A Planar Array for the Generation of Evanescent Waves

Wavenumber-frequency calibration of underwater, planar, receiving arrays requires the ability to generate single-wavenumber pressure fields over the surface of the array. When the wavenumber-frequency region of interest is evanescent, transmitting arrays previously constructed have been found to generate fields contaminated with harmonics, acoustic wavenumbers, and nonacoustic wavenumbers from the excitation of the antisymmetric Lamb waves. An array that greatly reduces contamination has recently been constructed using a sheet of polyvinylidene fluoride (PVDF) with independent rectangular electrode stripes. The array operates in the frequency range of 500 Hz to 2kHz and generates evanescent waves with phase speeds between 30 and 150 m/s. Contamination due to the excitation of antisymmetric Lamb waves is eliminated by shifting the phase speed of the Lamb wave out of the region of interest. This is accomplished by bonding the thin sheet of PVDF directly to a thick plate of LEXAN. Contamination from harmonics and acoustic wavenumbers is eliminated by driving the electrode stripes with suitably chosen shading coefficients generated by a simple-source numerical algorithm. Measured pressure fields and phase velocities compare favorably with numerical calculations. Keywords: Evanescent waves; Calibration; Sonar arrays; Acoustic waves; Equations; Flow noise. Pub. in Jnl. of the Acoustical Society of America, v87 n6 p2535-2540, Jun 1990.