The word ultrasonic combines the Latin roots ultra, meaning "beyond," and sonic, or sound. The field of ultrasonics thus involves the use of sound waves outside the audible range for humans. These sounds have applications for imaging, detection, and navigation—from helping prospective parents get a glimpse of their unborn child to guiding submarines through the oceans. Ultrasonics can be used to join materials, as for instance in welding or the homogenization of milk, or to separate them, as for example in extremely delicate cleaning operations. Among the broad sectors of society that regularly apply ultrasonic technology are the medical community, industry, the military, and private citizens.
HOW IT WORKS
In the realm of physics, ultrasonics falls under the category of studies in sound. Sound itself fits within the larger heading of wave motion, which is in turn closely related to vibration, or harmonic (back-and-forth) motion. Both wave motion and vibration involve the regular repetition of a certain form of movement; and in both, potential energy (think of the energy in a sled at the top of a hill) is continually converted to kinetic energy (like the energy of a sled as it is sliding down the hill) and back again.
Wave motion carries energy from one place to another without actually moving any matter. Waves themselves may consist of matter, as for instance in the case of a wave on a plucked string or the waves on the ocean. This type of wave is called a mechanical wave, but again, the matter itself does not undergo any net displacement over horizontal space: contrary to what our eyes tell us, molecules of water in an ocean wave move up and down, but they do not actually travel with the wave itself. Only the energy is moved.
Then there are waves of pulses, such as light, sound, radio, or electromagnetic waves. Sound travels by means of periodic waves, a period being the amount of time it takes a complete wave, from trough to crest and back again, to pass through a given point. These periodic waves are typified by a sinusoidal pattern. To picture a sinusoidal wave, one need only imagine an x-axis crossed at regular intervals by a curve that rises above the line to point y before moving downward, below the axis, to point y. This may be expressed also as a graph of sin x versus x. In any case, the wave varies by equal distances upward and downward as it moves along the x-axis in a regular, unvarying pattern.
Periodic waves have three notable interrelated characteristics. One of these is speed, typically calculated in seconds. Another is wavelength, or the distance between a crest and the adjacent crest, or a trough and the adjacent trough, along a plane parallel to that of the wave itself. Finally, there is frequency, the number of waves passing through a given point during the interval of one second.
Frequency is measured in terms of cycles per second, or Hertz (Hz), named in honor of the nineteenth-century German physicist Heinrich Hertz. If a wave has a frequency of 100 Hz, this means that 100 waves are passing through a given point during the interval of one second. Higher frequencies are expressed in terms of kilohertz
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(kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)
Clearly, frequency is a function of the wave's speed or velocity, and the same relationship—though it is not so obvious intuitively—exists between wavelength and speed. Over the interval of one second, a given number of waves pass a certain point (frequency), and each wave occupies a certain distance (wavelength). Multiplied by one another, these two properties equal the velocity of the wave.
An additional characteristic of waves (though one that is not related mathematically to the three named above) is amplitude, or maximum displacement, which can be described as the distance from the x-axis to either the crest or the trough. Amplitude is related to the intensity or the amount of energy in the wave.
These four qualities are easiest to imagine on a transverse wave, described earlier with reference to the x-axis—a wave, in other words, in which vibration or harmonic motion occurs perpendicular to the direction in which the wave is moving.
Such a wave is much easier to picture, for the purposes of illustrating concepts such as frequency, than a longitudinal wave; but in fact, sound waves are longitudinal. A longitudinal wave is one in which the individual segments vibrate in the same direction as the wave itself. The shock waves of an explosion, or the concentric waves of a radio transmission as it goes out from the station to all points within receiving distance, are examples of longitudinal waves. In this type of wave pattern, the crests and troughs are not side by side in a line; they radiate outward. Wavelength is the distance between each concentric circle or semicircle (that is, wave), and amplitude the "width" of each wave, which one may imagine by likening it to the relative width of colors on a rainbow.
