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I am trying to measure the "at rest" characteristics of a piezo disc. The piezo is of inexpensive variety, the type enclosed in a black plastic holder: http://www.sunrom.com/img/p/83/83_0_800.jpg

When I use my oscilloscope to measure the piezo, I attach the osc. ground to the piezo black cable, and the 10X selected probe to the piezo red cable. I am able to view the resulting wave at 50mV divisions on my oscilloscope screen.

I find that the piezo "at rest" produces a wave of about 30mV peak to peak, oscillating about 8.3mV. Does this seem like a typical piezo peak-to-peak ambient "output" voltage?

Also, however, there is approximately a 66Hz wave passing through the output signal. What accounts for this 66Hz wave? Is it the 60Hz hum that is somehow created by my oscilloscope, being connected to mains power?

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A piezo transducer produces no voltage when truly "at rest". Perhaps there are small vibrations where you are testing, like a fan running in equipment on the same bench. Or, you are picking up power line or another signal capacitively. Piezos have very high impedance, so capacitive pickup is quite likely.

To test this properly, shield the lines and mechanically isolate the piezo and the wires going to it. Set it on a folded up towel or similar, with a grounded piece of aluminum foil underneath the towel. If that significantly reduces the signal you are seeing, then you were clearly getting mechanical vibrations or capacitive pickup previously.

You really should use a shielded cable for the connection, like something intended for sensitive audio signals.

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