## 3 SADIE II Binaural Database
### Head Related Transfer Functions  
#### 3.2.1 Measurement  
An acoustically treated HRTF measurement rig was designed for the anechoic chamber at the Audio Lab, University of York, York, UK (see Figure 2a [47]). The set-up consisted of three static, vertical semi-circular arcs, each separated by 45\({}^{\circ}\) azimuth. Participants were sat centrally on a motor-controlled'saddle stool', selected for its minimal acoustic occlusion. Their feet were tucked underneath their body, supported by a footrest. Their inter-aural axis was laser aligned to the precise centre of the loudspeaker array. Their head position was tracked in real time via a multi-purpose restraint, shown in Figure 2b. The restraint could be attached to a rigid back rest to help prevent unintentional head movement, as in Figure 2a. The restraint supported 10 reflective markers: six positioned asymmetricaly above the head and four positioned around the head. Four Optitrack Flex-3 Infra-Red motion capture cameras (www.optitrack.com/products/flex-3) tracked the 10 point rigid-body to within \(<\)0.1\({}^{\circ}\) via optical motion capture software, Motive (www.optitrack.com/products/motive) (Version 1.8.0 Final 64-bit). Utilizing this data, participants were rotated in place about the horizontal plane to a series of predetermined azimuthal positions by means of a Yaesu G-2800 DXC motor and GS-232B serial interface (Yaesu. Cypress, CA, USA.). Twenty-three Genelec 8010 loudspeakers (www.genelec.com/8010) were installed at 23 unique elevations at a radius of 1.2 m. In each case, the loudspeaker was aligned to its acoustic axis [48]. The 8010 was chosen for its small footprint and reliable frequency response (\(\pm\)2.5 dB) from 74-20 kHz. Twelve elevations were measured at 15\({}^{\circ}\) intervals between \(-\)75\({}^{\circ}\) and 90\({}^{\circ}\). These were necessary to measure the regular lattitude-longitude distribution [45]. Source localisation in the median plane is reported to be significantly worse than that in the horizontal plane [3]. A localisation blur of \(\pm\)9  
Figure 1: (a) a Knowles FG-23329-C05 microphone housed inside a 3D printed capsule (scale in cm) and (b) the position of the capsule inside a participant’s ear. is reported for continuous familiar speech [49] whilst \(\pm 17^{\circ}\) is reported for continuous unfamiliar speech [50]. Measurement intervals of \(15^{\circ}\) were considered to be of fine enough resolution to give an accurate representation of perceptual localisation cues without oversampling the subject unnecessarily. A further 10 elevations were determined according to the common elevations coordinates of 11 typical Ambsonic loudspeaker layouts. An additional three layouts are composed of the same approximate elevations \(\pm 2^{\circ}\). A summary of these layouts and corresponding elevation coordinates is given in Table 3.  
At a distance of 1.2 m, an error of \(2^{\circ}\) translates to a speaker displacement of 4.2 cm. This is small with respect to the size of the loudspeaker (18.1 cm) and main driver (12 cm). Considering such a  
Figure 2: (a) a subject being prepared for HRTF measurements. They are sat on a motor-controlled rotating ‘saddle stool’ and their head movement has been restricted by a motion tracked restraint; (b) an example of the cross-axis laser guides used to align a subject’s interaural axis to the centre of the loudspeaker array.