It has recently become necessary to find a way to determine far-field characteristics of large underwater sound transducers from measurements made in the near field of the transducer (at ranges small compared to the transducer dimensions). It is possible to compute far-field directivity patterns and source levels from pressure amplitude and phase measurements made in the near field. This can be done by using Kirchhoff's formula with a simple approximation for the normal component of the pressure gradient. Pattern computations were made on line, plane-array, dipole, and line-and-cone transducers from measurements made in open water. Extensive pattern and source level computations were made on a large, multistave cylindrical transducer from measurements made in open water and in a highly reflective tank. Agreement with measured far-field patterns and source levels was good throughout and was within ±1 dB in the more recent work with the cylindrical transducer. Particularly significant is the fact that successful measurements were made in a tank of diameter only 2.9 times that of the transducer. These measurements were made near the leading edge of the received pulses in order to avoid interference due to reflections. Preliminary work on a simplified near-field test for a cylindrical transducer has also been done. Such a test would enable one to use near-field data to make certain judgments about the far-field performance of a transducer without computations.
A sparse equivalent source method for near-field acoustic holography
This paper compares several digital measurement approaches which have been used for underwater acoustical measurements and experiments. The advantages of the digital measurements systems include (1) flexibility in data reduction, storage, and presentation; (2) flexibility in interaction with sophisticated signal processing and display systems; and (3) the capability for close comparison between theoretical and experimental results because experimental parameters can be used as inputs to a theoretical calculation. In this paper, three digital systems are considered: (1) a calenlator-based system, (2) a mini-computer based system with a minimum of support hardware, and (3) a larger mini-computer system, which includes a disk. The calculator-based system uses primarily existing measurement instruments with digital outputs; in other words, most of the signal processing is done with conventional analog instruments. The smaller computer systems use fixed routines that are linked to a BASIC program. This system allows for operator interaction by using the BASIC interpretative langauge. Subroutines written in FORTRAN and machine language can be included to enhance the speed and performance of the measurement system, but such subroutines are not easily modified in the field. The computer/disk system provides the most flexibility in the FORTRAN as well as BASIC programs can be written on-line by the operator. The application of these three systems to specific acoustical measurements is described.
The directivity pattern and source level of a large cylindrical transducer were computed using the Kirchhoff formula with a simple approximation for the normal pressure gradient. The integration was performed numerically over a finite cylinder. Nearfield pressure amplitude and phase data were made over the surface of integration by rotating the transducer and measuring the output of a line hydrophone less than one wavelength from the active face. These measurements were made in a highly reflective sheet-steel tank of diameter 2.9 times that of the transducer and wall thickness much less than a wavelength. The amplitude and phase were measured on the leading portion of the received pulses. Directivity patterns were computed in a plane normal to the axis of the transducer. They were compared with the equivalent patterns measured in open-sea conditions. Agreement was good both in pattern shape and source level. It is felt that this work shows that it is possible to calibrate such a transducer in a tank that is relatively small and highly reflective.
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