piezoelectric sensor

For 31 mode piezoelectric plate. I have measured it's electric impedance spectrum (curve of Log IZI~ Frequency). I found that, when I used this plate for a piezoelectric sensor to receive mechanic signals. the maximum output  occured at antiresonance frequency;   For example, for piezoelectric hydrophone or receiving-type piezoelectric transducers, It works at antiresonance frequency (at the maximum electric impedance).