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. Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasonic imaging (sonography) is used in human and veterinary medicine. In non-destructive 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. Organisms such as bats and porpoises use ultrasound for locating prey and obstacles. Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications A fetus in its mother's womb, viewed at 12 weeks of pregnancy (bidimensional-scan) An ultrasonic examination in East G
rmany, 1990 Ultrasonics is the application of ultrasound. Ultrasound can be used for imaging, detection, measurement, and cleaning. At higher power levels ultrasonics are useful for changing the chemical properties of substances.Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example, if a newborn baby's heart beats at a frequency of 120 times a minute, its period (the interval between beats) is half a second. Hearing range usually describes the range of frequencies that can be heard by an animal or human, though it can also refer to the range of levels. In humans the audible range of frequencies is usually 20 to 20,000 Hz, although there is considerable variation between individuals, especially at high frequencies, where a gradual decline with age is considered normal. Sensitivity also varies with frequency, as shown by equal-loudness contours. Routine investigation for hearing loss usually involves an audiogram which shows threshold levels relative to a standardised norm.