## 2 Experiment One: Localisation Thresholds for Natural Sound Sources
### Materials and Methods
#### 2.1.4 Test Method  
For each stimulus, subjects were presented with a 'test' and'reference' sound. The'reference' was the stimulus presented from the main layer only. The 'test' sound was the stimulus presented as a vertically-oriented phantom image with one of the three test ICTDs applied to the height layer. For each 'test' sound, subjects were required to reduce the amplitude of the height layer until they perceived the location of the resultant phantom image to be matching that of the'reference'. To ensure the localisation threshold was found in each case, they were asked to set the amplitude of the height layer to the highest possible point at which this condition was met. The threshold detection method used in the current study was based on the method of adjustment (MOA). This is an indirect scaling method that requires subjects to reduce the amplitude of a stimulus until it is equivalent to that of a reference [16]. [PERSON] [17] asserted that the principal application of MOA is in situations whereby stimuli differ from one another by more than one attribute. This was applicable to the present study, as, although subjects were tasked with identifying localisation shifts, there would inevitably be some timbral changes due to the use of ICTD. However, despite such benefits, the MOA is limited in that it presents subjects with a large range from which to find the threshold, making it difficult for answers to be precise [18]. This limitation was addressed for the present experiment by requiring subjects to complete a three-stage MOA for each stimulus. Each stage was designed to be a more refined version of the previous stage as follows:  
* Stage 1: The amplitude of the height layer could be adjusted from 0 to \(-25\) dB in 5-dB steps. The localisation threshold was therefore found to within 5 dB. * Stage 2: The amplitude of the height layer could be adjusted in the 5-dB range determined by the previous stage. The step size was 1 dB. * Stage 3: The amplitude of the height layer could be adjusted in the 1-dB range determined by the previous stage. The step size was 0.25 dB. This method can be considered as combining the standard MOA with adaptive testing. Adaptive threshold detection methods, which include Parameter Estimation by Sequential Testing (PEST) [19] and Up-Down [20], present stimuli to subjects at amplitudes determined by the history of the test run. This allows testing to be made at levels closer to the threshold, increasing efficiency [20]. It was however decided against using an adaptive method outright based on research conducted by [PERSON] [21], which indicated that the subsequent duration of the test is between three and five times longer compared to when MOA is used. As the method used for the present experiment used elements of both MOA and adaptive testing, it was named as an 'adaptive method of adjustment' (AMOA). It was considered that this fusion of methods would improve the accuracy of the test, whilst still making it relatively quick and easy for subjects to complete. The graphical user interface for the AMOA task was created using the Max7 software (Cycling74, Walnut, CA, USA). During the test, subjects were strictly instructed to face forwards, keeping their head still and using only their eyes to look at the test interface. The heads of subjects were not fixed, however head movements were monitored using a motion tracker device [22]. The tracker instructed subjects if their head position had deviated from an acceptable range of natural motion (10 mm in any direction). Additionally, a guide point for the ear height and distance was placed on the right hand side of the subject to help maintain the correct listening position throughout the test. Prior to the start of eachtest, all subjects sat a supervised practice, which utilised a speech source, in order to ensure that the instructions were understood. The test was completed in two sittings, each of which contained 15 stimuli and lasted around 20 min.