## 3 Perceptual Evaluation
### Spatial Sub-Sampling  
The main aim of the evaluation was to investigate the occurrence and perception of unanticipated events while walking. By this we assumed to find errors caused by different spatial resolutions of the provided grids. One sound source position outside the walkable area was synthesized playing a music or speech audio signal. The creation of the BRIRs for each grid cell was based on the ITDG-shaping approach described in Section 3.2.3. A BRIR data set from the mid position of the walkable area with a yaw resolution of 5 degrees was applied. A KEMAR dummy head was used for the measurement. The perceptual evaluation was performed for different quality features and for four different grids with different spatial resolutions as shown previously in Figure 4. In addition to single quality features, the evaluation also included the rating of the overall impression. The individual features included localization, externalization, and timbre perception. The ratings of 21 test persons with a mean age of 29.1 years (standard deviation 8.2) have been used for the evaluation. Fifteen of 22 participants (five women and 17 men) were experienced in listening tests and ten persons had special experience with binaural synthesis systems.  
The task for the test persons was to walk twice along a given path through the accessible area. The mean walking speed was at 0.4 m/s, with a length of the whole path of 48 meters. After passing the path, the test persons had to evaluate the quality features externalization, ability to localize the auditory event (LA), the stability of the position of the reproduced audio object (LS), the coloration during movement (CO), and the overall impression (OI). Quasi-continuous rating scales have been used for OL, LA, and LS with the extremes \"poor\" as minimum and \"very good\" as maximum. For CO the endpoints are \"strong coloration\" and \"no coloration\". A discrete three-point scale with \"in-head\", \"outside but close to the head\", and \"outside the head\" was used for externalization.  
Figure 10 shows the ratings for the externalization as index. The index is the ratio of the number of ratings for \"outside the head\" and the overall number of ratings. When using the grid with the highest resolution (Grid 1), high externalization indices close to 1 are visible. The indices decrease continuously but slightly to a value of around 0.75 when less high-resolution grids are used. Figure 11 shows the ratings for the individual quality features and for the overall impression on the music signal. The best ratings can be found  
for Grid 1. There is a tendency for lower valuations to be given for the less resolved grids. The interquartile distances tend to rise slightly for Grids 3 and 4. This indicates that the test persons are less in agreement if grids with lower resolutions are used. Only the results for the music signal are shown here, as this was evaluated as more critical in comparison to the speech signal. The ratings for the speech signal show a very similar trend, with slightly better ratings across all quality features.  
Overall, it must be noted that relatively high quality ratings have been given for both the overall impression and the individual quality features with respect to the reduced spatial resolution of the grids. However, it must be made clear that perceived quality also depends on the type of application and test paradigm. For example, a test scenario with a direct comparison of real and virtual sources with the intention of testing for authenticity would most likely lead to a much more critical evaluation [1].