Sound moves quite slowly. An echo demonstrates how slowly it moves – shout at a distant building and you can hear your voice bounce back at you with a slight delay. It moves slowly enough (330m/s) that your brain can detect a time difference between the sound arriving at your left ear and your right ear. This is called the interaural time difference.
Our ears can detect interaural time differences as small as 10 microseconds. So, if someone speaks on your left, your brain knows the sound is coming from the left because it takes a fraction of a second longer for the same sound to reach your right ear.
Stereo music uses this to give the illusion that sound is coming from different directions, and headphones provide the best way to experience that illusion as they push the music directly to each ear. But to hear stereo properly, you need the headphones on the right way round, otherwise, all the sounds will be reversed – left will come from the right and vice versa, and sounds from the front will sound like they’re coming from behind you.
You won’t notice any difference for mono audio, but backwards stereo sound makes watching movies a bit disconcerting.
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Asked by: Adam Marsh, London
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