A pressure responsive diaphragm or capsule having a variable self-resonant frequency characteristic is excited to vibrate in a mode other than the fundamental or f01 mode, that is, vibration occurs in a mode wherein at least first and second portions of the diaphragm are simultaneously moving 180* out of phase with respect to one another. This is accomplished by suitably coupling first and second electromechanical transducers to the first and second diaphragm portions respectively. A feedback amplifier whose output frequency is determined by the frequency at which the first transducer is vibrating is connected to drive the second transducer.