Audition: Temporal Factors

Time, Frequency, and Modulation

Sound is a pressure waveform that is generated by a vibrating object. With sound a waveform that varies over time, temporal processing by the auditory system is a central basis for determining the information conveyed by sound. The vibrations that generate most sounds repeat in some manner or another. These periodicities are described in terms of repetition frequency. Repetitions per second are defined in units of hertz (Hz). For example, a simple or sinusoidal vibration that repeats one thousand times a second has a frequency of 1,000 Hz.

Representation of the periodicities in the time waveform in terms of frequency illustrates the relationship between the temporal and frequency-based descriptions of sound. The implication of this relationship, formalized by the Fourier theorem, is that change in one domain affects the other. The auditory system codes both the temporal and frequency characteristics of sound. Time is coded by the temporal pattern of the response of auditory neurons, and frequency by both response pattern and by which neurons respond among the many thousands in the auditory nervous system. A consequence of the interrelationship between time and frequency, and the direct coding of each aspect by the auditory system, is that studies of auditory temporal factors must ensure that the basis of measured effects is indeed temporal. Thus, studies of [Page 155]temporal factors often use stimuli with wideband frequency content or include maskers to restrict the influence of unintended frequency variations.

A further effect that ties together time and frequency is their inverse relationship in resolution. That is, enhancement of frequency selectivity results in a sluggish temporal response, and a rapid temporal response results in broad or poorly tuned frequency selectivity. The complexity of the auditory system no doubt arose partly to meet the conflicting demands of temporal and frequency resolution.

Variations in the time waveform of sound can generally be described in terms of modulation. Modulation simply refers to some pattern of change conveyed by a carrier. For auditory processing, the relevant variations are amplitude modulation (AM) and frequency modulation (FM). At slow rates of variation, listeners can follow the modulation, with AM perceived as fluctuation in loudness and FM as fluctuation in pitch. At faster rates, both types of modulation change the stimulus quality, timbre, or pitch, often adding a richness or roughness to the sound. Modulated stimuli play an important role in assessing temporal resolution and discrimination abilities, and also temporal-pattern processing.

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