The PING is supersonic. So why do we hear it?

[Keelbuster]

Kindof a cool question. Not sure, but - from the single-ping echo ranging link:

4. Keep your eyes on the revolving slit and listen intently for the echo. Disregard the rolling reverberations, and concentrate on catching the clear note of the returning echo from the target. At the instant it comes, note the reading on the scale. This is the range.

But this 'audible' ping is through a transducer - i.e. a hydrophone of some kind. So why do we hear it inside the sub? Well, I think long-range sonar pings are high amplitude. The hull probably acts as a kind of transducer (like a big drum) and you get some kind of sound from the transformed ping energy - even if it was originally a frequency that humans can't hear audibly, the hull may act as a low-pass filter/transducer that warps a high frequency ping into the audible range. Not sure - just a guess. I know that modern subs also use High-frequency sonar that doesn't travel a long distance and is not detectable except at close range. I feel like the ping-echo we hear has something to do with the amplitude of signal required to travel long distances. But I'm not sure. I don't even know how a hydrophone works.

Edit: Also - think of diffraction of light - when it passes from a good conductor to a poor conductor, it's frequency gets warped downward (i think). So the good sonar medium is water, carrying a high frequency signal, and the air inside the sub is a poor medium, resulting in the high frequency signal being warped into a lower frequency signal that happens to be in the audible range.



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