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Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz". In air at atmospheric pressure, ultrasonic waves have wavelengths of 1.9 cm or less.
Ultrasound can be generated at very high frequencies; ultrasound is used for sonochemistry at frequencies up to multiple hundreds of kilohertz.[6][7] Medical imaging equipment uses frequencies in the MHz range.[8] UHF ultrasound waves have been generated as high as the gigahertz range.[9][10][11]
Characterizing extremely high-frequency ultrasound poses challenges, as such rapid movement causes waveforms to steepen and form shock waves.[12]The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear. Auditory sensation can occur if high‐intensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear.[13]
Children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to decrease with age.[14] An American cell phone company has used this to create ring signals that supposedly are only audible to younger humans,[15] but many older people can hear the signals, which may be because of the considerable variation of age-related deterioration in the upper hearing threshold.
Animals
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Bats use ultrasounds to navigate in the darkness.
A dog whistle, which emits sound in the ultrasonic range, used to train dogs and other animals
Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz.[16]
Many insects have good ultrasonic hearing, and most of these are nocturnal insects listening
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