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Sonar operators are pretty cool. I saw a clip where they identified another ship (or sub) because they heard a valve open or close. It sounded and looked like static to me but they said it was pretty clear to the trained ear.

And then the sonar operator records the sound onto a cassette tape and plays it at 10x speed for the captain to convince them that they really are chasing another submarine.

I love that movie so much...

A low repetition thump is not the same as a low frequency sound.

A thump can be a pulse of high frequency sound.

Right, but IIUC it's the difference between a "transient" [0] and a "tonal" [1], which AFAIK is an important distinction for sonar operators.

[0] https://en.wikipedia.org/wiki/Transient_(acoustics)