Human hearing peaks in sensitivity at 3–4 kHz due to ear canal resonance

The outer ear canal is a resonant tube approximately 2.5 cm long that resonates near 3–4 kHz. This creates a natural peak in hearing sensitivity at those frequencies — the ear is most sensitive there and least sensitive to very low and very high frequencies at the same SPL. Equal-loudness contours (Fletcher–Munson / ISO 226) show that at low listening levels the ear requires much more bass and treble energy to perceive them as equally loud as midrange. A-weighting filters applied to sound level meters mimic this sensitivity curve for low-level measurements.

Examples

A mix that sounds bass-heavy at high volume often sounds thin when played quietly, because the low-frequency sensitivity drops off steeply at low SPL (the ‘bass rolloff at night’ problem). A-weighted SPL readings (dBA) de-emphasize lows and highs to model this effect.

Assessment

Explain why you might add low-end boost to a quiet background music system that sounds balanced at full concert volume. What curve shapes this effect?