As noted previously, the key function of the auditory periphery is to code for the frequency, intensity, and temporal structure of sound. Auditory systems of all animals are sensitive to only a limited range of frequencies. Figure 10 displays the auditory thresholds of humans to sinusoidal sounds of different frequencies. These thresholds of audibility can be obtained in one of two ways: The sounds can be presented over headphones [the minimal audible pressure (MAP) method] or over loudspeakers in a room with the listener [the minimal audible field (MAF) method]. When thresholds of audibility are measured, on average there is a 6-dB difference between MAP and MAF thresholds, which is due to diffraction of sound around the head and the calibration procedures used to obtain the MAP and MAF thresholds. The upper limit of hearing (e.g., the level at which sound becomes uncomfortably loud) is about 130 dB SPL. Thus, the dynamic range of hearing is about 130 dB in the frequency region between 500 and 4000 Hz. Humans can detect sound with frequencies ranging from 20 to 20,000 Hz, although hearing sensitivity, starting with the high frequencies, decreases with age.
The thresholds of hearing depend on the sound's duration. For durations less than approximately 300 msec, the energy of the sound determines threshold. Thus, long sounds require less sound power for detection than do short sounds. For durations greater than 300 msec, the ability to detect sinusoidal sound remains constant for signals of constant power. The duration at which the power of the signal no longer changes for detection threshold is used as an estimate of the integration time for detection.
Humans are sensitive to about a 0.2% change in frequency over a significant part of the range of hearing. For instance, one can discrimination between a 1000- and a 1002-Hz sinwave based on the frequency difference. An approximately 0.5-dB change in level is
also discriminable over a significant part of the range of hearing. Thus, sensitivity to frequency and sound intensity (not expressed in decibels) is approximately proportional to frequency or level. This proportionality is referred to as Weber's fraction or law, and to a first approximation the auditory system displays a constant Weber fraction for frequency and level.
The ability to discriminate sounds with changing levels from steady sounds is described by the temporal modulation transfer function (TMTF). The TMTF is measured by asking listeners to discriminate between a noise whose amplitudes are sinusoidally modulated and a noise with no amplitude modulation. The TMTF describes the depth ofmodulation (the difference in the changing amplitude) required to discriminate amplitude-modulated noise from unmodulated noise as a function of the rate of the amplitude modulation. The TMTF has a low-pass characteristic because as the rate of amplitude modulation increases beyond about 50 Hz the threshold for detecting amplitude modulation decreases. That is, it is more difficult to detect changes in the amplitude of modulated noise when the amplitude is changing faster than 50 times per second.
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