## 4 Listening Test: HRTF Preference  
A listening test was conducted to investigate the existence of quantifiable timbral and/or spatial attributes within individual and non-individual HRTF measurements. Sixteen participants (13 male, 1 female, 2 non-binary, ages: 20-63 (majority 20-30)) all of whom were admitted to the SADIE II database were re-recruited for the test. All subjects gave their informed consent for inclusion before they participated in the study. The protocol was approved by the University of York Physical Sciences Ethics Committee. Participants were presented with a set of auditory stimuli over headphones and were asked to rate each one based on four attributes as defined in Section 2: brightness, richness, externalisation and preference. The terms brightness and richness were described to each participant along with example audio files containing filtered exerts of music. Three examples were filtered according to frequency response plots shown in Figure 8.  
A high boost simulated a bright signal, a high cut simulated a dark signal and a low and high cut simulated a thin signal. One further example was left unfiltered to simulate a rich signal. Figure 8: Frequency responses of the filters used to generate stereo anchor stimuli. (**a**) bright; (**b**) dark; (**c**) thin. The term externalisation is not so easy to conceptualise. To verify the effectiveness of any example file would have required the verification of the spatial filters used to create such a file. This was in part the purpose of this study. Participants were instead provided with a graphical depiction of the soundfield and were advised that effective externalisation would be as if they were hearing the sources in real life. Preference required not only a sense on externalisation, but also a pleasant timbre and overall feeling of realism. Participants were given the opportunity to ask any questions relating to the definitions of each term. During the test, participants were able to freely switch between the set of stimuli over a continuous looped playback. Ratings were performed using the graphical interface shown in Figure 9.  
Participants were required to drag a marker corresponding to a stimuli to a point on a graph. Two graphs were used to represent the four attributes on continuous scales. Brightness and richness were represented by the \(x\)- and \(y\)-axes of one graph and preference and externalisation the axes of the other. The interface allowed participants to easily compare and adjust the ratings they were giving to each stimuli. They were instructed to make use of the entire range. Noise bursts and other broadband signals are common stimuli used throughout listening tests; however, such unfamiliar and unnatural audio is inappropriate for this type of study. A common alternative is to use speech [13; 15; 62]. Whilst this is a more ecological than noise, it is relatively band limited and lacks low frequencies especially. The stimuli used should represent examples of everyday bianural audio and as such should elicit the same or at least similar perceptual characteristics [36]. For example, whilst it would be quite unusual to discuss the brightness of radio static, a similar discussion about the sound of a piano would be relatively common. With this in mind, approximately a minute and a half of music was composed in a jazz style using a range of non-reverberant VST MIDI samplers. These included a stereo drum set, stereo piano, flute, trumpet, trombone and double bass for a well balanced mix covering a large range of frequencies. The ensemble was binaurally spatialised by convolving individual audio stems with HRTFs spaced at 45\({}^{\circ}\) increments around the horizontal plane, starting at 0\({}^{\circ}\), and summing the results. Stereo sources were convolved with adjacent HRTFs to mimic phantom source phenomena in real world listening [63]. 20 binaural signals were produced using each of the Twenty HRTF measurement sets admitted to the SADIE II binaural database, discussed in Section 3.1.