Electromechanical Coupling and Composite Transducers

Abstract

Measurements with composite types of transducers indicate that both the classical definition of the electromechanical coupling factor and the equivalent circuit for a piezoelectric transducer require revisions for improved consistency. The effective coupling coefficient depends on the proportion of the piezoelectric material that is employed electromechanically rather than being independent of it as implied by existing definitive formulations. Furthermore, the inherently usable coupling decreases below the fundamental resonance frequency so that it approaches zero at very low frequencies. At higher frequencies the inherent coupling factor may approach the ultimate value at a frequency specified by the dimensions and characteristics of the piezoelectric specimen in a transducer. For the equivalent circuit, the measurements favor a parallel rather than a series form for the motional or transformed mechanical impedance because the frequency for parallel resonance remains invariant with the proportion of the piezoelectric specimen that is excited electromechanically.