Ultrasound as high-frequency longitudinal waves

Steps:

• Sound waves, including ultrasound, are longitudinal, involving particle compression and rarefaction parallel to propagation.
• Human audible range is 20 Hz to 20 kHz.
• Ultrasound exceeds the upper audible limit, defined as frequencies greater than 20 kHz.
• Thus, ultrasound matches longitudinal waves above 20 kHz.

Why B is correct:

• Ultrasound is defined as longitudinal sound waves with frequencies greater than 20 kHz, beyond human hearing.

Why the others are wrong:

• A: Frequencies smaller than 20 Hz describe infrasound, below human hearing.
• C: Sound waves are longitudinal, not transverse; smaller than 20 Hz is infrasound.
• D: Ultrasound waves are longitudinal, not transverse, despite the correct frequency threshold.

Final answer: B