Auditory localization

Sound localization is a listener's ability to identify the location of origin of a detected sound or the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space (see binaural recording).

There are two general methods for sound localization, binaural cues and monaural cues.

Binaural cues
Binaural localization relies on the comparison of auditory input from two separate detectors; most evolved auditory systems feature two ears, one on each side of the head. The primary biological binaural cue is the split-second delay between the time when sound from a single source reaches the near ear and when it reaches the far ear. This is often technically referred to as the "inter-aural time difference" (ITD). A less biologically important binaural cue is the reduction in loudness when the sound reaches the far ear, or the "inter-aural amplitude difference" (IAD). This is also referred to as the "inter-aural level difference" (ILD) or "inter-aural intensity difference" (IID).

Note that these cues will only aid in localizing the sound source's azimuth (the angle between the source and the sagittal plane), not its elevation (the angle between the source and the horizontal plane through both ears), unless the two detectors are positioned at different heights in addition to being separated in the horizontal plane. In animals, however, rough elevation information is gained simply by tilting the head, provided that the sound lasts long enough to complete the movement. This explains the evolved behavior of cocking the head to one side when trying to localize a sound precisely. To get instantaneous localization in more than two dimensions from time-difference or amplitude-difference cues requires more than two detectors.

In vertebrates, inter-aural time differences are known to be calculated in the superior olivary nucleus of the brainstem. The calculation is believed to rely on delay lines: neurons in the superior olive accept innervation from each ear with different connecting axon lengths. Some cells are more directly connected to one ear than the other, thus they are specific for a particular inter-aural time difference.

The tiny parasitic fly Ormia ochracea has become a model organism for studying sound localization in animals too small for ITDs to be calculated in the usual way. In this animal, the tympanic membranes of opposite ears are directly connected mechanically, allowing resolution of nanosecond time differences and requiring a new neural coding strategy.

Monaural (filtering) cues
Monaural localization mostly depends on the filtering effects of external structures. In evolved auditory systems, these external filters include the head, shoulders, torso, and outer ear or "pinna", and can be summarized as the head-related transfer function. Sounds are frequency filtered specifically depending on the angle from which they strike the various external filters. The most significant filtering cue for biological sound localization is the pinna notch, a notch filtering effect resulting from destructive interference of waves reflected from the outer ear. The frequency that is selectively notch filtered depends on the angle from which the sound strikes the outer ear. Instantaneous localization of sound source elevation in evolved systems primarily depends on the pinna notch and other head-related filtering. These monaural effects also provide azimuth information, but it is inferior to that gained from binaural cues.

In order to enhance filtering information, many animals have evolved large, specially shaped outer ears. Many also have the ability to turn the outer ear at will, which allows for better sound localization and also better sound detection. Bats and barn owls are paragons of monaural localization in the animal kingdom, and have thus become model organisms.

Processing of head-related transfer functions for biological sound localization occurs in the auditory cortex.

Distance cues
Neither inter-aural time differences nor monaural filtering information provides good distance localization. Distance can theoretically be approximated through inter-aural amplitude differences or by comparing the relative head-related filtering in each ear: a combination of binaural and filtering information. The most direct cue to distance is sound amplitude, which decays with increasing distance. However, this is not a reliable cue, because in general it is not known how strong the sound source is. In case of familiar sounds, such as speech, there is an implicit knowledge of how strong the sound source should be, which enables a rough distance judgment to be made.

In general, humans are best at judging sound source azimuth, then elevation, and worst at judging distance. Source distance is qualitatively obvious to a human observer when a sound is extremely close (the mosquito in the ear effect), or when sound is echoed by large structures in the environment (such as walls and ceiling). Such echoes provide reasonable cues to the distance of a sound source, in particular because the strength of echoes does not depend on the distance of the source, while the strength of the sound that arrives directly from the sound source becomes weaker with distance. The ratio of direct-to-echo strength alters the quality of the sound in a way to which humans are sensitive. In this way consistent, although not very accurate, distance judgments are possible. This method fails outdoors, due to a lack of echoes.