Ultrasound – Wikipedia, the free encyclopedia

Ultrasound is an oscillating 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 by differences in physical properties, only by 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 20kHz up to several gigahertz.

Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasonic imaging (sonography) is used in both veterinary medicine and human medicine. In the nondestructive testing of products and structures, ultrasound is used to detect invisible flaws. Industrially, ultrasound is used for cleaning and for mixing, and to accelerate chemical processes. Animals such as bats and porpoises use ultrasound for locating prey and obstacles.[1]

Ultrasonics is the application of ultrasound. Ultrasound can be used for medical imaging, detection, measurement and cleaning. At higher power levels, ultrasonics is useful for changing the chemical properties of substances.

Acoustics, the science of sound, starts as far back as Pythagoras in the 6th century BC, who wrote on the mathematical properties of stringed instruments. Sir Francis Galton constructed a whistle producing ultrasound in 1893. The first technological application of ultrasound was an attempt to detect submarines by Paul Langevin in 1917. The piezoelectric effect, discovered by Jacques and Pierre Curie in 1880, was useful in transducers to generate and detect ultrasonic waves in air and water.[2]Echolocation in bats was discovered by Lazzaro Spallanzani in 1794, when he demonstrated that bats hunted and navigated by inaudible sound and not vision.

Ultrasound is defined by the American National Standards Institute as "sound at frequencies greater than 20 kHz."

The upper frequency limit in humans (approximately 20kHz) is due to limitations of the middle ear. Auditory sensation can occur if highintensity ultrasound is fed directly into the human skull and reaches the cochlea through bone conduction, without passing through the middle ear.[3]

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.[4] An American cell phone company has used this to create ring signals supposedly only able to be heard by younger humans;[5] 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. The Mosquito is an electronic device that uses a high pitched frequency to deter loitering by young people.

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100kHz, possibly up to 200kHz.[6]

Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats. This includes many groups of moths, beetles, praying mantids and lacewings. Upon hearing a bat, some insects will make evasive manoeuvres to escape being caught.[7] Ultrasonic frequencies trigger a reflex action in the noctuid moth that cause it to drop slightly in its flight to evade attack.[8]Tiger moths also emit clicks which may disturb bats' echolocation,[9][10] but may also in other cases evade being eaten by advertising the fact that they are poisonous by emitting sound.[11][12]

Dogs with normal hearing can hear ultrasound. A dog whistle exploits this by emitting a high frequency sound to call to a dog. Many such 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 1822kHz.

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Ultrasound - Wikipedia, the free encyclopedia