In their study, test subjects listened to pure tones at various frequencies and over 10 dB increments in stimulus intensity. Within this range, the human ear is most sensitive between 2 and 5 kHz, largely due to the resonance of the ear canal and the transfer function of the ossicles of the middle ear.įletcher and Munson first measured equal-loudness contours using headphones (1933).

The human auditory system is sensitive to frequencies from about 20 Hz to a maximum of around 20,000 Hz, although the upper hearing limit decreases with age. It is now better to use the generic term equal-loudness contours, of which the Fletcher–Munson curves are now a sub-set, and especially since a 2003 survey by ISO redefined the curves in a new standard. Until recently, it was common to see the term Fletcher–Munson used to refer to equal-loudness contours generally, even though a re-determination was carried out by Robinson and Dadson in 1956, which became the basis for an ISO 226 standard. 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 Fletcher–Munson curves have been superseded and incorporated into newer standards.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.

Loudness equalization