A New Method for the Calibration of a Plane Hydrophone

Abstract

The calibration of a plane-disk hydrophone of 112-inch diameter over a wide frequency range by the usual plane-wave reciprocity method would be a very difficult task, requiring a large anechoic tank and precise mechanical alignment gear. A new and absolute method has been developed utilizing the radiation pressure of a standing wave system. The air-backed barium titanate hydrophone was suspended in oil above a quartz crystal projector by means of a sensitive Joly balance spring. The hydrophone thus acted simultaneously as a reflector to set up standing waves, as an electromechanical transducer, and as a device to measure radiation pressure. The weight, buoyant force, spring force, and radiation pressure combined to trap the hydrophone in any one of a number of discrete equilibrium positions, while the radiation pressure tended to align the hydrophone face parallel to the face of the quartz projector. The acoustic pressure was calculated from the spring extension, and the electrical output of the hydrophone was measured with a Ballantine voltmeter. In this manner the sensitivity of the hydrophone was determined over the frequency range 300 kc/sec to 5 Mc/sec. For a hydrophone of 1 Mc/sec resonant frequency, the calibration showed peaks at 1 Mc/sec, 3 Mc/sec, and 5 Mc/sec, and the sensitivity varied from a high of −116 db to a low of −156 db ref 1 volt/(dyne/cm2).