The same sound reaches right and left ear at different intensity levels {interaural level difference} (ILD). Level difference can be as small as 1 dB. Intensity difference reflects stimulus distance, approaching or receding sounds, and body sound damping. Slight head movements are enough to eliminate direction ambiguity. Intensity differences due only to sound distance, or to approaching or receding sounds, are useful up to one or two meters. Beyond two meters, differences are too small to detect.
damping
Pinnae and head bones absorb sounds with frequencies higher than 1500 Hz, according to their frequency-related dampening function. Pinnae and head-bone damping differs on right and left, depending on source location, and hearing uses the intensity differences to determine space directions and distances beyond one or two meters.
brain
Lateral superior olive detects intensity-level differences between left-right ears and right-left ears, to make opponent systems. To find distance, two receptor outputs go to two different neurons, which both send to difference-finding neuron. Opposite-ear output goes to trapezoid-body medial nucleus, which lies beside pons lateral superior olive and inhibits same-ear lateral-superior-olive output. Interaural time difference and interaural level difference work together.
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Date Modified: 2022.0224