(word processor parameters LM=8, RM=75, TM=2, BM=2) Taken from KeelyNet BBS (214) 324-3501 Sponsored by Vangard Sciences PO BOX 1031 Mesquite, TX 75150 There are ABSOLUTELY NO RESTRICTIONS on duplicating, publishing or distributing the files on KeelyNet! March 7, 1991 SONIC1.ASC -------------------------------------------------------------------- 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 (see SOUND AND ACOUSTICS) 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). Generation of Ultrasonics 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. Page 1 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. Wave Properties. 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), respectively. 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. Applications. Perhaps the most widespread use of ultrasound has been in the detection of obstacles in materials that do not transmit light (optically opaque materials). Thus, ultrasound is used in underwater signaling because a low- frequency ultrasonic beam can penetrate many kilometers of the ocean and be reflected back from any obstacle. This is the principle of SONAR, which can be used to identify submarines and map the ocean bottom. Sonar can even be used to measure the thickness of ice packs by submarines traveling under the polar ice caps. If a short pulse of ultrasound is sent into a metal, it will be reflected from any cracks or minute defects such as blowholes. A system generating such pulses is widely used in flaw detection in solids (nondestructive testing). Page 2 Because different solids and liquids reflect at different rates, the reflection of an ultrasonic pulse can also be used in medical examinations, especially of unborn fetuses, and in the detection of brain tumors and breast cancers. Echocardiography, the study of heart motions by ultrasonic means, is another medical application. An important physical property of ultrasound is the vigorous small-scale vibration of the medium that it represents. This property has led to many industrial applications. The vibrations can be used to shake dirt or other deposits off metals (ultrasonic cleaning). Such vibrations can also be used in soldering or welding. The ultrasonic transducer serves to remove oxide from the outer surface of the material, making more efficient the use of heat in the joining process. Plastic powders can be molded into small cylinders by similar techniques. -------------------------------------------------------------------- If you have comments or other information relating to such topics as this paper covers, please upload to KeelyNet or send to the Vangard Sciences address as listed on the first page. Thank you for your consideration, interest and support. Jerry W. Decker.........Ron Barker...........Chuck Henderson Vangard Sciences/KeelyNet -------------------------------------------------------------------- If we can be of service, you may contact Jerry at (214) 324-8741 or Ron at (214) 242-9346 -------------------------------------------------------------------- Page 3