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Like loudness (Section 4.3.2), pitch is another subjective aspect of hearing. Just as people have invented scales to express loudness, others have invented scales for pitch. Stevens et al. [32] were the first to produce a scale in mels. A pure tone of 1000 Hz at a sound pressure level of 40 dB has a pitch of 1000 mels. As a result of subjective experiments, the pitch scale is found to be approximately linear with frequency below 1000 Hz, but approximately logarithmic above 1000 Hz. It has been suggested by some that noise measurements should be made in bands of equal mels (mel was named after the musical term, melody). However, this suggestion has not been adopted. A formula to convert frequency, f (in hertz) into mel, m, is [33]

(4.3)

Masking noise may change the pitch of a tone. If the masking noise is of a higher frequency, the pitch of the masked tone is reduced slightly; if the masking noise is of a lower frequency the pitch is increased slightly. This can be explained [34, 35] by a signal/noise ratio argument. The locus of the position on the basilar membrane at which the tone is normally perceived could be changed by the masking noise [34].