Fletcher-Munson Curves (equal loudness contours)
The ear is not equally sensitive to all frequencies. In addition, its non-linear response is dissimilar at different loudness levels. Fletcher and Munson researched this in 1933 and presented their finding in a – now famous - set of curves showing the sound pressure levels of pure tones that are perceived as being equally loud at various intensity levels. These “equal loudness contours” are illustrated below. (Note that the Phon scale is used here because we are talking about subjective loudness.) In the reference, the curves are plotted for each 10 dB rise in level from 0dB, defined as the “just-discernable” limit of perception up to 130dB, the “threshold of pain” although I have only illustrated the results up to 110dB which is the prudent limit for music listening! The Fletcher-Munson curves illustrate that human hearing is extremely dependent upon loudness, being most sensitive to sounds in the 3 kHz to 4 kHz area and very much less sensitive at the frequency extremes, especially at lower levels.
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