What is this self-made piezo sensor actually measuring?

[Xanthippe]

Hello everyone,


I made a primitive "sensor" by soldering a small piezo to a set of wires connected to a BNC connector and pluging them into an oscilloscope. Photo included.


My objective was to detect ultrasound waves under water to get a better understanding of the acoustic field, similar to a hydrophone.
And at first my oscilloscope showed a periodic sinusoidal wave when I would dip the piezo into water with a transducer nearby.
But then i accidentally put it on the optical table and I saw another wave appear. It turns out that only when I hold the piezo in air, there is a flat line. As soon as it touches skin, liquids and metal objects a wave appears.
I am not sure what actually is being measured here.
It can't be temperatur, because the optical table and the water had the same temperature yet their waves didn't resemble.
Some weak waves would also appear when I would whistle near the piezo but they were nowhere as defined and as periodic as when the piezo would come into touch with an object. However no waves would appear when i put the piezo on plexiglass.
I am confused. Does anyone understand this?

[langwadt]

the mystery wave wouldn't happen to be exactly 60Hz ?

[cdev]

Its a mechanical sensir, i.e. a microphone. There also meay be some conductivity inherent to skin and ground loops.