Which of the following describes sound waves with frequencies below 20 Hz?

Sound waves with frequencies below 20 Hz are referred to as infrasonic waves. These low-frequency sounds are below the threshold of human hearing, which typically ranges from approximately 20 Hz to 20,000 Hz. Infrasonic waves can occur naturally, for example, during earthquakes or volcanic eruptions, and can also be generated by man-made sources like machinery.

Ultrasound refers to sound waves with frequencies above the audible range, typically above 20 kHz, and therefore does not apply here. Audible sound encompasses frequencies that humans can detect, which excludes any sound below 20 Hz. Vocal sounds typically refer to the sounds produced by humans when speaking, which also fall within the audible frequency range. Hence, the term infrasonic is specifically accurate for categorizing frequencies below 20 Hz.