Having identified its shape, it is reasonable to ask what, exactly, a sound wave is. Simply put, sound waves are changes in pressure, or an alternation between condensation and rarefaction. Imagine a set of longitudinal waves—represented as concentric circles—radiating from a sound source. The waves themselves are relatively higher in pressure, or denser, than the "spaces" between them, though this is just an illustration for the purposes of clarity: in fact the "spaces" are waves of lower pressure that alternate with higher-pressure waves.
Vibration is integral to the generation of sound. When the diaphragm of a loudspeaker
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pushes outward, it forces nearby air molecules closer together, creating a high-pressure region all around the loudspeaker. The diaphragm is pushed backward in response, thus freeing up a volume of space for the air molecules. These then rush toward the diaphragm, creating a low-pressure region behind the high-pressure one. As a result, the loudspeaker sends out alternating waves of high pressure (condensation) and low pressure (rarefaction). Furthermore, as sound waves pass through a medium—air, for the purposes of this discussion—they create fluctuations between density and rarefaction. These result in pressure changes that cause the listener's eardrum to vibrate with the same frequency as the sound wave, a vibration that the ear's inner mechanisms translate and pass on to the brain.
THE SPEED OF SOUND: CONSIDER THE MEDIUM
The speed of sound varies with the hardness of the medium through which it passes: contrary to what you might imagine, it travels faster through liquids than through gasses such as air, and faster through solids than through liquids. By definition, molecules are closer together in harder material, and thus more quickly responsive to signals from neighboring particles. In granite, for instance, sound travels at 19,680 ft per second (6,000 mps), whereas in air, the speed of sound is only 1,086 ft per second (331 mps). It follows that sound travels faster in water—5,023 ft per second (1,531 mps), to be exact—than in air. It should be clear, then, that there is a correlation between density and the ease with which a sound travels. Thus, sound cannot travel in a vacuum, giving credence to the famous tagline from the 1979 science fiction thriller Alien: "In space, no one can hear you scream."
When sound travels through a medium such as air, however, two factors govern its audibility: intensity or volume (related to amplitude and measured in decibels, or dB) and frequency. There is no direct correlation between intensity and frequency, though for a person to hear a very low-frequency sound, it must be above a certain decibel level. (At all frequencies, however, the threshold of discomfort is around 120 decibels.)
In any case, when discussing ultrasonics, frequency and not intensity is of principal concern. The range of audibility for the human ear is from 20 Hz to 20,000 Hz, with frequencies below that range dubbed infrasound and those above it referred to as ultrasound. (There is a third category, hypersound, which refers to frequencies above 1013, or 10 trillion Hz. It is almost impossible
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for hypersound waves to travel through most media, because its wavelengths are so short.)
WHAT MAKES THE GLASS SHATTER
The lowest note of the eighty-eight keys on a piano is 27 Hz and the highest 4,186 Hz. This places the middle and upper register of the piano well within the optimal range for audibility, which is between 3,000 and 4,000 Hz. Clearly, the higher the note, the higher the frequency—but it is not high frequency, per se, that causes a glass to shatter when a singer or a violinist hits a certain note. All objects, or at least all rigid ones, possess their own natural frequency of vibration or oscillation. This frequency depends on a number of factors, including material composition and shape, and its characteristics are much more complex than those of sound frequency described above. In any case, a musician cannot cause a glass to shatter simply by hitting a very high note; rather, the note must be on the exact frequency at which the glass itself oscillates. Under such conditions, all the energy from the voice or musical instrument is transferred to the glass, a sudden burst that overloads the object and causes it to shatter.
To create ultrasonic waves, technicians use a transducer, a device that converts energy into ultrasonic sound waves. The most basic type of transducer is mechanical, involving oscillators or vibrating blades powered either by gas or the pressure of gas or liquids—that is, pneumatic or hydrodynamic pressure, respectively. The vibrations from these mechanical devices are on a relatively low ultrasonic frequency, and most commonly they are applied in industry for purposes such as drying or cleaning.
An electromechanical transducer, which has a much wider range of applications, converts electrical energy, in the form of current, to mechanical energy—that is, sound waves. This it does either by a magnetostrictive or a piezoelectric device. The term "magnetostrictive" comes from magneto, or magnetic, and strictio, or "drawing together." This type of transducer involves the magnetization of iron or nickel, which causes a change in dimension by forcing the atoms together. This change in dimension in turn produces a high-frequency vibration. Again, the frequency is relatively low in ultrasonic terms, and likewise the application is primarily industrial, for purposes such as cleaning and machining.
