system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do two headphones make the music play in the top of my head? A left or right headphone on its own is noticeably left or right, but as you place the second one in, rather than it sounding like it’s coming into each ear simultaneously, it just sort of like, swims up in the top. Why?
gpt
The brain uses loudness and time-based cues to determine whether a sound is coming from left, right, or center. If only your left ear hears, say, a cymbal crash, your brain concludes it must be coming from straight to your left. If both ears hear the strike at equal volume and at the same time (as opposed to the noise reaching one ear before the other), your brain concludes it's coming from the center.

As for why music might seem to be coming from the top instead of in front of you, another stereo imaging cue you hear would be the set of resonant peaks and dips in the treble caused by sound waves being reflected off the bowls of your outer ears at various angles before reaching your eardrum. For whatever reason (maybe tuning, maybe headphone cup angling, maybe something else), your brain interprets the highs your headphones generate as coming from the top.