Ultrasonics is the term used to describe the study
of all soundlike waves whose frequency is above the range of normal
human hearing. Audible sound frequencies extend from about 30
to 20,000 hertz (1 Hz = 1 cycle per second). The actual waves
and the vibrations producing them are called ultrasound. As late
as 1900 ultrasound was still a novelty and studied only with a
few specially made whistles; by 1930 it had become an interesting
but small area of physics research. In the 1960s and '70s, however,
it became an important research tool in physics, a far-ranging
instrument for flaw detection in engineering, a rival to the X
ray in medicine, and a reliable method of underwater sound -signaling.
The range of frequencies available has been extended to millions
and even billions of hertz (megahertz and gigahertz).
The principal modern sources of ultrasound are specially cut crystals of materials such as quartz or ceramics such as barium titanate and lead zirconate. The application of an alternating electrical voltage across the opposite faces of a plate made of such a material produces an alternating expansion and contraction of the plate at the impressed frequency. This phenomenon in crystals, known as piezoelectricity, was first discovered in the 1880s by Paul-Jacques and Pierre Curie. If the frequency of alternation f is such that f = c/2l, where c is the speed of sound in the material and l is the thickness of the plate, the size of the alternating expansions and contractions becomes very large, and the plate is said to exhibit resonance.
Similar effects are observed in ceramics. Ceramic
objects have the added advantage of being able to be cast in the
form of plates, rings, cylinders, and other special shapes that
are convenient for engineering applications. In addition, some
materials, such as cadmium sulfide, can be deposited in thin films
on a solid medium. Such material can then serve as a transducer.
Still other ultrasonic transducers are produced in ferromagnetic
materials by varying the magnetic-field intensity in the material.
Ultrasonic waves travel through matter with virtually the same speed as sound waves-hundreds of meters per second in air, thousands of meters per second in solids, and 1,500 m/sec (5,000 ft/sec) in water. Most of the properties of sound waves (reflection, refraction, and so forth) are also characteristic of ultrasound. The attenuation of sound waves increases with the frequency, however, so that ultrasonic waves are damped far more rapidly than those of ordinary sound. For example, an ultrasonic wave of 1 MHz frequency passing through water will lose half of its intensity over a distance of 20 m (66 ft) through absorption of the energy by the water; in air, the distance over which the intensity falls by half would be a few centimeters. At the audio frequency of 20,000 Hz, the corresponding distances for water and for air would be about 50 km (30 mi) and 5 m (16.5 ft).
In addition to waves that travel through the bulk
of a material, it is also possible to send waves along the surface
of a solid. These waves, called Rayleigh waves, can be produced
and detected by minute metallic "fingers" deposited
on the surface of a piezoelectric substrate. Techniques utilizing
surface waves have been widely exploited in signal processing.
Ultrasound is widely used in the detection of obstacles in materials that do not transmit light. For example, a low-frequency ultrasonic beam can penetrate many kilometers of the ocean and be reflected back from an obstacle there. This is the principle of sonar, which is used to identify submarines, map the ocean bottom, and measure the thickness of ice packs. Industrially, this same principle is used to detect flaws in solids. When a short pulse of ultrasound is sent into a metal, for example, it is reflected back from any cracks or minute defects such as blowholes. The concept is further extended by the combined use of light and sound in what is called acoustic holography. In this method, an ultrasound beam traveling through water agitates the water's surface. A light beam reflected from this surface is modulated by the pattern of agitation, and the modulated beam can then be used to reconstruct the original sound beam. If the sound beam is first directed through an object, it is thus possible to reconstruct images of the object's interior.
The intense small-scale vibrations of ultrasound
are also used industrially to shake dirt or other deposits off
metals. The ultrasonic transducer, by removing oxides from metal
surfaces, aids in the processes of soldering and welding. Plastic
powders can be molded into small cylinders by similar techniques.
Ultrasound is further used in the atomization of liquids and
even metals, and in the precipitation of smoke particles before
they enter the environment.
Ultrasonic transducers also serve in medicine in numerous ways. The vibrations of a transducer can be conveyed to a cutting edge in ultrasonic drills and saws-for use in surgery and dentistry, for example. Pulses of ultrasound can be sent into the body without the need for surgery, as well, to shatter kidney stones and gallstones.
More generally, ultrasonic transducers have come to be widely used in medical imaging for the diagnosis of disease states and the evaluation of internal organs and structures. Because different portions of the body reflect and scatter sound waves at different rates, the returning echo can be formed into a picture of these structures. A handheld transducer is moved across the part of the body to be scanned, coupled to the skin with a gel or liquid to prevent air from interfering with the transmission of sound waves. The painless procedure takes anywhere from a few minutes to an hour.
Ultrasound provides information about tumors and cysts that cannot be obtained by conventional X-ray studies. Echocardiography, the study of heart motions by ultrasonic means, is another medical application. Ultrasonics plays a significant role in observing pregnancy problems, as well, and is often the first screening examination for many medical conditions that affect children.
The dissipation of sound energy in a medium results in local heating and motion of any fluid present. These phenomena are useful in ultrasonic therapy or massage. Highly concentrated beams of ultrasound can also be used to destroy cells and have had some medical applications, as in treatment of secondary glaucoma.