11.5 Summary of sections 8 to 11
In these sections we have described some of the quantitative relationships between the physical dimensions of simple sounds and their subjective psychological dimensions. The physical dimension of intensity, or pressure amplitude, given in decibels (dB), directly affects loudness. Frequency of pressure changes, in hertz (Hz), mainly determines pitch.
The lowest threshold value and hence the maximal sensitivity for humans is in the region of 3000 Hz.
The quantitative relationship between intensity and loudness is that loudness grows more slowly than intensity. Equal loudness contours indicate that humans are more sensitive to frequencies between 1000 and 4000 Hz than other frequencies within the hearing range. When intensity is held constant, sounds in the region of 3000 Hz appear louder than sounds of other frequencies. The minimum change in intensity of a sound that produces a perceptual difference is 1 to 2 dB.
A number of different mechanisms play a role in intensity discrimination. Intensity changes can be signalled by both changes in the firing rates of neurons at the centre of the excitation pattern, and by the spreading of the excitation pattern. In addition, cues related to phase locking may also play a role in intensity discrimination. This may be particularly important for complex stimuli, for which the relative levels of different components may be signalled by the degree of phase locking to components.
The relationship between frequency and pitch is investigated using the mel scale. This shows that pitch is not linearly related to frequency. Pitch increases more rapidly than frequency for tones below 1000 Hz and less rapidly for tones above 1000 Hz.
The difference threshold for frequencies up to 1000 Hz is about 3 Hz, whereas for frequencies between 1000 and 4000 Hz the Weber fraction remains constant at about 0.002. Intensity affects the difference threshold: the lower the intensity, the higher the difference threshold.
Masking experiments support the place theory of frequency selectivity on the basilar membrane.
Although the sound heard depends primarily on its frequency and intensity, both its pitch and its loudness are secondarily affected by the duration of the sound. Within limits, loudness and pitch recognition increases as the duration of a brief burst of sound is lengthened.