Ultrasound

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Ultrasound is sound with a frequency higher than the upper limit of human hearing, this limit being approximately 20 kilohertz (20,000 hertz).

[edit] Ability to hear ultrasound

Some animals, such as dogs, dolphins, bats, and mice, have an upper frequency limit that is greater than that of the human ear, so they can hear ultrasound. Children can hear some high-pitched sounds that older adults cannot hear, as in humans the upper limit pitch of hearing gets lower as a person gets older. This frequency limit is caused by the middle ear that acts as a low-pass filter. If ultrasound is fed directly into the skull bone and reaches the cochlea without passing through the middle ear, much higher frequencies (up to about 200 kHz) can be heard. This effect (sometimes called ultrasonic hearing) was first discovered by divers exposed to a high-frequency (ca. 50 kHz) sonar signal.

[edit] Diagnostic sonography

A foetus, aged 29 weeks, in a "3D ultrasound"
A foetus, aged 29 weeks, in a "3D ultrasound"

Medical sonography (ultrasonography) is a useful ultrasound-based diagnostic medical imaging technique used to see muscles, tendons, and many internal organs, their size, structure and any pathological lesions. They are also used to see foetuses during routine and emergency prenatal care. Ultrasound scans are performed by medical health care professionals called sonographers. Obstetric sonography is commonly used during pregnancy. Ultrasound is generally regarded as a "safe test" because it does not use ionising radiation like x-rays, nuclear medicine, or CT.

[edit] Ultrasonic cleaning

Ultrasonic cleaners, sometimes mistakenly called supersonic cleaners, are used at frequencies from 20-40 kHz for jewellery, lenses and other optical parts, watches, dental instruments, surgical instruments and industrial parts. An ultrasonic cleaner works mostly by energy released from the collapse of millions of microscopic cavitations near the dirty surface. The bubbles formed by cavitation collapse forming tiny jets directed at the surface.