system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how it's possible that I can hear if a sound is coming from in front of or behind me.

Or top/bottom for that matter.  It's easy to understand how it works for left/right - if a sound is coming from my left, it reaches my left ear slightly before my right and is also slightly louder in my left.  But how does it work for the other 2 dimensions?
gpt
I had a demonstration of this in a physiology class once - put 4 people around a test subject: one in front, one behind, one left and one right. Test subject closes their eyes, a person claps, and they have have to point at who they thought clapped. If the person was in front or behind the chances of getting it right are 50:50, you can't tell, as the sound reaches each ear at the same time and volume. The same should go for above and below if you couldn't feel a vibration coming through your feet or whatever. In real life you may hear a noise an instinctively turn your head a little to counter this problem and locate the source. Add visual stimuli and often movement from the object that suggests where it's coming from/going to, and the crudity of our hearing is masked. In the little experiment above the guesses about the direction of the clapping were often confident, the brain seemed to fill in the missing data and jump to a conclusion.