system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do some stereo sounds sound like they are coming from both sides rather than the middle?

Example: when listening to music on headphones, the vocal is usually in the center, inside your head, whereas instruments sound like they're outside your head on both sides, even though the volume is the same in both channels.
gpt
So the way our brains interpret the direction a sound is coming from is to listen for differences in the sounds reaching each ear. So when stereo speakers (placed at an equal distance) are playing the EXACT same sound, our brain interprets that as coming from directly in front of us.

When an audio engineer is mixing music, it is sometime desirable to have some instruments sound like they are coming from the sides, but not necessarily one side or the other. To accomplish this, they have to make one side (either the left or the right channel) SLIGHTLY different from the other.

One way this is accomplished is by a “Stereo Spread” which will take a stereo audio file and spread certain sections of the frequency band to either the left or right. That way the left speaker will be playing some of the frequency bands, and the right speaker will be playing the other frequency bands of the same sound.

The other way this can be accomplished is by using the Haas effect. This is done by simply “delaying” one of the sides (either the left or right channel) by up to about 30 milliseconds; so one side is playing the exact same sound as the other side, but just slightly later than the other. By only delaying it as much as 30ms, our brain doesn’t detect it as being “out of time” and can barely tell the difference. However our brain DOES definitely hear two separate and distinct sounds in each ear making it interpret the sounds as coming from each side instead of directly in front of us.