An investigation of the performance of a ribbon and small planar magnetic transducer, made for use in air, as an underwater acoustic velocity sensor

Abstract

The use of vector sensors in sonar receiving array applications is an area of active investigation by the U.S. Navy. A vector sensor combines the outputs of a monopolar (pressure) sensor co-located with one or more orthogonal dipolar (velocity or, more commonly, pressure-gradient) sensors to achieve a modest (5–6 dB) amount of directivity at a single point. The directivity of an array formed using such sensors is also modestly improved by the same amount. More importantly, though, the azimuthal angle ambiguity that occurs in the beam pattern of a line array of point pressure sensors is removed by employing vector sensors instead. This thesis describes an investigation into the underwater use of a pressure-gradient transducer technology that has been employed in airborne acoustic applications for many years (mostly as a source), but, to our knowledge, has heretofore never been employed under water. The devices examined include planar-magnetic and true ribbon transducers. The planar-magnetic transducer is very closely related to what was developed first (i.e., the true magnetic ribbon transducer). The same as an ordinary magnetic loudspeaker or microphone, both the planar-magnetic and true ribbon transducers employ a mutually orthogonal electrical conductor, magnetic field, and axis of motion of a diaphragm. We present the results of measurements made both in air and water of the transmitting and receiving responses of a planar-magnetic and a true ribbon transducer. We also present a procedure for the free-field reciprocity calibration of a pressure-gradient transducer.