ULTRASONICS

CONCEPT

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.

Sound Waves

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 (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 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 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.

REAL-LIFE APPLICATIONS

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 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 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 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 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.

Medical Ultrasound WWW Directory (Web site). <http://www.ultrasoundinsider.com> (February 16, 2001).

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.

KEY TERMS

FREQUENCY:

The number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength.

HERTZ:

A unit for measuring frequency, equal to one cycle per second. If a sound wave has a frequency of 20,000 Hz, this means that 20,000 waves are passing through a given point during the interval of one second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Hence 20,000 Hz—the threshold of ultrasonic sound—would be rendered as 20 kHz.

INFRASOUND:

Sound of a frequency between 20 Hz, which places it outside the range of audibility for human beings. Its opposite is ultrasound.

LONGITUDINAL WAVE:

A wave in which the individual segments vibrate in the same direction as the wave itself. This is in contrast to a transverse wave, or one in which the vibration or harmonic motion occurs perpendicular to the direction in which the wave is moving. Waves on the ocean are an example of transverse waves;by contrast, the shock waves of an explosion, the concentric waves of a radio transmission, and sound waves are all examples of longitudinal waves.

TRANSDUCER:

A device that converts energy into ultrasonic sound waves.

ULTRASOUND:

Sound waves with a frequency above 20,000 Hz, which makes them in audible to the human ear. Its opposite is infrasound.

WAVELENGTH:

The distance, measured on a plane parallel to that of the wave itself, between a crest and the adjacentcrest, or the trough and an adjacent trough. On a longitudinal wave, this is simply the distance between waves, which constitute a series of concentric circles radiating from the source.

WAVE MOTION:

Activity that carries energy from one place to another without actually moving any matter.

Dental Drill

Background

The dental drill is a tool used by dentists to bore through tooth enamel as well as to clean and remove plaque from the tooth's surface. It is composed primarily of a handpiece, an air turbine, and a tungsten carbide drill bit. Since its development began in the mid 1700s, the dental drill has revolutionized the field of dentistry. The modern dental drill has enabled dentists to work more quickly and accurately than ever before, with less pain for the patient.

The teeth are composed of both living and nonliving tissue. The soft tissue inner layer, called the dentin, is similar in composition to skeletal bones. Enamel, the outer layer of teeth, which is highly calcified and harder than bone, cannot be regenerated by the body. Tooth decay, which damages to the enamel, is caused by various oral bacteria. One type of bacteria that resides in the mouth breaks down residual food particles that remain on teeth after eating. A byproduct of this bacteria's metabolism is plaque. Other bacteria attach themselves to this plaque and begin secreting an acid which causes small holes to form in the tooth enamel. This allows still other types of bacteria to enter these holes and crevices and erode the softer tissue below. The process weakens the tooth by creating a cavity. The breakdown of the soft tissue is responsible for the pain that is typically associated with cavities. Beyond the initial holes, the outer enamel is left primarily intact. Untreated, cavities can result in diseases such as dental caries and abscesses.

To prevent these diseases, dentists use a dental drill or other tools to remove the plaque from a cavity. As the tooth is drilled, the tiny diamond chips that cover its tip wear away the plaque and damaged enamel. Only by drilling into a tooth can dentists' ensure that all of the plaque is removed. With the plaque gone from the teeth, the enamel-damaging bacteria have nowhere to reside and cannot cause cavities. After the drilling is complete, the hole that is left is filled with a suitable material which strengthens the tooth and helps prevent further damage.

History

The earliest examples of dental drills were developed by the Mayans over 1,000 years ago. They used a stone tool made of jade, which was shaped as a long tube and sharpened on the end. By twirling it between the palms, a hole could be drilled into the teeth. They used this tool primarily in conjunction with a religious ritual for putting jewels in the teeth. Though this technology was ahead of its time, it was not known throughout the rest of the world. The early Greek, Roman, and Jewish civilizations also developed versions of a dental drill. While these early examples of tooth drilling are found, during the Middle Ages the technology was lost. In the mid 1600s, doctors discovered that temporary relief from dental diseases could be achieved by filling the natural holes in teeth with various substances. These early dentists even used a chisel to chip away bits of the damaged enamel. However, it was not until Pierre Fauchard came on the scene that dental drill technology was rediscovered.

