I recently bought a K&K "Big Shot" piezo pickup, to mount in my acoustic/electric bass. I was looking to add to the sound that I get from the undersaddle pickup.
If you don't already know, let me explain that piezos react to vibration, so you have to mount in such a way that it either A) picks up a lot of sonic energy from the sound source (in this case, the strings), picks up a lot of resonance from the instrument's body, or C) all the above.
The problem with "A" is that it can also create a lot of feedback if the piezo is too close to an opening... much like putting a mic near the opening of an instrument... so you have less trouble if you keep the piezo buried some place safe.
The problem with "B" is that the piezo will pick up ANY vibration, including the sound of your arm rubbing against the guitar body... much like trying to record vocals while holding a Neumann U87 studio mic with your hand.
Another interesting property is that piezos respond best when there is a little bit of pressure on them; they pick up more of the low frequencies and do a better job of picking up all frequencies as you increase the pressure. Well, up to a point. If you apply too much pressure, then the piezo cannot vibrate, which keeps it from doing anything.
So, with all of those properties in mind, I designed and built a chassis that would hold the piezo tight, shield it from direct sound, and pick up more sonic energy.
Here is a simple graphic of what I built:
The "soundboard" is a piece of solid hardwood that is about 1/8" thick. The "pedestals" are two hardwood dowels. I haven't included dimensions, because that would really depend on what you mount this in; the amount of space you have for mounting, and the amount of space you have for slipping this in so you can mount it.
The two bolts (that hold the piece of wood that holds the piezo) can be tightened until you get just the right amount of pressure to provide the best sound. I also added a thin rubber pad between the piezo and the wood*, which seems to enhance the piezo's ability to vibrate properly.
Over the course of several days, I tried sticking the piezo to the guitar's body, clamping it to the guitar's body, and then mounting it in this chassis. The chassis worked the best by a long shot, and provided incredibly deep lows... the kind that shake subs.
However, it's not the sound that I want for this guitar, so I'm looking into mounting a mic in the body. However, I am holding onto this chassis for other guitar projects, and I'm thinking about making a similar rig for mounting a piezo inside a djembe.
I just thought I would share it, in case anybody wants to experiment.
D~s
*I failed to mention that only used one rubber pad on one side of the piezo.