A prototype cylindrical wave-number calibration array.

Abstract

There has been considerable interest in recent years in the acoustic interference of the turbulent boundary layer (TBL) produced on the surface of a vehicle moving through water. The ‘‘flow noise’’ produced by the TBL is generally characterized by pressure waves that are transported along the surface at speeds much less than the sound speed in water and are, therefore, evanescent. Planar array devices have been constructed previously that generate evanescent waves with desired spatial wave numbers in a region near the front surface of the array. This paper describes a prototype cylindrical wave-number calibrator array designed to generate the desired evanescent waves inside the free-flooded calibrator. This geometry is suitable for the evaluation of line acoustic sensors such as in towed sonar arrays. The prototype calibrator consists of a cylindrical tube of PVDF with an electrode pattern along the tube length that allows the PVDF to be operated as 40 independent bands each of which can be driven at a specified amplitude and phase. The required drive voltages are calculated by measuring the electroacoustic transfer matrix of the calibrator at the desired drive frequency. Measured evanescent pressure fields inside the calibrator are compared with desired and predicted fields at selected spatial wave numbers for frequencies of 800 and 1600 Hz. [Work jointly supported by ONR and ONT.]