## 3 Materials and Methods
### Experimental Procedures  
In this work, three tests were conducted to validate the developed prototype. These tests were designed for normal-sighted, untrained individuals. The objective is to assess whether the subjects could decipher the data encoding of GbVAS, as well as to gather information regarding the development and usage of a mental representation of a scenario only perceivable through the prototype's output. The tests were carried out on flat surfaces of at least \(40\times 20\) m, such as soccer fields (Figure 5a). Each test has its own virtual scenario which was loaded within the field bounds (Figure 6). The first two tests consist of the free exploration of an unknown scenario (Figure 6a,b) within a time limit. The blindfolded subjects were told to move freely, make their own judgements about the audio spatial cues, and afterwards draw a sketch of how they perceived the scene. The sketch must be a bird's eye view of the scene and must be drawn within a square representing the football field (scale reference). No prior information of the scenario was provided as it could be used to decipher the data output. The time limit was fixed to three minutes, because in previous experiments there were no signs of major improvements in the output data decoding, nor in the knowledge of the scenario, after that time. Figure 5: Experiment design. (**a**) User in the soccer field with a loaded virtual scenario (**b**) Screenshot of the server’s graphical interface while the user moves through the scenario. Figure 6: Scenarios for tests 1 (**a**), 2 (**b**) and 3 (**c**). The default material is “brick”. The starting position and orientation are represented by a red dot and red arrow, respectively. Thereafter, the sketches are meant to provide data on the subjects' perceptions through the sensory substitution algorithm, as well as any resulting survey/configurational knowledge of the scenario. In the third test, subjects were shown a map of a virtual scenario (Figure 6c). Thereafter, they were blindfolded and immersed in the scenario at an unknown start position and were told to find the exit. The time limit was also three minutes. The subjects received six minutes training in GbVAS' usage through Tests 1 and 2. Also, subjects were familiarized with the scenario, which is meant to further improve output decoding. Subjects must then use that information to periodically estimate their position in relation to the exit, plan a route, and follow it. Regarding the scenarios' design, all three were meant to ease GbVAS' output decoding, as well as evaluate orientation and mobility performance. GbVAS partially uses natural data encoding, e.g., 3D location through HRTF, or the presence of physical elements and their material composition, by reproducing impact sounds. Starting from the premise that the subjects effectively and efficiently assimilate such data with no prior training, they must then search for the causes that trigger those stimuli. To that end, they need to observe the contingencies between body movements and the resulting changes in acoustic stimulation. Taking this into consideration, the first scenario was designed as an open area with few elements of a reduced size, specifically three columns which are also intended to serve as landmarks (Figure 6a). This is meant to ease the discrimination of when an element enters or exits the GbVAS virtual area, i.e., the identification of spatial boundaries. Thereafter, the subjects are free to make estimations about the range of such boundaries. Given that the scenario is unknown, this serves to test if the kinesthetic sense is able to assimilate GbVAS' virtual area and disentangle its shape from those of the elements in the vicinity. The second scenario is a rectangular room of \(4.5\times 9\) m (Figure 6b). In Test 1, the users were provided with basic cues of the data output, emphasizing whether there is an element or not. In this closed area, minimum perception and consequent mobility tasks can be assumed if the subjects move without leaving the room.