I recently noticed something interesting*.
My laptop has an aluminium case. When I slide my finger
over it, I get a buzzing sensation, but only when the
charger is connected. On batteries, no buzz.
This is presumably because of mains hum making it
through to the (earthed, I guess) metal
case. Perhaps the
supply to this wing of the estate isn't well-earthed.
In a quiet room, I can actually hear this buzz, and it is
indeed the familiar 50Hz mains frequency. It only happens
as I stroke the metal case, and the frequency is
independent of the speed of stroking.
I presume, then, that the mains ripple is modulating the
friction between my finger than the aluminium, resulting
in a slip/stick cycle at 50Hz.
So. If a similar voltage ripple were applied to a thin,
sonorous metal sheet, I'd expect to be able to make a
somewhat louder 50Hz hum by stroking it.
But why stop at 50Hz? An array of metal sheets, each fed
with a modest voltage at a different frequency, would be a
musical instrument slightly akin to a glass harp.
[*Six out of ten customers agree]