Tunable cylinders for sound control in water

Abstract

Long wavelength effective medium properties are achieved using arrays of closely spaced tunable cylinders. Thin metal shells provide the starting point: for a given shell thickness h and radius a, the effective bulk modulus and density are both proportional to h/a. Since the metal has large impedance relative to water it follows that there is a unique value of h/a at which the shell is effectively impedance matched to water. The effective sound speed cannot be matched by the thin shell alone (except for impractical metals like silver). However, simultaneous impedance and speed matching can be obtained by adding an internal mass, e.g., an acrylic core in aluminum cylindrical tubes. By varying the shell thickness and the internal mass, a range of effective properties is achievable. Practical considerations such as shell thickness, internal mass material, and fabrication will be discussed. Arrays made of a small number of different tuned shells will be described using numerical simulations: example applications include focusing, lensing, and wave steering. [Work supported by ONR.]