Classic psychoacoustic experiments showed that, when excited with simple sine
waves, the hearing system uses two strong cues for estimating the apparent
direction of a sound source. Namely, interaural intensity and time differences
(IID and ITD) are jointly used to that purpose. IID is mainly useful above
1500Hz, where the acoustic shadow produced by the head becomes effective,
thus reducing the intensity of the waves reaching the contralateral ear. For this
high-frequency range and for stationary waves, the ITD is also far less reliable,
since it produces phase differences in sine waves which often exceed 360