What defines infrasound?

Infrasound is defined as sound waves that have frequencies below 20 Hz. This range is below the threshold of human hearing, which typically starts at around 20 Hz and extends to 20,000 Hz. Infrasound can be produced by natural events, such as earthquakes or volcanic eruptions, as well as by man-made sources, like machinery and explosions.

The other options describe different aspects of sound. Sound waves above 20,000 Hz are known as ultrasound, which is utilized in various applications including medical imaging. Erratic, intermittent, or random sounds do not pertain to a specific frequency range and thus do not define infrasound. Lastly, the quality of sound refers to attributes like timbre or tonal quality rather than frequency. Understanding that infrasound specifically relates to frequencies below 20 Hz helps clarify its distinction from other types of sound.