What determines the frequency of a sound beam from a pulsed transducer?

• A. Voltage applied to PZT
• B. PRF of the pulser
• C. PZT thickness
• D. Frequency of the system

Answer: (C)

The frequency of the emitted sound is set by the transducer’s physical resonance, which is determined mainly by the thickness of the piezoelectric element. In a thickness mode, the crystal vibrates most strongly at a frequency where the thickness corresponds to about half the wavelength of the sound in the crystal, so f ≈ v/2t, with v being the speed of sound in the PZT. Therefore, thinner crystals produce higher frequencies, and thicker crystals produce lower frequencies. The driving voltage affects how strongly the crystal vibrates (amplitude), not the frequency. The pulse repetition frequency controls how often you fire pulses, not the frequency of each pulse. The system’s nominal frequency relates to design and bandwidth, but the actual beam frequency is set by the PZT thickness.