Infrasound sometimes referred to as low-frequency sound, lower in frequency than 20 Hz or cycles per second of the “normal“ limit of human hearing (20 kHz).

The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.1 Hz and rarely to 0.001 Hz. This frequency range can be used for monitoring earthquakes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart.

Infrasound is characterized by an ability to get around obstacles with little dissipation.