How To Measure Performance of Piezo Contact Mics (3.4MB pdf)

How can we define and measure standard performance characteristics of piezo contact mics? Condenser and dynamic mics measure sound pressure in the atmosphere. We can standardize the medium and the environment in which we test those mics. But piezo contact mics pick up vibrations in a solid medium. How can I measure the frequency response of a peizo when it, in turn, is actually measuring the frequency response of the object it is attached to? Is there an acoustically perfect solid substance, one that we know will transmit faithfully and without color all the frequencies with which I excite it? Of course there is not: all solid objects resonate. So how can I measure frequency response characteristics of piezos? The article says, I can’t, not in any absolute sense, but at least I can measure a mic’s response at any given frequency, and compare it to others. So the article describes our mic testing rig, shown below — how we designed and built it, what it’s good for and what it’s useless for.

What do you think? How would you measure the frequency response characteristics of a piezo contact mic? Can you think of a system for driving the piezo that does not resonate? Or can you think of a way to asses the system’s resonant properties so as to correct for them in the piezo test results?