11 The perception of frequency
11.1 The relationship between frequency and pitch
Although the perception of sound involves the interaction of frequency and intensity, many aspects of frequency reception can be analysed separately.
For normal or typical hearing, the limits of hearing for frequency fall between 20 and 20 000 Hz. Below 20 Hz only a feeling of vibration is perceived; above 20 000 Hz, only a ‘tickling’ is experienced.
As well as loudness, the other most obvious characteristic of a sound is its pitch. Pitch is a subjective dimension of hearing. It is the sound quality most closely related to the frequency of a pure tone. High-frequency tones are perceived as being of high pitch while low-frequency tones are said to be of low pitch. The relationship between pitch and frequency is however, not a simple linear one. In order to investigate how the two are related, pitch has been assigned the arbitrary unit the mel. The pitch of a 1000 Hz tone at 40 dB SPL has been given a fixed value of 1000 mel. In order to determine the number of mels that are associated with different frequency tones, a subject is presented with a 1000 Hz tone and told that the pitch is 1000 mels. The subject is then asked to manipulate the frequency of a tone until that tone has a pitch that is one half as high as the 1000 mel tone. This tone is then assigned a value of 500 mel. The subject is then asked to find a frequency that is half the pitch of the 500 mel tone which is then assigned a value of 250 mel. In this way a function relating frequency to mels can be generated (Figure 36). Figure 36 shows that pitch is not related to frequency in either a linear fashion or a logarithmic fashion (note that frequency is plotted on a logarithmic scale); the relationship is more complex. In general, pitch increases more rapidly than frequency for tones below 1000 Hz and less rapidly for tones above 1000 Hz. That is, for frequencies above 1000 Hz a greater change in frequency is needed to produce a corresponding change in pitch.