Hi, I am making a capacitor with Aluminum plate and Mylar film. Al plate surface has tiny groove arrays, each groove forms a cavity and the mylar film is sitting on top of the groove to allow its vibration under input AC voltage. I am putting in AC of ~300Vpp with frequency of 60KHz. The AC signal input is pretty noise free (observed from OSC). But I can hear some high frequency noise coming from the Capacitor, frequency ranges from 5K~10K. Is anybody know where this noise coming from ? Is this Al alloy dependent ? Because when I use 6061-T6 Al plate, the noise is gone, when I use 3003 (not sure the temper) and 6063-T5 plate, the noise is quite obvious. If anybody has any related knowledge I would be greatly appreciated. Thanks a lot.