# The Potential of Focusing Acoustic Retroreflectors for Architectural Surface Treatment
## 4 Discussion
### Questions and Challenges  
This paper shows that, in principle, it is possible to strongly concentrate reflected sound at arbitrary unknown source positions within an assumed area using focusing retroreflectors. Focusing on its own is an acoustic design hazard (domes have been called an \"acoustician's nightmare\" [68])--but when combined with space-reversal, focusing could support talkers without relying on reverberation, concentrating reflected sound in the area where it is wanted rather than other areas. However, more scientific research and practical experience with built designs are needed for us to determine whether or not focusing retroreflectors can go from an acoustician's dream to realistic architectural designs. Four important questions are outlined and briefly discussed in the following paragraphs. _Question 1_: How can focusing retroreflector arrays be best designed for high acoustic quality in terms of their spectral, spatial and temporal characteristics, especially considering the oral-binaural impulse response? This question includes considerations such as how the body of a talking listener creates, scatters, and receives the sound field. The mouth and ears are not collocated, and speaking and listening both have directional characteristics. Strong reflections can have positive or negative effects for a talker; delayed auditory feedback can make it very difficult to talk resulting in raised speaking level [69; 70], whereas early reflections that are integrated in the auditory system are supportive [71]. The high frequency spectral bias in focusing retroreflection may introduce sound quality issues, although [PERSON] et al. [72] and the authors' experience with built retroreflective environments [51] provide some indication that this might not be a problem for speech. These multiple issues feed into the question of how focusing retroreflectors should be practically designed for envisaged environments. There are many possible variations in focus design, such as broadband or frequency-selective focusing/defocusing, which may prove to be fruitful in refining spectral and spatial distributions of the reflected sound field. For example, the simulations in this study used ideal retroreflectors focused onto the source-receiver plane, but focusing farther might better balance the contributions of multiple patches. _Question 2_: Behaviorally, how do people respond to an acoustic environment strongly supported by focusing retroreflectors? Studies at the time of writing provide only indirect evidence that acoustic support in such an environment will lead to useful voice regulation effects. This is because no space with a large human/speech-scale focusing acoustic retroreflector array has been built anywhere, let alone used for human voice regulation experiments. Indirect indicators of likely voice regulation benefits come from multiple studies using sidetone gain or various room acoustic conditions (not including retroreflection). While these indirect indications are encouraging--given the high voice support values that this study's results suggest--the particular spectral, spatial, temporal and broader contextual factors of focusing retroreflector treatments may affect behavioral response. _Question 3_: How should focusing retroreflector arrays be combined with established design techniques such as the use of sound absorption, scattering, and reverberation-based room acoustic design? This question is both practical and theoretical. Design parameter targets and calculation methods may need to be developed and refined for theoretical support of room design incorporating focusing retroreflectors. The hypothesis is that shorter reverberation times combined with greater retroreflected energy can provide improved acoustic conditions for certain types of room, but the theoretical tools to support such design have not been developed. _Question 4_: Can space-efficient, acoustically efficient, cost-effective, and visually innocuous focusing retroreflectors be designed for common situations in buildings?