Retroreflection is rarely used as a surface treatment in architectural acoustics but is found incidentally with building surfaces that have many simultaneously visible concave right-angle trihedral corners. Such surfaces concentrate reflected sound onto the sound source, mostly at high frequencies. This study investigated the potential for some Indian stepwells (stepped ponds, known as a kund or baori/baoli in Hindi) to provide exceptionally acoustically retroreflective semi-enclosed environments because of the unusually large number of corners formed by the steps. Two cases—Panna Meena ka Kund and Lahan Vav—were investigated using finite-difference time-domain (FDTD) acoustic simulation. The results are consistent with retroreflection, showing reflected energy concentrating on the source position mostly in the high-frequency bands (4 kHz and 2 kHz octave bands). However, the larger stepped pond has substantially less retroreflection, even though it has many more corners, because of the greater diffraction loss over the longer distances. Retroreflection is still evident (but reduced) with non-right-angle trihedral corners (80°–100°). The overall results are sufficiently strong to indicate that acoustic retroreflection should be audible to an attuned visitor in benign environmental conditions, at least at moderately sized stepped ponds that are in good geometric condition.

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.