Ultrasonics

Type of physical science: Classical physics

Field of study: Acoustics

Ultrasound is the region of high-frequency sound above the highest frequencies that can be perceived by humans. Since the 1960's, ultrasonic technology has developed to such an extent that ultrasound is now recognized as a significant branch of physics with numerous important applications.

Overview

Although the human ear can detect sounds in the frequency range from 16 hertz through 20 kilohertz, the maximum sensitivity lies between 2 and 4 kilohertz. Above this range, the aural sensitivity decreases rapidly to complete insensitivity at 20 kilohertz. Sound waves having frequencies greater than 20 kilohertz are called ultrasonic waves. Although, by definition, ultrasonic sound waves cannot be perceived directly by human ears, the useful applications of ultrasound are as multitudinous as they are varied. Two important properties of sound waves become more pronounced in the ultrasonic region. First, their ability to be focused makes it easier to explore objects and structures of small dimensions. Second, the higher the frequency of a wave, the more rapidly the wave is absorbed. The frequency range of ultrasound is from 20 kilohertz through 1 gigahertz, at which frequency ultrasonic waves can no longer be propagated in air at standard temperatures and pressures. The ultrasonic region may be subdivided into low (20 to 100 kilohertz), medium (100 kilohertz to 10 megahertz), and high (10 megahertz to 0.1 gigahertz) frequencies. Signaling applications, such as range finding, are usually in the low-frequency region, while ultrasonic cleaners and acoustical imaging processors usually operate in the medium-frequency range.

Ultrasonic waves may be generated in one of two ways: by mechanical devices, such as sirens and whistles, or by electromechanical transducers. Although mechanical devices are simple and inexpensive, they are useful only for producing low-frequency ultrasonic waves.

Because of their broad spectrum, as well as some frequency and amplitude instability, these devices are seldom employed when accuracy is a criterion. Electromechanical devices use piezoelectric crystals subjected to an alternating voltage to produce medium and high-frequency narrow-band ultrasonic waves of constant amplitude and frequency.

Because ultrasonic waves have wavelengths that are short compared with the objects being irradiated, they can be focused easily. Acoustic lenses and mirrors focus narrow beams within a restricted space. Although X rays, which are highly penetrating electromagnetic waves, have an even shorter wavelength, they are virtually impossible to focus. An X-ray image is thus a shadowy silhouette, while an ultrasonic image shows details and structure.

Other advantages of using ultrasonic image formation over electromagnetic waves (such as light and X rays) are the greater range of wavelengths (frequencies) available with ultrasound, and the fact that many materials opaque to electromagnetic waves appear transparent to ultrasound. The disadvantages of ultrasound are that it cannot be directly seen or heard, it has no effect on photographic film, and, unlike electromagnetic waves, ultrasonic waves do not propagate through a vacuum. Additionally, designing an acoustic lens to focus the waves is a complex task. Achieving good resolving power requires the ratio of the ultrasonic wavelength to the diameter of the lens to be as small as possible. There are practical limits to how large a lens may be and using shorter wavelengths to improve resolution increases the attenuation of ultrasonic waves. Thus, even using an acoustic lens having a 0.50-centimeter diameter with a 0.1-megahertz frequency ultrasonic wave, the resolution is still two orders of magnitude less than for a 0.5-centimeter diameter optical lens system.

Ultrasonic images are made visible to the human eye by an ultrasonic image converter.

Ultrasonic waves are produced, focused, and directed through the object to be investigated. The waves strike a metalized surface of a quartz disk, which forms the front plate of a cathode-ray tube (television picture tube). The variation in ultrasonic intensity across the disk surface--resulting from the waves passing through the object--produces a corresponding variation in the (alternating) charge pattern on the disk's rear surface. When this rear surface is scanned by an electron beam, the beam will be modulated in accordance with the surface distribution, which produces a visible image (by the usual means) on the face of the television picture tube. This picture may be considered as an ultrasonic "X ray" of the object.

Applications

The unique properties of ultrasonic waves has led industry and medicine to develop many practical applications for ultrasound. Because these waves have very short wavelengths and can be focused in a manner similar to light waves, they have proved to be a versatile and safe alternative in many situations where electromagnetic waves could not be employed. The applications can be divided into two broad categories: high-intensity ultrasound and low-intensity ultrasound. High-intensity ultrasonics is used to effect physical or chemical changes in some material substance. Low-intensity ultrasonics is concerned with information that can be gathered by means of coherent ultrasonic waves. High-intensity ultrasonic applications include ultrasonic cleaning, mixing, soldering, welding, drilling, and a variety of chemical procedures. The principal applications of low-intensity ultrasonics are the detection of flaws in solid objects, medical sonograms, and acoustical holography.

