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Introduction vibrations of frequencies greater than the upper limit of the audible range for humans—that is, greater than about 20 The term sonic is applied to ultrasound waves of very high Hypersound, sometimes called praetersound or microsound, is sound waves of frequencies greater than 1013 At such high frequencies it is very difficult for a sound wave to propagate efficiently; indeed, above a frequency of about 25 × 1013 hertz, it is impossible for longitudinal waves to propagate at all, even in a liquid or a solid, because the molecules of the material in which the waves are traveling cannot pass the vibration along rapidly TableMany animals have the ability to hear sounds in the human ultrasonic frequency Some ranges of hearing for mammals and insects are compared with those of humans in the T A presumed sensitivity of roaches and rodents to frequencies in the 40 kilohertz region has led to the manufacture of “pest controllers” that emit loud sounds in that frequency range to drive the pests away, but they do not appear to work as Transducers An ultrasonic transducer is a device used to convert some other type of energy into an ultrasonic There are several basic types, classified by the energy source and by the medium into which the waves are being Mechanical devices include gas-driven, or pneumatic, transducers such as whistles as well as liquid-driven transducers such as hydrodynamic oscillators and vibrating These devices, limited to low ultrasonic frequencies, have a number of industrial applications, including drying, ultrasonic cleaning, and injection of fuel oil into Electromechanical transducers are far more versatile and include piezoelectric and magnetostrictive A magnetostrictive transducer makes use of a type of magnetic material in which an applied oscillating magnetic field squeezes the atoms of the material together, creating a periodic change in the length of the material and thus producing a high-frequency mechanical Magnetostrictive transducers are used primarily in the lower frequency ranges and are common in ultrasonic cleaners and ultrasonic machining By far the most popular and versatile type of ultrasonic transducer is the piezoelectric crystal, which converts an oscillating electric field applied to the crystal into a mechanical Piezoelectric crystals include quartz, Rochelle salt, and certain types of Piezoelectric transducers are readily employed over the entire frequency range and at all output Particular shapes can be chosen for particular For example, a disc shape provides a plane ultrasonic wave, while curving the radiating surface in a slightly concave or bowl shape creates an ultrasonic wave that will focus at a specific Piezoelectric and magnetostrictive transducers also are employed as ultrasonic receivers, picking up an ultrasonic vibration and converting it into an electrical Applications in research One of the important areas of scientific study in which ultrasonics has had an enormous impact is When water is boiled, bubbles form at the bottom of the container, rise in the water, and then collapse, leading to the sound of the boiling The boiling process and the resulting sounds have intrigued people since they were first observed, and they were the object of considerable research and calculation by the British physicists Osborne Reynolds and Lord Rayleigh, who applied the term cavitation to the process of formation of Because an ultrasonic wave can be used carefully to control cavitation, ultrasound has been a useful tool in the investigation of the The study of cavitation has also provided important information on intermolecular Research is being carried out on aspects of the cavitation process and its A contemporary subject of research involves emission of light as the cavity produced by a high-intensity ultrasonic wave This effect, called sonoluminescence, is believed to create instantaneous temperatures hotter than the surface of the S The speed of propagation of an ultrasonic wave is strongly dependent on the viscosity of the This property can be a useful tool in investigating the viscosity of Because the various parts of a living cell are distinguished by differing viscosities, acoustical microscopy can make use of this property of cells to “see” into living cells, as will be discussed below in Medical Ranging and navigating Sonar (sound navigation and ranging) has extensive marine By sending out pulses of sound or ultrasound and measuring the time required for the pulses to reflect off a distant object and return to the source, the location of that object can be ascertained and its motion This technique is used extensively to locate and track submarines at sea and to locate explosive mines below the surface of the Two boats at known locations can also use triangulation to locate and track a third boat or The distance over which these techniques can be used is limited by temperature gradients in the water, which bend the beam away from the surface and create shadow One of the advantages of ultrasonic waves over sound waves in underwater applications is that, because of their higher frequencies (or shorter wavelengths), the former will travel greater distances with less Ranging has also been used to map the bottom of the ocean, providing depth charts that are commonly used in navigation, particularly near coasts and in shallow Even small boats are now equipped with sonic ranging devices that determine and display the depth of the water so that the navigator can keep the boat from beaching on submerged sandbars or other shallow Modern fishing boats use ultrasonic ranging devices to locate schools of fish, substantially increasing their Even in the absence of visible light, bats can guide their flight and even locate flying insects (which they consume in flight) through the use of sonic Ultrasonic echolocation has also been used in traffic control applications and in counting and sorting items on an assembly Ultrasonic ranging provides the basis of the eye and vision systems for robots, and