Acoustic-Backing Techniques for Transducers and Radiators

Abstract

The acoustic stiffness of a liquid within a small enclosure may be reduced greatly by the introduction of compressible matter, which may be in the form of compliant tubes. This effect is due in part to the added compressibility of the compliant tubes and in part to volume resonance. The mixture can be tuned or detuned by changing either the number of compliant tubes or the type of liquid in the fill. Such flexibility permits the resonant frequency of an acoustic transducer to be altered. Additional investigations demonstrate that a simple liquid fill can also be employed successfully if the diaphragm compliance of a transducer is designed to be comparable to that presented by the fluid. The resonance width of a transducer may be broadened by reducing the thickness of a metallic diaphragm and using the liquid fill for the essential stiffness with a lesser amount of mass. However, the effective electromechanical coupling factor may increase or decrease by an amount that depends on both the relative stiffness and the specific transducer design.