## 4 Discussion
### Sound Source Dependency  
One of the key aims of the present study was to determine whether or not there existed a sound source dependency of localisation thresholds. The results of Experiment One showed that, although the median localisation threshold varied for different sound sources, these differences were not significant, which agrees with the results reported in [8; 9]. Additionally, in Experiment Two the non-significant effect of sound source on the localisation threshold was demonstrated further. In this regard, the results showed that, when the same threshold was applied to each source, the resultant phantom image position was not significantly different from that for main layer only presentation. In order to explain the reasons for these results, considerations were given to three different aspects of vertical localisation: (1) the spectral energy distribution of the ear input signal; (2) the so-called 'pitch-height' effect [29]; and (3) the effect of vertical image spread (VIS) on vertical localisation. Since the primary cue for elevation perception is known to be the spectral filtering of the pinnae in the 4-10 kHz range [30], it was first considered how the ear input spectra are affected by the presence of the height layer. In Figure 10, the difference in spectral energy between main and height layer only presentation has been shown for both presentation methods as delta spectra. The head-related impulse responses (HRIRs) used for this exercise were taken from the Massachusetts Institute of Technology (MIT)'s HRIR database created using the KEMAR dummy head microphone [31]. Any points where the line falls below 0 dB represents dominance of the main layer over the height layer and vice versa. From both delta spectra, it can be seen that the predominant difference in energy between the two layers is a peak (height layer dominant) in the range of 7-9 kHz, a region that is associated with localisation above the subject [30; 32]. It is therefore reasonable to suggest that the increased energy in this region is a key reason that stimuli presented from the height layer only were perceived as being more elevated than those presented from the main layer only in Experiment Two. Since the resultant spectrum for phantom image presentation will depend on the relative strengths of each layer, the following can be suggested. When the ICLD is small, the contribution of each layer to the resultant spectrum will be similar. As a consequence of this, a given source presented as a vertically oriented phantom image will feature more energy in the 7-9 kHz region compared to the same source presented from the main layer only. This will manifest in differences in perceived elevation between the two conditions, as was demonstrated in [33]. However, as the ICLD increases, the main layer becomes more dominant in determining the resultant ear input spectrum, which means that the differences between the phantom image and main layer only conditions in the 7-9 kHz region would decrease. At the localisation threshold then, this difference is not sufficient enough to be interpreted as an elevation difference and therefore the main layer only and phantom image conditions are perceived as being in the same location. Based on this hypothesis, it can be argued that the primary mechanism used for the subject's localisation threshold judgment might be the relative spectral energy weighting between the main and height layers in head-related transfer function (HRTF), rather than being related to the fine spectral details of the sound source, which agrees with a previous study conducted by the authors [10]. It should be noted, however, that the ear input spectra are dependent on the subject and that it is therefore difficult to generalise HRTF characteristics. Based on this, it is apparent that further study is necessary, using measured HRTFs of different subjects, before the importance of the 7-9 kHz region in particular in determining the localisation threshold can be confirmed.