 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.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.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.