Equal-loudness contour

An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and is arrived at by reference to equal-loudness contours. By definition, two sine waves of differing frequencies are said to have equal loudness level measured in phons if they are perceived as equally loud by the average young person without significant hearing impairment.

The Fletcher–Munson curves are one of many sets of equal-loudness contours for the human ear, determined experimentally by Harvey Fletcher and Wilden A. Munson, and reported in a 1933 paper entitled "Loudness, its definition, measurement and calculation" in the Journal of the Acoustical Society of America. Fletcher–Munson curves have been superseded and incorporated into newer standards. The definitive modern curves are those defined in ISO 226 from the International Organization for Standardization, which are based on a review of modern determinations made in various countries.