Spatial sound perception

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

1500Hz the IID becomes smaller due to head diffraction which overcomes the
shadowing effect. In this low-frequency range it is possible to rely on phase
differences produced by the ITD. IID and ITD can only partially explain the
ability to discriminate among different spatial directions. In fact, if the sound
source would move laterally along a circle (see figure 10) the IID and ITD would
not change. The cone formed by the circle with the center of the head has been
called cone of confusion. Front-back and vertical discrimination within a cone
of confusion are better understood in terms of broadband signals and Head-
Related Transfer Functions (HRTF). The system pinna - head - torso acts like
a linear filter for a plane wave coming from a given direction. The magnitude
and phase responses of this filter are very complex and direction dependent, so
that it is possible for the listener to disambiguate between directions having the
same, stationary, ITD and IID. In some cases, it is advantageous to think about
these filtering effects in the time domain, thus considering the Head-Related
Impulse Responses (HRIR) [13, 82].