ultrasound

Pressure waves, known as ultrasonic waves, similar in nature to sound waves but occurring at frequencies above 20,000 Hz (cycles per second), the approximate upper limit of human hearing (15–16 Hz is the lower limit).

Ultrasonics is concerned with the study and practical application of these phenomena.

Some animals, such as dogs and bats, and insects are able to detect ultrasonic waves. Bats, as well as detecting ultrasonic waves, also emit them. Sound waves reflected from objects allow the bat to estimate the distance of objects, from the time taken for the echo to return.

Ultrasound can also be used for cleaning. A material is immersed in water through which ultrasonic waves are passed. The high-frequency vibrations cause dirt particles to be removed. Another use of ultrasound is in developing emulsion-based products in the cosmetics and healthcare industries. High-frequency sounds used in a mixture of oil and water form cavitations (formation of large bubbles). This helps the oil to mix with water to form an emulsion.

The earliest practical application was to detect submarines during World War I, but recently the field of ultrasonics has greatly expanded. Frequencies above 80,000 Hz have been used to produce echoes as a means of measuring the depth of the sea or to detect flaws in metal, and in medicine, high-frequency pressure waves are used to investigate various body organs. High-power ultrasound has been used with focusing arrangements to destroy deep-lying tissue in the body, and extremely high frequencies of 1,000 MHz (megahertz) or more are used in ultrasonic microscopes.