An experimental investigation of the parabolic reflector as a nearfield calibration device for underwater sound transducers

Abstract

Results are presented of an experimental investigation of the use of thin metal parabolic reflectors to form underwater quasiplane wave regions in the nearfield of the reflector. This quasiplane wave volume is then used to calibrate underwater sound transducers in terms of directivity and sensitivity. Such reflectors, mass produced for use with electromagnetic wave devices, are readily available, inexpensive, and rugged. Experiments were conducted with reflectors ranging from 45 to 122 cm in diameter at frequencies from 30 to 300 kHz. It might be noted that at the higher frequencies the acoustic wavelength is approximately an order of magnitude shorter than that for which the reflectors were designed. Test transducers, typically half the size of the reflectors, were calibrated both in the nearfield of the reflector and in the farfield (using conventional techniques) and the results were compared. It was found that for the major lobe there was excellent agreement and that the levels of the minor lobes were in acceptable agreement but that the fine detail of the minor lobe structure differed for the two types of measurements. Sensitivity measurements differed by about ±1.0 dB. For low-frequency operation the metal reflector needs to be backed by air or other pressure release material in order to be an efficient reflector. It is concluded that the parabolic reflector technique has useful accuracy for many applications.