## 4 Discussion
### Sound Source Dependency
An interesting point of note with respect to the non-significant effect of sound source relates to the results of Experiment Two, which showed that localisation judgments for the oboe source were generally consistent regardless of how the source was presented to subjects. A potential explanation as to why this result was obtained is offered thus. According to the literature, narrowband stimuli incident from the median plane are localised on the basis of frequency, with increases in frequency corresponding to increases in perceived elevation [11; 30; 34]. This phenomenon is known as the 'pitch-height effect' [29]. As can be seen from Figure 2, the spectrum for the oboe source was notably  
Figure 10: The differences in spectral energy between the main and height layer only conditions for both vertical stereophonic and vertical quadraphonic source presentation. narrow, with a bandwidth ranging from around 500 Hz to 4 kHz and with its predominant energy focused around 1 to 2 kHz. According to the literature, band-limited stimuli in this frequency range are localised at a similar vertical position regardless of which loudspeaker layer presented the source for both vertical stereophonic [11; 29] and vertical quadraphonic [12] loudspeaker arrangements. Therefore, it might be that localisation judgments for the obee were determined by the pitch-height effect rather than the HRTF-based vertical localisation mechanism discussed above. This would indicate that the latter mechanism is predominantly applicable to broadband sources, with less relevance to sources that are both narrowband and absent in high frequency energy. That ICLD was always necessary to reach the localisation threshold for the obee source, despite there being no significant difference in perceived elevation between the 0 dB and main layer only conditions, might be explained by an alternative hypothesis proposed by the authors [10]. When sound source presentation shifts from main layer only to vertical phantom image, a key difference is an increase in perceived VIS. Such an increase contributes to an increase in localisation blur [35], which inevitably makes the sound source more difficult to localise in vertical space. Therefore, given that the test conditions required a direct comparison between the positions of stimuli presented using the main layer only and vertical phantom image conditions, it is possible that differences in perceived VIS were perceived as elevation differences. Further, these differences would have decreased as the amplitude of the height layer was reduced. At the localisation threshold then, the difference in VIS is sufficiently small for stimuli presented using the two conditions to be perceived as being in the same location. As with the hypothesis regarding the importance of the spectral energy weighting in determining the localisation threshold, this hypothesis is able to explain why the effect of sound source was not significant in Experiment One. Simply, the difference in perceived VIS between main layer only and vertical phantom image presentation is primarily a function of ICLD and is not affected by the sound source itself. Based on these discussions, it is clear that the exact mechanisms that determine whether or not the localisation threshold has been met requires further study.