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study and application of the energy of soundsound,
any disturbance that travels through an elastic medium such as air, ground, or water to be heard by the human ear. When a body vibrates, or moves back and forth (see vibration), the oscillation causes a periodic disturbance of the surrounding air or other medium that
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 waves vibrating at frequencies greater than 20,000 cycles per second, i.e., beyond the range of human hearing. The application of sound energy in the audible range is limited almost entirely to communications, since increasing the pressure, or intensity, of sound waves increases loudness and therefore causes discomfort to human beings. Ultrasonic waves, however, being inaudible, have little or no effect on the ear even at high intensities. They are produced, commonly, by a transducertransducer,
device that accepts an input of energy in one form and produces an output of energy in some other form, with a known, fixed relationship between the input and output. One widely used class of transducers consists of devices that produce an electric output signal, e.g.
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 containing a piezoelectric substance, e.g., a quartz-crystal oscillator that converts high-frequency electric current into vibrating ultrasonic waves. Ultrasonics has found wide industrial use. For nondestructive testing an object is irradiated with ultrasonic waves; variation in velocity or echo of the transmitted waves indicates a flaw. Fine machine parts, ball bearings, surgical instruments, and many other objects can be cleaned ultrasonically. They are placed in a liquid, e.g., a detergent solution or a solvent, into which ultrasonic waves are introduced. By a phenomenon called cavitation, the vibrations cause large numbers of invisible bubbles to explode with great force on the surfaces of the objects. Film or dirt is thus removed even from normally inaccessible holes, cracks, and corners. Radioactive scale is similarly removed from nuclear reactor fuel and control rods. In medicine ultrasonic devices are used to examine internal organs without surgery and are safer to genetic material than X rays. The waves with which the body is irradiated are reflected and refracted; these are recorded by a sonograph for use in diagnosis (see ultrasoundultrasound
or sonography,
in medicine, technique that uses sound waves to study and treat hard-to-reach body areas. In scanning with ultrasound, high-frequency sound waves are transmitted to the area of interest and the returning echoes recorded (for more detail, see
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 for further description of medical uses). Metals can be welded together by placing their surfaces in contact with each other and irradiating the contact with ultrasound. The molecules are stimulated into rearranged crystalline form, making a permanent bond. Ultrasonic whistles, which cannot be heard by human beings, are audible to dogs and are used to summon them.


The science of sound waves having frequencies above the audible range, that is, above about 20,000 Hz. Original workers in this field adopted the term supersonics. However, this name was also used in the study of airflow for velocities faster than the speed of sound. The present convention is to use the term ultrasonics as defined above. Since there is no marked distinction between the propagation and the uses of sound waves above and below 20,000 Hz, the division is artificial. See Sound

Ultrasonic generators and detectors

Ultrasonic transducers have two functions: transmission and reception. There may be separate transducers for each function or a single transducer for both functions. The usual types of generators and detectors for air, liquids, and solids are piezoelectric and magnetostrictive transducers. Quartz and lithium niobate (LiNbO3) crystals are used to produce longitudinal and transverse waves; thin-film zinc oxide (ZnO) transducers can generate longitudinal waves at frequencies up to 96 GHz. Another class of materials used to generate ultrasonic signals is the piezoelectric ceramics. In contrast to the naturally occurring piezoelectric crystals, these ceramics have a polycrystalline structure. The most commonly produced piezoelectric ceramics are lead zirconate titanate (PZT), barium titanate (BaTiO3), lead titanate (PbTiO3), and lead metaniobate (PbNb2O6). Composite transducers are transducers in which the radiating or receiving element is a diced piezoelectric plate with filler between the elements. They are called “composite” to account for the two disparate elements, the piezoelectric diced into rods and the compliant adhesive filler. See Magnetostriction, Piezoelectricity

High-power ultrasound (typically 600 W) can be obtained with sonicators, consisting of a converter, horn, and tip. The converter transforms electrical energy to mechanical energy at a frequency of 20 kHz. Oscillation of piezoelectric transducers is transmitted and focused by a titanium horn that radiates energy into the liquid being treated. Horn and tip sizes are determined by the volume to be processed and the intensity desired. As the tip diameter increases, intensity or amplitude decreases.

Engineering applications

The engineering applications of ultrasonics can be divided into those dealing with low-amplitude sound waves and those dealing with high-amplitude (usually called macrosonics) waves.

Low-amplitude applications

Low-amplitude applications are in sonar (an underwater-detection apparatus), in the measurement of the elastic constants of gases, liquids, and solids by a determination of the velocity of propagation of sound waves, in the measurement of acoustic emission, and in a number of ultrasonic devices such as delay lines, mechanical filters, inspectoscopes, thickness gages, and surface-acoustic-wave devices. All these applications depend on the modifications that boundaries and imperfections in the materials cause in wave propagation properties. The attenuation and scattering of the sound in the media are important factors in determining the frequencies used and the sizes of the pieces that can be utilized or investigated.

High-amplitude applications

High-amplitude acoustic waves (macrosonic) have been used in a variety of applications involving gases, liquids, and solids. Some common applications are mentioned below.

A liquid subjected to high-amplitude acoustic waves can rupture, resulting in the formation of gas- and vapor-filled bubbles. When such a cavity collapses, extremely high pressures and temperatures are produced. The process, called cavitation, is the origin of a number of mechanical, chemical, and biological effects.

Cavitation plays an integral role in a wide range of processes such as ultrasonic cleaning and machining, catalysis of chemical reactions, disruption of cells, erosion of solids, degassing of liquids, emulsification of immiscible liquids, and dispersion of solids in liquids. Cavitation can also result in weak emission of light, called sonoluminescence. See Cavitation

One of the principal applications of ultrasonics to gases is particle agglomeration. This technique has been used in industry to collect fumes, dust, sulfuric acid mist, carbon black, and other substances.

Another industrial use of ultrasonics has been to produce alloys, such as lead-aluminum and lead-tin-zinc, that could not be produced by conventional metallurgical techniques. Shaking by ultrasonic means causes lead, tin, and zinc to mix.

Analytical uses

In addition to their engineering applications, high-frequency sound waves have been used to determine the specific types of motions that can occur in gaseous, liquid, and solid mediums. Both the velocity and attenuation of a sound wave are functions of the sound frequency. By studying the changes in these properties with changes of frequency, temperature, and pressure, indications of the motions taking place can be obtained. See Sound absorption

Medical applications

Application of ultrasonics in medicine can be generally classified as diagnostic and therapeutic. The more common of these at present is the diagnostic use of ultrasound, specifically ultrasonic imaging. See Nonlinear acoustics

Ultrasonic fields of sufficient amplitude can generate bioeffects in tissues. Although diagnostic ultrasound systems try to limit the potential for these effects, therapeutic levels of ultrasound have been used in medicine for a number of applications. Conventional therapeutic ultrasound is a commonly available technique used in physical therapy. High-frequency acoustic fields (typically 1 MHz) are applied through the skin to the affected area in either a continuous wave or long pulses.

Extracorporeal shock-wave lithotripsy (ESWL) disintegrates kidney stones with a high-amplitude acoustic pulse passing through the skin of the patient. The procedure eliminates the need for extensive surgery. Bioeffects are limited to the location of the stone by using highly focused fields which are targeted on the stone by imaging techniques such as ultrasound or fluoroscopy.


The science of ultrasonic sound waves.


the branch of physics concerned with ultrasonic waves
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