Ultrasonic cleaning is achieved when the extremely high accelerations of ultrasound waves vibrate small particles of grit off dirty objects. High-intensity ultrasonic waves in a liquid produce regions of intense compression and expansion, which literally tears the liquid apart. The small bubbles, or cavities, formed by this process (known as cavitation) produce shock waves that impinge on the dirty surface and scour it clean. Since cavitation decreases with frequency, ultrasonic cleaners use high-intensity signals of low-ultrasonic frequency, adjusted to produce maximum cavitation in a liquid bath.

Ultrasonic mixing is used for homogenizing and emulsifying substances. When two nonmiscible liquids, such as oil and water, are simultaneously irradiated by a high-intensity ultrasonic signal, an emulsion (a fine dispersion of oil in water, or vice versa) will be formed.

Many cosmetics, polishes, paints, and some food products are emulsions. Ultrasonic homogenization is used for condensed milk, baby food, soup, margarine, catsup, mayonnaise, and peanut butter.

Aluminum cannot be soldered by conventional means because it reacts very readily with oxygen, forming a thin film of aluminum oxide on the surface. By propagating ultrasonic waves through a tinning bath, the oxide is torn off the surface by cavitation, thus allowing soldering. In order to weld aluminum, the two metal surfaces are placed in contact and made to vibrate against each other at ultrasonic frequencies. Although no surface preparation is necessary and no heat is required, the two metal surfaces bond together firmly. The ultrasonic vibration removes the dirt and oxide films and produces a localized high temperature at the junction, causing a plastic flow, without melting, of the aluminum. A weld produced by this process is neater, stronger, and requires less power than conventional heat welding.

Another high-intensity ultrasonic application includes ultrasonic drilling, which is used to drill unusually shaped holes in an assortment of materials, many of which are difficult to drill by ordinary means. An example is the drilling of square holes in glass. An ultrasonic transducer is used to vibrate a bit having the desired shape and size. An abrasive slurry, containing small particles of a powdered material, transmits the vibrations to the work piece, which is literally chipped away by the intensely oscillating powder.

Chemical applications of high-intensity ultrasonics include electroplating, atomization of liquids into fogs, and as a catalyst in chemical reactions. When the electrode is irradiated by ultrasound during electroplating processes, the high-frequency waves clean the surface and remove the bubbles, which interfere with the plating process by accumulating on the electrode.

Low-intensity ultrasonic waves in the low- and medium-frequency region are used for the nondestructive testing of materials. A transducer emits a pulse of ultrasonic energy into the solid object to be tested. The boundaries of the material will reflect the pulse, but flaws will be detected as an earlier reflection. Ultrasonic testing is both vital and effective for inspections requiring the highest possible quality control, such as spacecraft components and nuclear pressure vessels. If ultrasonic surveillance were used to reduce further the risk of failure, and ultrasonic testing were used during routine maintenance procedures, nuclear reactors could be made nearly fail-safe. Inspection time would also be shortened, thus reducing reactor downtime, and components difficult to access by other means could be checked for integrity.

It is known that objects subjected to stress will emit ultrasonic waves, which increase in intensity as the body begins to crack. The condition of the stressed object may be evaluated by listening to and interpreting these signals in terms of the deformation processes at work. A related application is to use ultrasound to monitor natural gas stored under pressure in holding tanks of porous rock. The maximum pressure that the impervious rock cap can withstand is known from ultrasonic emission testing. The unique advantage of ultrasonic emission testing is that large structures can be monitored in their entirety in one operation, using a small number of stationary transducers.

Ultrasonics has proved to be an extremely useful tool for medical diagnostics, where it can be employed in two different manners, known as echolocation and imaging. Echolocation is the process by which ultrasonic waves can be passed through body tissues and reflected from internal organs that are moving, such as the heart. When sound bounces off a moving target, the reflected wave frequency is shifted as a result of the Doppler effect. The shifted frequency will interact with the original wave to produce beats (slow variations in amplitude equal to the difference of the two frequencies). By listening to these beats, a trained diagnostician can hear the moving structure. This process is often used during pregnancy because the fluid in the uterus provides an unobstructed path for sound, and as sound waves are not electromagnetic, ultrasound will not harm a fetus as would X rays. The placenta can be located by listening for the sound of blood flowing through it, and the fetus is located by listening to its heart movements.

