Equal-loudness contours (Fletcher-Munson curves) show that perceived loudness varies with frequency at the same SPL

The human ear is not equally sensitive at all frequencies. Equal-loudness contours map out how much sound pressure is needed at each frequency to achieve a given perceived loudness. The ear is most sensitive around 3,500 Hz (the resonant frequency of the auditory canal) and substantially less sensitive at bass frequencies. This means that the same piece of music sounds differently balanced at different playback volumes: bass appears quieter at lower volumes. Mix engineers must account for this by monitoring at multiple volume levels. The bass-boost built into some amplifiers compensates for low-volume equal-loudness rolloff.

Examples

The Fletcher-Munson curves. The loudness button on older amplifiers compensates for low-volume bass rolloff. The BBC uses EBU R128 loudness normalisation to ensure consistent perceptual loudness.

Assessment

Why does a mix that sounds well-balanced when played loudly lose its bass when played quietly? At approximately what frequency is the human ear most sensitive, and what anatomical feature causes this?