This paper describes a materials system for forming rubber composites with selectable acoustic properties.It is particularly suitable for laboratory use in molding small articles with longitudinal sound speeds and impedances less than (or not much greater than) those of water, and with sound absorption coefficients which may be varied over a wide range. The composites are formed using a common silicone rubber resin (RTV-602) to which various fillers may be added in controlled amounts. In this study, we measured the sound speed, density, and attenuation coefficient for more than 100 samples containing various concentrations and types of these fillers. These data were then reduced to determine the best-fit coefficients in a set of descriptive equations. Thereafter these equations could be used to calculate the filler concentrations needed to form composites with specific required properties. To demonstrate the usefulness and predictability of these materials in an application of general interest, these composites are used to construct several broadband anechoic coatings which reduce tank wall reflections by 20-35 dB at frequencies from 400 kHz to greater than 7 MHz. One-and two-layer coatings were designed using equations derived from Brekhovskikh's treatment of sound propagation in multilayer media. These coatings were then molded as tiles, and their actual performance is compared with that theoretically predicted. When applied to the inside walls of ultrasonic testing tanks, these tiles significantly reduce the tank size needed for various laboratory and biomedical applications. PACS numbers: 43.20.Fn, 43.20.Tb, 81.20.Ti, 43.55.Ev

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A general need exists for inexpensive finite-area transducer arrays which Intrinsically combine acoustic or vibration sensing with area actuation. Such combination transducers are particularly needed in active sound and vibration control and smart-materials applications. Commercial areas of Interest Include advanced underwater, aerospace or robotic-sensing applications. To be economically attractive they must be relatively simple to manufacture from reasonable cost materials. One promising new technology for such applications is Injection-molded 1-3 composite piezo-ceramics, pioneered by Material Systems Inc. (MSI). This transducer material is well suited for use as the low-cost actuator component of such a smart actuator. The challenge of this study was to design an Inexpensive accelerometer which could be Injection molded along with the actuator as an Interspersed array. This paper describes a monolithic accelerometer which is suitable for fabrication by injection-molding as an integrated co-formed actuator component. Experimental results are presented for actuator I accelerometer arrays and issues related to the design and use of accelerometers In close proximity to an actuator are discussed.

We have had a recurring need for molded articles (wave guides) and thin anechoic coatings, composed of rubbers with well defined and continuously variable acoustic properties. To fill this need, we have measured the dependence of various acoustic properties (sound speed, density, absorption) on the amount and type of filler added to RTV 602 silicone rubber. Both high impedance (ferric oxide, lead) and low impedance (glass and backlite microballons) modifiers were added, in two and three component mixes. A diluent (toluene) was found, which had no observed effect on the acoustic properties of the resulting rubber. Acoustic measurements were made using thick (molded) slabs, as well as thin (sprayed) coatings. In the latter cases, sound speed and absorption is evaluated using the measured frequency dependence of the coating's impedance. In all cases, we find that density and sound speed is well described using ideal mixture theory, providing the geometry of the microballons is taken into account. Absorption is described by simple phenomenological equations, and compared with theoretical predictions.

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