## 2 Experiment One: Localisation Thresholds for Natural Sound Sources
### Materials and Methods
#### 2.1.2 Test Stimuli  
The test stimuli used for the experiment were anechoically-recorded guitar, speech, conga, quartet and oboe excerpts (Figure 2). These stimuli were chosen primarily due to their variations in spectral content. The predominant energy of the oboe source, for example, was in the range of 600 Hz-2 kHz, with notable peaks around 700 Hz and 1.5 kHz, whilst that for the conga ranged from 150 to 500 Hz. Given that in [10] it was reported that the localisation thresholds for low frequency octave bands were significantly higher compared to those for the mid-high frequency bands, it was thought that these two sources in particular would be beneficial in analysing the source dependency of localisation thresholds for natural sound sources. The speech source was chosen due to its broadband nature. It was reasoned that if a source dependency could be identified based on frequency then the inclusion of a wideband source would potentially make it possible to identify the frequency region that is more dominant when determining the localisation threshold for broadband sources. The guitar and quartet sources were chosen due to their varying balance between low and high frequency content, which was greater for the quartet compared to the guitar (although both were more dominant in the region below 1 kHz). This again was due to the aim of analysing whether or not the frequency dependency of localisation thresholds could translate to complex sound sources. It should also be noted that the sources contained a varied blend of continuous and transient characteristics, as can be seen in Figure 2, although it has not yet been reported in the literature how this would affect the localisation threshold.  
Figure 1: The physical setup used for Experiment One. C: centre loudspeaker; L: left loudspeaker; R: right loudspeaker; HC: height centre loudspeaker; HL: height left loudspeaker; HR: height right loudspeaker.  
The test stimuli were presented to subjects as vertically oriented phantom images using the following two conditions (Figure 3):  
1. Vertical stereophonic: stimulus presentation from the C (main layer) and HC (height layer) loudspeakers.
2. Vertical quadraphonic: stimulus presentation from the L, R (main layer), HL and HR (height layer) loudspeakers.  
For each condition, the resultant phantom image was formed directly in front of the subject (i.e., on the median plane). The height layer was delayed with respect to the main layer for both conditions by 0, 1 and 10 ms. The delay times were chosen to emulate different spacings between the main and height microphone layers in the context of concert hall recording, with 10 ms being a likely maximum spacing (3.4 m path difference between the direct sound arriving at the main and height layers, respectively). In total, there were 30 stimuli (five sources, three delay times and two presentation methods). The amplitude of each stimulus at the listening position when presented from the main layer only (either C or L and R) was 70 dB LAeq. The amplitude of the stimulus when presented as a phantom image was dependent on the amplitude of the height layer relative to the main, which was to be varied by the subject as described in Section 2.1.4.  
Figure 3: Presentation methods for test stimuli.  
Figure 2: Long-term average spectra and waveforms of test stimuli used for Experiment One.