A modified Prony method is developed to provide a complex exponential signal representation for use with underwater acoustic calibration waveforms. The approach is tested using simulated waveforms, and the simulation results are validated with limited experiments. Results show the complex exponential is an ideal signal representation for underwater acoustic calibration because of its remarkable ability to extrapolate calibration waveforms beyond the actural observation period. Results are shown of real reciprocity calibration experiments that are valid down to 25 Hz where the calibration waveforms are generated from a 5-ms current ramp and where a 5-ms observation period is used. The period of the low-frequency limit is eight times the observation period used in the calibration.
When two transducers are coupled together by a sound channel such as a water-filled steel tube and independently driven at a common frequency, dynamic control may be exerted over the acoustic impedance presented by the channel to either of the transducers. The determination of this load in the practical case, however, presents considerable experimental difficulty. For the ideal rigid-walled channel excited by symmetrically mounted piston sources, the load impedances may be expressed in terms either of the ratio of the source velocities or of their blocked driving forces. At frequencies below those for which radial mode propagation occurs within the channel medium, the load impedances may be also related to the standing-wave ratio of the longitudinal mode. This latter relationship seems better suited to practical measurements. Of particular interest for underwater acoustic calibration is the use of the active-load technique to produce plane progressive waves inside a practical water-filled steel tube.
A new application of the reciprocity principle has been developed for calibrating electroacoustic transducers in a dosed vessel at static pressures to 8500 psi and frequencies from 100 to 1500 Hz. The necessary plane progressive wavefield is provided by sound propagation in the longitudinal mode within a sound channel terminated at both ends with active impedances. The technique is particularly well suited to the calibration of underwater-sound transducers because the high static pressure under which many of them must operate, as well as their size, mechanical construction, and operating frequency, often prevents the use of more conventional methods. The case of a rigid-walled, water-filled tube is analyzed theoretically. Results of measurements made by this method in a practical high-pressure calibration chamber are shown.
A unique tube facility for the calibration of sonar transducers under hydrostatic pressures to 8500 psi is described. The pressure vessel consists of the modified liner from a 16-in. gun. The liner is 50 ft long and has an inside diameter of 15 in. An active-impedance principle is applied to establish plane progressive waves within the tube. Resonant transducers can be calibrated in the frequency range 100–1500 Hz; nonresonant, hard transducers can be calibrated in the range 40–1500 Hz.
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