# The Potential of Focusing Acoustic Retroreflectors for Architectural Surface Treatment
## 1 Introduction  
Sound reflection in rooms, when designed well, can make a substantial contribution to acoustic quality. For some types of room, sound reflected back to the source--a talker or musician--can be beneficial. This is well-established in auditorium acoustics for music, where the stage support parameters indicate aspects of the acoustic quality for performers [1; 2; 3; 4]. In rooms for speech, reflections back to the talker can often be beneficial, leading to a more comfortable environment for talking and affecting the characteristics of speech [5; 6; 7; 8]. Straightforward approaches to increasing reflected sound returned to a talker include using directed or local reflections and relying on reverberation. However,those approaches have limitations: directed/local reflections rely on a designer knowing where the talker(s) will be; and reverberation has hazards of reduced intelligibility and noise build-up in rooms. This paper explores the possibility of providing strong acoustic reflections back to a talker, without knowledge of the talker's position, and without relying on reverberation. The concept introduced is arrays of focusing acoustic retroreflectors, which are shown, in principle, to potentially provide reflected sound energy comparable to that of the reverberation of a supportive room. The voice regulation phenomenon of reduced vocal output caused by greater electroacoustic support is sometimes known as'sidetone compensation' [8,9], but has also been called the 'room effect' to signify the role of support from room reflections [6,10]. It is closely related to the better-known Lombard effect [11], whereby people instinctively raise their voices in the presence of background noise--both can be explained by a talker instinctively maintaining the audibility of their own voice [9,12,13]. However, the extent of these effects depends on the communication scenario/task [14,15,16,17] (e.g., reading to oneself, holding a conversation, lecturing, etc. ), and voice regulation is affected by multimodal perception [18]. Benefits of acoustic support for talkers have been investigated in particular for teachers. Sustained speech by teachers using high levels of projection is associated with voice strain and discomfort [19]. Classrooms with somewhat greater acoustic support have been shown to lead to reduced voice projection [6,20], especially for teachers who experience voice strain [21], but limited to a workable reverberation time [22]. Hence, although generally established intelligibility-oriented recommendations are for reverberation time in the vicinity of 0.4 s or less [23], a field study by [PERSON] et al. [22] found 0.7 s to provide optimum acoustic support; and [PERSON] et al. [24] propose similar recommendations, noting that some national classroom recommendations include relatively long reverberation times appropriate for acoustic support of teachers. Room types other than classrooms also could potentially benefit from strong acoustic support for talkers. Multi-talker spaces could be candidates, considering that a reduction in each person's acoustic power would reduce babble noise build-up--this problem has been studied particularly in large public rooms and eating establishments [25,26,27,28,29,25,24,26,29,27,28,25,29,30,31,32]. In work environments such as open-plan offices, speech distraction from irrelevant intelligible speech is a widespread problem [33,34,35,36,37]. In such contexts, reflecting a talker's sound back to them might not only have a voice regulation effect, but also ameliorate unwanted transmission to others, potentially improving the masking of unwanted speech by ambient sound. Another possible context is outdoor areas, which typically have very little acoustic support unless near buildings. Some studies have investigated open air environments using room acoustics concepts [38,39,40,41], and certain outdoor activities might benefit from enhanced acoustic support whilst preserving their openness. This paper leaves the development and assessment of such applications to future studies.