Most widely used is a transducer equipped with a specially cut piezoelectric quartz crystal. Piezoelectricity involves the application of mechanical pressure to a nonconducting crystal, which results in polarization of electrical charges, with all positive charges at one end of the crystal and all negative charges at the other end. By successively compressing and stretching the crystal at an appropriate frequency, an alternating electrical current is generated that can be converted into mechanical energy—specifically, ultrasonic waves.
Scientists use different shapes and materials (including quartz and varieties of ceramic) in fashioning piezoelectric crystals: for instance, a concave shape is best for an ultrasonic wave that will be focused on a very tight point. Piezoelectric transducers have a variety of applications in ultrasonic technology, and are capable of acting as receivers for ultrasonic vibrations.
PETS AND PESTS: ULTRASONIC BEHAVIOR MODIFICATION
Some of the simplest ultrasonic applications build on the fact that the upper range of audibility for human beings is relatively low among animals. Cats, by comparison, have an infrasound threshold only slightly higher than that of humans (100 Hz), but their ultrasound range of audibility is much greater—32,000 Hz instead of a mere 20,000. This explains why a cat sometimes seems to respond mysteriously to noises its owner is incapable of hearing.
For dogs, the difference is even more remarkable: their lower threshold is 40 Hz, and their high end 46,000 Hz, giving them a range more than twice that of humans. It has been said Paul McCartney, who was fond of his sheepdog Martha, arranged for the Beatles' sound engineer at Abbey Road Studios to add a short 20,000-Hz tone at the very end of Sgt. Peppers' Lonely Hearts Club Band in 1967. Thus—if the story is true—the Beatles' human fans would never hear the note, but it would be a special signal to Martha and all the dogs of England.
On a more practical level, a dog whistle is an extremely simple ultrasonic or near-ultrasonic device, one that obviously involves no transducers. The owner blows the whistle, which utters a tone nearly in audible to humans but—like McCartney's 20,000-Hz tone—well within a dog's range. In fact, the Acme Silent Dog Whistle, which the company has produced since 1935, emits a tone that humans can hear (the listed range is 5,800 to 12,400 Hz), but which dogs can hear much better.
There are numerous products on the market that use ultrasonic waves for animal behavior modification of one kind or another; however, most such items are intended to repel rather than attract the animal. Hence, there are ultrasonic devices to discourage animals from relieving themselves in the wrong places, as well as some which keep unwanted dogs and cats away.
Then there is one of the most well-known uses of ultrasound for pets, which, rather than keeping other animals out, is designed to keep one's own animals in the yard. Many people know this item as an "Invisible Fence," though in fact that term is a registered trademark of the Invisible Fence Company. The "Invisible Fence" and similar products literally create a barrier of sound, using both radio signals and ultrasonics. The pet is outfitted with a collar that contains a radio receiver, and a radio transmitter is placed in some centrally located place on the owner's property—a basement, perhaps, or a garage. The "fence" itself is "visible," though usually buried, and consists of an antenna wire at the perimeter of the property. The transmitter sends a signal to the wire, which in turn signals the pet's collar. A tiny computer in the collar emits an ultrasonic sound if the animal tries to stray beyond the boundaries.
Not all animals have a higher range of hearing than humans: elephants, for instance, cannot hear tones above 12,000 Hz. On the other hand, some are drastically more sensitive acoustically than dogs: bats, whales, and dolphins all have an upper range of 150,000 Hz, though both have a low-end threshold of 1,000. Mice, at 100,000 Hz, are also at the high end, while a number of other pests—rodents and insects—fall into the region between 40,000 and 100,000 Hz. This fact has given rise to another type of ultrasonic device, for repelling all kinds of unwanted household creatures by bombarding them with ear-splitting tones.
