Equal Loudness

Simple Explanation

Our ears don’t hear all frequencies equally. We are naturally much better at hearing human voices (mid-range) than very deep bass or high-pitched whistles, especially when the volume is low. As you turn the volume up, our ears start to hear the low and high sounds more clearly and "equally."

Concise Technical Definition

The measure of sound pressure level (SPL) across the frequency spectrum for which a listener perceives a constant loudness. While historically based on the Fletcher-Munson curves, it is now defined by the ISO 226:2003 standard.

Layman-Friendly Analogy

Think of it like a flashlight beam in a dark room. The center of the beam (mid-range frequencies/voices) is very bright and easy to see. The edges of the beam (bass and treble) are dim and hard to see. If you get a much more powerful flashlight (turn up the volume), the edges become bright and clear, just like the center.

Industry Usage Summary

Used in audio system calibration to ensure a balanced tonal experience at different listening levels. It is the principle behind the "Loudness" button on consumer stereos, which boosts bass and treble at low volumes. In professional mixing, engineers often work at a calibrated level (typically 85 dB SPL) because that is where human hearing is most "flat" or balanced across the spectrum.

Engineering Shortcut

The ISO 226 Curves. It explains why a mix that sounds "bassy" and "crisp" at high volumes often sounds "thin" or "mid-heavy" when played quietly.

Full Technical Explanation

Equal Loudness describes the frequency-dependent sensitivity of the human primary auditory system. The curves plot SPL (measured in decibels) against frequency, where each curve represents a constant level of perceived loudness, measured in Phons. By definition, the Phon level of a sound is equal to its SPL at 1 kHz.

Human hearing is most sensitive between 2 kHz and 5 kHz due to the natural resonance of the ear canal. At lower intensities, the ear requires significantly more energy to perceive low-frequency sounds (bass) compared to mid-frequencies. As intensity increases, the "curves" flatten out, meaning our perception of bass and treble becomes more proportional to the actual physical energy. Modern engineering relies on ISO 226:2003 data, which updated the original 1933 Fletcher-Munson and 1956 Robinson-Dadson research to account for more accurate measurements of human hearing thresholds.