Sunday, November 13, 2005
We really ought to have three ears.
The brain calculates the origin of a sound in part by measuring the time elapsed between the sound’s arrival at one ear and its arrival at the other. Other procedures accompany that one, such as spectral analysis. (For instance, your brain knows that distant sounds lose treble, even if you don’t.) However, the elapsed-time calculation is a fundamental one.
The brain’s calculations of the origins of sound work astonishingly well; however, they IS prone to error. For instance, a sound reaching your skull from directly behind you can sometimes sound as if it comes from somewhere else. It’s an unsettling illusion, because it persists even after you’ve recognized it, as long as you don’t move your head.
If we had three noncollinear ears, we could triangulate, and such ambiguities would vanish.
(Sticklers for the geometric truth might gripe that, were the elapsed-time calculation the brain’s only method for determining the origin of a sound, then every sound not originating from the plane spanned by the three ears could be interpreted as having one of two distinct origins. We would need four ears in general position to truly make no mistakes. However, choosing from among two choices is so much easier than choosing from a continuous circular array that I think our brains could adequately handle the task using spectral analysis or other tricks. That is, I don't think a fourth ear would be worth the trouble.)
The argument that we need more than two receivers to adequately determine the spatial origin of signals applies only to ears, not eyes. If we were content with knowing the direction of a signal, then a single eye would be enough; we use two of them so that we can determine how far something is from the receiver. (Astonishingly, we manage the same task with our ears. I don't think an extra ear would help with the distance calculation at all.)