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Wow, that is super fun. There is a story which I can neither confirm nor deny that a company with large data centers and drive firmware that exported PES data was able to correlate data from drives in different locations of said data center and make a seismic interferometer which could "image" traffic on a freeway nearby. :-)

I had no idea you could get as much as 4KHz of frequency bandwidth out of those sensors. That is a pretty cool result.

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That's a very credible story. Pulling signals from arrays of noisy microphones is computationally intensive but very very powerful. On a small scale, that's what allows Alexa to hear people clearly from across the room.

...which leads immediately to the question of whether they're using the fact that HDDs have multiple platters.

I would, but I'm not a good enough mathematician to say whether you'd get better results from an average or a differential. You'd need to ID the drive model then figure out the # of platters and the distance between them and then take the speed of sound into account while getting rid of small peaks, and...

I've often wondered what sort of physical problems the people who engineer hard disks deal with, whether they have problems with micro-turbulence and suchlike.

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