FIG. 8A is a plan view showing a practical example of acoustic coupler 300 and FIGS. 8B and 8C are cross-sectional views along section lines 8B-8B and 8C-8C, respectively, shown in FIG. 8A. The same reference numerals are used to denote the elements of acoustic coupler 300 in FIG. 7 and in FIGS. 8A-8C.