Concentration of reflected sound in a room treated with cube corner retroreflectorsa)

A room was treated to be predominantly retroreflective in the high frequency range by introducing arrays of cube corner retroreflectors (CCRs) over most surfaces (excluding the floor). In a small room (volume 55 m3), 156 CCRs in the form of square trihedra with 350 mm edge lengths were used as wall and ceiling treatment. The horizontal plane distribution of reflected energy was measured from omnidirectional sources, and a head and torso simulator was used to measure voice support. Results show a high concentration of reflected energy returned to omnidirectional source positions in high frequency octave bands (2-8 kHz). Finite-difference time-domain (FDTD) simulations of the room yielded similar distributions to the omnidirectional measurements, showing greater sound concentration when more CCRs are introduced. By contrast, FDTD simulation of an equivalent flat-surfaced room yielded no reflected sound concentration at the source, with results close to diffuse field theory in high frequency octave bands. Measured voice support values derived from oral-binaural room impulse responses exceed diffuse theory expectations by 5 dB. Thus, the paper demonstrates that retroreflective array treatment can change room acoustical conditions, concentrating reflected energy onto an arbitrarily located source.