Auditory Localization: Physiology

Cues for Localization

Sound localization is provided for by sensitivity to a combination of acoustic cues that arise from the interaction of sounds with the head. These include binaural (or “two-eared”) cues of interaural time and level differences (ITDs and ILDs, respectively). ITDs arise because the ears are separated in space by the distance of the head. Sound sources directly in front (or behind) generate zero ITD because the sound sources are equidistant from the two ears; for sound-sources located to one side or the other, however, the sound arrives later at the further ear than at the nearer ear, generating an ITD. In humans, these ITDs are maximally in the order of several hundred microseconds (millionths of a second) for sources located directly to one side or the other. ITDs are useful for localizing low-frequency sounds (< 1,500 Hz; approximately the note G two octaves above middle C on the piano).

For frequencies higher than 1,500 hertz (Hz), the head creates an “acoustic shadow” such that the sound arriving at the ear further from the source is less intense than the sound arriving at the nearer ear, creating an ILD. As well as depending on the location of the source, the magnitude of the ILD depends on the frequency of the sound and the size of the head (ILDs are bigger at higher frequencies and bigger for bigger head sizes). This dichotomy of ITDs for low-frequency localization and ILDs for high-frequency localization is referred to as the duplex theory of sound localization.

A final cue for sound localization arises because of the convoluted shape of the outer ear, or pinna. The various nooks and crannies of the pinna lead to sound waves interacting with each other, with the result that sound energy is reduced at some frequencies and increased at others. This effect is different for different frequencies, depends on the location of the sound source, and leads to a different pattern of sound energy reaching the eardrum compared with that arriving at the outer ear. As well as contributing to the perception of source location above and below the horizon, these “spectral cues” are necessary for disambiguating sound sources located behind from those in front.

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