Ultrasonic waves are sound waves transmitted above the human-detectable frequency range, usually above 20,000 Hz. They are used by some animals and in medical or industrial technological devices.
Some ultrasound-emitting devices are used to silence barking dogs without hurting human hearing, because only dogs find the sounds intolerable. Similarly, some medical equipment for diagnosing health complications can produce ultrasonic sounds of 10 MHz and above. Such sounds do not harm human tissue.
Bats use high-frequency sounds to navigate and catch prey. The animals emit ultrasonic waves while moving at high speed and in pitch darkness. When the sounds are reflected back, bats can tell what exactly is in front of them.