## 3 Perceptual Evaluation
### BRIR Synthesis
#### 3.2.1 Constant Reverberation  
The idea to keep the whole reverberation part of the BRIRs constant when the listener changes the position is a very simple approach which is very efficient with regard to the required calculation power and memory. The physics actually occurring for listeners walking through a sound field are roughly approximated. This is likely to cause audible drawbacks and inaccuracies. In contrast, it is also known that listeners are not sensitive to all the small physical details in such scenarios. Moreover, a perceptually indistinguishable replication of the real sound field is not required in all applications. For this reason, it is necessary to define the desired perceptual quality before designing a reproduction system with an adequate, efficient synthesis method. In addition, a suitable test method for a perceptual evaluation of the proposed approach for simplified binaural rendering is required. The minimum demand in terms of perceptual quality is the creation of a plausible auditory illusion. Plausibility refers to the agreement of the auditory impression with  
Figure 11: Ratings of quality features for the different grids and for the music signal as violin plots including box-plots; Ol... Overall Impression with 0 = poor and 100 = very good, LA... Localization Ability with 0 = poor and 100 = very good, LS... Localization Stability with 0 = poor and 100 = very good, CO... Coloration with 0 = strong coloration and 100 = no coloration. Figure 10: Externalization as index with 95% conf. interval; **left**: speech, **right**: music. an internal reference [39]. [PERSON] and [PERSON] [40] proposed a method to evaluate the plausibility of auditory illusion created with binaural synthesis by randomly providing the binaural version or its corresponding real version and asking the participants in a Yes/No test paradigm whether they are listening to the simulation or not. However, for practical reasons this method requires taking the occlusion effect of the headphones into account. This can affect the quality of the auditory image of the real sound source as well [41]. The occlusion effect of the headphones could also dilute the perceptual differences between different approaches to simplify a BRIR data set for a perceptually optimized reproduction. For this reason, a different method to evaluate plausibility was needed. One idea was to ask for it directly. Especially inexperienced subjects often indicate that they are not sure about their own criteria for plausibility. For this reason, further questions were added to evaluate the quality of the auditory illusion [17; 32]. Besides plausibility the participants had to rate externalization, continuity, sound source stability, and impression of walking towards (and away from) a sound source in a multi-attribute absolute category rating. Direct comparison between the different test cases was not possible within this experiment. In the experiment, [PERSON] et al. [17] realized an interactive approaching motion towards a virtual loudspeaker and away from it with a BRIR data set measured with a positional resolution of 25 cm. Additionally, systematic simplifications were applied to the originally measured BRIR data set. One of the approaches was to replace the reverberation parts of all BRIRs in the data set by the reverberation tail of BRIRs from one specific position on the line. Thus, the naturally occurring changes in the early reflection pattern between the different listening positions were removed in this reproduction. Another approach, further simplifying the synthesis, used the BRIRs from one position and kept their reverberation constant over the various listening positions. Only the energy of the direct sound was adjusted according to the energy progress of the direct sound in the measured data set. Consequently, the changes of the ITDG and of the early reflection patterns between the different listening positions were removed in this realization. Still, the listeners perceived this scene as plausible as the originally measured data set.