the speed of sound in the gas in the balloon is less than it is in the surrounding air.

Tom

(published on 10/22/2007)