Let's say you have a sound source, in the ocean, at a frequency around 50 kHz. It's a point source, so the sound waves emanate in all directions. Some will go straight up. Do these straight up sound waves reflect off the surface and go straight down?

If no, then where does the sound energy go? Into the air? How could the much less dense air absorb the sound so quickly?

If yes, why? What is it about the surface that reflects sound? I know a steel hull of a ship reflects sound waves, and steel is much more dense than water, so why would a much less dense entity (air) reflect anything?

I guess another way to ask this is, what is the critical angle in this situation? I've only ever heard that term in reference to light waves, though. I don't know if underwater sound waves also have total internal reflection in some circumstances, or what the critical angle would be.


1 Answer 1


Yes, sound waves from below do reflect at the water surface. This can lead to standing waves. One can see these when air bubbles remain at nodes of the sound wave.

It is like a string reflecting at a free end. The restoring force is half at that point, the amplitude twice as high. The boundary conditions can be matched by a wave coming in from beyond the free end, as in this animation.

Any change in wave impedance will lead to reflection. That is how sonar and dolphins see fish. A large part of the reflection comes from scattering by air in the swim bladder of the fish.

  • $\begingroup$ So wait, is it really reflected back straight downwards, or is it scattered back in all directions? $\endgroup$
    – DrZ214
    Commented Feb 26, 2018 at 10:13
  • 1
    $\begingroup$ @DrZ214 Depends on surface roughness (in relation to wavelength). A perfectly plane level surface reflects like a mirror. $\endgroup$
    – user137289
    Commented Feb 26, 2018 at 10:49

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