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It is very difficult to hear underwater sounds from above the surface of a body of water. This is suggested to be due to the speed of sound being very different in air versus water, leading to most underwater sound being reflected back into the water, and not passing into the air.

Am I supposed to think of the air-water interface as some sort of plane mirror then? I have heard of Snells law for refraction, but am not sure how it applies to reflection in this case. If I am to think of the interface as a mirror, I might expect a very thin layer of water to have equal reflection/reflectivity compared to a thick one. Is this the case?

If not, how can I better think about what is going on? Also, is there interplay between the wavelength of the sound and the thickness of the water layer required for reflection?

user391339
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2 Answers2

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Within reason, a thick sheet of water and a thin one will affect sound mostly the same.

With extremely large thicknesses, sound actually transmits better.

When the thickness approaches a wavelength of the sound in question, the equations get a bit more complicated. I'd be wary of venturing a guess in this regime.

joseph h
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Cort Ammon
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Yes but not directly because of the speed of sound. It's analogous to Snell's Law in optical reflectivity, or to VSWR in microwave components. The effective "index of refraction" for sound changes drastically at a water-air interface, thus leading to a lot of reflection and not much transmission.

This holds true for interfaces between depths of water with significantly different temperatures for the same reasons. Submarines take advantage of this to limit their sound "signature" from propagating to bad places like enemy sonar receivers.

Carl Witthoft
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