## 3 Perceptual Evaluation
### BRIR Synthesis
#### 3.2.2 Acoustical Shaping  
A listening test has been conducted to evaluate the ITDG-based BRIR synthesis approach described in Section 3.2.3. The ability of the technical system to create a plausible auditory augmented reality is indicated by evaluating single quality features. Figure 12 shows the rating scales and names of the single features. The feature externalization describes the perception of an auditory event outside the head of a listener. This feature is assumed as critical if a spatial fusion of the virtual audio object and a real (visual) object in the room is aspired. Furthermore, this feature is strongly affected by the so-called room divergence effect which describes a decrease of externalization if the synthesized audio object does not fit to the expected auditory environment [22]. The localization ability is defined as the ability of the test participants to perceive a direction of the auditory event. The localization stability describes if the event is stable at a fixed position in the real room during the movement of the participant. An effect on stability is expected by the relatively low grid and yaw resolution. The individual overall impression can be rated as a further feature. A discrete three-point scale is used for externalization with the naming \"in-head\", \"outside but close to the head\", and \"outside the head\". An extended continuous scale from Jekosch and Bodden is used for the other features (scale details in [42]). This scale type takes into account the fact that test participants tend to avoid using extremes on rating scales. The scale therefore provides extended scale categories at the lower and upper ends. The test persons have to move on predefined paths through the auralized grid and rate each of the four paths (see Figure 13). The paths are selected to cover close/far distances to the virtual audio object, rapid/slow change of the relative angle between head and source position, front/back head orientations to the audio object, and most critical switching of the filter functions for the different grid positions. Figure 12: Rating scales for externalization, overall impression, localization ability, and stability. Scale design on the right from Jekosch-Bodden named in [42]. The designation of the upper scale with two quality features is only for this figure. In the evaluation, each feature was evaluated with its own scale. Figure 13: Auralized setup used for quality evaluation; **Left**: marked are the measurement points of BRIRs used for ITDG shaping and for the interpolation approach. **Right**: walkable paths (I to IV) used in the evaluation (data from the work in [43]). An empty office room with dimensions 4.8 m \(\times\) 4.7 m \(\times\) 3 m and a reverberation time of \(\sim\)0.5 s has been used for the recordings and as listening room for the test on system viability [43]. A HTC VIVE-based tracking using the HTC VIVE tracker [44] has been used for position and pose estimation. An extra-aural headphone has been used for playback [45]. The transfer characteristic of the positioned headphone was equalized using a KEMAR dummy head (same one as for the BRIRs measurement). Figure 13 shows the setup used in the quality evaluation of the BRIRs synthesis. The left part of the figure shows the measurement points of BRIRs used in the ITDG shaping. Furthermore, an additional approach was investigated, which creates new BRIRs from a weighted interpolation of three measuring points. In this approach, the BRIRs are shifted in their traveling time to the new position. The time range of the early reflections is separated from all three BRIRs and is interpolated in time domain. The weighting of the interpolation results from the distance of the new position to the measured positions. The direct sound component as well as the late reverberation for the new position is taken from the nearest measured BRIR. Finally, an energy adjustment of the direct sound component, the new interpolated early reflections, and the late reverberation takes place.