ABSTRACT
In distinction to light and laser, it is customary to describe acoustic energy by its frequency rather than its wavelength. Acoustic waves with frequencies in the range of 20 hertz (Hz) to 20 kilohertz (kHz) are perceived by the human ear. Frequencies above 20 kHz are in the ultrasound range. Frequencies below 20 Hz constitute infrasound, which is used by geologists to monitor movement and by whales to communicate across long distances.
