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Fig. 4. Echo features of a hollow hemisphere (r=25 mm) for 90 directions
(-90° to 90°, in 2° increments) measured at a distance of 20 cm.
(A) Relative amplitude. (B) Directional pattern of the impulse response
function (white: positive amplitudes; black: negative amplitudes) Edges, 1st,
2nd reflections, see text and Fig.
5D. (C) Spectral directional pattern between 20 and 140 kHz.