Reflections arriving from about 50-70 degrees lateral maximize perceived spaciousness.

Evidence

Figure 7.3(a) shows inverted IACC data from Ando (1977) so that it relates to “preference” on the vertical scale. This is calculated as 1 − IACC. This quantity is also the one shown for the Hidaka et al. (1997) measurements of IACCE3 so that it relates to the ASW (image broadening) component of spaciousness. It is only important to observe that all curves start at 0 at 0°, reach a broad maximum around 60°, and then slowly decline towards 180°; differences in vertical scaling are unimportant in this context as we are looking for trends. … A circle of constant effect is shown, confirming that for a direct sound arriving from 0°, reflections from about 50° to 70° are likely to generate the strongest impression of ASW/image-broadening, although those from many other angles make substantial contributions. — Toole, Floyd E, Sound reproduction- the acoustics and…, 698

Categorization

• DEFINE: Clarifies that 1 minus IACC (and 1 minus IACCE3) serves as a proxy for ASW/image-broadening preference.
• XREF: Connects to the reader’s general knowledge of binaural decorrelation metrics (IACC) as the standard correlate of perceived spaciousness.