## 3 Materials and Methods
### The Virtually Enhanced Senses System
#### 3.1.3 Geometry-Based Virtual Acoustic Space
It firstly generates a fast, low-resolution acoustic image of the environment in \(T_{1}=\lfloor MN/K\rfloor T_{s}\) seconds. The original acoustic image is then generated after \(T_{2}=NMT_{s}-\ T_{1}\) seconds. This is meant to ease active sensing by reducing the motor-sensory loop latency. However, it assumes that orientation and mobility tasks are associated to relatively large elements, i.e., low spatial frequency data. The original recordings have been weighted according to the size of each element and their relevance from the user perspective. Large background elements such as walls or ceiling are mapped to short, low-volume audio tracks. Conversely, tables, cupboards, etc., are mapped to longer, higher-volume audio tracks. This is oriented to reinforce the figure-background discrimination, to reduce the background noise, and to draw the user's attention to relevant features of the environment even through a single acoustic pixel (Figure 4b). Finally, S(t) is converted to spatialized audio in the server with Resonance Audio (i.e., directivity of sound sources, reflections, occlusions, HRTF, etc.) and conveyed to the earphones. The second sub-mode, Aural VAS (AVAS), spawns one virtual sound source per primitive which collides with a virtual area surrounding the user (Figure 3). The sound source is positioned at the closest point of the collided primitive. Each acoustic material is mapped to an AVAS audio track. These audio tracks are modulated according to the relative motion of the user in relation to the collided objects, which offers a rudimentary illusion of friction. Alternatively, non-modulated sounds are applied. These could be triggered when the user first collides with an object, or periodically while the virtual area overlaps with a nearby object. To avoid masking simultaneous sound sources, split-second audio tracks are reproduced periodically. The third sub-mode, Flashlight, implements PVAS in combination with a speech synthesis module. In this mode, the virtual field of view of PVAS originates from the hand-held device, thus decoupling head movements from sound source spawning. This is meant to ease sound source localization, as well as promote faster and wider scanning movements. By default, the virtual field of view is configured as a narrow beam to ease pointing accurately at a single element, similarly to Lazzus [10] and previous projects. All elements of the virtual scenario are tagged with a verbal description, which is reproduced when the user points to each object and taps the screen. The identification of an element through a verbal message, e.g., \"table\", then conditions the induced perceptions and the development of mental representations of the scene. All these modes include a filtering mask which allows the user to define a subset of perceivable objects of the virtual scenario, e.g., table, doors. Analogously to the audio track weighting, this feature was used to filter unnecessary information, and to help users in object-background discrimination. From the perspective of vision-acoustic sensory substitution, these methods favor figure-ground perception.