A powerful cue for segregating one sound source from a mixture of other sounds is temporal asynchrony. If the sound from one source begins or ends at a different time than other sounds from other sources, the differences in onsets or offsets allow listeners to determine the various sound sources. For instance, in the case described previously in which listeners may not perceive two complex pitches (e.g., 300 and 410 Hz) when two different harmonically related stimuli are mixed, listeners report perceiving the two pitches if one complex harmonic stimulus is presented slightly before the other stimulus. Although both stimuli may be played together for most of their duration, a small onset asynchrony can assist complex pitch segregation.
The onset (attack) and offset (decay) characteristics of the playing of many musical instruments are crucial variables for the characteristic timbre of the instruments. These timbrel differences can also aid sound source segregation. The ability to represent the proper attack and decay properties is a key element in musical instrument synthesis.
Thus, the auditory system appears to compare information across the spectrum of sound and over time to process complex sounds so that sound sources can be determined and segregated one from the other. Interaural differences, amplitude modulation, harmonicity, temporal regularity, relative amplitude differences (spectral profiles), and common onsets and offsets are some of the variables that allow the auditory system to determine sound sources in our environment, especially when several sound sources exist at the same time.
See Also the Following Articles
AUDITORY AGNOSIA • AUDITORY CORTEX • BRAINSTEM • HEARING • SENSORY DEPRIVATION • SPEECH • VISION: BRAIN MECHANISMS
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