[Hallicrafters] Piezo

[Wilson Lamb]

I expect the material in disc caps is similar to that in piezoelectric 
transducers.
Bias is not required to generate signals.  Just put a .01 disc on a scope 
and tap it.  I haven't, but you may well see a little spike.

I think the microphonics are "reduced" when signals are strong because you 
can't hear them.  The presence of audio would not generally affect the 
generation of the piezo signal.

73,
Wilson
W4BOH