## 5 Discussion
### The Role of Spectral Details in Direct Parts on Perceived Externalization  
The results from the study in [17] revealed that the magnitude spectra in direct parts of BRIRs contributed to sound externalization in reverberant environments, while the spectral details of the reverberant parts had little impact on externalization. It can be assumed that the spectral details in the reverberation of either the left or the right ear BRIRs could not noticeably affect perceived externalization. Therefore, our study focused on the role of the magnitude spectra of direct parts in contralateral and ipsilateral ear signals on perceived externalization. The experimental results of the \"SN\" and \"SB\" conditions confirmed the conclusions reported in [17], i.e., the magnitude spectra in the direct parts of BRIRs were important to perceived externalization, the degree of externalization decreased significantly by smoothing the magnitude spectra in BRIRs of both ears. It can be observed that the degree of externalization decreased significantly by smoothing the magnitudes spectra of direct parts in either left or right ear signals. This means that an externalized sound source required complete spectral details in direct parts of both ear signals; maintaining the correct magnitude spectra in direct sounds only of the left or the right ear was not sufficient to externalize a sound image well. As a source moved from frontal to lateral incidence angles, the relative influence of spectral details in direct parts of contralateral versus ipsilateral ear signals on perceived externalization could be observed. Again, the contralateral ear signals denote the left and right ear signals for the positive and negative azimuth angles, respectively. In contrast, the ipsilateral ear signals are the left and right ear signals for the negative and positive azimuth angles, respectively. For azimuth angles of \(0^{\circ}\) and \(\pm 30^{\circ}\), the spectral details in direct parts of left and right ear signals had almost the same influence on externalization. The contribution of the spectral details in direct parts of ipsilateral ear signals on perceived externalization increased as the source moved laterally. In the case of large azimuth angles of \(\pm 60^{\circ}\) and \(\pm 90^{\circ}\), the externalization ratings were significantly higher for smoothed magnitude spectra in direct parts of the contralateral ear signals than those of the ipsilateral ear signals. During this experiment, all listeners reported noticeable localization biases of sound images for the \"SL\" and \"SB\" conditions at \(-30^{\circ}\) and \(-60^{\circ}\) and for the \"SR\" and \"SB\" conditions at \(30^{\circ}\) and \(60^{\circ}\), i.e., the perceived sound sources were located in ipsilateral sides of heads, very close or immediately adjacent to their ipsilateral ears. It appeared that a strong smoothing of the magnitude spectra in direct parts of ipsilateral ear signals led to the problem of source localization, which was consistent with the results from the studies in [37; 38]. Since our study focused on perceived externalization, the listeners were asked only to evaluate the degree of externalization for the stimuli presented. They found that the sound source was diffuse and very close to their heads under the conditions mentioned above. None of them reported that the sound was well externalized and perceived far away from their heads. [PERSON] [37], [PERSON] and [PERSON] [38] illustrated that the contribution of the ipsilateral ear on the sound source localization increased as the source moved laterally. In contrast, the contribution of the contralateral ear decreased by increasing the source angle. For the azimuth angles larger than \(60^{\circ}\), the contralateral ear did not have contributions to determine the source localization. This result revealed that the spectral information in the ipsilateral ear signal was more important to the sound localization than that in the contralateral ear signal.