Auditory Localization: Psychophysics

Horizontal Dimension

A sound from a source that lies to one side of a listener in the azimuth dimension such as a source off to the right will arrive at the ear closer to the sound before it reaches the ear on the other side of the head. This small interaural (between ears) time difference in the arrival of the sound at the two ears produces an interaural time difference (ITD) cue for locating sound sources in the azimuth dimension. The sound will also be more intense (have a higher sound level) at the ear closer to the sound source relative to that at the opposite ear. Thus, there is also an interaural level difference (ILD) cue for sources that lie in the horizontal dimension (see Figure 1). When the sound source is directly in front of a listener, the ITD and ILD cues are both zero because the sound source is midway between the two ears. As the location of the sound source is moved to one side or the other, the ILD and ITD cues increase. Thus, large ITD or ILD values indicate sound sources that are located well to one side of the listener.

The ILD cue is based largely on the fact that the head and body shield the sound on the ear opposite the sound source relative to that on the side of the sound source. That is, the head provides a sound shadow for the sound that arrives at the ear opposite the location of the sound source. This sound shadow produces a lower level sound at the far ear. The amount of attenuation of the level of the sound at the far ear depends on the sound's frequency. Higher frequencies receive more attenuation than do lower frequencies (the short wavelengths of high-frequency sounds allow high-frequency sounds to be reflected from the head), resulting in large ILDs for high frequencies compared with low frequencies. Thus, ILDs provide a better cue for sound source location in the azimuth dimension for high frequencies relative to low frequencies. For several reasons, the ITD cue provides most accurate information about the location of a sound source in the azimuth dimension when the sound has low frequencies (below about 1,200 hertz [Hz]). Above about 1,200 Hz, sounds such as sinusoidal and other narrow-band sounds cannot be located on the basis of an ITD. Thus, ITDs provide the best cue for sound source location in the azimuth dimension for low frequencies.

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