## 3 Experiment Two: Verification of the Localisation Thresholds
### Results  
Levene and Shapiro-Wilk tests were first conducted, using the SPSS Statistics 22 software (IBM, New York, NY, USA), in order to determine the suitability of the collected data for parametric statistical analysis. The Shapiro-Wilk test showed that not all scores in each condition featured normal distribution, although the results of the Levene test showed homogeneity of variance for all sound sources. For these reasons, non-parametric tests were chosen for the statistical analysis. Figure 9 shows the median perceived elevation of each of the test stimuli, plotted with notch edges. Consideration of the data reveals the following. Firstly, the localisation thresholds derived in Experiment One resulted in perceived elevation judgments similar to those for the same source presented from the main layer only. This was the case for all sources, with the median difference in perceived elevation between the main layer only and localisation threshold conditions ranging between \(-4.0^{\circ}\) and \(-0.8^{\circ}\) for the 0-ms ICTD and between \(3.9^{\circ}\) and \(0.0^{\circ}\) for the 1-ms ICTD. In addition to this, the notch edges for all the localisation threshold conditions overlap with those for main layer-only presentation. It is also interesting to note that, for the 0-ms ICTD, the median perceived elevation for the stimuli with the localisation threshold applied was slightly lower than the main layer only condition for all sources. In order to further determine whether or not the localisation thresholds derived from Experiment One were successful in preventing vertical interchannel crosstalk from affecting the perceived location of the main channel signal, Wilcoxon tests were conducted. The results suggested that, generally, there were no significant differences between the elevation judgments for the localisation threshold and main layer-only conditions. However, the data did suggest that the difference was significant for the quartet at 0 ms (\(p\) = 0.041) and for both the guitar (\(p\) = 0.002) and speech (\(p\) = 0.025) at 1 ms. Despite this, there is a clear overlap between the notch edges for each of these stimuli. In addition, the Pearson's correlation coefficient did not show a large effect in any case (\(r\) = 0.25 for quartet at 0 ms, \(r\) = 0.39 for guitar at 1 ms, \(r\) = 0.28 for speech at 1 ms). It can therefore be suggested that the difference in median perceived elevation between the localisation threshold and main layer only conditions was not significant. Consequently, the localisation thresholds derived in the present study are appropriate in preventing vertical interchannel crosstalk from affecting the perceived location of the main channel signal. Figure 9: Medians and associated notch edges for the results of the verification test showing the perceived elevation of each of the test stimuli. The dotted lines at 0 and \(30^{\circ}\) represent the physical positions of the main and height layers respectively. ICLD: interchannel level difference. With respect to the 0-dB ICLD conditions, a series of interesting results can be seen. Firstly, for the oboe it is clear that perceived elevation was not significantly affected by changes in how the stimulus was presented to subjects. In all cases, the perceived elevation was similar, which would indicate that this source was less affected by the migration of the main channel signal from the main layer as a result of vertical interchannel crosstalk. This result might suggest that the application of localisation thresholds would not always be necessary. Furthermore, for the other stimuli it is clear that the median perceived elevation was greater for the 0-dB ICLD condition compared to the main layer only condition. For 0 ms, the difference in median perceived elevation ranged from 5.5\({}^{\circ}\) to 7.2\({}^{\circ}\), whilst for 1 ms the difference ranged from 7.9\({}^{\circ}\) to 11.4\({}^{\circ}\).