The low-frequency sounds elephants use to communicate are called infrasound. These calls occupy frequencies below, or partly below, the lower limit of typical human hearing.
African elephants produce infrasonic rumbles commonly measured across roughly 15–35 hertz. Because low-frequency sound can travel long distances through suitable environments, these calls help elephants remain in contact when family groups or individuals are separated by kilometers. The calls can carry information about contact, movement, greeting, mating, and alarm.
Elephant communication is more varied than infrasonic rumbles alone. Elephants also trumpet, touch one another with their trunks, use visual gestures such as ear spreading, and create vibrations that can travel through the ground. Infrasound is therefore one channel in a broad communication system, especially useful for long-range contact.
The common mistake is to call these sounds “ultrasound.” Ultrasound means frequencies above the human hearing range, while infrasound means frequencies below it. “Subsonic” usually refers to speed below the speed of sound, so it is not the correct biological term here.