What are the frequencies below 20 Hz and above 20,000 Hz referred to as?

Frequencies below 20 Hz are referred to as infrasonic, while frequencies above 20,000 Hz are labeled ultrasonic. This terminology is widely used in acoustics to categorize sounds that fall outside the range of human hearing. Infrasonic waves can include natural phenomena such as earthquakes and can be used in various scientific applications, while ultrasonic waves find applications in technology, such as in medical imaging (ultrasound) and cleaning. The choice aligns with established definitions in the field of acoustics, making it the correct response.

The other options reference different concepts. "Subsonic and supersonic" pertain to speeds relative to the speed of sound. "Infrared and ultraviolet" refer to regions of the electromagnetic spectrum, not sound. "Subaudible and audible" mistakenly categorizes sound; subaudible is not the proper term to define frequencies below 20 Hz, as it is related more to the perceptual ability to hear, whereas the established terms infrasonic and ultrasonic provide precise frequency classifications.