For the second method, an image of the interior of the body, a sonogram, can be obtained by ultrasonic means. In ultrasonic tomography (from two Greek words meaning "writing a cut"), a complete cross-sectional picture through part of the body is taken by recording a scanning set of ultrasonic waves that are used to modulate an oscilloscope screen. The resulting sonogram--unlike an X ray, which shows only bone and calcified substances--reveals all the tissues in the cross-section. Naturally, this procedure is widely used in obstetrics because the pregnant uterus is an ideal transmitting medium, and the danger of X rays is avoided.

A final interesting application of ultrasonics is in conjunction with holography.

Coherent ultrasound used with a laser can produce acoustical holograms in a manner similar to that used to produce optical holograms. An object is irradiated by one ultrasonic beam, which combines with a reference beam to form an interference pattern. One method used for recording an acoustical hologram creates an interference pattern in the form of standing waves on the surface of a tank of water. The resultant ripples can be illuminated by laser light and observed by telescope. By adjusting the focus, one can look at different depths inside the object. As in conventional holography, the object can be recorded on a photographic plate for later reconstruction. Illuminating the plate with laser light re-creates an image of the original object.

Context

The emergence of ultrasound as a subbranch of acoustics can be traced to the invention of the ultrasonic whistle by Francis Galton in 1883. Galton used the whistle to investigate the upper-frequency threshold of human and animal hearing. Another possible application of ultrasonic waves was suggested to Lewis Fry Richardson by the sinking of the Titanic in 1912. He proposed that underwater ultrasonic waves could detect submerged obstacles, such as icebergs. This idea was developed by French scientist Paul Langevin during World War I as a means of detecting enemy submarines. By World War II, this primitive system had fully evolved into standard naval equipment for submarine detection.

The advances in electronic technology necessitated by the war led to further refinements in ultrasonic applications, such as sonar systems for underwater sound.

Subsequently, the same ultrasonic echolocation techniques were adapted to operate in solids, liquids, and gases, which led to a wide variety of applications.

Considerable potential exists for further developments in both low- and high-energy ultrasound. New materials are being developed for high-intensity generators, and new technology is evolving for better and more sensitive receivers. Although the existence of ultrasonic waves has been known for many years, ultrasonic technology began to develop in the post-World War II period. That this development has been rapid and diverse only exemplifies that scientists are on the threshold of the silent world of ultrasound.

Principal terms

FREQUENCY: the number of oscillations per second of a vibrating object, defined so that one vibration per second is equal to one hertz; in the ultrasound region, frequencies are measured in kilohertz (thousand hertz), megahertz (million hertz), and gigahertz (billion hertz)

TRANSDUCER: a device that changes energy from one form to another; acoustic transducers are of two types: acoustic generators that produce acoustic waves and acoustic receivers that detect them

ULTRASOUND: literally, beyond sound; vibrations of a material substance similar to audible sound but having frequencies too high to be detected by human ears

WAVELENGTH: the distance between successive crests, or other identical parts, of a wave train; wavelength is inversely related to frequency, thus, the higher the frequency, the shorter the wavelength, and vice versa

Bibliography

Asimov, Isaac. UNDERSTANDING PHYSICS. Vol 1. New York: New American Library, 1966. This excellent volume includes a short discussion of ultrasound found in the detailed presentation on sound waves and acoustics.

Cracknell, A. P. ULTRASONICS. London: Wykeham, 1980. In part 1, this important book summarizes the physics of the generation, propagation, attenuation, and detection of ultrasound. Part 2 treats the applications of ultrasonics in science, industry, and medicine, as well as the uses made of ultrasonics by mammals, insects, and birds.

Hewitt, Paul G. CONCEPTUAL PHYSICS. 6th ed. Scranton, Pa.: HarperCollins, 1989. This amusing and easy-to-read book includes discussions of many aspects of sound as well as a treatment of holography.

Kock, Winston E. SOUND WAVES AND LIGHT WAVES. Garden City, N.Y.: Doubleday, 1965. An original approach that synthesizes sound and light by emphasizing their similarities and applications.

Strong, W. J., and George R. Plitnik. MUSIC, SPEECH, AND HIGH FIDELITY. 2d ed. Provo, Utah: Soundprint Press, 1983. A nontechnical treatment of all aspects of sound, including discussions of ultrasonics and infrasonics.

Producing and Detecting Sound