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Barn owls use interaural level differences for vertical sound source localization: their feathers on their head are asymmetric leading to differences in their ears' sensitivities to sounds from above and below.

The difference in intensity between one ear and the other, the interaural level difference (ILD), is one cue used in biological sound-source localization.

In mammals, different neurons in the lateral superior olive (LSO) are tuned to different ILDs.

ITD and ILD are most useful for auditory localization in different frequency ranges:

  • In the low frequency ranges, ITD is most informative for auditory localization.
  • In the high frequency ranges, ILD is most informative for auditory localization.

The granularity of representations of ITDs and ILDs in MSO and LSO reflects the fact that ITD and ILD are most useful for auditory localization in different frequency ranges: ITDs for high frequencies are less densely represented in MSO and ITDS are less densely represented in LSO.

Liu et al. model the LSO and MSO as well as the integrating inferior colliculus.

Their system can localize sounds with a spatial resolution of 30 degrees.

Interaural time and level difference do not help (much) in localizing sounds in the vertical plane. Spectral cues—cues in the change of the frequencies in the sound due to differential reflection from various body parts—help us do that.

A head-related transfer function summarizes ITD, ILD, and spectral cues for sound-source localization.

Sound source localization based only on binaural cues (like ITD or ILD) suffer from the ambiguity due to the approximate point symmetry of the head: ITD and ILD identify only a `cone of confusion', ie. a virtual cone whose tip is at the center of the head and whose axis is the interaural axis, not strictly a single angle of incidence.

Spectral cues provide disambiguation: due to the asymmetry of the head, the sound is shaped differently depending on where on a cone of confusion a sound source is.

Sound-source localization using head-related impulse response functions is precise, but computationally expensive.

In Casey et al.'s system, ILD alone is used for SSL.

In Casey et al's experiments, the two microphones are one meter apart and the stimulus is one meter away from the center between the two microphones. There is no damping body between the microphones, but at that interaural distance and distance to the stimulus, ILD should still be a good localization cue.

Yan et al. perform sound source localization using both ITD and ILD. Some of their auditory processing is bio-inspired.

Aarabi use ITD (computed using cross-correlation) and ILD in an array of 3 microphones for auditory localization.

There are fine-structure and envelope ITDs. Humans are sensitive to both, but do not weight envelope ITDs very strongly when localizing sound sources.