## 4 Discussion
### Practical Implications  
A primary aim of the present study was to obtain localisation thresholds that could be used to influence both the placement of microphones and the rendering of 3D images in the context of 3D audio. The purpose of this was to prevent vertical interchannel crosstalk from affecting the perceived location of the main channel signal. With respect to 3D microphone configurations, a series of techniques have already been proposed in [8]. In that study, it was suggested that, since the localisation threshold needs to be applied to the height-layer microphone signals in order for the source image to be located at the position of the main layer, directional microphones should be used for the height layer rather than omni-directional ones. The results of the present study support this suggestion. In addition, it was proposed that, in case of using microphones with an 'ideal' cardioid polar pattern (i.e., \(-6\) dB attenuation at \(90^{\circ}\)), the necessary ICLD could be achieved for both vertically coincident and spaced configurations by angling the height layer of microphones at least \(90^{\circ}\) away from the direct sound. However, the data reported in the present study indicates that a minimum angle of \(105^{\circ}\) would be necessary in the case that the main and height layers are spaced apart, whilst for a coincident configuration the angle should be \(115^{\circ}\). This would provide attenuation of direct sounds in the height layer at the localisation thresholds for \(0\) ms and \(1\) ms found in the present study: around \(-7\) and \(-9.5\) dB, respectively. The results of the current study are considered to be also useful for vertical image rendering in 3D sound mixing and upmixing applications. They indicate that direct sounds can be present in the height layer provided they are attenuated with respect to those in the main layer by either \(9.5\) dB (in the case of \(0\) ms ICTD) or \(7\) dB (in the case of \(1\)-\(10\) ms ICTD) without the perceived location of the main channel signal being affected. Such a technique could have potentially pleasing effects such as an increase in perceived VIS. However, it is currently not clear how the timbre of the main channel signal would be affected by such a technique and, further, if the end result would be pleasing. It can be seen from Figure 11, for example, that the resultant spectrum of the signal is different at the localisation threshold compared to main layer-only presentation, with a notable peak in the \(7\)-\(9\) kHz range. Alongside this, [PERSON] [40] suggested that secondary vertical sources would result in orchestral music sounding 'boxy', whilst [PERSON] and [PERSON] [41] indicated that timbral colouration as a result of vertical reflections are more audible than for lateral reflections. It would be necessary then to evaluate first of all what the perceptual differences are between the main layer only and vertical phantom image conditions with the localisation thresholds applied and further if the threshold conditions are considered as being preferable. Such a study would make it possible to determine whether the localisation threshold should be applied or, conversely, if the direct sound in the height layer should be either masked or absent entirely. This would provide further insights on both image rendering and microphone techniques in the context of 3D audio production. It should also be noted that there are some, limited, applications with respect to the vertical panning of sound sources. It is indicated by the results that, depending on the ICTD, the threshold value for a source to be fully panned to the main loudspeaker layer is in the range of \(7\)-\(9\) dB, which agrees with the vertical localisation studies of both [PERSON] [5] and [PERSON] et al. [6].