An example of this device is the Transonic 1X-L, which offers three frequency ranges: "loud Page 324 | Top of Article mode" (1,000-50,000 Hz); "medium mode" (10,000-50,000 Hz); and "quiet mode" (20,000-50,000 Hz). The lowest of these can be used for repelling pest birds and small animals, the medium range for insects, and the "quiet mode" for rodents.
ULTRASONIC DETECTION IN MEDICINE
Medicine represents one of the widest areas of application for ultrasound. Though the machinery used to provide parents-to-be with an image of their unborn child is the most well-known form of medical ultrasound, it is far from the only one. Developed in 1957 by British physician Ian Donald (1910-1987), also a pioneer in the use of ultrasonics to detect flaws in machinery, ultra-sound was first used to diagnosis a patient's heart condition. Within a year, British hospitals began using it with pregnant women.
High-frequency waves penetrate soft tissue with ease, but they bounce off of harder tissue such as organs and bones, and thus send back a message to the transducer. Because each type of tissue absorbs or deflects sound differently, according to its density, the ultrasound machine can interpret these signals, creating an image of what it "sees" inside the patient's body. The technician scans the area to be studied with a series of ultrasonic waves in succession, and this results in the creation of a moving picture. It is this that creates the sight so memorable in the lives of many a modern parent: their first glimpse of their child in its mother's womb.
Though ultrasound enables physicians and nurses to determine the child's sex, this is far from being the only reason it is used. It also gives them data concerning the fetus's size; position (for instance, if the head is in a place that suggests the baby will have to be delivered by means of cesarean section); and other abnormalities.
The beauty of ultrasound is that it can provide this information without the danger posed by x rays or incisions. Doctors and ultrasound technicians use ultrasonic technology to detect body parts as small as 0.004 in (0.1 mm), making it possible to conduct procedures safely, such as locating foreign objects in the eye or measuring the depth of a severe burn. Furthermore, ultrasonic microscopes can image cellular structures to within 0.2 microns (0.002 mm).
Ultrasonic heart examination can locate tumors, valve diseases, and accumulations of fluid. Using the Doppler effect—the fact that a sound's perceived frequency changes as its source moves past the observer—physicians observe shifts in the frequency of ultrasonic measurements to determine the direction of blood flow in the body. Not only can ultrasound be used to differentiate tumors from healthy tissue, it can sometimes be used to destroy those tumors. In some cases, ultrasound actually destroys cancer cells, making use of a principle called cavitation—a promising area of ultrasound research.
Perhaps the best example of cavitation occurs when you are boiling a pot of water: bubbles—temporary cavities in the water itself—rise up from the bottom to the surface, then collapse, making a popping sound as they do. Among the research areas combining cavitation and ultrasonics are studies of light emissions produced in the collapse of a cavity created by an ultrasonic wave. These emissions are so intense that for an infinitesimal moment, they produce heat of staggering proportions—hotter than the surface of the Sun, some scientists maintain. (Again, it should be stressed that this occurs during a period too small to measure with any but the most sophisticated instruments.)
As for the use of cavitation in attacking cancer cells, ultrasonic waves can be used to create microscopic bubbles which, when they collapse, produce intense shock waves that destroy the cells. Doctors are now using a similar technique against gallstones and kidney stones. Other medical uses of ultrasound technology include ultrasonic heat for treating muscle strain, or—in a process similar to some industrial applications—the use of 25,000-Hz signals to clean teeth.
SONAR AND OTHER DETECTION DEVICES
Airplane pilots typically use radar, but the crew of a ocean-going vessel relies on sonar (SOund Navigation and Ranging) to guide their vessel through the ocean depths. This technology takes advantage of the fact that sound waves travel well under water—much better, in fact, than light waves. Whereas a high-powered light would be of limited value underwater, particularly in the murky realms of the deep sea, sonar provides excellent data on the water's depth, as well as the location of shipwrecks, large obstacles—and, for Page 325 | Top of Article commercial or even recreational fishermen—the presence of fish.
