Fig. 8. Spectral directional pattern of a hollow hemisphere (A), a paraboloid (B) and an ellipsoid (C). All three hollow forms had the same circular opening with a diameter of 36 mm but different depths (for exact parameters, see text). Above each spectral pattern the relative intensity of the echo, averaged over the frequency range from 20 kHz to 140 kHz, is plotted as a function of the angle of sound incidence.