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Comment by cosmodisk | Hacker News Reader

Not sure how it's nowadays, however during the cold war, those serving in Soviet submarines often wore slippers to avoid making any loud noises so their counterpart won't pick it up with their equipment.

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I saw an older show about submarines that said they had to use exercise bikes instead of treadmills. Repetitive running was easy to pick up.

This doesn't surprise me.

In general, underwater sound attenuates more quickly as you increase its frequency. Foot-fall thumping has a pretty strong low-frequency component. It's also repetitious, which (I'm guessing but don't know for sure) probably makes it easier for a sonar operator to notice.

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)

This is not true.

Source: I've used a treadmill on a submarine.

And the US built a global network of microphones to track them called SOSUS.

https://en.wikipedia.org/wiki/SOSUS

The uniform hasn't changed so slippers are there, guys in Northern fleet usually don't have conditions to wear those.

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