THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

16Ultrasonics16.1 IntroductionAs a subcategory of acoustics, ultrasonics deals with acoustics beyond the audiofrequency limit of 20 kHz. Although ultrasonics has been employed for most ofthe twentieth century, the tempo of new and improved applications has reachedvirtually explosive proportions only in the past few years, particularly in medicaldiagnostics and therapeutics.Applications of ultrasonics fall into two categories—low intensity and high intensity.Low intensity applications carry the purpose of simply transmitting energythrough a medium in order to obtain information about the medium or to conveyinformation through the medium. Nondestructive testing, medical diagnostics,acoustical holography, and measurements of elastic properties of materials fallinto this category. Even marine applications are included in this category, despitethe large energy input into operating sonar submarine detectors, depth sounders,echo ranging processors, and communication devices. 1High-intensity applications deliberately affect the propagation medium or itscontents. Uses of high intensities include medical therapy and surgery, atomizationof liquids, machining of materials, cleaning, welding of plastics and metals,disruption of biological cells, and homogenization of materials.Human beings are not alone in the use of ultrasonics, and even in this respect theyhave been preceded by thousands of years by other species in the animal kingdom.Certain animals are capable of generating and detecting ultrasonic signals in orderto locate and identify food, navigate their way through their environment, anddetect danger. In fact, the study of these animals have helped and is still helpingscientists to develop and improve techniques in the application of ultrasonic energy.Bats are known to emit pulses in the 30–120 kHz range, and it has been hypothesizedthat the bats judge range by sensing the time delay between an emittedpulse and the echo. Small bats can fly at full speed through barriers constructedof 0.4 mm vertical wiring spaced only one wingspan apart. They are capable of1 Emsinger (1988) suggested that a third category not based on intensity be designated to specificallycover underwater applications.443