## 4 Discussion  
This paper gives an insight into an audio system for the creation of an Auditory Augmented Reality (AAR) environment. Single system components were described, which are motivated by human spatial hearing. The objective is to minimize the technical effort while maintaining sufficiently good spatial auditory perception in an AR scenario. Specifically, methods for BRIR synthesis were presented that create new listening positions in a room based on very few measurements. The evaluations of these methods show that the spatial audio quality remains comparable to reference measurements and allows a high plausibility of the generated listening environment for a moving listener. Furthermore, a method was presented that optimizes the spatial deployment of BRIRs in space based on spatial auditory perception. This makes it possible to significantly reduce the number of BRIRs required without causing disruptive effects on auditory perception. While the techniques described in this paper show significant progress towards the goal of a plausible synthesis of auditory events in an augmented reality, another important goal of such developments is not yet reached: \"What is necessary to enable AAR systems, e.g., for consumer devices in a way that the illusion is perfect for every user?\" While the authors are not aware of any technical systems which fulfill this goal, there are known limitations in our work, too. The methods presented here interfere strongly with the structure of the BRIRs. Nevertheless, the evaluation shows that the perceived spatial audio quality is only moderately or partly not affected. It seems that essential characteristics and features are preserved even in the modified BRIRs. Further research in this area to determine relevant acoustic features and feature combinations which are used to build up a cognitive auditory model of our environment seems essential. Our proposed algorithms can never extrapolate the first reflections correctly, but the negative impact seems to be weak in our listening tests. This observation questions the importance of first reflections in perception of the room. First reflections can have an influence on the perceived direction, timbre perception and coloration, apparent source width, and others [47]. A study by [PERSON] et al. claims that rendering the first six reflections is sufficient to minimize the overall difference compared to an auralization containing all reflections [48]. However, these study data are based on image source models and shoe-box-shaped rooms which may differ from the acoustics in real rooms. The perceptual metrics in our listening tests suggest that the effect of acoustically authentic reproduction of the first reflection on externalization is small. This could be an explanation why our approaches are good regarding this quality feature. However, it remains open to what extent these results can be generalized. However, it is a strong conjecture that  
Figure 17: Perceived overall difference between the synthesized BRIRs and the measured BRIRs for two synthesis methods, without (1M) and with (D-1M) inclusion of the SSD, and for the synthesized positions “A2”, “C5”, and “D2” for an incidence angle of 30\({}^{\circ}\). A value of “0” indicates no perceptual difference and “5” a very high difference.—***\(p\leq 0.001\), *\(p\leq 0.05\) ([PERSON] signed rank test). essential patterns of the auditory space are preserved in the adapted BRIRs to ensure a high match with the internal representation of the room in the auditory system (cognitive model of the room). The endeavor to answer these questions are of interest for our further research to cover also larger areas to be synthesized, rooms with complex geometries, and for all types of audio signals. Observations may also differ with the tasks set to the listener and the complexity of the listening scenarios. The presented work still leaves other open questions:  
* What is the influence of room modes regarding the measurement positions (avoided in the current measurements)?