Harmonicity and Temporal Regularity

Many sound sources vibrate with a fundamental frequency having many harmonics (e.g., voiced speech, musical instrument, and sounds from motors). These harmonically related spectra usually have temporally regular waveforms. As discussed previously, stimuli with harmonic structure or temporal regularity often produce a complex pitch. This unitary pitch can be used to characterize a complex sound. However, it is not always the case that mixing two complex sounds, each with a different fundamental, results in the perception of two pitches that could be used to segregate the two complexes. For instance, a sound source consisting of a 300-Hz fundamental and its harmonics could be mixed with a sound source consisting of a 410-Hz fundamental and its harmonics. In many situations, the auditory system does not analyze the complex mixture into two sources, one with a 300-Hz pitch and one with a 410-Hz pitch. In these cases, the auditory system appears to process the complex mixture synthetically.

In other conditions, a change in the fundamental component of a complex sound can be an aid to sound source segregation. For instance, if two different speech vowels are computer generated so that they have the same fundamental frequency (reflecting the periodicity of the vibrating vocal cords), listeners may have difficulty identifying the two vowels. However, when the two vowels also have different fundamental frequencies, vowel identification is possible. Thus, harmonic relationships and temporal regularity can be cues for sound source determination, but not in all cases.

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