Question about energy moving through one object to another I think this question is part physics, and maybe part chemistry.
I'm making electronic drums. On the top of the drum there's a flat surface (the drum head) you hit with a stick anywhere on the head.  The diameter is say 14 inches.   A couple of inches below the head, is the bottom of the drum - a piece of flat metal.  Adhered to the center of the metal bottom, is a flat, round piezo which is wired to the electronics that make the drum sound.  The piezo is about 1 inch in diameter.  Sandwiched between the bottom of the drum (with the piezo) and the drum head, is "some type of foam".
The idea is that you hit the drum head and the energy is transferred to the head, through the foam, and then hopefully enough to trigger the piezo.  [b]I'm trying to figure out what type of foam would be best for this, so that no matter where you hit the drum head it's able to trigger the piezo.[/b]  I've spent a small fortune on various foams so far and I'm getting closer, but there's got to be a more scientific approach!
Why not just mount the piezo to the drum head?  Because hitting it with the sticks would break it quickly.  Why foam?  Because you want to be able to play using headphones and not have others hear you - so you can't use anything hard in the middle, and you still want a little bounce.  So what type of foam would be best for transferring the energy from the large head, to the small piezo? Low density? High Density?  I don't understand how the kinetic energy (?) moves from one thing to another.  If something is good at absorbing shock does that mean that it converts energy to some other form that dissipates the energy which wouldn't be good for triggering the piezo, or does it mean that its good at passing the energy from one end to the other (from the top to the bottom) which would be good for triggering the piezo? I guess this question is part physics (energy), part chemistry (foams), part electronics etc.  And sorry for being so long winded....
 A: Part of the problem here is that you want two things:


*

*regardless of where you hit the large surface, you want the small piezo to record the "event"

*you would like the force on the piezo to scale with the force of the hit


The obvious material to give you both these properties would be an incompressible fluid - preferably a non-conductive one that doesn't degrade the materials of the drum. Unfortunately, any hole in the container would make a big mess quickly...
If we don't use water, we need to somehow do acoustic impedance matching: the air behind the drum skin will simply reflect off the piezo, transferring almost no energy. A denser material is a better choice, since acoustic impedance is given by
$$Z = \sqrt{\rho c}$$
where $\rho$ is the density, and $c$ the speed of sound in the medium. Adding high density foam gives you a material with "intermediate" acoustic impedance (between air and piezo) which is why it might be a good choice.
You might look up "acoustic impedance matching" for some more ideas on how to make this project of yours work - without a lot more details about your construction it's hard to get very specific with advice.
