sensor choice for "gong detect"? & invisible light switches

["Tracker" <trackgoogle@xxxxxxxxxxxx>]

Hello, all.  Long time no read etc.

Anybody ever *actually done* a sensor on a heavy bell?  (It's a
large-size "Mission Bell" by Tom Torrens, if you care...  To be used as
a doorbell, well, actually as a button replacement for a DSC door box.)
 Specifically triggered by the resonating from the gong itself, no
stuff like mallet switchhooks or motion sensor on the swinging mount...
 I thought about an old-style glass break, but getting the frequency
right would be a trick.

Also does anybody know if commercially cheap density sensors can be
used to determine if a finger is placed on the other side of sheetrock?
 I'm gonna go do some experiments, so don't shoot me for asking first.
But if someone has *already* done it, please let me know.  Why did this
come up?  Well, I was looking at a Leviton "Acenti" ad the other day,
and thought, "Why on earth does anybody want a larger, more prominent
light switch?!"  Remember the Pass&Seymour/Legrand ones?  Ugly. as.
sin.  Anyway, shouting "lights" at some comput er is silly, and motion
sensors don't have the positive control (or on-location dimming)
needed.  Again, don't flame me about greasy spots on the wall, or
repairability of buried switches -- just, *if* you've done it, pretty
please tell me what you learned.

Thanks in advance!