Some interesting features of spherical cap transducers

Interesting features of spherical caps have been identified and investigated as a result of the availability of large computers which has overcome the problem of poor convergence of the wave-harmonic series for spherical cap directivity at large ka. Caps which have radial velocity excitation exhibit Fresnel diffraction effects for ka ⩾ 2π. The presence of these effects has been verified experimentally with a spherical transducer which shows an axial loss of sensitivity for conditions corresponding to the 2nd Fresnel half-period zone. Fresnel theory predicts the occurrence of directivity extrema for caps which are hemispherical or smaller. Data for prediction of diffraction effects for larger caps is presented for 2π ⩽ ka ⩽ 20π. Axially excited caps do not exhibit diffraction effects. The velocity coefficients for the wave-harmonic series for axial velocity excitation are developed. Axially excited hemispherical caps were observed to exhibit constant directivity index for ka ⩾ 0.8π. [Work partially supported by NAVSEA and NCSC.]