We have, of course, two ears which jointly allow us to localize the direction of incoming sound. But our ears are much more than simple point hearing devices; there must be some more advanced mechanisms contained within. Why? Because we can tell whether a sound is coming from our back or our front.
Imagine the case in our ears were replaced with two simple mono microphones of appropriate sensitivity. Any sound would be picked up by the two microphones, though at different delays. Knowing the speed of sound, it is simple to calculate the distance of the source from each microphone and hence, the intersections (locii). Note the word intersections. With two point microphones, there are two intersections (in a 2-D scenario) and infinite intersections (dispersed in a circle in a 3-D scenario).
If our ears were only point microphones, we would not be able to differentiate sounds coming from behind us.
But of course, we can. That is because our ears are directionally sensitive, or at least, structures in our ears serve as directional filters.
One interesting result of this line of thought is that with headphones, it is not possible to duplicate full directional sound. Headphones are merely two point sources of sound; there will be ambiguity of direction, leading us to confuse front-back sounds.
1 comment:
Although I see your point that the ear can differentiate the source of the sound from front and back and that it is calculated from the difference in the time of picking up those sounds between ears, I believe that there's evidence that runs contrary to your proposition that headphones cannot emulate full directional sound.
http://www.youtube.com/watch?v=8IXm6SuUigI
They do claim in their video that the illusion of directional sound is created through the contrast of the output of sound through the different sources, and it seems that they're able to create the illusion of sound coming from left and right, as well as upwards and downwards.
Somehow the brain is able to figure out a 3D sound field from two intersections.
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