Edelmen's Sonography Principles and Instrumentation (SPI) Practice Exam

The emitted ultrasound frequency is determined by the transducer’s physical resonator properties, especially the thickness of the PZT crystal. The piezoelectric element vibrates most strongly in a thickness-mode resonance, so the center frequency is roughly f ≈ v_pzt / (2t), where v_pzt is the speed of sound in the PZT and t is its thickness. In short, thinner elements produce higher frequencies, thicker elements yield lower frequencies.

The voltage applied to the PZT mainly changes how loudly (the amplitude) the crystal vibrates, not the frequency. The pulse repetition frequency sets how often pulses are sent, affecting imaging speed and depth, but not the center frequency of each pulse.