
If you've ever wondered why a mix sounds different at low versus high playback volumes, you're bumping into a century-old psychoacoustic phenomenon: the Fletcher-Munson curves, also known as equal-loudness contours. These curves describe how our ears perceive loudness differently across frequencies, depending on sound pressure level (SPL). While they have evolved over time into more refined ISO standards, the original Fletcher-Munson data remains a seminal contribution to both audio engineering and psychoacoustics.