A hydrophone is used to measure which aspect of an ultrasonic signal?

• A. Velocity
• B. Frequency
• C. Intesity
• D. Phase

Answer: (B)

A hydrophone directly senses acoustic pressure fluctuations in a fluid and converts them into an electrical signal. The information you get from that signal is the rate at which the pressure oscillates, so the frequency of the ultrasonic wave is determined from the time-varying pressure waveform (for example, by analyzing its spectrum or counting oscillations). Particle velocity isn’t measured directly by a hydrophone, and intensity would require calibration to relate pressure to power per area, while phase requires comparing with a reference or separate measurement. So the most straightforward attribute obtained from the hydrophone output is the frequency.