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

42 3. Sound Wave Propagation and CharacteristicsThe phenomenon of sound wave reflection finds many applications. The time ittakes for a sound wave pulse to travel from a transducer at sea level to the oceanbottom and for the echo to travel back gives a measure of the depth of the water.Further, comparison of the spectral characteristics of the reflected wave with thoseof the generated waves provides an ample measure of the geological compositionof the ocean bottom, for example, silt, rock, sand, coral, and so on. Reflected soundis also used in an analogous way by geologists to gauge the depth and compositionof stratified layers in the earth crust, to locate oil, natural gas, and mineral deposits.3.8 RefractionA phenomenon more familiar in optics than in acoustics is that of refraction, inwhich the direction of the advancing wavefront is bent away from the straightline of travel. Refraction occurs as the result of the difference in the propagationvelocity as the wave travels from one medium to a different medium.In the optical situation, refraction occurs suddenly when light waves cross thesharp interface between the atmosphere and glass at the surface of a lens, becauselight travels more slowly in glass than it does in air. At audible frequencies ofsound waves, the wavelengths are so long that the apparatus would have to beextremely large in order to render observable acoustic refractions. However, at ultrasonicfrequencies, which correspond to extremely short wavelengths, refractionconstitutes the operating principle of the acoustic microscope. The device functionsas indicated in Figure 3.8. A piezoelectric transducer P z , under the impetusMFigure 3.8. A schematic of an acoustic microscope. Voltage V causes the piezoelectriccrystal P t to launch a short train of waves into lens L. The propagation velocity of thewaves slows down in liquid medium M, and the waves are focused toward point S onthe surface of the specimen. Reflected waves follow the same paths in reverse, reachingthe piezoelectric transducer that now has been switched into the detector mode.