Michael M MerzenichLocalisation of a sound source depends upon cues that result from having
two ears separated in space by the head. The binaural cues consist of an
interaural time delay (ITD) caused by a longer sound path to one ear and,
depending on the frequency, an interaural level difference (ILD) resulting
from the shadowing effect of the head. Head shadowing effects are minimal
for low-frequencies, for which the wavelength is longer than the head width,
whilst for higher frequencies the interaural phase differences (IPDs), that
result from the ITD, present an ambiguous cue. For high-frequency or
complex sounds, the different transmission delays to the ears also create a
difference in the arrival times of the sounds at the ears, and an on-going
delay of the envelope. In addition to these cues, interference between
reflections of sound waves from the torso and within the outer ear results in
frequency-dependent spectral colouring of broadband sounds. Lowfrequency
sounds can be localised using IPDs alone, high-frequency sounds
by their ILDs, and for complex sounds the envelope delay may also be used.
Spectral cues permit accurate monaxiral sound localisation of complex
sounds and provide cues for sound elevation. In practice, in real listening
environments with broadband signals ITDs, ILDs and spectral cues will all
be simultaneously present and consistent with each other.