Consider the phenomenon of a beaver signalling alarm by
smacking his tail against the surface of the water. It
makes a sound like a gunshot. Bang! Loud!
When I was in elementary school, I read a book that said
beavers use this method to signal to their buddies, and that
it works particularly well if their buddies are under water,
since sound travels so well through water.
I never questioned that until recently. I was in the pool,
teaching my young nephew how to make a really big bang by
smacking a small boogie-board against the surface. Then I
went under the water and found that the sound was nearly
imperceptible.
Evidently
a) the sound-producing process takes place in the air, and
b) does not couple well to the water.
I could have told you part (b) in advance; it's well known
that there's a terrible impedance mismatch between water and
air. This has all sorts of practical manifestations and
consequences. Also, on a related note, low-temperature physics
folks are daily faced with the Kapitza boundary resistance,
i.e. the mismatch between helium and (almost) everything else,
which makes it hard to cool metal by soaking it in helium,
even if the helium is reeeeally cold, because the transport of
thermal phonons is impaired.
Part (a) came as a surprise to me. That's partly (but only
partly) because I had "known" the opposite since puppyhood.
Even in 20/20 hindsight in can only barely kinda sorta
convince myself I understand it. I now imagine the sound-
producing mechanism just involves the air being expelled from
the rapidly-decreasing space between the smacker and the water.
Such a process taking place in the air will not couple much to
the water. OTOH if you think of it as violently hitting
something against "the water" you might expect it to have *some*
significant effect on the water.
Whenever I get fooled like this I like to reflect on it, and
see if I can somehow become smarter, so that I won't get fooled
again.
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BTW as for beavers, I don't think they need to signal to their
buddies who are underwater. A beaver underwater is already safe.
It is the buddies up on the hillside who need to be notified.
So a method that makes a big bang _in the air_ might suit the
purpose just fine.