Psychoacoustics and "perceptual audio coding" are the search terms you were looking for, here's a couple of links:

https://www.izotope.com/en/learn/psychoacoustics-how-percept...

https://www.i-programmer.info/babbages-bag/1222-mp3.html

http://www0.cs.ucl.ac.uk/teaching/GZ05/05-music-coding.pdf