Investigation. A type of imaging technique which uses high-frequency sound waves. This is highly operator-dependent and is thought to be useful in diagnosis but not particularly accurate in the assessment of tumour response. For the latter, CT or MR imaging are more accurate. 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 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 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. Organisms such as bats and porpoises use ultrasound for locating prey and obstacles. thumb|right|Ultrasound image of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional-scan) thumb|right|An ultrasonic examination in East Germany, 1990 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. (An OSH glossary used in safety and health at work which is, adopted by ILO {102}.. The term definition has been obtained from the secondary source