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.