If instead the piezo film is placed on a flexible back, (see Figure 7) and thumped with a finger a much higher (up to 200x greater) voltage is produced. The reason being the finger pressure causes the film to stretch upon impact.
Piezo-Film Transducers
To begin experimenting, there are three inexpensive piezo film transducers to choose from, PZ-01, PZ-02, and PZ-03, see figure 8. These transducers are suitable for making microphones, acoustical instrument pick-ups, piezo demonstrations, and CMOS switches. The piezoelectric material is encased in a protective urethane jacket.