What would happen if a piezoelectric unimorph was made with two piezo discs instead of one?

Question

Is there a particular reason unimorphs only have just one thin piezo disc? Could two or more such piezo discs be used instead of just one to get greater sound output? Can a unimorph be made to produce low ultrasonic frequencies as well?

Answer 1

Two in mechanical series should provide twice the amplitude of motion, except that the mass of one loads the other, so the frequency response will be poorer at the high end. (Since I don't know the stiffnesses and masses, I don't know how high the high end is, and don't know about ultrasonic response either.)
Two in parallel of course would just be two unimorphs, with the same near-field amplitude.