## 5 General Discussion  
This section provides some general discussion which applies to both parts of this study. With respect to the correct perception of the target elevation sign, the performance for virtual sources was poorer than for real sources. On average, virtual target sources both inside and outside the horizontal plane were often perceived as higher than they were. The tendency to perceive virtual sources with a positive bias was reported in some previous studies using non-individual HRTFs [28; 29; 30]. The missing individual features in BRIRs could not have been the only reason for the elevation bias in the present study, since otherwise this phenomenon should not have occurred for horizontal virtual sources generated with the VAHs that included horizontal directions in the calculation of the spectral \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**BRIRs** & **V11** & **V21** & **V23** & **HTK** & **V11\({}_{\text{s}}\)** & **V21\({}_{\text{s}}\)** & **V23\({}_{\text{s}}\)** & **HTK\({}_{\text{s}}\)** \\ \hline Reversal rate & 0.47\% & 1.90\% & 1.42\% & 0.47\% & 15.24\% & 23.33\% & 30\% & 20\% \\ \hline \end{tabular}
\end{table} 
Table 3: Reversal rate when localizing virtual sources generated with V11, V21, V23 and HTK in TestDynamic and with V11\({}_{\text{s}}\), V21\({}_{\text{s}}\), V23\({}_{\text{s}}\) and HTK\({}_{\text{s}}\) in TestStatic. weights (V11 and V21), because the monaural spectral and temporal features of individual horizontal BRIRs could be preserved well with these syntheses. However, the elevation bias observed in the present study was an average tendency and did not apply to all subjects. One subject, for instance, perceived over 60% of all virtual target elevations as negative, whereas the two other subjects mostly gave elevation responses zero, independently of target's elevation or the BRIR. The pattern of elevation perception was not consistent among all subjects. The clear trend is that the elevation localization was more difficult with virtual sources than with real sources. One can argue that when listening to speech signals, the main elevation cues related to the pinna reflections (above about 5 kHz [31; 32]) cannot be presented well. However, the elevation localization performance was quite impressive with the same speech signal when listening to real sources. It is not clear whether the poor elevation localization with virtual sources was solely related to the synthesis inaccuracy of VAH BRIRs and the non-individuality of KEMAR BRIRs, nor whether also factors related to the rendering process played a role. The localization experiment should be repeated with individually measured BRIRs in order to provide more details. Despite the generally unconvincing elevation perception with the VAH BRIRs, the target elevation sign could be better perceived with VAH2 with horizontal and non-horizontal directions included (V23 in TestVR) compared to other VAH BRIRs. This was, however, not the case for VAH1 with horizontal and non-horizontal directions included (V13 in TestVR). The three-dimensional topology of VAH2 was suspected to have contributed to the better elevation perception compared to VAH1. It seemed that the sound incidence from different elevations could be better recognized with microphones having different positions along the \(z\)-axis than with microphones distributed at the same height. The additional seven microphones of VAH2 compared to VAH1 could also have been advantageous. Further investigations with VAH1 and VAH2 or other array topologies are required to confirm these statements. An important point to be mentioned is that the results achieved in both parts of this study were obtained using speech signals. In general, the synthesis accuracy of the VAHs decreases towards higher frequencies [11; 13]. In addition, in case of measured BRIRs of a conventional artificial head, the non-individuality gets more prominent at higher frequencies.