Localization of sound sources

The ways in which auditory space is perceived are highly dependent on the nature of the acoustic environment. An important distinction is made between the direct wave, free of any interference, and the reverberant field, observed in enclosed spaces and resulting from a large number of reflections off the walls.

For each situation, the ear can evaluate the position of sound sources based on specific cues.

The article begins by presenting free-field indices:

• interaural differences in intensity, arrival time and spectrum ;

• ways of integrating spatial information :

  • azimuth,

  • elevation,

  • in depth.

The second part is devoted to examining the effects of spatial impression induced by the various characteristics of the room, and to specific localization cues for listening in volumes limited by walls.

Just as the eye is able to evaluate the position of objects in space on the basis of certain cues, sometimes monocular, sometimes binocular, the ear is able to explore sound space on the basis of monaural or binaural cues.

However, there are two essential differences from vision:

• the extent of the auditory (spatial) field is much greater than that of the visual (frontal) field;

• auditory localization modes use different indices depending on whether the sources are in the open (open air) or in enclosed spaces (rooms).

In free field, the ear takes advantage of the shifts between the signals perceived by each ear to deduce the position of the sound source that generated them. These shifts are mainly due to the differences in pitch imposed by the contour of the head, and to masking effects which differ according to whether the ear is on the side of the source or on the opposite side.

In rooms, the direct field still comes into play, but it is generally very weak compared to the reverberant field, and the ear evaluates the distribution of sources using other perceptive properties.