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Re: [Phys-l] frequency and wavelength of sound in air



Jack Uretsky wrote:
Hi all-
This is a teacher's net. Whith that in mind, I have read the various explanations of the physics of sound crossing a boundary - a topic that I think I have thoroughoy understood for at least 50 years. Reading the explanations, and trying to understand them from the viewpoint of a twenty-year old college student, left me torouglhly confused. I urge others to reread these explanations from a similar viewpoint and see if they can't come up with more picturesque and compelling explanations.
Regards,
Jack
Julie Quah wrote:
Just wondering, if the speed of sound in water is higher than that in air,
Which of its, frequency or wavelength, (or both) increases in water?
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A person might ask this question, if the available insights are not clear-cut.
What sort of comparable observations might come to mind?

On the one hand, seeing a person inhaling a chestful of Helium then
speaking in a high voice might suggest that frequency is the variable in question.
Or recalling the heyday of Cowboys and Indian movies, one might imagine the faithful tracker with ear to the iron-horse rail, hearing the distant thunder of the locomotive, because its sound travels faster and more effectively through steel, and here, suppose that the wavelength would be the variable.

Hmm...yes I can see why this question might arise.

How could one arrange an informal experiment, to distinguish the factors involved?

An experiment that could in principle answer this particular question might go like this....
Place an electric bell in a plastic jar, with the DC leads exiting through the lid
so that the bell can be heard outside the jar. Place a lapel microphone in a rubber balloon, and arrange the leads to exit via the knot.

It turns out that the frequencies emitted by the bell sound much the same, if the microphone picks up the sound in air, or if both are immersed in a swim-pool.
Yet the speed of sound in water is faster - as a depth sounder operating in air and water
will confirm.
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At this point I paused supposing that this sort of experimental demonstration was far too muddy.

Then I read John D's and Curtis's posts, which point out that pulses of compression are shared at the air/water interface at identical moments, so that frequency is invariant in both media.
This is no doubt compelling. Still, I was sympathetic to Jack's call for vivid clarification.

Dressing this argument up for dramatic effect might have one tapping on a flexible diaphragm at one pulse per second. If there is water on the far side, it would feel one pulse per second too, no matter at what speed that pulse is transmitted.
( A concrete example is SO helpful, always....)

Brian W