The Potential of Focusing Acoustic Retroreflectors for Architectural Surface Treatment

How much sound can a building surface reflect to a source, the location of which is not exactly known? This paper considers this question particularly for a planar surface acting as an array of retroreflectors, or of focusing retroreflectors. The question is investigated using finite-difference time-domain acoustic simulation, using ideal retroreflective patches achieved by space-reversal, and focusing achieved by delays. Extensive (7.2 × 7.2 m) and local (2.4 × 2.4 m) ideal planar reflector arrays were investigated at distances of 1.5 to 4 m from sources that were within a 2.4 × 2.4 m square plane. Patch sizes ranged from 0.3 m squares to the full reflector size. Physically realizable non-ideal focusing retroreflectors based on parabolic trihedra were also investigated. With sufficiently large patches, ideal focusing retroreflector arrays consistently outperform non-focusing retroreflector arrays. A large focusing retroreflector array has the potential to provide retroreflected energy levels (speech and A-weighted) from the first reflection to a source at 2 m distance comparable to the diffuse field energy level of acoustically supportive reverberant rooms. A small focusing retroreflector array returns less sound, but still much more than a single reflection from an equivalent specularly reflecting surface. Results from parabolic trihedra demonstrate that retroreflected energy levels similar to those from ideal surfaces can be achieved by architectural form. Challenges in translating these concepts to practical design solutions are discussed.