INTERAURAL TIME- AND LEVEL DIFFERENCES (ITD AND ILD)
The perception of the horizontal direction in which a sound source is located is based on analysis of time and level differences between the signals arriving at the two ears.
Sound pressure differences during lateral sound incidence occur due to the obstructive effect of the head (shadowing). For speech, the sound pressure difference between the two ears in the horizontal plane in the angular range from about 45° to 135° is about 6 to 8 dB.
The emergence of a time delay between the ears is sketched on the attached figure, for sound incidence in the horizontal plane. Up to a sound incidence angle of about 45°, a time difference of 35 μs (travel distance difference approx. 1 cm) is already sufficient to perceive a perceptible change in the direction of sound incidence. This corresponds to an angular deviation of only about 3°. For sound incidence angles of more than 45°, this just noticable difference (JND) increases.
