Fletcher-Munson Curve (Equal Loudness Curve)

Simple Explanation

A famous set of graphs that proved our ears are "biased." They show that we are naturally tuned to hear human voices clearly, while deep bass and high-pitched sounds need to be physically louder for us to think they are at the same volume.

Concise Technical Definition

The original set of equal-loudness contours established in 1933. They map the relationship between the actual physical sound pressure level (SPL) and the loudness perceived by the human ear across the frequency spectrum.

Layman-Friendly Analogy

Imagine a choir where everyone is singing at the exact same physical strength. To your ears, the middle-voiced singers (tenors and altos) would sound like they are shouting, while the deep basses and high sopranos would sound like they are whispering. The Fletcher-Munson Curves are the "cheat sheet" used to tell the basses to sing louder so the whole choir sounds balanced to the audience.

Industry Usage Summary

These curves provided the foundation for A-weighting in sound level meters and the "Loudness" compensation circuits in vintage hi-fi gear. While modern standards have updated the data, the term "Fletcher-Munson" is still used by audio engineers as shorthand for the phenomenon of frequency-dependent hearing sensitivity.

Engineering Shortcut

Human hearing is most sensitive at 3 kHz to 4 kHz (the resonance of the ear canal). At low playback volumes, you must boost the low and high frequencies (the "smiley face" EQ) to maintain a perceived tonal balance.

Full Technical Explanation

Developed by Harvey Fletcher and Wilden A. Munson at Bell Labs, these curves were the first to quantify psychoacoustic loudness. Using headphones, they asked listeners to match the loudness of various frequencies to a 1 kHz reference tone. The resulting contours showed that the ear's sensitivity is non-linear: it is highly sensitive in the 3–4 kHz range (critical for speech intelligibility) and significantly less sensitive to low frequencies (bass).

The curves also demonstrate that as the volume (SPL) increases, the ear's response becomes "flatter," meaning we perceive bass more accurately at higher volumes. While the original 1933 data has been superseded by the ISO 226 standard—which used more modern calibrated speakers and larger test groups—the Fletcher-Munson research remains the bedrock of how we understand human hearing thresholds and loudness perception.