Speech intelligibility increases in a reverberant space as the ratio of direct to reverberant sound energy (D/R) becomes high. The increase in D/R is investigated and a loudspeaker (or microphone) array for improving speech intelligibility in a reverberant space is proposed. The D/R at the center of a spherical surface (or circle) increases with the number of loudspeakers arrayed on the surface. It is shown theoretically that the D/R for a sphere has a limit that is independent of the radius of the surface. This limit is given by the ratio of the equivalent sound absorption area in the room to the half-wavelength squared (uncorrelated minimum distance). The D/R may be further improved as the number of spherical layers on which the loudspeakers are arrayed increases when proper delays and level adjustments are applied. The D/R reaches a final limit when the room space is fully occupied by uncorrelated spherical layers. Rapid speech transmission index (RASTI) measurements using concentric circular loudspeaker arrays demonstrate that the RASTI becomes high at the center in a highly reverberant space. The increase in D/R is demonstrated for circular arrays as well as for linear loudspeaker arrays with time delays.
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