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Comment by dmos62 | Hacker News Reader

That's awesome. What sensors have you considered?

I'm interested in extremely weak, high-frequency vibrations of everyday things while in resting state (which is sort of the opposite of what you're after, as I understand), but have not gotten far in acquiring the sensors. I'd love to get a laser doppler vibrometer, but they're pricey.

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Well, I was just going to put a contact mic in my cellar. My understanding is that this is the best way to pick up low-frequency vibrations.

I know very little about audio engineering. I wonder what else I might be able to use to pick up the vibration signature?

I completely understand your drive to pick up those high-frequency vibrations! There's a whole secret world of vibrations out there, and we can analyze it!

You might struggle using microphones if it's very low frequency because most human audio microphones intentionally filter frequencies that are below (and above) human hearing range. Perhaps fixing to he contact mic to the right resonant object might serve to translate the low frequency to a higher frequency?

Could you use the accelerometer on a smart phone to 'hear' the vibrations?

I don't know if a contact mic really has any filtering though; it's just a piezo element that I plug into a pre-amp.

However, what you're saying is completely legit. I read that I might want to lay a large, sturdy thing on the floor and put the mic on that, for the same reason that you gave. I used a large old pane of glass that I found in the cellar.

All that came to my mind is making sure to physically bond it to the floor. But then you still need to have something that moves relative to the magnet and then I got confused again.

What do seismographs do? People also use acoustic pings to measure soil and rock density underground. What are those devices using for their “microphone”?

And yeah, would an accelerometer work? I don’t know what the temporal resolution is on your standard Amazon / AliExpress accelerometer is but it’s probably pretty decent?

"Geophone" is the device you seek.

Amplifying and digitizing the signal is straightforward for an experienced EE, but you might look into acquiring some Raspberry Shake hardware which has it all already done.

Oh, snap. Thank you. Guess I’m about to be $350 poorer!

Stupid question, but how far can you get with a laser, a mirror, and a light sensor?

That's actually a really cool idea. I am not sure what variance there would be on a light sensor though.

I remember building a toy interferometer in high school - a laser, splitter and a mirror. You could definitely see the interference patterns when you'd press on a wall with a finger.

Maybe something like that could be turned into a reasonable microphone?

I have pondered adding 4 microphones to the corners of my house. I suspect with some processing and filtering you could likely tell quite a bit about the house. When the front door or garage opens, what rooms are occupied, when it's raining or windy, what appliances are on, when someone parks in the driveway, when the mail comes, duty cycle of the fridge and furnace, when it's sunny out (expansion noises on the sunny side), when someone is showering (and which shower), when toilets are used, etc. Certainly should be able to detect when a window is broken or a door forced.

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