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I think we can credit Darwin's natural-selection process for our sense of aural directionality. The ability of our remote ancestors to detect the precise direction from which a saber-toothed tiger was coming (as a guide to the direction they should be going) contributed significantly to our species' survival. The ability to localize sound contributed to the survival of many other species as well.
Human sound localization has been under study since about the middle of the 19th century. Ultimately, it became clear that we use the differences between the sounds reaching our two ears to derive our perception of directionality. English physicist Lord Rayleigh (1842-1919) was the first to determine that a sound coming from the far right or left is significantly more intense in the ear closest to the sound source. ("Significant" means that the sound pressure level, or SPL, difference provides sufficient raw material for the brain to work with.)
The SPL difference is easy to explain: The listener's head partially deflects and/or blocks the impinging sound wave from …