Click here to purchase the entire book in PDF format. Lord Rayleigh to Wightman and Kistler How do you localize sound? For example, if you close your eyes and you hear a sound and you point and say ``the sound came from over there,'' how did you know? And, how good are you at doing this? Well, you have two general things to sort out Which direction is the sound coming from? How far away is it? We determine the direction of the sound using a couple of basic components that rely on the fact that we have two ears Which ear got the sound first? Which ear is the sound louder in? What is the timbre of the sound? How do things change if I turn my head a little bit? The first thing you rely on is the interaural time difference6.2 (or ITD) of the sound. If the right ear hears the sound first, then the sound is on your right, if the left ear hears the sound first, then the sound is on your left. If the two ears get the sound simultaneously, then the sound is directly ahead, or above, or below or behind you. The next thing you rely on is the interaural amplitude difference (or IAD). If the sound is louder in the right ear, then the sound is probably on your right. Interestingly, if the sound is louder in your right ear, but arrives in your left ear first, then your brain decides that the interaural time of arrival is the more important cue and basically ignores the amplitude information. You also have these things sticking out of your head which most people call their ears but are really called your pinnae. These things bounce sound around inside them differently depending on which direction the sound is coming from. They tend to block really high frequencies coming from the rear (because they stick out a bit...) so rear sources sound ``darker'' than front sources. These are of a little more help when you turn your head back and forth a bit (which you do involuntarily anyway). A long time ago, a guy named Lord Rayleigh wrote a book called ``The Theory of Sound''[Rayleigh, 1945a][Rayleigh, 1945b] in which he said that the brain uses the phase difference of low frequencies to sort out where things are, whereas for high frequencies, the amplitude differences between the two ears are used. This is a pretty good estimation, although there's a couple of people by the names of Wightman and Kistler that have been doing a lot of research in the matter.