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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?

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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?

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