Measurement Conditions Influencing the Front-To-Back Discrimination of an Underwater Hydrophone

In almost every application of an underwater listening unit it is necessary to listen by means of some predetermined scanning or searching technique for some external noise source in the presence of a nearer noise source caused by the propulsion mechanism of the carrier vessel. In order to listen more advantageously for external sources, it is general practice to locate the listening hydrophone in the forward part of the vessel, so as to remove it as far as possible from the so-called “self-noise” source in the aft part of the vessel. In order to reduce further the reception of this unwanted self-noise, it is desirable to design the listening hydrophone to discriminate against such noise coming in from the rear, without materially affecting the front-side sensitivity. The term “front-to-back discrimination” is used to describe this particular quality of a hydrophone. It is the purpose of this report to discuss a few of the measurement conditions which influence the value of the “front-to-back ratio” of a hydrophone. Emphasis is placed on those factors involved in any measurement which are external to the hydrophone itself; for example, the conditions of the medium and the surrounding region, the mounting conditions and the acoustic baffling of the hydrophone, the directionality characteristics of the transmitting transducer, or projector, with which the hydrophone is measured, and the distance separating the hydrophone and the projector. The data here presented were made with various types of hydrophones under various sets of conditions at the Harvard Underwater Sound Laboratory measurement station at Spy Pond during the last two or three years of the war.