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Because water is denser than air, sound waves travel faster and with more energy in water than air. However, we are worse at hearing in water than in air. Why is this?

To clarify, I was comparing these two:

  1. Having both the sound source and the listener (human) underwater
  2. Having both the sound source and the listener (human) above water

Supposedly, sound waves are 'better' with denser media, but we humans cannot hear very well underwater.

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    $\begingroup$ Are you comparing hearing an underwater sound source while underwater and an above water source while out of water? Or are you asking about hearing a sound that must pass into or out of water? $\endgroup$ Aug 30, 2022 at 15:26
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    $\begingroup$ As a side topic, you cannot tell the direction a sound is coming from underwater, because sound travels so much faster in the water. You are able to tell direction in air because of the time difference between when the vibrations of a sound hit one ear and then the other. When underwater, the time difference is too short for the air calibrated human ear to register a difference. $\endgroup$ Aug 30, 2022 at 16:03
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    $\begingroup$ @MichaelRichardson i don't think that's true. when i rebalance sound in a digital mixer that does not adjust the timing at all. it merely adjusts the relative volume in each ear, yet that definitely gives the impression of the sound source moving from left to right (or vice-versa) $\endgroup$ Aug 30, 2022 at 21:10
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    $\begingroup$ @jamesturner Then your digital mixer is not fully modelling the biophysics of hearing - both relative phase (i.e. timing) and intensity are used to determine the azimuth of a sound. Like phase, simple relative intensity is not sufficient as it is ambiguous in determining whether sounds are coming from in front or behind - this requires modelling an individual-specific head-related transfer function (HRTF), which modulates relative intensity by the shape of one's external ear (pinna) - this is what allows us to determine if a sound is coming from in front or behind us. $\endgroup$ Aug 30, 2022 at 21:43
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    $\begingroup$ @MichaelMacAskill while your insight contains interesting details about the subtle mechanics of human hearing, your claim that one absolutely cannot tell the direction of sound underwater seems dubious. are you claiming that there is no discernible loss of volume underwater when your ear is facing away from the sound source? that does not seem consistent with my personal experience, although i've never properly tested the hypothesis myself with a randomized blind trial. $\endgroup$ Aug 31, 2022 at 14:13

2 Answers 2

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Impedance mismatch.

The impedance ratio or the admittance ratio (admittance = inverse of impedance) describes how much of a wave is reflected or transmitted at the boundary of two media depending on the frequency. In principle, the ear is an impedance transducer that converts sound waves hitting the eardrum into smaller, more powerful vibrations by means of the auditory ossicles, which act on the cochlea. If the medium acting on the ear is water instead of air, to which the eardrum is optimised, there is an impedance mismatch and the waves are largely reflected instead and lead to only minor vibrations of the eardrum.

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    $\begingroup$ Are there any animals that have optimized hearing underwater? $\endgroup$ Aug 30, 2022 at 12:52
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    $\begingroup$ When swimming, does water contact the eardrum? I would have assumed a bubble stays trapped and this is the interface where the reflection would occur. $\endgroup$ Aug 30, 2022 at 14:56
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    $\begingroup$ @JohnAlexiou. Whales most likely $\endgroup$ Aug 30, 2022 at 16:54
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    $\begingroup$ @JohnAlexiou At least among mammals, cetaceans definitely do, sirenians probably do, some pinnipeds might, most other aquatic mammals don't. $\endgroup$ Aug 30, 2022 at 21:32
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    $\begingroup$ @evildemonic Generally yes, but even if you manage to get rid of all that air (it actually is doable for some people), you still have an impedance mismatch between the water and the eardrum. It’s not as bad as if the air is still there, but it’s still enough that hearing underwater is difficult. $\endgroup$ Aug 30, 2022 at 21:33
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TL; DR: most of the sound (energy) is reflected from water surface.

If we think about an actual setup of an experiment when we cannot hear somebody while underwater, the person that we want to hear is not in the water: they are speaking in the air and the sound waves have to propagate through the boundary between the air and the water. Water, being denser than air, reflects most of the sound incident on it (like a mirror reflecting EM waves) - only small fraction of the energy of the initial signal penetrates.

The situation is quite different, if we are dealing with sound created underwater, by animals or devices adapted to creating underwater sound.

This could be described as impedance mismatch, and, in this sense, my answer is complimentary to that by @Bulbasaur, who focused on the impedance mismatch between the medium (water) and the ear (adapted to match the acoustic impedance of air.)

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    $\begingroup$ Sounds originating from underwater are also difficult to hear $\endgroup$ Aug 30, 2022 at 23:43
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    $\begingroup$ @BlueRaja-DannyPflughoeft difticult in which sense? Difficult in comparison to what? I think the original question is too vague about the nature of the difficulty. $\endgroup$
    – Roger V.
    Aug 31, 2022 at 5:20
  • $\begingroup$ The same effect happens where sound waves cross from the water into the air inside your ear. $\endgroup$
    – w123
    Aug 31, 2022 at 19:11
  • $\begingroup$ @w123 this has been discussed in the answer by at_Bulbasaur, which is also explicitly mentioned in my answer. Although, the air that remains inside the ear is probably doesn't play much role here, since the water is in direct contact with eardrums, skin, etc. $\endgroup$
    – Roger V.
    Aug 31, 2022 at 19:35
  • $\begingroup$ Also, don't forget that any process that produces sound will function differently underwater, further difficultifying any attempt at making sense of a "direct comparison" (you can't just submerge the sound source and expect that it makes the "same sound" as it does in air, whatever that means). $\endgroup$
    – Arthur
    Sep 1, 2022 at 8:58

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