Ultrasonics
Ultrasonic waves are sound waves too high-pitched for human ears to detect. Their frequencies range from 20,000 hertz to about 10 trillion hertz (the maximum that modern ultrasonic generators are able to produce). Because ultrasonic waves are compressional, or pressure waves, with the ability to both shake and penetrate many materials, they have tremendous applications in industry and medicine.
A compression wave is created when a very dense material vibrates very fast. This causes the air around the material to be alternately pushed and pulled, producing regular pressure variations. At very high frequencies, such as the ultrasonic range, the compression wave can be focused into a fine "beam" which can then be used to vibrate particles in its path.
In order to generate an ultrasonic beam one must use a transducer—a device that converts electrical or magnetic energy into kinetic, or mechanical, energy. There are three methods for generating ultrasonic waves: piezoelectric transducers, magnetostrictive transducers, and electrostrictive transducers. All three are used to set up ultrasonic vibrations.
The piezoelectric effect was discovered by Pierre Curie in the 1880s. Curie found that certain crystals--quartz, in particular--would expand slightly when an electrical current was passed through it and contract slightly when the current was applied in the opposite direction. By using an alternating current he caused the crystal to expand and contract very rapidly, vibrating at a fixed frequency. Through later experiments Curie found that the frequency of the quartz's vibration could be altered by using a crystal of a different size and shape, and that the piezoelectric frequency was always equal to the natural frequency of the quartz.
In ultrasonics, crystals are selected whose natural frequencies are greater than 20,000 hertz. As they vibrate, ultrasonic compression waves are formed. Because scientists often require waves of a precise frequency, piezoelectric crystals made of ceramic have been developed; these man-made crystals can be engineered to vibrate at a particular frequency and are often more compact than natural crystals.
The second type of transducer is based upon the phenomenon called magnetostriction, which, like piezoelectricity, it causes a vibration when an electrical current is applied. However, a coil-wrapped metal rod is used instead of crystal. When electricity is passed through the coil a magnetic field is generated, and, because of magnetostriction, the rod will change in length. As the magnetic field alternates, the rod will vibrate. Magnetostriction transducers are more popular than quartz transducers because they can be manufactured in any size and shape.
Electrostrictive transducers are essentially the same as the magnetostrictive variety, but uses an electrical rather than a magnetic field. Electrostriction uses materials such as calcium, lead, barium titanates, and ceramics. Like magnetostrictive devices they are easily manufactured and can be machined into many shapes. Also, they require much lower voltages to produce ultrasonic waves than do piezoelectric transducers.
There are numerous practical applications for ultrasonics. The first widespread use was in marine technology, where ultrasonic waves proved to be an excellent method for determining the depth of water. By improving echo sounding equipment, ultrasonics were ultimately used to map the topography of lake and ocean floors. During the war years, ultrasonic waves were often a medium for secret contact between submarines.
In industry, ultrasonic waves have been used in the testing of machinery and machine parts. Using a narrow beam of sound, engineers have been able to look inside metal parts in much the same way physicians use X-rays to examine the human body. Using ultrasonic technology, flaws can be detected and repaired before they cause irreparable damage to the machine.
Similar ultrasonic diagnostic methods have been developed for use on the human body. As an ultrasonic beam passes through the human body it encounters different types of tissue such as flesh, bone, and organs. Each type of tissue causes the beam to reflect in a different way. Using a computer to assimilate these reflections, physicians can accurately map the interior of the body. Unlike X-rays, there is no risk of harmful overexposure with ultrasonics; thus, they have become a useful alternative to X-rays, and are often used on sensitive organs, such as kidneys, as well as to monitor the progress of pregnancies.
Because of their ability to vibrate the particles they pass through, ultrasonic waves are often used to shake, or even destroy, certain materials. A good example of this is ultrasonic emulsification, through which two liquids which do not normally mix (such as oil and water) are made to vibrate until they are blended. This method is also used to remove air bubbles from molten metals before casting so that the finished piece will be free of cavities. Ultrasonic vibration can also be used to kill bacteria in milk and other liquids. Finally, some inventors are attempting to perfect an "ultrasonic laundry," using high-frequency vibrations to shake dirt and other particles out of clothing.
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