Human sound localization in the azimuthal plane is primarily cued by the interaural time difference (ITD) and the interaural level difference (ILD). For sine tones with frequencies greater than about 1500 Hz, the ILD is the only steady-state cue available to listeners. In free field, plane waves incident on a listener diffract around the head which results in an acoustical bright spot on the opposite side of the head as the source. This results in the ILD cue being non-monotonic with azimuth. Listeners' localization responses to stimuli in free field are highly correlated with this misleading ILD cue, as listeners err dramatically in localization of sources with large azimuths. Discrimination experiment results also confirm the misleading nature of the ILD cue. Training and feedback provide very minimal performance improvements for listeners. However, the introduction of either amplitude modulation, or narrow-band noise of the same bandwidth helped some listeners greatly, and others to a lesser extent. The non-monotonic cue remains prominent for noise with bandwidths as wide as 4 octaves when centered around 1500 kHz.Amplitude modulation introduces an envelope interaural time difference (EITD) as a localization cue for listeners. Sinusoidally-modulated amplitude modulation signals become altered by a listener's anatomy. The EITD cue is often misleading and in conflict with the ILD cue. Nevertheless, the two cues together usually result in accurate localization. The quality of the amplitude modulation in these experiments was not degraded significantly and does not impact the weight of the EITD.
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