HRTF
Head-Related Transfer Function — the filter that your head, torso and outer ear impose on a sound, and that the brain reads to tell where it came from.
Before a sound reaches the eardrum it is coloured by the body's shape: the folds of the pinna, the shadow of the head, the reflection off the shoulders. That colouring varies with direction, and the brain uses the difference to place sounds in elevation and front/back — not just left/right. Convolve a mono source with a measured HRTF pair and you get convincing 3D audio over ordinary headphones, so-called binaural sound. The technique drives Steam Audio, Meta's XR audio and Dolby Atmos for Headphones. HRTFs are measured in anechoic chambers; generic sets work passably, but personal ones are better, because every ear is shaped like a fingerprint.