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.