In human beings, hearing is performed by the ears, which also perform the function of balance, a sense in itself but not one of the traditional list (due to Aristotle). This is in common with most mammals. Many other organisms also have some form of hearing, either by some sort of ear, or by other structures, or by a combination.
Normal human ears are said to be sensitive in the range of frequency of 20 Hz to 20 kHz. Some individuals are able to hear up to 22 kHz, and high-quality sound reproduction equipment often goes up to 16 kHz or even beyond. Frequencies capable of being heard by humans are called audio. Frequencies higher than audio are called ultrasonic, while frequencies below audio are called infrasonic.
Some organisms are able to hear ultrasound and/or infrasound. Some batss use ultrasound for echo location while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their bellies, and there is evidence that whales and elephants may use it for communication. See sound for hearing ranges of various organisms.