At the bottom of the craft's hull is a transducer, which emits an ultrasonic pulse. These sound waves travel through the water to the bottom, where they bounce back. Upon receiving the echo, the transducer sends this information to an onboard computer, which converts data on the amount of time the signal took, providing a reading of distance that gives an accurate measurement of the vessel's clearance. For instance, it takes one second for sound waves from a depth of 2,500 ft (750 m) to return to the ship. The onboard computer converts this data into a rough picture of what lies below: the ocean floor, and schools of fish or other significant objects between it and the ship.
Even more useful is a scanning sonar, which adds dimension to the scope of the ship's ultrasonic detection: not only does the sonar beam move forward along with the vessel, but it moves from side-to-side, providing a picture of a wider area along the ship's path. Sonar in general, and particularly scanning sonar, is of particular importance to a submarine's crew. Despite the fact that the periscope is perhaps the most notable feature of these underwater craft, from the viewpoint of a casual observer, in fact, the purely visual data provided by the periscope is of limited value—and that value decreases as the sub descends. It is thanks to sonar (which produces the pinging sound one so often hears in movie scenes depicting the submarine control room), combined with nuclear technology, that makes it possible for today's U.S. Navy submarines to stay submerged for months.
Sonar is perhaps the most dramatic use of ultrasonic technology for detection; less wellknown—but equally intriguing, especially for its connection with clandestine activity—is the use of ultrasonics for electronic eavesdropping. Private detectives, suspicious spouses, and no doubt international spies from the CIA or Britain's MI5, use ultrasonic waves to listen to conversations in places where they cannot insert a microphone. For example, an operative might want to listen in on an encounter taking place on the seventh floor of a building with heavy security, meaning it would be impossible to plant a microphone either inside the room or on the window ledge.
Instead, the operative uses ultrasonic waves, which a transducer beams toward the window of the room being monitored. If people are speaking inside the room, this will produce vibrations on the window the transducer can detect, although the sounds would not be decipherable as conversation by a person with unaided perception. Speech vibrations from inside produce characteristic effects on the ultrasonic waves beamed back to the transducer and the operative's monitoring technology. The transducer then converts these reflected vibrations into electrical signals, which analysts can then reconstruct as intelligible sounds.
Much less dramatic, but highly significant, is the use of ultrasonic technology for detection in industry. Here the purpose is to test materials for faults, holes, cracks, or signs of corrosion. Again, the transducer beams an ultrasonic signal, and the way in which the material reflects this signal can alert the operator to issues such as metal fatigue or a faulty weld. Another method is to subject the material or materials to stress, then look for characteristic acoustic emissions from the stressed materials. (The latter is a developing field of acoustics known as acoustic emission.)
Though industrial detection applications can be used on materials such as porcelain (to test for microscopic cracks) or concrete (to evaluate how well it was poured), ultrasonics is particularly effective on metal, in which sound moves more quickly and freely than any other type of wave. Not only does ultrasonics provide an opportunity for thorough, informative, but nondestructive testing, it also allows technicians to penetrate areas where they otherwise could not go—or, in the case of ultrasonic inspection of the interior of a nuclear reactor while in operation—would not and should not go.
BINDING AND LOOSENING: A HOST OF INDUSTRIAL APPLICATIONS
Materials testing is but one among myriad uses for ultrasonics in industry, applications that can be described broadly as "binding and loosening"—either bringing materials together, or pulling them apart.
For instance, ultrasound is often used to bind, or coagulate, loose particles of dust, mist, or smoke. This makes it possible to clean a factory smokestack before it exhales pollutants into the atmosphere, or to clear clumps of fog and Page 326 | Top of Article mist off a runway. Another form of "binding" is the use of ultrasonic vibrations to heat and weld together materials. Ultrasonics provides an even, localized flow of molten material, and is effective both on plastics and metals.
Ultrasonic soldering implements the principle of cavitation, producing microscopic bubbles in molten solder, a process that removes metal oxides. Hence, this is a case of both "binding" (soldering) and "loosening"—removing impurities from the area to be soldered. The dairy industry, too, uses ultrasonics for both purposes: ultrasonic waves break up fat globules in milk, so that the fat can be mixed together with the milk in the well-known process of homogenization. Similarly, ultrasonic pasteurization facilitates the separation of the milk from harmful bacteria and other microorganisms.
