Which type of sound waves are associated with frequencies below human hearing?

The correct answer is infrasound, which refers to sound waves with frequencies below the range of human hearing, typically regarded as frequencies below 20 Hz. These low-frequency sound waves can travel long distances and are often utilized in various scientific fields, including meteorology and seismology.

In contrast, ultrasound refers to sound waves that have frequencies above the upper limit of human hearing, typically above 20 kHz, and are commonly used in medical imaging applications. Audible sound encompasses frequencies within the human hearing range (approximately 20 Hz to 20 kHz). The term subsonic sound is not commonly used in the context of sound wave frequencies; it may refer to sound speeds below the speed of sound rather than specifically addressing frequency ranges. Therefore, the term infrasound specifically denotes the correct category of low-frequency sound waves that fall below human hearing capabilities.