Fig. 5. (A) Impulse response of a hollow hemisphere (r=38.5 mm) for an angle of sound incidence of 10° (edges, 1st, 2nd to nth reflections, see text and D). The time gap between first and second reflection (gap_1-2) is marked with a grey bar. (B) Duration of the gap between first and second reflection plotted over the radius of the hollow hemispheres. The gaps were measured (black triangles) from the impulse response functions, and calculated (open squares) from Eqn 2 in the text. (C) Relative amplitude of the echoes of hollow hemispheres of different sizes measured from an angle of 10° in a distance of 40 cm (D) Sound reflections within a hollow hemisphere, for a sound source at infinite distance such that incoming and outgoing rays are parallel and assuming that wavelength is small in comparison to the radius.