Fauchard is said by some to be the father of modern dentistry. He first mentions the use of a bow drill on teeth for root canals in a book published in 1746. This device consisted of a long metal rod with a handle and a bow that was used to power it. During this time, many innovations were developed. One of these was the 1778 introduction of a near-mechanical drill, which was powered by a hand crank that activated a rotating gear. Soon afterward, an inventor added a spinning wheel to power the drill head. The motion in this device was created by the dentist pushing a foot pedal to move a spinning wheel, which in turn moved the drill head. Other attempts at mechanical drills were made during the 1800s, but they were hard to handle and inefficient, so most dentists used simple, hand-operated steel drills.

Drill technology steadily improved over time, resulting in faster and more efficient drills. New types of foot-powered engines were attached to dental drills by 1870. Electrically powered drills soon followed, and the time it took to prepare a cavity was decreased from hours to less than 10 minutes. High speed drills were developed in 1911, but it was not until 1953 that the modern dental drill with its air turbine engine was introduced. These drills were over 100 times faster than their predecessors and significantly reduced the pain associated with tooth drilling. To accommodate these faster speeds, tungsten carbide drill bits were introduced. Since then, manufacturers have made many modifications, such as adding fiber optic lights and cameras, incorporating sophisticated cooling systems, and making highly durable handpieces.

Design

There are various designs of dental drills available, however, each have the same basic features, including motors, a handpiece, couplings, and a drill bit. The high speed drilling is activated by an air turbine. These devices convert highly pressurized air into mechanical energy, enabling drill bits to rotate over 300,000 rpms. Slower speeds are also necessary for things such as polishing, finishing, and soft tissue drilling, so dental drills are typically equipped with secondary motors. Common types include electric motors and air-driven motors.

The handpiece is typically a slender, tube-shaped device which connects the drill bit with the driving motor. It is often light-weight and ergonomically designed. It also has an E-shaped attachment that ensures that the drill bit is properly angled for maximum system stability. These components of the dental drill were once quite delicate. However, recent health concerns have forced designers to develop handpieces that can withstand high-pressure steam sterilization. The couplings are used to connect the drill unit to the electric or air power sources and cooling water. They can either consist of two or four holes, depending on the type of fitting.

The drill bit, or bur, is the most important part of the dental drill. It is short and highly durable, able to withstand high speed rotation and the heat that is subsequently generated. Many bur shapes are manufactured, each with varying cutting and drilling abilities. Some burs are even designed with diamond cutting flutes. Additional features may be added, such as coolant spray systems or illumination devices. The most sophisticated dental drill has an internal cooling system, an epicyclic speed-increasing gearbox, and fiberoptic illumination.

Raw Materials

Dental drills are constructed from a variety of raw materials, including metals and polymers. The handpiece, which houses the motors, gears, and drive shaft, can be made from either lightweight, hard plastics or metal alloys such as brass. The most advanced handpieces are made with titanium. The bur is made of tungsten carbide, one of the hardest substances known. Other materials such as steel are used for the internal motors. The tubing that connects the drill to the main power sources is made of a flexible material, such as polymeric silicone or polyvinyl chloride (PVC).

The Manufacturing
Process

The production of a dental drill is an integrated process in which individual components are first made and then assembled to make the final product. While manufacturers could make each part individually, they usually depend on outside suppliers for many of the parts. A typical production method would include constructing the motors and the drill bits, forming the handpiece, final assembly, and packaging.

Handpiece

• 1 Although numerous designs and materials are used to make the handpiece, they are all typically made using a pre-formed mold. For plastic handpieces, this involves injection molding, a process in which the plastic is melted, injected into a mold, and released after it forms. Metal handpieces also use a similar molding process.

Drill bit

• 2 The drill bits are formed from tungsten carbide particles. They are made by first taking tungsten ore and chemically processing it to produce tungsten oxides. Hydrogen is then added to the system to remove the oxygen, resulting in a fine tungsten metal powder. This powder is then blended with carbon and heated, producing tungsten carbide particles of varying sizes. These particles are further processed to form the appropriately shaped drill bit.

Air turbine engine

• 3 The air turbine engine is constructed from small steel components. In one design, the turbine is sandwiched between two sets of ball raced bearings and connected directly to the drill bit. The entire unit is encased in the drill head, with openings for incoming air and exhaust air. Other types of turbine engines are farther up in the handpiece and are connected to the drill bit by a series of driveshafts and gears.

Low-powered motors

• 4 The low-powered motors are put together much like the air turbine engines. The rotary vane air-powered motor consists of a core structure with sliding vanes protruding outward. It is placed in the handpiece and connected to the main driveshaft of the drill. It also has an opening for incoming and outgoing air. Electric motors are significantly more complex, consisting of a set of bearings, magnets, brushes, and armature coils.

