Auditory localization: a comprehensive practical review

Abstract

Auditory localization is a fundamental ability that allows to perceive the spatial location of a sound source in the environment. The present work aims to provide a comprehensive overview of the mechanisms and acoustic cues used by the human perceptual system to achieve such accurate auditory localization. Acoustic cues are derived from the physical properties of sound waves, and many factors allow and influence auditory localization abilities. This review presents the monaural and binaural perceptual mechanisms involved in auditory localization in the three dimensions. Besides the main mechanisms of Interaural Time Difference, Interaural Level Difference and Head Related Transfer Function, secondary important elements such as reverberation and motion, are also analyzed. For each mechanism, the perceptual limits of localization abilities are presented. A section is specifically devoted to reference systems in space, and to the pointing methods used in experimental research. Finally, some cases of misperception and auditory illusion are described. More than a simple description of the perceptual mechanisms underlying localization, this paper is intended to provide also practical information available for experiments and work in the auditory field.

Keywords: HRTF; ILD; ITD; acoustics; action perception coupling; auditory localization.

Figure 1

Reference system. The most commonly used reference system for locating a sound source in three-dimensional space is the polar coordinate system. The reference system is centered on the listener and divides space according to two angular coordinates (azimuth in the horizontal plane, elevation in the vertical plane) and one linear coordinate (distance or depth), as shown in the figure.

Figure 2

ILD, ITD and IPD. The fundamental binaural cues for auditory localization are based on the difference in perception between the two ears in terms of both intensity and time. A source located in front of the listener produces a sound wave that arrives at both ears identically (the direct wave arrives at the same time and with the same intensity). By contrast, a lateral source results in a difference in signal intensity between the right and left ears, respectively i_R and i_L (Δi, Panel A), and in arrival time (Δt, Panel B). (A) In the case of a lateral source, the sound stimulus arriving at the more distant ear is less intense, due to its greater distance from the source and the shadow effect produced by the head itself. Interaural Level Difference (“ILD”) is the perceptual mechanism that estimates the position of the source as a function of the intensity difference between the two ears. (B) The ear more distant from the source receives the sound with a time delay. The Interaural Time Differences (“ITD”) is the perceptual mechanism for localizing the sound source based on the time delay between the two ears. Fine variations in azimuth localization are also measured as Interaural Phase Differences (“IPD”), based on the phase differences between the waves reaching each ear.

Figure 3

Cone of confusion. A sound emitted from any point on the dotted line will give rise to the same ITD because the difference between the distances to the ears is constant. This set of points forms the “cone of confusion.”

Figure 4

HRTF. (A) Arrival of a sound wave at the outer ear and the generation of a series of secondary waves due to reflection in the auricle. (B) Each sound wave that reaches the ear thus generates a different set of reflected waves, depending on its original orientation. Using this relationship, our auditory system is able to reconstruct the origin of the sound by analyzing the set of waves that reach the eardrum.