Cube-corner retroreflectors for acoustic support outdoors: A comparison of simple and optimised designs

Abstract

Activities conducted outdoors have reduced risk of disease transmission and yield positive mental and physical effects, but outdoor environments commonly suffer from acoustic problems such as urban noise, rapid spatial sound decay and insufficient acoustic support. This paper addresses the last problem, i.e., people's poor self-hearing due to lack of reflecting surfaces, which can reduce speaking comfort. The performance of easily fabricable, portable and deployable cube-corner retroreflectors (CCRs) is investigated through finite-difference time-domain (FDTD) acoustic simulations, omnidirectional measurements and oral-binaural measurements of physical prototypes. The CCRs' geometry is optimised to reduce sound dispersion using a process integrating parametric modelling, FDTD and genetic algorithms. Results show that optimisation can significantly boost the performance of CCRs by concave deformation of several centimetres, leading to more than doubling of the speech-weighted retroreflected energy of CCRs of 1 m edge lengths, and that of 0.5 m CCRs also substantially increased. Greatest improvements are achieved at high frequencies. Four CCRs of 0.5 m or one of 1 m, either simple or optimised, achieve speech-weighted voice support (STV) ranging from −19.1 to −15 dB over a 3 × 3 m area, better than common outdoor elements such as the ground or a wall and similar to typical enclosed rooms. CCRs are more effective for spectra emphasizing high frequencies, e.g., loud speech over normal speech, and larger CCRs have more balanced retroreflective spectra. This study provides a practical solution to improving weakly supported environments and thus contributes to improving the feasibility and acoustic quality of outdoor activities.