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

15.11 Sonar Transducers and Their Properties 425and its variant, electrostriction, and the other type functions on the principle ofmagnetostriction. Certain crystalline substances, such as quartz, ammonium dihydrogenphosphate (ADP), and Rochelle salt, generate a charge between certaincrystal surfaces when they are subject to pressure. Conversely, when a voltage isapplied to these substances, this causes stresses to occur in them. Such materialsare said to be piezoelectric. Electrostrictive materials are polycrystalline ceramicsthat produce the same effect and these have to be properly polarized in a strongelectrostatic field. Barium titanate and lead zirconate titanate are examples of suchmaterials.A magnetostrictive material such as nickel exhibits the same effect as piezoelectricitybut it does so under the influence of a magnetic field rather than appliedstresses. It changes its dimensions when subjected to a magnetic field and, conversely,changes the magnetic field within and around it when it becomes stressed.In other words, when a properly designed nickel element is subjected to an oscillatingmagnetic field, a mechanical oscillation is produced which generates acousticwaves in water. Magnetostrictive materials are also polarized in order to avoidfrequency doubling and to achieve a higher efficiency.Piezoelectric and magnetostrictive types of transducers are more suitable thanother kinds of transducers for use underwater due to better impedance matchingwith water. Because they are relatively inexpensive and can be readily fashionedinto the desired shapes, ceramic materials are finding increasing applicationsas underwater devices. Other types of units now include the thin-film transducers(Hennin and Lewiner, 1978) and fiberoptic hydrophones (Hucaro et al.,1977).ArraysWhile single piezoelectric or magnetostrictive elements are normally used in hydrophonesfor research or measurement purposes, much of the applications ofhydrophones entail hydrophone arrays that use a number of properly spaced elements.The following reasons exist for the use of arrays:1. The array is more sensitive, as a number of elements will generate more voltage,if connected in series, or more current, if connected in parallel.2. The array provides directivity that enables it to discriminate between soundscoming from different directions.3. An improved signal-to-noise ratio SNR over that of a single hydrophone isprovided, because the array discriminates against isotropic or quasi-isotropicnoise to favor a signal arriving from a direction that the array is pointingto.Because of the above advantages, most practical applications of underwater soundmake of arrays. Moreover, the first and second benefits listed above also apply toprojectors as well as to hydrophones.