## 3 Part I: Localization of Real and Virtual Sources in the Absence of Visual Cues
### Discussion  
According to the results, average azimuth errors and externalization and reversal rates were similar among the real sources and virtual sources generated with V11 and V21. In the two previous studies which reported a good perceptual performance for the VAH [12; 13], not only could subjects see the sound sources, but the experiment task was also different. In those studies, subjective ratings were given for the matches between positions of real and virtual sources rather than giving direct judgments about the perceived azimuth angles and elevation. Nevertheless, the results in the present study showed that in the absence of visual cues and for a more challenging localization task, it is possible to generate virtual sources with the VAH with comparable externalization and azimuth localization performances as real sound sources. In line with the previous study [13], VAH BRIRs synthesized with spectral weights with only horizontal directions included (V11 and V21) performed better than the syntheses with horizontal and non-horizontal directions included (V13 and V23). As already discussed in Section 2.2, the inclusion of horizontal and non-horizontal directions introduced increased spectral and temporal distortions, which impacted the localization performance negatively. The degradation in binaural cues, due to temporal and spectral distortions, led to a lack of externalization with V13 and V23. In addition, regarding the important dynamic cues provided by low frequency interaural time differences (ITDs) when listening to speech signals [22; 23], the deteriorated low frequency ITDs could also explain the higher reversal rates of V13 and V23, despite a dynamic presentation with head tracking.  
According to the results, azimuth accuracy, externalization and reversal rates were very convincing for virtual sources generated with BRIRs of the KEMAR artificial head (HTK), despite the non-individuality of these BRIRs. Listening to non-individual recordings can degrade the externalization and increase front-back reversals in static scenarios without head tracking [1; 2; 3]. In contrast, it has been shown that when head movements are enabled, externalization is improved and reversal rates can be reduced, regardless of whether the subject is listening to individual or non-individual binaural signals [3; 4; 24]. The fact that the non-individual signals of HTK could be presented dynamically was advantageous for improving externalization and reducing the reversals. In general, the results confirm previous studies of the authors [12; 13], in that for speech signals and with head tracking, individual binaural signals do not constitute a major advantage over non-individual signals of a conventional artificial head. However, the effort to acquire the non-individual BRIRs of the artificial head for different head orientations should not be neglected.  
Elevation perception with virtual sources was in general not very convincing in comparison to the results with real sources. When listening to virtual sources, subjects seemed to have difficulty dealing with the task of vertical localization. This will be discussed more thoroughly in Section 5.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Condition** & **Real Source** & **V11** & **V13** & **V21** & **V23** & **HTK** \\ \hline Reversal rate & 1.4\% & 1.4\% & 4.2\% & 0.47\% & 1.9\% & 0.95\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Reversal rate when localizing real sources in TestReal (Real Source) and virtual sources in TestVR (generated with V11, V13, V21, V23 and HTK).