## 1 Introduction
[9] was generally similar, although for that study phantom images were formed between diagonally-arranged loudspeakers (e.g., the left loudspeaker in the main layer and right loudspeaker in the height layer). In addition, their test stimuli also included male speech alongside the cello and bongo sources. The results of both studies revealed the following with respect to localisation thresholds. Firstly, for delays in the range of 0-10 ms, they were not significantly affected by interchannel time difference (ICTD). In addition to this, the effect of sound source was not significant. The thresholds reported by each study then, for ICTDs up to 10 ms, were in the range of \(-6\) to \(-7\) dB, which shows good agreement with the amplitude panning experiments of [PERSON] [5] and [PERSON] et al. [6]. From a practical standpoint, the results presented in [8] and [9] can be used to influence techniques both for the rendering of 3D images and for the design of microphone configurations for recording in 3D audio formats. In either case, the results are informative as to the maximum levels of direct sound that can be present in the height layer without the perceived location of the main channel signal being affected. For example, with respect to microphone techniques, it can be seen that the direct sound in the height layer must be attenuated by a minimum of 6 dB when the spacing between the main and height layers of microphones is less than 3.4 m (corresponding to an ICTD of 10 ms). This can be achieved through the use of cardioid microphones in the height layer. In the case that a vertically-coincident configuration is used, angling the height microphones at least 90\({}^{\circ}\) away from the sound source should provide the necessary attenuation, as was suggested in [8]. The microphones in the main layer should be positioned on axis with respect to the sound source. In a more recent study conducted by the authors [10], it was demonstrated that localisation thresholds have a frequency dependency. Octave bands of pink noise, with centre frequencies ranging from 125 Hz to 8 kHz, as well as broadband pink noise, were presented to subjects from vertically-arranged stereophonic loudspeakers in an anechoic chamber. Delays ranging from 0 to 10 ms were applied to the height layer with respect to the main. The results of the study showed that the localisation thresholds were not significantly affected by ICTD, which agreed with the results reported in [8; 9]. The thresholds for the 125 and 250 Hz bands were in the range of \(-3\) to \(-5\) dB, which was significantly higher than the \(-9\) to \(-11\) dB thresholds found for the 1, 2 and 8 kHz bands. In addition,the threshold for the broadband source was the lowest of all stimuli tested, being \(-11.5\) dB. These results seem to provide an implication for the analysis of localisation thresholds for natural sound sources with different spectral balances; it might be suggested that the threshold for a high frequency dominant source would be lower than that for a low frequency dominant source. As mentioned above, previous studies [8; 9] reported that the localisation threshold was not source dependent. However, since the sources used in those studies were somewhat limited (cello and bongo in [8], cello, bongo and speech in [9]), a wider range of sources would need to be tested in order to confirm the source dependency of the localisation threshold. Of further interest in the present study is how localisation thresholds are affected by the way in which the test stimuli are presented to subjects (the presentation method). In two recent localisation experiments conducted by the authors [11; 12], continuous broadband pink noise was presented to subjects from loudspeakers arranged in two layers. The loudspeakers positioned on the main layer were not elevated with respect to the listening position, whilst those in the height layer were elevated by 30\({}^{\circ}\). In the first experiment [11], each layer consisted of a single loudspeaker positioned with 0\({}^{\circ}\) azimuth.