## 5 Conclusions  
The present study carried out an analysis of localisation thresholds for natural sound sources. The study was divided into two experiments. In the first (Experiment One) the effects of sound source, ICTD and presentation method were examined. Anechoically recorded conga, quartet, speech, guitar and oboe sources were presented to subjects in a natural listening environment using two conditions: vertical stereophonic and vertical quadraphonic. For each condition, the loudspeakers were divided into two layers, being 'height' (\(30^{\circ}\) elevation) and'main' (\(0^{\circ}\) elevation). Delays ranging from \(0\) to \(10\) mswere applied to the height layer with respect to the main. Subjects sat a listening test in which the minimum amount of attenuation necessary in the height layer for the resultant phantom image to match the position of the same source presented from the main layer alone was considered. The results of the experiment showed that the localisation thresholds were affected only by ICTD. For delays of 0 ms the threshold was \(-9.5\) dB, which was significantly lower than the \(-7\) dB found for 1 and 10 ms. That less ICLD was necessary in the presence of a delay was initially interpreted based on the existence of a localisation dominance effect. In addition, attempts to explain the non-significant effect of sound source were made based on the hypothesis that the primary mechanism to determine whether or not the localisation threshold had been met was the balance of spectral energy provided by the main and height layer, particularly in the 7-9 kHz range, which is not related to the spectrum of the source itself. This hypothesis also explained the non-significant effect of presentation method, with it being demonstrated that the reduction in the difference in energy between the main layer only and phantom image conditions in the 7-9 kHz region was similar for both methods for a given ICLD. In Experiment Two, the localisation thresholds obtained in Experiment One were applied to natural sound sources, with localisation tests being conducted in order to verify that they were effective at preventing vertical interchannel crosstalk from affecting the perceived location of the main channel signal. Stimuli were presented using the vertical quadraphonic condition, with the main and height layer-only, 0-dB ICLD and localisation threshold conditions all being tested. ICTDs of 0 and 1 ms were applied to the height layer with respect to the main. Subjects used a light strip, which was controlled by a handheld knob, in order to identify the perceived location of each stimulus. For all stimuli, there was no significant difference in perceived elevation between the main layer only and localisation threshold conditions. A key result from Experiment Two was that no evidence was found to support the existence of a localisation dominance effect, with the perceived elevation of the sources with 1-ms ICTD being higher than those with 0-ms ICTD. It is therefore unclear why less level reduction was necessary in Experiment One in the case that an ICTD was present. The results also showed evidence of the phantom image elevation effect, which was used to suggest that the perceived difference in elevation between the main layer only and phantom image conditions would be similar for both presentation methods for a given ICLD. This therefore further explained why the effect of presentation method was not significant in Experiment One. In addition, the results implied that the oboe source was localised based on the pitch-height effect. This meant that the hypothesis regarding the balance of spectral cues provided by the main and height layers did not adequately explain the localisation thresholds obtained for this source. As a result of this, it was suggested that the results might be explained based on differences in perceived VIS between the main layer-only and phantom image conditions. The practical implications of the results obtained in the study were also discussed.