The uses of ultrasonics to "loosen" include ultrasonic humidification, where in ultrasonic vibrations reduce water to a fine spray. Similarly, ultrasonic cleaning uses ultrasound to break down the attraction between two different types of materials. Though it is not yet practical for home use, the technology exists today to use ultrasonics for laundering clothes without using water: the ultrasonic vibrations break the bond between dirt particles and the fibers of a garment, shaking loose the dirt and subjecting the fabric to far less trauma than the agitation of a washing machine does.
As noted earlier, dentists use ultrasound for cleaning teeth, another example of loosening the bond between materials. In most of these forms of ultrasonic cleaning, a critical part of the process is the production of microscopic shock waves in the process of cavitation. The frequency of sound waves in these operations ranges from 15,000 Hz (15 kHz) to 2 million Hz (2 MHz). Ultrasonic cleaning has been used on metals, plastics, and ceramics, as well as for cleaning precision instruments used in the optical, surgical, and dental fields. Nor is it just for small objects: the electronics, automotive, and aircraft industries make heavy use of ultrasonic cleaning for a variety of machines.
Ultrasonic "loosening" makes it possible to drill though extremely hard or brittle materials, including tungsten carbide or precious stones. Just as a dental hygienist cleaning a person's teeth bombards the enamel with gentle abrasives, this form of high-intensity drilling works hand-in-hand with the use of abrasive materials such as silicon carbide or aluminum oxide.
A WORLD OF APPLICATIONS
Scientists often use ultrasound in research, for instance to break up high molecular weight polymers, thus creating new plastic materials. Indeed, ultrasound also makes it possible to determine the molecular weight of liquid polymers, and to conduct other forms of investigation on the physical properties of materials.
Ultrasonics can also speed up certain chemical reactions. Hence, it has gained application in agriculture, thanks to research which revealed that seeds subjected to ultrasound may germinate more rapidly and produce higher yields. In addition to its uses in the dairy industry, noted above, ultrasonics is of value to farmers in the related beef industry, who use it to measure cows' fat layers before taking them to market.
In contrast to the use of ultrasonics for electronic eavesdropping, as noted earlier, today ultrasonic technology is available to persons who think someone might be spying on them: now they can use ultrasonics to detect the presence of electronic eavesdropping, and thus circumvent it. Closer to home is another promising application of ultrasonics for remote sensing of sounds: ultrasonic stereo speakers.
These make use of research dating back to the 1960s, which showed that ultrasound waves of relatively low frequency can carry audible sound to pinpointed locations. In 1996, Woody Norris had perfected the technology necessary to reduce distortion, and soon he and his son Joe began selling the ultrasonic speakers through the elder Norris's company, American Technology Corporation of San Diego, California.
Eric Niiler in Business Week described a demonstration: "Joe Norris twists a few knobs on a receiver, takes aim with a 10-inch-square gold-covered flat speaker, and blasts an invisible beam…. Thirty feet away, the tinny but easily recognizable sound of Vivaldi's Four Seasons rushes over you. Step to the right or left, however, and it fades away. The exotic-looking speaker emits 'sound beams' that envelop the listener but are silent to those nearby. 'We use the air as our virtual speakers,' says Norris…." Niiler went to note several other applications suggested by Norris: "Airline passengers could listen to their own Page 327 | Top of Article music channel sans headphones without disturbing neighbors. Troops could confuse the enemy with 'virtual' artillery fire, or talk to each other without having their radio communications picked up by eavesdroppers."
WHERE TO LEARN MORE
Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.
Crocker, Malcolm J. Encyclopedia of Acoustics. New York: John Wiley & Sons, 1997.
Knight, David C. Silent Sound: The World of Ultrasonics. New York: Morrow, 1980.
Langone, John. National Geographic's How Things Work. Washington, D.C.: National Geographic Society, 1999.
Niiler, Eric; edited by Alex Salkever. "Now Here This—IfYou're in the Sweet Spot." Business Week, October 16, 2000.
Meire, Hylton B. and Pat Farrant. Basic Ultrasound. Chichester, N.Y.: John Wiley & Sons, 1995.
Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.