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OK this is probably a really dumb question, but I haven't been able to google my way out of this.

I have some headphones I want to ensure do not have a mic. I'm 99% sure they don't, but to be cautious I plugged them into the microphone jack to see if I could get the "Test your microphone" bar to show output.

In Windows 10 sound settings, I was able to make it show output when I thumped the headphone speaker. Odd I thought. So I tried it again with plain old ear buds that definitely can't have a mic (no little bar along the wires). Same thing happened, thumping the ear bud showed output.

Does anyone know why this could happen? Am I crazy and maybe these devices due have a mic embedded?

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Speakers & microphones operate on the exact same principle. One is merely the inverse of the other.
Each uses a permanent magnet, a wire coil & a diaphragm.

If you send an AC current through a wire coil it generates a magnetic field. In proximity to a permanent magnet, this will cause cycles of attraction & repulsion. This is what makes your speaker diaphragm/cone move to make sound.

Inversely, if you move a wire coil in the presence of a permanent magnet it will generate a current. This is how your mic transmits sound as AC current to the mic input.

Banging on a speaker will generate signal in much the same way as speaking into your microphone.

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