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Frequency is measured by the number of cycles, per second, by which a pattern of air pressure changes is repeated. Pitch is a sensation that results from psychological response to frequency. The range of audible frequencies for humans falls roughly between 20 and 20,000 Hz. A Hz (herz) is a measure of the number of cycles per second.
Much research has been extended in finding correlations between pitch and frequency, where pitch is understood as a response pattern to the frequency of a sound. The goal is to be able to scale perceptual response to stimulus. S. S. Stevens conducted experiments in the 1930s in which subjects were given a keyboard with five keys similar to a piano keyboard. The lowest key was tuned to 200 Hz and the highest was tuned to 6500 Hz. Subjects were asked to adjust the frequency for each of three intermediate keys until the difference, in pitch, between each key was judged to be the same. In plotting the results on a cartesian graph, the x-coordinates represent the frequencies (in Herz) selected, while the y-coordinates are defined such that the vertical distance between the five points are equidistant. The units so-represented are called mels. Stevens and his colleagues ran three groups of experiments: the first one tested subjects within the range 40-1000 Hz, the second within the range of 200-6500 Hz, the third within the range 3000-12000 Hz.