Elephants use infrasound, very low-frequency sound, to communicate over long distances.
Many elephant rumbles contain frequencies below the usual lower limit of human hearing, about 20 hertz. These calls can travel through air over considerable distances, especially in open habitats, allowing separated animals to exchange information about contact, movement, mating and threats. The elephant’s large vocal tract helps it produce these deep sounds.
Infrasound is not the same as seismic communication. Elephants can also create vibrations that travel through the ground, and their feet and trunks can detect some ground-borne signals. Those are separate channels within a broader communication system.
People sometimes assume that every elephant call is inaudible to humans. That is incorrect: elephants also make trumpets, rumbles and other sounds that people can hear. Infrasound refers specifically to the frequency range, not to all elephant vocalization.