Re: Sound Cancellation - Thanks!

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 2 May 2002, Polvani, Donald G. wrote:

> Now I understand.  If we could create them, my imagined perfectly canceling
> waves would violate energy conservation, but there is no way to physically
> achieve such a situation.

I believe JD mentioned, or at least implied, this in one of his
previous posts, but you certainly *can* set up "perfectly
cancelling waves" without violation of energy conservation.  Use
one source to create the first wave and another that is somewhere
along the wave propagation direction to create the second.  If you
arrange for the second source to produce a precisely cancelling
wave, you will find that it gets precisely the energy it needs for
its operation by absorbing the energy from the first source.

That is to say, the process of absorption can be viewed as
"setting up a perfectly cancelling wave."

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm