## 2 Materials and Methods
### Measurement of Localization Accuracy  
The binaural sound system designed for use in the localization accuracy experiment was implemented using a high-quality open-type headphone set, a high-quality external sound card, a PC, a wired head-tracker with a PC application for sending tracking data via OSC protocol [23], REAPER v6.75 as the digital audio workstation (DAW) software of choice [24], and a Sparta Binauralizer VST plug-in [25; 26]. Before taking part in the experiment, the participants were given an informed consent form as well as written information and instructions about the research and the experiment. Both the headphone set and the head tracker were securely placed on the participant's head. To determine the direction of sound arrival, participants were seated on a rotating chair and allowed to freely pivot their bodies and heads. To localize the sound source and establish its azimuthal and elevation directions, two paper tapes were stretched symmetrically over a 180\({}^{\circ}\) arc around the listening position in the horizontal and vertical planes, with markings of azimuthal and elevation angles in 1\({}^{\circ}\) resolution (as seen in Figure 2). The radius of the paper arc was 1.5 m, and the precision of the markings was checked with the digital protractor. To avoid negative values of azimuth and elevation, which would be more difficult for the subjects to read, the frontal direction was assigned an azimuth and elevation of 90\({}^{\circ}\), thus yielding a range from 0\({}^{\circ}\) to 180\({}^{\circ}\) for both parameters. The participants were asked to  
Figure 1: HRTF measurement in the anechoic chamber: (**a**) back view of a participant; (**b**) front view of a participant. Figure 2: Horizontal and vertical paper tapes used for determining the azimuth and the elevation of the sound source: (**a**) front view; (**b**) side view. determine the starting and final azimuth (or elevation) of a virtual sound source moving in the horizontal (or vertical) plane within the chosen range of azimuth (elevation) angles. The sound stimulus employed in this experiment was a single knock on a piece of wood that repeated itself at a rate of 3.53 knocks per second. The total duration of the stimulus was randomized, ranging from 8 to 12 s. The direction of arrival of the sound source continuously changed from a starting value to an ending value at a non-consistent angular velocity (ranging from 4\({}^{\circ}\) to 10\({}^{\circ}\) per second). The angular velocity of the virtual sound source was randomized (by no more than 3\({}^{\circ}\) per second from the starting angular velocity) to avoid automatic turning of the head after adjusting to the constant speed of the sound source (in the case of a constant angular velocity). As an additional measure implemented to avoid automatic turning of the head, in some cases the sound stimulus in the horizontal plane was allowed to have a turning azimuth as the position at which the virtual source changes its direction of movement, i.e., starts moving back towards the starting position. The vertical and horizontal planes were tested independently, and the participants were informed which paper tape they should focus on before every individual measurement. The participants were instructed to verbally indicate the initial direction of the virtual sound source immediately upon localizing it, as well as the final direction from which they perceived the sound following the conclusion of the sound stimulus (as seen in Figure 3). Before the actual testing began, two test examples were presented to each participant to verify the functionality of the binaural system and ensure that the participants understood the experimental protocol. In total, each participant listened to 18 test cases and indirectly evaluated the effectiveness of three different HRTFs.