Ultrasound as High-Frequency Longitudinal Waves

Steps:

• Sound waves are mechanical waves that propagate as longitudinal waves, with particles oscillating parallel to the direction of travel.
• The human audible range is frequencies from 20 Hz to 20 kHz.
• Ultrasound refers to sound waves beyond the upper limit of human hearing.
• Thus, ultrasound is longitudinal waves with frequency greater than 20 kHz.

Why B is correct:

• Ultrasound is defined as longitudinal sound waves with frequencies exceeding 20 kHz, the threshold for human hearing.

Why the others are wrong:

• A: Frequencies less than 20 kHz are audible sound, not ultrasound.
• C: Sound waves, including ultrasound, are longitudinal, not transverse.
• D: Transverse waves do not describe sound or ultrasound, regardless of frequency.

Final answer: B