# Small-size solid material vibration sensor?

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#### Mr RB

##### Well-Known Member
Hi, does anyone know a small microphone or piezo sensor that can detect vibration when a solid material is tapped (imagine "rapping" on a chunk of wood with a fingernail etc).

Ideally the sensor should be small, and couple directly to the wood so it senses the vibration moving inside the wood, with a good rejection of surrounding ambient sound.

Thanks!

#### cowboybob

##### Well-Known Member
Did a project to detect loggerhead turtle hatch-out noise with a tiny FM xmitter. Had a 3/16" X 3' "all-thread" touching the FM PCB (with an onboard mic) and got incredible sensitivity to mechanical vibrations through the sand.

Had to replace one of the mics with one like This. It's response was identical to the original.

You might want to physically attach the mic to a metal "pick-up" of some sort, maybe stuck into the wood, to enhance the sensitivity.

Rejection of ambient, unwanted noise(s) would be problematic but doable.

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#### ericgibbs

##### Well-Known Member
hi Roman,
The old stylus from a vinyl record player will work well, a decent level output signal.

E.

##### Well-Known Member
You may want to find an acoustic guitar pickup transducer. Look for the inexpensive versions. I see them for about $30 USD but years ago Radio Shack sold a line of them for about$5.00 USD.

I also use the much more expensive Endevco (Meggitt) line of accelerometers for noise (bang) pickup and detection. I have several old Endevco Model 2242c accelerometers but can't find a data sheet for these old units. If you can find a data sheet and think it would work it's yours for the asking.

Then as CB mentions maybe a electoret mic glued face down to a small metal strip.

Finally, many automobile engines use an anti-knock sensor. You may want to look into them from a junk yard. I just am not sure of their frequency response.

<EDIT> I really like Eric's suggestion which escaped me. </EDIT>

Ron

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#### colin55

##### Well-Known Member
Just use any piezo diaphragm.

That's wht they use in window-smash detectors.
Or you can use an electret microphone.

#### Mr RB

##### Well-Known Member
Wow a heap of really good suggestions! Thanks guys.

I really didn't want to go much larger than say an electret mic unit, maybe 1/2" cube total size. That probably rules out the stylus head due to size.

Ok it looks like a modified electret mic insert or some piezo disc. The window break detector also sounds possible, I have seen quite small ones on windows.

Thanks again for the suggestions, I'll check my junkbox and some catalogues.

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