The first research on the topic of how the ear hears different frequencies at different levels was conducted by Fletcher and Munson in 1933. Boosting these frequencies produces a flatter equal-loudness contour that appears to be louder even at low volume, preventing the perceived sound from being dominated by the mid-frequencies where the ear is most sensitive. These are intended to offset the apparent loudness fall-off at those frequencies, especially at lower volume levels.

The definitive 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.Īmplifiers often feature a 'loudness' button, known technically as loudness compensation, that boosts low and high-frequency components of the sound. Fletcher–Munson curves have been superseded and incorporated into newer standards. Munson, and reported in a 1933 paper entitled 'Loudness, its definition, measurement and calculation' in the Journal of the Acoustical Society of America. 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. 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 unit of measurement for loudness levels is the phon and is arrived at by reference to equal-loudness contours.

ISO equal-loudness contours with frequency in Hz.Īn 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.

Iso standard equalizer frequencies