Psychoacoustics
The study of how sound is perceived rather than how it is measured — and the reason an MP3 can throw away most of the audio without anyone noticing.
The ear is not a microphone. Its sensitivity varies with frequency, as the equal-loudness contours show, and it divides the spectrum into critical bands that Eberhard Zwicker mapped onto the Bark scale in the 1960s. A loud sound hides quieter ones nearby, in frequency and in time, and what is masked goes unheard even though it is in the signal. Direction comes from timing and level differences between the ears plus the head's own filtering, described by the HRTF.
Perceptual coding is built entirely on this. MP3, AAC and Opus compute a masking threshold and only spend bits where the ear is listening. The Haas effect — the first-arriving sound sets the direction — is exploited by every PA system with delayed fill speakers.