it has a number of important medical applications (see below) The Doppler effect If an ultrasonic wave is reflected off a moving obstacle, the frequency of the resulting wave will be changed, or Doppler- More specifically, if the obstacle is moving toward the source, the frequency of the reflected wave will be increased; and if the obstacle is moving away from the source, the frequency of the reflected wave will be The amount of the frequency shift can be used to determine the velocity of the moving Just as the Doppler shift for radar, an electromagnetic wave, can be used to determine the speed of a moving car, so can the speed of a moving submarine be determined by the Doppler shift of a sonar An important industrial application is the ultrasonic flow meter, in which reflecting ultrasound off a flowing liquid leads to a Doppler shift that is calibrated to provide the flow rate of the This technique also has been applied to blood flow in Many burglar alarms, both for home use and for use in commercial buildings, employ the ultrasonic Doppler shift Such alarms cannot be used where pets or moving curtains might activate Materials testing Nondestructive testing involves the use of ultrasonic echolocation to gather information on the integrity of mechanical Since changes in the material present an impedance mismatch from which an ultrasonic wave is reflected, ultrasonic testing can be used to identify faults, holes, cracks, or corrosion in materials, to inspect welds, to determine the quality of poured concrete, and to monitor metal Owing to the mechanism by which sound waves propagate in metals, ultrasound can be used to probe more deeply than any other form of Ultrasonic procedures are used to perform in-service inspection of structures in nuclear Structural flaws in materials can also be studied by subjecting the materials to stress and looking for acoustic emissions as the materials are Acoustic emission, the general name for this type of nondestructive study, has developed as a distinct field of High-intensity applications High-intensity ultrasound has achieved a variety of important Perhaps the most ubiquitous is ultrasonic cleaning, in which ultrasonic vibrations are set up in small liquid tanks in which objects are placed for Cavitation of the liquid by the ultrasound, as well as the vibration, create turbulence in the liquid and result in the cleaning Ultrasonic cleaning is very popular for jewelry and has also been used with such items as dentures, surgical instruments, and small Degreasing is often enhanced by ultrasonic Large-scale ultrasonic cleaners have also been developed for use in assembly Ultrasonic machining employs the high-intensity vibrations of a transducer to move a machine If necessary, a slurry containing carborundum grit may be used; diamond tools can also be A variation of this technique is ultrasonic drilling, which makes use of pneumatic vibrations at ultrasonic frequencies in place of the standard rotary drill Holes of virtually any shape can be drilled in hard or brittle materials such as glass, germanium, or Ultrasonic soldering has become important, especially for soldering unusual or difficult materials and for very clean The ultrasonic vibrations perform the function of cleaning the surface, even removing the oxide layer on aluminum so that the material can be Because the surfaces can be made extremely clean and free from the normal thin oxide layer, soldering flux becomes Chemical and electrical uses The chemical effects of ultrasound arise from an electrical discharge that accompanies the cavitation This forms a basis for ultrasound's acting as a catalyst in certain chemical reactions, including oxidation, reduction, hydrolysis, polymerization and depolymerization, and molecular With ultrasound, some chemical processes can be carried out more rapidly, at lower temperatures, or more The ultrasonic delay line is a thin layer of piezoelectric material used to produce a short, precise delay in an electrical The electrical signal creates a mechanical vibration in the piezoelectric crystal that passes through the crystal and is converted back to an electrical A very precise time delay can be achieved by constructing a crystal with the proper These devices are employed in fast electronic timing Medical applications Although ultrasound competes with other forms of medical imaging, such as X-ray techniques and magnetic resonance imaging, it has certain desirable features—for example, Doppler motion study—that the other techniques cannot In addition, among the various modern techniques for the imaging of internal organs, ultrasonic devices are by far the least Ultrasound is also used for treating joint pains and for treating certain types of tumours for which it is desirable to produce localized A very effective use of ultrasound deriving from its nature as a mechanical vibration is the elimination of kidney and bladder Diagnosis Much medical diagnostic imaging is carried out with X Because of the high photon energies of the X ray, this type of radiation is highly ionizing—that is, X rays are readily capable of destroying molecular bonds in the body tissue through which they This destruction can lead to changes in the function of the tissue involved or, in extreme cases, its One of the important advantages of ultrasound is that it is a mechanical vibration and is therefore a nonionizing form of Thus, it is usable in many sensitive circumstances where X rays might be Also, the resolution of X rays is limited owing to their great penetrating ability and the slight differences between soft Ultrasound, on the other hand, gives good contrast between various types of soft Ultrasonic scanning in medical diagnosis uses the same principle as Pulses of high-frequency ultrasound, generally above one megahertz, are created by a piezoelectric transducer and directed into the As the ultrasound traverses various internal organs, it encounters changes in acoustic impedance, which cause The amount and time delay of the various reflections can be analyzed to obtain information