## 4 Part II: The Impact of Head Tracking on the Localization Performance
### Discussion  
In line with previous studies [3; 4; 5; 20; 25], the dynamic presentation in TestDynamic improved the localization performance of virtual sources with respect to azimuth, externalization and the reduction of front-back reversals compared to TestStatic. Horizontal head movements limited to the frontal hemisphere were also advantageous for the localization accuracy of virtual sources located to the rear.  
No effect of head movement on the vertical localization accuracy could be observed. Some previous studies reported a positive impact of horizontal head movements on vertical localization. Such an advantage, however, was only found for sources limited to the median plane or the left lateral plane [23; 26] or only for low-frequency noise signals [27]. In the current study, target sources were distributed all around the listener and a more broadband signal (speech) was used. Therefore, the results were more in line with localization studies using speech signals [4] or a wider range of source positions [25], which also reported no specific effect of head movements on the elevation error. The unconvincing performances in vertical localization in TestDynamic and TestStatic were similar to the performance in TestVR in Part I, and suggested that independently of the presentation method, subjects had difficulty dealing with the task of vertical localization.  
Apart from the generally poor elevation results, the localization performance of virtual sources generated with the non-individual BRIRs (HTK or HTK\({}_{\text{s}}\)) was comparable to the localization performance of virtual sources generated with the individually synthesized VAH BRIRs (V11 and V21 or V11\({}_{\text{s}}\) and V21\({}_{\text{s}}\)). When considering the results of TestDynamic and TestStatic separately, the individually synthesized VAH BRIRs offered no special advantage over the non-individual KEMAR BRIRs. It should, however, be kept in mind that the dynamic presentation of KEMAR BRIRs was artificially enabled in this study. In standard applications of conventional artificial heads, dynamic presentation of binaural recordings is not possible; in contrast, the VAH straightforwardly allows one to dynamically present recordings. Therefore, from a practical point of view, one should compare the localization performance of VAH BRIRs in TestDynamic with KEMAR BRIRs in TestStatic. Not only was the reversal rate higher for HTK\({}_{\text{s}}\) in TestStatic (20%) compared to VAH BRIRs in TestDynamic (below 2%), but also the average azimuth error for HTK\({}_{\text{s}}\) was significantly higher than for V11 (t(13) = 3.21, \(p\) = 0.006) and V21 (t(13) = 2.94, \(p\) = 0.014). Furthermore, according to the Wilcoxon signed-rank test, the average externalization rate for HTK\({}_{\text{s}}\) was significantly lower compared to V11 (\(p\) = 0.002) and V21 (\(p\) = 0.034). This analysis indicated that besides individualization, an important advantage of the VAH technology over conventional artificial heads is the dynamic presentation of recorded signals, a feature shown in this study to largely improve the localization performance with virtual sources.