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Acoustics

Applications

The applications of acoustical devices are far too numerous to describe; one only has to look around our homes to see some of them: telephones, radios and television sets, compact disc players and tape recorders; even clocks that "speak" the time. Probably one of the most important from the human point of view is the hearing aid, a miniature microphone-amplifier-loudspeaker that is designed to enhance whatever range of frequencies a person finds difficulty hearing.

However, one of the first large-scale industrial uses of sound propagation was by the military in World War I, in the detection of enemy submarines by means of sonar (for sound navigation and ranging). This was further developed during the period between then and World War II, and since then. The ship-hunting submarine has a sound source and receiver projecting from the ship's hull that can be used for either listening or in an echo-ranging mode; the source and receiver are directional, so that they can send and receive an acoustic signal from only a small range of directions at one time. In the listening mode of operation, the operator tries to determine what are the sources of any noise that might be heard: the regular beat of an engine heard underwater can tell that an enemy might be in the vicinity. In the echo-ranging mode, a series of short bursts of sound is sent out, and the time for the echo to return is noted; that time interval multiplied by the speed of sound in water indicates (twice) the distance to the reflecting object. Since the sound source is directional, the direction in which the object lies is also known. This is now such a well developed method of finding underwater objects that commercial versions are available for fishermen to hunt for schools of fish.

Ultrasonic sources, utilizing pulses of frequencies in the many millions of cycles per second (and higher), are now used for inspecting metals for flaws. The small wavelengths make the pulses liable to reflection from any imperfections in a piece of metal. Castings may have internal cracks which will weaken the structure; welds may be imperfect, possibly leading to failure of a metal-to-metal joint; metal fatigue may produce cracks in areas impossible to inspect by eye. The use of ultrasonic inspection techniques is increasingly important for failure prevention in bridges, aircraft, and pipelines, to name just a few.

The use of ultrasonics in medicine is also of growing importance. The detection of kidney stones or gallstones is routine, as is the imaging of fetuses to detect suspected birth defects, cardiac imaging, blood flow measurements, and so forth.

Thus, the field of acoustics covers a vast array of different areas of use, and they are constantly expanding. Acoustics in the communications industry, in various phases of the construction industries, in oil field exploration, in medicine, in the military, and in the entertainment industry, all attest to the growth of this field and to its continuing importance in the future.


Resources

Books

Deutsch, Diana. Ear and Brain: How We Make Sense of Sounds New York: Copernicus Books, 2003.

Kinsler, Lawrence E., et. al. Fundamentals of Acoustics, 4th ed. New York: John Wiley & Sons, 1999.

Organizations

The Acoustical Society of America. 2 Huntington Quadrangle, Suite 1NO1, Melville, NY 11747–4502 Phone: (516) 576–2360. <http://asa.aip.org/index.html>

Other

The University of New South Wales. "Music Acoustics" [cited March 10, 2003]. <http://www.phys.unsw.edu.au/music/>.


David Mintzer

KEY TERMS


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Amplitude

—The maximum displacement of the material that is vibrating.

Condensations

—When air is the vibrating medium there are alternate small increases and decreases of the density of the air; the increases are called condensations.

Cycle

—A single complete vibration.

Cycles per second

—The number of complete vibrations per second.

Frequency

—The rate at which vibrations take place (number of times per second the motion is repeated), denoted here by f and given in cycles per second or in hertz.

Fundamental

—The lowest frequency of vibration of a sound-producing body (also called the first harmonic).

Harmonics (first, second, etc.)

—The various frequencies of vibration of a sound-producing body, numbered from the one of lowest frequency to higher frequencies.

Hertz

—A hertz (abbreviated as Hz) is one cycle per second.

Infrasonic vibrations

—When the rate of vibration is below the range of human hearing, e.g., below about 10 cycles per second.

Longitudinal wave

—The case where the motion of the vibrating body is in the wave propagation direction.

Loudspeaker

—A device to produce sounds from an electric current, by electrical and mechanical means, in the range of frequencies around the sonic range (that is produced by humans).

Medium

—A material body that carries the acoustic vibrations away from the body producing them.

Microphone

—A device to change sound waves (pressure waves), by electrical and mechanical means, into an electric current having the same frequencies as the sound, in the range of frequencies around the sonic range (that is produced by humans).

Nodes

—Places where the amplitude of vibration is zero.

Overtones

—The set of harmonics, beyond the first, of a soundproducing body.

Period

—The length of time for one cycle to be completed; the reciprocal of the frequency.

Propagation direction

—The direction in which the wave is traveling.

Rarefactions

—When air is the vibrating medium there are alternate small increases and decreases of the density of the air; the decreases are called rarefactions.

Sonar

—A device utilizing sound to determine the range and direction to an underwater object.

Supersonic

—Refers to bodies moving at speeds greater than the speed of sound (not normally involved in the study of acoustics).

Transverse wave

—The case where the motion of the vibrating body is perpendicular to the wave propagation direction.

Ultrasonic vibrations

—When the rate of vibration is above the range of human hearing, e.g., above about 20,000 cycles per second.

Wave

—A motion, in which energy changes are propagated, is carried away from some source, which repeats itself in space and time with little or no change.

Wavelength

—The distance, at any instant of time, between parts of a vibrating body having the identical motion, denoted here by L.

Additional topics

Science EncyclopediaScience & Philosophy: 1,2-dibromoethane to AdrenergicAcoustics - Vibrations Of A String, Vibrations Of An Air Column, Sound Production In General, Transmission Of Sound - Production of sound