This paper presents a simple one-dimensional model for describing the influence of backing impedance on the performance of sensors and projectors. It uses a center-of-mass model, where the influence of backing can be largely extracted into a separable term. As an example, it includes consideration of the familiar underwater case of an air-backed steel plate with an outer compliant (decoupler) coating. This approach is principally useful in large numerical structural acoustical studies, where the use of complete transducer formulations would become computationally impractical.

Applications requiring underwater sound absorption usually make use of the high damping capability of polymeric materials. This paper describes the trade-offs necessary in selecting materials for sound absorbing coatings for use in reducing extraneous wall echoes in water filled pools and tanks. Coatings for this purpose are called anechoic, literally meaning no echoes, or non-reflective. Satisfactory anechoic coatings have been developed using a variety of different approaches including: simple and multi-layer absorbers; wedgeshaped designs; and resonant cavity approaches. This paper reviews the advantages of each and the material requirements involved. Data is presented on various commercially available underwater sound absorbing coatings, including three new coatings which have been only recently developed. Two examples of typical coating applications are presented: a tank for use in medical studies at ultrasonic frequencies (0.5 to 10 MHz) and one designed for calibration use at lower frequencies (6 to 100 kHz).Underwater acoustics is routinely used in laboratory-scale test facilities for flaw detection, transducer calibration, material property evaluations, and acoustic visualization. In a typical underwater acoustic study, an object of interest is submerged in a water filled tank and acoustically illuminated (insonified). The acoustic signals scattered by the object are then measured and analyzed. If the tank used is not sufficiently large, these measured acoustic signals will include spurious echo components due to extraneous wall reflections. Since the effect of these contaminating echoes usually cannot be removed from the resulting data set by post analysis, they must be prevented from occurring at their source. One cost 0097-6156/90/0424-O208$O6.25/0

The performance of acoustic transducers (sensors and actuators) can be degraded or enhanced by the structure on which they are mounted. For various numerical structural acoustics studies, a simple computationally efficient model is required for estimating the change in transducer performance resulting from variations in the front and back face impedances. The approach used here is based on simple physical principles and intuition, which makes it well suited for incorporation in more-complicated structural models since it is conducive to direct extraction of physically meaningful forcing functions. As an example of its use, the performance of pressure sensors, velocity sensors, and acoustic drivers on an air-backed steel plate are estimated, and then the change in performance when a layer of compliant material is inserted between the transducer and the plate is calculated. Results are compared with a Mason-type equivalent circuit model.

Active feedback in a hydrophone-projector system is used to modify the transmit or receive efficiency of the system. Hence, the output from the hydrophone layer can be used to broaden the bandwidth of the projector or, conversely, the driver can be used to correct the response of the hydrophone. The principal features of the system can be represented by a one-dimensional model resembling a four-media system, consisting of water, a receiver layer, a projector layer, and a backing impedance. Since the output of the receiver layer is amplified and applied to the projector layer, an active acoustic feedback system results. Experimental data using a large-area projector bonded to a transparent large area polymer (LAP) hydrophone are presented.