FletcherMunson

Fletcher–Munson curves, also known as equal-loudness contours, are sets of curves that describe the sound pressure level in decibels required at each frequency for a sound to be perceived as equally loud as a reference tone at a given loudness. They illustrate how human hearing sensitivity varies with frequency, being most sensitive in the midrange and less sensitive at very low and very high frequencies.

Originating in the 1930s at Bell Telephone Laboratories, the curves were developed by researchers including Fletcher and Munson. They were based on psychophysical measurements of perceived loudness and formed an influential framework for understanding how loudness depends on both level and frequency. The concept is tied to the phon, a unit of loudness level defined by the SPL of a 1 kHz reference tone that matches the perceived loudness of the test signal.

Interpretation of the curves shows that the ear requires higher sound pressure levels to achieve the same perceived loudness at low frequencies (below roughly 200 Hz) and at high frequencies (well above a few kilohertz), with a pronounced sensitivity peak around 2–5 kHz. This frequency dependence explains why music may sound attenuated or colored when played back at low levels unless compensations are applied.

Applications and evolution: Fletcher–Munson data informed early audio engineering practices, loudness compensation, and broadcast standards. Later measurements, including Robinson and Dadson data, refined the contours, and modern standards such as ISO 226:2003 provide updated equal-loudness contours. Today, the curves underpin concepts of loudness normalization and are a reference point in headphone and loudspeaker design, mastering, and listening-room calibration.