Why do low bass notes become harder to hear when music is played softly?

The human ear’s response to sound is nonlinear, meaning it does not perceive all frequencies with equal sensitivity. This phenomenon is illustrated by Fletcher-Munson curves, which map the varying intensities required for different frequencies to be perceived as having the same loudness.1

Because the threshold of hearing rises significantly at lower frequencies, the ear is naturally less sensitive to bass than to mid-range frequencies. When music is played at a high volume, these low frequencies remain above the threshold of audibility. However, when the overall volume is reduced, the intensity of all frequencies drops by the same amount. Because the threshold for bass is higher, these notes fall below the level of human perception more quickly than higher-pitched sounds. This is why audio systems often include a loudness control to boost low frequencies, ensuring the music maintains a balanced proportion of bass and treble at lower listening levels.1