E6 Binaural Hearing: Spatial Localization

Topic

When a sound source is located to one side of the head rather than exactly in the center, the sound pressure it produces does not reach both ears at the same time or with the same intensity: it arrives sooner and more intensely at the ear closer to the source, and later and more weakly at the ear farther away. The auditory system utilizes this asymmetry between the two ears—rather than treating it as a nuisance—to calculate the location of the sound source that produced it.

The difference in arrival time of the same sound pressure at each ear is known as the interaural time difference; the central auditory pathway relies on this primarily for lower frequencies, as the wavelength of a low frequency is long enough for the pathway to clearly distinguish the exact moment each cycle of sound pressure reaches each ear. The further the sound source moves from the center of the head toward one side, the greater the interaural time difference between the ears; conversely, an interaural time difference of zero indicates that the sound source is exactly centered, equidistant from both ears.

For higher frequencies, however, the wavelength is so short that the head itself—positioned between the two ears—partially blocks the sound pressure before it reaches the farther ear; this phenomenon is known as the head's acoustic shadow. This acoustic shadow causes the ear farther from the source to receive weaker sound pressure than the closer ear, a difference in intensity known as the interaural level difference. The further the sound source moves from the center of the head toward one side, the greater the head's acoustic shadow over the farther ear and the greater the interaural level difference between the two ears. Both interaural time difference and interaural level difference leave an ambiguity that neither can resolve on its own: many distinct positions in the vertical plane—or in front of and behind the head—produce exactly the same interaural time and level differences, because both depend solely on the sound source's lateral distance from the center of the head, rather than whether it is positioned above, below, in front, or behind. It is precisely the spatial filtering of the pinna that resolves this ambiguity: since the pattern of frequency reinforcement and cancellation produced by the pinna differs for every sound arrival direction—even for directions sharing the same interaural time and level differences—the central auditory pathway can utilize this frequency pattern, alongside the two interaural differences, to localize the sound source in the vertical plane and distinguish whether it originates from the front or the back.

By combining interaural time difference, interaural level difference, and the spatial filtering of the pinna, the central auditory pathway is able to localize a sound source in both the horizontal and vertical planes using only the input received by the two ears. This same ability to compare inputs from each ear also aids in speech comprehension within noisy environments: when the speech signal and background noise arrive from different directions, the central auditory pathway uses interaural time and level differences to distinguish which sound comes from which direction, thereby improving the effective signal-to-noise ratio beyond what either ear could achieve in isolation.

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