Final assembly

• 5 After all the components are available, final assembly can begin. Depending on the design, the air turbine can be placed directly into casing of the handpiece or it can be attached along with the drill bit. The other parts of the drill are put into the handpiece, including air or electric motors, driveshaft, gears, and control switches. Other accessories are added, such as the cooling hoses and fiber optic lighting devices. The coupler is placed on one end of the handpiece, and the drill bit is attached to the other.
• 6 After an array of quality checks, the finished drills are placed in the appropriate packaging, along with accessories, manuals, and replacement parts, and are then shipped to distributors.

Quality Control

The quality of each drill component is checked during each stage of manufacturing. Since many parts are made each day, inspecting all of them is impossible. Therefore, line inspectors typically take random samples at certain time intervals and check to ensure that those samples meet set specifications for size, shape, and consistency. During this phase of quality control, the primary testing method is visual inspection, although more rigorous measurements can also be made.

The Future

During much of the developmental history of the dental drill, the focus of research had been on increasing the speed of the drill bits and correcting the problems related to these greater speeds. However, studies have shown that there is no benefit to increasing the drill bit speed any higher than it is today. Therefore, the focus of research has shifted to developing altematives to conventional drills altogether. Two recent introductions are noteworthy and may be indicative of the direction dentistry is headed.

A new method of treating cavities is known as "air-abrasive" technology. Using this technique, a dentist blasts away parts of the tooth surface without using a drill. Small particles of alumina are forced by a stream of air, and the plaque is literally knocked off the tooth. Another technology that may replace the dental drill is the laser. The FDA has recently approved the use of a laser drill for use on the soft tissue of teeth. However, approval for use on hard tissue is pending. This technology may allow for quicker and more accurate drilling. The result of both of these new technologies is optimal patient comfort as the pain and noise associated with conventional drilling are eliminated.

Where to Learn More

Books

Jedynakiewicz, Nicolas. A Practical Guide to Technology in Dentistry. Wolfe Publishing, 1992.

Glenner, Richard, Audrey Davis, and Stanley Burns. The American Dentist. Pictorial Histories Publishing Co., 1990.

Simonsen, Richard. Dentistry in the 21st Century A Global Perspective. Quintessence Publishing Co., 1989.

Periodicals

Ring, Malvin. "Behind the Dentist's Drill." Invention & Technology, Fall 1995, pp. 25-31.

—RandySchuellerand

PerryRomanowski

Dental drill

When a tooth develops a cavity, the decayed tissue must be removed. The earliest devices for doing this were picks and enamel scissors. Later, two-edged cutting instruments were designed that were twirled in both directions between the fingers. The father of modern dentistry, Frenchman Pierre Fauchard (1678-1761), described an improved drill in 1728. Its rotary (turning around a central point) movement was powered by catgut twisted around a cylinder, or by jewelers' bowstrings. A hand-cranked dental drill bit was patented by John Lewis in 1838. George Washington's dentist, John Greenwood (1760-1819), invented the first known "dental foot engine" in 1790. Greenwood adapted his mother's foot-treadle (pedal) spinning wheel to rotate a drill. Greenwood's dentist son continued to use the drill, but the idea went no further.

Early Drill Designs

Scottish inventor James Nasmyth used a coiled wire spring to drive a drill in 1829. Charles Merry of St. Louis, Missouri, adapted Nasmyth's drill by adding a flexible cable to it in 1858. The first motor-driven drill appeared in 1864, the design of Englishman George F. Harrington. This "motor drill" was a hand-held device powered by the spring action of a clock movement. In 1868 American George F. Green introduced a pneumatic (air-driven) drill powered by a pedal bellows. Fellow American James B. Morrison patented a pedal bur drill in 1871. A further improvement of the Nasmyth-Merry design, the bur drill featured a flexible arm with a handpiece to hold the drill, a foot treadle, and pulleys. Each of these developments increased the speed at which the drill operated.

In 1874, six years after he made his original contribution to drills, Green added electricity to the dental drill. Green's drill was powered by electromagnetic motors and worked well, but it was also heavy and expensive. Plug-in electric drills became available in 1908. By that time, most dental offices had electricity.

The Modern Dental Drill

Once efficient, mechanically-driven drills became widely available, teeth could be properly and accurately prepared for well-fitting crowns (an artificial substitute for the part of the tooth projecting beyond the gum line) and fillings. American teeth blossomed with the gold of inlays (a filling for a tooth made from a mold) and crowns. Modern dental drills are turbine-powered and rotate at speeds of 300,000 to 400,000 revolutions per minute. Drills generate a large amount of heat but are less irritating to the patient because the cutting action is smoother.

[See also Dental fillings, crown, and bridge ]