ZB and ZM are the backing and propagation medium
acoustic impedances, respectively. Ri (i ¼ 1; . . . ; 4), with
a very high value (requirement of the Spice analysis
program), have a negligible effect on the responses. Z0 ,
STd (transit time), h, and C0 are the usual piezoelectric
parameters of a modified transmission-line equivalent
circuit [10] for the transducer. Finally, the equivalent
force source (Feq ) is a function of the emitted force FM ,
ZM and the reflector impedance, ZRf :