## 3 Perceptual Evaluation
### BRIR Synthesis
#### 3.2.2 Acoustical Shaping
This approach is mentioned here only briefly, and it is referred to in the literature [2; 43]. In the experiment, BRIR data sets have been provided for a quadratic uniform grid with an edge length of 0.5 m. A reference system was established for which BRIR measurements at all intersections of the grid have been performed. The yaw resolution was 5\({}^{\circ}\) for all systems under test. The right part of the figure shows the walkable paths for evaluation. Eighteen test persons with a mean age of 29 years (SD = 9) participated in the test. The ratings for externalization are shown as indices in Figure 14 for the different systems under test and investigated paths. The index is the ratio of the ratings for a perception of an auditory event outside the head (outer circle in Figure 12) and the overall number of ratings. No differences in terms of a significance level with \(p<0.05\) between the used BRIR synthesis systems or evaluation paths have been observed. However, there is a trend towards higher indices for path II and smaller indices for path IV. Path II includes the largest distance from listening positions to the source position as well as the presentation of the sound source from a lateral direction. Effects due to the relatively low spatial resolution of the grid are less significant due to the high distance. Furthermore, the perceived externalization of lateral sound sources is higher compared to frontal presentation. Path IV, on the other hand, includes close distances to the source and a predominantly direct frontal or rear source position. The overall impression is rated in the range of \"good\" (scale points > 4) for the systems and the paths (see Figure 15). Bigger differences are observed for paths I and III. These paths are most critical in terms of jumps in the perceived position of the auditory event caused by the low grid resolution at close distances to the audio object position. The synthesis approach using interpolation between BRIRs from three measurement positions was evaluated compared to the ITDG approach and the reference. The interpolation of the early reflections causes a change of the early reflection pattern compared to measured BRIRs. This is especially reflected in changes in the timbre of the early reflections. Other approaches which consider a maintenance of the reflections give reason to expect better quality ratings. Figure 14: Externalization indices for the systems and paths with 95% binomial confidence intervals; Ref. : reference, ITDG: initial time delay gap, Intp. : interpolation (data from the work in [43]). The stability of the perceived position of the auditory event is shown in Figure 16. Differences are visible between the paths and some systems. Paths I and III show lower grades compared to the other paths. These paths are assumed as most critical because of the close distance to the source and the low grid resolution. Especially at small distances and when moving diagonally through the grid, high errors in the perception of distance and angle changes are to be expected here. No statistical differences (\(p<0.05\)) between the synthesis systems or evaluation paths are observed. Only slightly bigger interquartile distances for the interpolation approach compared to the ITDG approach are visible. The overall good localization ability at a scale value of \"4\" is the result of the usage of the unchanged (except of an intensity adaptation) direct sound of the recorded BRIRs.