Infrasound is sound with a frequency too low to be detected by the human ear (less than approximately 20 Hertz). Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
Whales and elephants are known to use infrasound to communicate over distances of several miles. However, since an animal must be exceedingly large to produce infrasound, they are probably the only kinds of animals that make use of it.
wikipedia.org dumped 2003-03-17 with terodump