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?
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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.
In the audio processing center of our brain there are nerves that respond to a difference in the time that a sound arrives at either one of our ears (these nerves are especially prevalent in the superior olivary complex of the medulla).
When a noise is discrete and simple, like a singular click, this difference is very noticeable as the sound creates a singular pressure wave that will arrive at each of our cochleas at a slightly different time. The time difference is a fraction of a second difference, but our brains can sense it.
With more complex noises, like music, our brains look for what is called a phase difference of the sound waves. Basically the sound wave arrives at each cochlea at a slightly different point along the waveform of the sound, meaning there will be a small difference in the amount of pressure exerted on either eardrum. Our brain can sense this difference.
Those neurons in the audio processing center of the brain then do some cool neural network calculation magic to roughly calculate the position that a sound originates from.
When you have headphones on/in one ear, the sound might not even arrive at the opposite ear at all, depending on how good the frames of your headphones are at keeping the sound waves in your ear space. This makes it very easy for our brain to place the music as clearly coming from one side.
When you put the headphones on/in both ears, there will be very little, if any phase difference between the sound waves arriving at either of your cochleas. Because there is almost 0 phase difference, the neural calculation magic will determine the sound is coming from a position either directly in front of or behind you, thus you get the sensation that the music is coming from the middle of your head.
While you detect whether sound is coming from your left or right by timing differences, you detect whether sound is coming from in front, behind, above or below you by the way the sound is distorted by the structure of your outer ears. Feeding the sound direct into your ear canal with a ear bud completely throws this out. So you can’t detect anything about where this sound is coming from, and your brain giving you the idea that it is coming from the top of your head is quite common.
Because that’s how you hear sounds in real life. You hear it through both ears, and your brains computes where it’s coming from.
Thank you all for your answers!
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