Therapeutic Ultrasound frequency is greater than which value in Hertz per second?

• A. Less than 1,000 Hertz
• B. Between 1,000 and 10,000 Hz per sec
• C. Greater than 20,000 Hertz per sec
• D. Exactly 40,000 Hertz per sec

Answer: (C)

Ultrasound is defined by frequencies above the range of human hearing. Humans can hear up to about 20,000 Hz, so any sound used as ultrasound, including therapeutic ultrasound, operates above that threshold. Therapeutic devices typically use frequencies around 1 MHz (1,000,000 Hz), which are far above 20,000 Hz. So the frequency is greater than 20,000 Hz per second.

The other options place the frequency in the audible range or pick a specific value rather than the threshold that separates audible sound from ultrasound, making them less accurate descriptions of what defines ultrasound.