Infrasound

Infrasound has the special characteristic of travelling well in ground or water; in fact, the waves of an earthquake can be thought of as a form of infrasound. Because sound travels much faster in ground than in air (or water), ground-borne vibrations, if perceived, can serve as an early-warning system, arriving well before sound from the same source arrives through the air. Infrasound also dissipates less rapidly in air, making it ideal for long distance communication.

Perception of infrasound, however, presents some specific problems. An object smaller than the distance between waves is a poor receiver for those waves. Thus infrasonic receivers need to be large. This is probably the reason that infrasonic communication is used by only a few animals, and the best understood infrasonic communication system is the African elephant's.

The large pinnae (external portion of the ear) in the African elephant may play an important role in the elephant's perception of low frequency sounds, which are significant in communication among elephants. Receiving structures whose size is matched to the wavelength of the sound perform better. Click for more on infrasonic communication in elephants.