What describes sound waves with frequencies lower than 20 Hz?

Sound waves with frequencies lower than 20 Hz are classified as infrasonic. This range of sound is below the threshold of human hearing, which typically starts at about 20 Hz and extends to around 20 kHz. Infrasonic waves are often associated with natural phenomena, such as earthquakes and volcanic eruptions, as well as some man-made sources, like heavy machinery.

Understanding this classification helps in distinguishing the types of sound waves based on their frequencies. For instance, sonorous typically refers to sounds that are deep and resonant but does not specifically denote their frequency. Ultrasonic refers to sounds above 20 kHz, and audible sounds are those within the range of human hearing. Thus, the designation of a wave as infrasonic specifically highlights its frequency range being below the human auditory threshold.