Duplex Theory of Localization
This is the combination of the two methods that describe human binaural sound localization first proposed by Lord Rayleigh around 1900.
Rayleigh observed that sound arriving at a listener from sources located away from the median (mid-sagittal) plane would result in differences in the signals observed at a listener's ears. He noted that the sound received at the far (contralateral) ear would be effectively shadowed by the head, resulting in a difference in the level, or intensity, of the sound reaching the two ears. Rayleigh correctly noted that the intensity difference would be negligible for frequencies below approximately 1000 Hz, where the wavelength of the sound is similar to, or larger than, the distance between the two ears.
Rayleigh also noted that human listeners are sensitive to differences in the phase of low frequency tones at the two ears. He concluded that the phase difference between the two ears caused by an arrival-time delay (Interaural Time Delay - ITD) might be used as a localization cue at low frequencies.