>In a message dated 6/17/2000 10:42:40 PM, Ric wrote: > ><<Hmm interesting--I suspect that when sound waves hit a temperature >differencial that they refract just as light does--and I seem to rememeber >that does affect frequency. I could have just imagined that--anyone know >for sure?>> > >Ric; > Absolutely on both counts. In water this phenomenon is called a >"thermocline" and is used constantly by submarines to hide their location. >The sound from a sub on one side of a thermocline will be either reflected >back to the same side it originates from or it is "shifted" as to make the >apparent location appear different than it really is. Frequencies going from >colder to warmer water tend to shift upward in pitch and from warmer to >colder lower in pitch. This is true in the world of sonar at any rate. There >is a name for this stuff but I can't remember that right now.....probably >three o'clock this morning though! >Jim Bryant (FL) Ric, Don, Jim... Once the sound is launched, if the signal speed changes over a path -- whether gradually or abruptly, due to changes in temperature, pressure, density, composition (as in a thermocline) -- the wavelength changes but the frequency doesn't. This is not the same question as whether the frequency of the tone-producing device changes with temperature! There, the physical properties of the source of sound are changing, and the strings themselves wind up vibrating at a different rate. Nor is it the same as the pitch rise in helium speech. All in good time... Once the wave is moving through a medium like air or water, the product of the frequency and local wavelength always equals the local speed. This is also true of electromagnetic radiation -- light. Both types of waves can be refracted when the propagation speed varies with position. It's refraction (along with total internal reflection -- TIR) that keeps the submarine sound confined to the thermocline waveguide, just as TIR keeps the light from leaking out of an optical fiber, but the refraction results from a change of signal speed, not frequency. The frequency is constant because of point-by-point and instant-by-instant (i.e., local) momentum conservation. If you count six pressure crests in succession crossing some interface from the left, you'd better count the same six leaving toward the right. There is no way to create or destroy cycles. I know this -- for sure -- because I paid my dues in 1968-1983 working on a Ph.D. in physics and astronomy (with time off for good behavior and child rearing). There were a bunch of Nobel prize winners hangin' around who gladly cleaned my clock every time I got it wrong, but they finally gave me the degree anyway ;) Regards, Marc Damashek Hampstead, MD
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