This paper presents a method of increasing the bandwidth of transducers feeding low acoustic impedance loads such as fluids and of electrically matching them over the full bandwidth. It consists of interposing between the transducer and the actual load, a wedge made out of a material having the desired acoustic impedance. The wedge angle is chosen such that no reflected wave from the wedge-load interface can return to the transducer. In this manner, the effective load seen by the transducer is that of the wedge itself, regardless of the actual load. The appropriate electrical matching is achieved by means of a hybrid band pass filter containing both electrical and mechanical components, with the transducer as an integral part.
Experimental results on a 4-inch diameter PZT-4 transducer, coupled to water through a brass wedge, demonstrate a bandwidth of 75 percent and an insertion loss of 8.5 dB, well in accordance with theory. Without the wedge, the bandwidth would be only 3.2 percent.