So imagine you're at the beach; you go into the water and the moment you enter the water you stop hearing anything from the outside world. The same happens vice-versa: your friend shouts at you from inside the water but you only hear the bubbles rising to the top.

So the questions are:

  1. Why does that happen?

  2. Would it also happen if you used a hydrophone?

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    $\begingroup$ It's not true. There's reflection and attenuation, but sound most certainly travels across boundaries. $\endgroup$ – Carl Witthoft Dec 11 '15 at 13:22
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    $\begingroup$ If you cannot hear a friend or child shouting on the beach, or a helicopter flying overhead, while your head is underwater near the shore, perhaps you might make an appointment to have your hearing checked. Certainly putting one's head underwater makes all abovewater sounds much quieter, but not inaudible. $\endgroup$ – Todd Wilcox Dec 11 '15 at 15:03

1: It's caused by the change of propagation medium, more specifically it's characteristic impedance given by it's density and sound propagation speed. In this point of view the difference between air and water is huge. At the border between those two media there are strong reflection caused by the impedance discontinuty (that's why you can only hardly hear in the water from the air outside and vice versa - the power is mainly reflected).

2. Yes, it would. The difference between hydrophone and you ear is only the difference between sensors, not the principle itself.

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  • $\begingroup$ In part 2 I meant using the hydrophone to produce the sound, because maybe it influenced the sound, but ok, that answer is really good. Thanks! $\endgroup$ – Pablowako Dec 11 '15 at 12:11
  • $\begingroup$ There will be some changes in wavelength, but again: the principle of the propagation remains the same. $\endgroup$ – Victor Pira Dec 11 '15 at 12:15
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    $\begingroup$ Liquids are roughly 1000 times denser than atmosphere. With light, you get large reflections when $n$ is only a few multiples of 1 - imagine the reflectivity of an $n = 1000$ medium... $\endgroup$ – Jon Custer Dec 11 '15 at 15:50

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