AUDITORY SPACE PERCEPTIO

The perception of spatial qualities through the sense of hearing.Space perception occurs in audition as well as vision since hearing gives us cues both to the direction and distance of the source of sound. It is of little use, however, in determining the size or exact position of objects.There are three auditory cues to direction: time difference, phase difference, and intensity difference. We judge where a sound comes from partly by the fact that it usually reaches our two ears at different times. We know that a sound comes from our left because our left ear receives it a fraction of a second (about .0002 second) before our right. We can even tell, roughly, how far it is toward our left. When both ears hear the sound at the same time, we know it comes from a point midway between our two ears, but we cannot tell whether it is above or below, in front or behind us. The second directional cue is intensity, or loudness. Intensity decreases as the square of the distance from the source. When one ear is closer to the source, it receives the sound at greater intensity than the farther ear. The farther ear is also in the “sound shadow” of the head, and consequently the sound waves it receives are weakened. Without realizing we are doing so, we learn to judge direction by these differences. We are even less consciously aware of the third type of cue, phase difference. This is the difference in pressure (positive or negative) between two tones at any particular instant. If one ear faces the source of sound more directly than the other, the maximum positive pressure of the sound wave reaches this ear before it reaches the more distant ear. The tones received by the two ears are then out of phase, and the extent of the difference helps us determine the direction of the sound source. Time, phase, and intensity differences provide a fairly good direction location when the head and source are stationary, but they are usually more accurate when the head and source are moving. The reason is that we can then perceive how these cues change with changes in the relative position of the head and the sound source. This is why we automatically move our heads in determining where a sound comes from. In contrast to the perception of direction, auditory perception of distance requires the use of only one ear, since it depends wholly on the monaural cues of intensity, frequency, and complexity. If we are familiar with a sound we can usually judge its distance in terms of its loudness, for we know that the closer it is the louder it will be. A distant train whistle may sound no louder than a nearby clock chime, but we know from experience that train whistles are louder than chimes, and conclude that the train must therefore be far away. We also base estimates of distance on the fact that low frequency, low- pitched sounds can be heard much further than high sounds even though we may not be consciously aware of this fact. This helps us recognize that a foghorn is actually far away even when the sound is loud. It also helps us know whether an orchestra is playing in the distance or nearby. If it is in the distance we will hear far more of the low notes than the high notes and this will alter the total effect, or composition, of the music being played.