Equal Loudness Contours (Fletcher-Munson)

Equal loudness contours, historically called Fletcher-Munson curves, represent one of the most important discoveries in psychoacoustics. They demonstrate that human loudness perception is not flat across frequency — it varies dramatically with both frequency and level. At quiet listening levels (20-30 phons), we are profoundly insensitive to bass frequencies: a 50 Hz tone must be nearly 40 dB louder than a 1 kHz tone to sound equally loud. At higher levels (80+ phons), the contours flatten significantly.

The modern ISO 226:2003 standard, based on research by Suzuki and Takeshima, replaced the original 1933 Fletcher-Munson data with more accurate measurements. The revised contours show somewhat less bass insensitivity at low levels than the originals but confirm the fundamental frequency-dependent nature of loudness perception. The A-weighting filter used in sound level meters is an approximation of the inverse of the 40-phon equal loudness contour.

For sound system tuning, equal loudness contours explain why a "flat" system sounds thin at low playback levels — the ear simply requires more bass energy at quiet levels. Many sound systems apply a "loudness" compensation curve at lower volumes. During measurement and calibration, engineers must account for listening level when judging the subjective quality of a frequency response curve that measures flat.

SonaVyx's SPL meter implements A-weighting (derived from equal loudness contours) alongside C and Z weightings. The RTA can display A-weighted spectra that approximate perceived loudness per band. When the AI diagnostic evaluates frequency response, it considers equal loudness contours in its assessment of subjective quality, weighting deviations in the sensitive 2-5 kHz region more heavily than equivalent deviations at frequency extremes.