The Ear's Uneven Frequency Response
Human hearing is far from a flat microphone. We are exquisitely sensitive around 2–5 kHz — the frequency range of speech consonants and infant cries — yet remarkably insensitive at low frequencies. A 30 Hz bass note must be 60 dB louder than a 3 kHz tone to sound equally loud. Fletcher and Munson first mapped this uneven landscape in 1933, and their curves (refined by Robinson-Dadson and ISO 226) remain foundational to audio engineering.
Phon and Sone: Perceptual Units
Because decibels do not correspond to perceived loudness, psychoacousticians use the phon (equal-loudness level) and sone (linear loudness) scales. The phon anchors to 1 kHz, while the sone scale captures the important finding that perceived loudness doubles every 10 dB — not with every doubling of sound pressure. A 70 dB conversation sounds twice as loud as 60 dB, not 1.12 times.
Loudness at Different Levels
The equal-loudness contours are not parallel — they flatten at higher SPL. At 90+ dB, the ear responds more uniformly across frequency. This is why music sounds 'fuller' at louder volumes and why audio systems include 'loudness' buttons that boost bass and treble at low volumes to compensate for the ear's reduced sensitivity at moderate levels.
Engineering Applications
Equal-loudness data drive A-weighting (the standard noise measurement filter), headphone equalization, hearing aid fitting, loudness normalization in broadcasting (EBU R128, ATSC A/85), and architectural acoustics. Understanding these curves means designing audio systems that account for human perception rather than just physical sound pressure.