a subsidiary unit for the pitch of a sound that is used mainly in musical acoustics. The quantitative evaluation of sound with respect to pitch is based on statistical treatment of a great deal of data regarding the subjective impression of pitch for audible tones. Research results indicate that the pitch of a sound is mainly dependent on the frequency of the vibrations but that it also depends on the loudness and timbre.

Acoustic vibrations at a frequency of 1000 hertz (Hz) and an effective sound pressure of 2 X 10⁻³ newton per sq m (that is, a loudness of 40 phons) coming from in front of an observer with normal hearing create for him an impression of a pitch fixed by definition at 1,000 mels. A sound at a frequency of 20 Hz and a loudness level of 40 phons has, by definition, zero pitch (0 mel). The relationship between the pitch of pure tones and the frequency for constant loudness are shown in Figure 1. It is a nonlinear function, especially at low frequencies (for “low” sounds).

Figure 1