## 4 Discussion
### The Effect of Presentation Method  
A further aim of the present study was to analyse how the localisation thresholds would be affected by changing the presentation method from vertical stereophonic to vertical quadraphonic, with the results of Experiment One showing that the effect was not significant. In Section 4.1 it was discussed how a key mechanism in determining whether or not the localisation threshold had been met might be the balance of spectral cues provided by the main and height layers respectively. The results of Experiment One would seemingly indicate that this balance was not affected when source presentation changed from vertical stereophonic to vertical quadraphonic. This is apparent when Figure 10 is considered, which shows that the difference in spectral energy between the main and height layer-only conditions were somewhat similar for both presentation methods. In order to gain further objective insights into this result, the influence of the height layer on the frequency spectrum of the ear-input signal was analysed for each presentation method. For this, the spectral magnitude of the ear-input signal resulting from the main layer only was subtracted from that from both the main and height layers, using the MIT's KEMAR HRIR database [31]. Figure 11 plots the analysis results obtained when the ICLD was 0 dB (no height layer level reduction applied) and when the localisation threshold (\(-9.5\) dB) was applied. From the plots, the following can be observed. Firstly, the spectral energy in the 7-9 kHz range for the 0-dB ICLD condition was dominant over that for the main layer-only condition, for both presentation methods, in a manner similar to that hypothesised in Section 4.1. In addition to this, when the localisation threshold was applied for each method, the difference in energy in this region was decreased between 8 and 10 dB, whereas that below this region was about 5 dB or less. This therefore supports the hypothesis that decreases in spectral energy in the 7-9 kHz region will result in the localisation threshold being met, with similar reductions for both presentation methods for a given ICLD likely being the reason for the non-significant effect of presentation method. There is also good agreement with the results of a previous study conducted by the present authors [10], which showed that the main layer only and two-layer conditions did not have to have equal energy in the frequency range in which the spectral cues for elevation lie for the localisation threshold to be met. The non-significant effect of presentation method is interesting when the results of previous localisation studies are considered. As shown in [11; 28; 29], for a single loudspeaker placed in front of the listener in the median plane, the perceived image of broadband noise tends to be localised accurately at the physical position of the loudspeaker. Conversely, the phantom centre image of the noise produced from stereophonic loudspeakers at the ear height (i.e., the main layer of the quadraphonic condition in the current study) would be elevated with respect to the physical position of the loudspeaker as reported in [12; 13; 14] (this was also observed for natural sound sources in Experiment Two of the present study). Furthermore, a similar degree of difference between real and phantom image conditions in perceived elevation would be observed also for elevated loudspeakers (i.e., the height layers of the current study), based on data presented in [11; 12]. From the above, it can be inferred that, for 0-dB ICLD, sound sources presented using the vertical quadraphonic condition would be elevated with respect to those presented using the vertical stereophonic condition, with the difference in perceived elevation being similar to that for the same sources presented from the main layer only.