Why Do Humans Can't Hear Ultrasound?

Ultrasound is a cyclic sound pressure wave with a frequency greater than the upper limit of the human hearing range. Ultrasound is thus not separated from "normal" (audible) sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is approximately 20 kilohertz (20,000 hertz) in healthy, young adults. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz. The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear, which acts as a low-pass filter.

Ultrasonic hearing can occur if ultrasound is fed directly into the skull bone and reaches the cochlea through bone conduction without passing through the middle ear. Children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to become lower with age. A cell phone company has used this to create ring signals supposedly only able to be heard by younger humans; but many older people can hear it, which may be due to the considerable variation of age-related deterioration in the upper hearing threshold.

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Many animals—such as dogs, cats, dolphins, bats, and mice—have an upper frequency limit that is higher than that of the human ear and thus can hear ultrasound. Dogs can hear sound at higher frequencies than humans can. A dog whistle exploits this by emitting a high frequency sound to call to a dog. Many dog whistles emit sound in the upper audible range of humans, but some, such as the silent whistle, emit ultrasound at a frequency in the range 18–22 kHz.

Humans can’t also hear the ultrasound because it is more than 20,000 hertz. The waves are too small and fast for the inner ear to transmit the vibrations. 20,000 Hertz is the typical upper limit of hearing, there may be individuals who can detect some frequencies slightly above this range, but not much higher. If the sound is too low or too high, our ears are not sensitive enough to hear the sound. Ultrasound is called "ultra" sound because it is beyond the normal range of human hearing in the same way that ultraviolet light is beyond violet light and outside the normal range of human vision. While ultrasound has frequencies above the range of human hearing, infrasound has frequencies below the range of human hearing, in this case just as infrared light has frequencies below those of red light, and therefore outside the normal range of human vision.

References:

http://en.wikipedia.org/wiki/Talk%3AUltrasound
http://wiki.answers.com/Q/Why_can%27t_humans_hear_ultrasound http://en.wikipedia.org/wiki/Ultrasound
http://www.nlm.nih.gov/medlineplus/ultrasound.html