The original Fletcher-Munson curves (1933) have been refined into ISO 226:2023 equal-loudness contours. The key finding: at low volumes, bass and treble sound significantly quieter than midrange. At higher volumes, our ears become more linear — all frequencies sound closer to equal loudness.
Practical implication: mix at moderate levels (~85 dB SPL) where the ear's response is reasonably flat. Mixing too quietly leads to bass-heavy mixes (you compensate for the ear's reduced bass sensitivity). Mixing too loudly leads to thin mixes. Many engineers periodically check their mix at very low volume — if the vocal and bass still sound balanced, the mix is working.