[sdiy]Touch pads?

[Alex Dickey]

> I'd use Piezo Sensors and sense impact. Almost all cheap-o
> drum sensors use this technology. It is velocity sensitive. The
> hard part is preventing interaction from mechanical coupling.
> Roland cured this by summing in an extra trigger mounted in the
> frame. Other methods work also.

can you say more about this?  maybe something about the problem of
mechanical coupling as a start?  

thanks,
alex