This is a recording I made of a 12-volt relay wired to a function generator. This causes the metal plate to vibrate, buzzing against the metal contacts. I swept the frequency up and down, I think through a couple of hertz to about 120. Anything higher than that did not supply the power to drive the coil (the generator was only rated to 10 volts peak-to-peak.) Furthermore, the relay is sitting on the metal case of the generator, acting as a soundboard and amplifying the buzz for the microphone only a few inches away.
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