Equal-loudness contour
Curves showing how loud a tone must be at each frequency to sound equally loud — the ear is nowhere near linear.
Harvey Fletcher and Wilden Munson at Bell Labs measured in 1933 how listeners perceived tones against a 1 kHz reference. The curves show the ear is most sensitive around 2–5 kHz, where the ear canal resonates, and drops off sharply in the bass, especially at low volume. The unit is the phon. Robinson and Dadson redid the measurements in 1956, and the current standard is ISO 226.
That is why old amplifiers have a loudness button that boosts bass and treble at low volume, and why a mix sounds different depending on how loud you play it. A-weighting is based on the 40-phon curve, and the K-weighting inside LUFS is a cruder take on the same insight. Hearing does not submit to a voltmeter.