Perception of sound: Localization of sound sources

In the previous figure we can see how the distance of the two ears from the sound source is different. This is in fact a difference in arrival time (called inter-arrival time) of each signal to the ears (in the practical case shown in the figure, the signal arrives at the right ear first and then at the left ear).

Naturally this implies a phase difference, seeing that time-delay and phase-difference are intrinsically correlated [Relationship between delay and phase ] .

The amplitude of the two signals are different both because amplitude decreases with an increase in distance, and because the signal that has to reach the furthest ear has to avoid the obstacle of the head and in doing so, loses energy. Furthermore, the higher frequencies won't even be able to cross the obstacle thus resulting in a difference in frequency levels entering the ears. This is reason why it is difficult to identify where low frequency sounds come from: the latter are capable of crossing obstacles without losing relevant amounts of energy and therefore the sounds that reach the two ears are almost identical. In circumstances when the sound source is right behind the listener, its direction can be identified because a lack of high frequencies is perceived, the latter having been blocked by the pinna.

Referring to the previous figure, we can see that one of the two waves has to take a detour round the head to reach the furthest ear. This implies a loss of high frequencies as a result of diffraction [Diffraction ] .