regarding the internal In the B-scan mode, a linear array of transducers is used to scan a plane in the body, and the resultant data is displayed on a television screen as a two-dimensional The A-scan technique uses a single transducer to scan along a line in the body, and the echoes are plotted as a function of This technique is used for measuring the distances or sizes of internal The M-scan mode is used to record the motion of internal organs, as in the study of heart Greater resolution is obtained in ultrasonic imaging by using higher frequencies—, shorter A limitation of this property of waves is that higher frequencies tend to be much more strongly Because it is nonionizing, ultrasound has become one of the staples of obstetric During the process of drawing amniotic fluid in testing for birth defects, ultrasonic imaging is used to guide the needle and thus avoid damage to the fetus or surrounding Ultrasonic imaging of the fetus can be used to determine the date of conception, to identify multiple births, and to diagnose abnormalities in the development of the Ultrasonic Doppler techniques have become very important in diagnosing problems in blood In one technique, a three-megahertz ultrasonic beam is reflected off typical oncoming arterial blood with a Doppler shift of a few kilohertz—a frequency difference that can be heard directly by a Using this technique, it is possible to monitor the heartbeat of a fetus long before a stethoscope can pick up the Arterial diseases such as arteriosclerosis can also be diagnosed, and the healing of arteries can be monitored following A combination of B-scan imaging and Doppler imaging, known as duplex scanning, can identify arteries and immediately measure their blood flow; this has been extensively used to diagnose heart valve Using ultrasound with frequencies up to 2,000 megahertz, which has a wavelength of 75 micrometre in soft tissues (as compared with a wavelength of about 55 micrometre for light), ultrasonic microscopes have been developed that rival light microscopes in their The distinct advantage of ultrasonic microscopes lies in their ability to distinguish various parts of a cell by their Also, because they require no artificial contrast mediums, which kill the cells, acoustic microscopy can study actual living Therapy and surgery Because ultrasound is a mechanical vibration and can be well focused at high frequencies, it can be used to create internal heating of localized tissue without harmful effects on nearby This technique can be employed to relieve pains in joints, particularly in the back and Also, research is now being carried out in the treatment of certain types of cancer by local heating, since focusing intense ultrasonic waves can heat the area of a tumour while not significantly affecting surrounding Trackless surgery—that is, surgery that does not require an incision or track from the skin to the affected area—has been developed for several Focused ultrasound has been used for the treatment of Parkinson's disease by creating brain lesions in areas that are inaccessible to traditional A common application of this technique is the destruction of kidney stones with shock waves formed by bursts of focused In some cases, a device called an ultrasonic lithotripter focuses the ultrasound with the help of X-ray guidance, but a more common technique for destruction of kidney stones, known as endoscopic ultrasonic disintegration, uses a small metal rod inserted through the skin to deliver ultrasound in the 22- to 30-kilohertz frequency Infrasonics The term infrasonics refers to waves of a frequency below the range of human hearing—, below about 20 Such waves occur in nature in earthquakes, waterfalls, ocean waves, volcanoes, and a variety of atmospheric phenomena such as wind, thunder, and weather Calculating the motion of these waves and predicting the weather using these calculations, among other information, is one of the great challenges for modern high-speed TableAircraft, automobiles, or other rapidly moving objects, as well as air handlers and blowers in buildings, also produce substantial amounts of infrasonic Studies have shown that many people experience adverse reactions to large intensities of infrasonic frequencies, developing headaches, nausea, blurred vision, and On the other hand, a number of animals are sensitive to infrasonic frequencies, as indicated in the T It is believed by many zoologists that this sensitivity in animals such as elephants may be helpful in providing them with early warning of earthquakes and weather It has been suggested that the sensitivity of birds to infrasound aids their navigation and even affects their One of the most important examples of infrasonic waves in nature is in Three principal types of earthquake wave exist: the S-wave, a transverse body wave; the P-wave, a longitudinal body wave; and the L-wave, which propagates along the boundary of stratified L-waves, which are of great importance in earthquake engineering, propagate in a similar way to water waves, at low velocities that are dependent on S-waves are transverse body waves and thus can only be propagated within solid bodies such as P-waves are longitudinal waves similar to sound waves; they propagate at the speed of sound and have large When P-waves propagating from the epicentre of an earthquake reach the surface of the Earth, they are converted into L-waves, which may then damage surface The great range of P-waves makes them useful in identifying earthquakes from observation points a great distance from the In many cases, the most severe shock from an earthquake is preceded by smaller shocks, which provide advance warning of the greater shock to Underground nuclear explosions also produce P-waves, allowing them to be monitored from any point in the world if they are of sufficient The reflection of man-made seismic shocks has helped to identify possible locations of oil and natural-gas Distinctive rock formations in which these minerals are likely to be found can be identified by sonic ranging, primarily at infrasonic