Sound localization is a very complex phenomenon and in fact your brain uses a few methods to localize sound, some of which work better at high frequencies and some at low frequencies. The basic bits of information that your brain makes use of is interaural time differences (the difference in time taken for a sound to reach different ears), as well as interaural level differences (one ear hears a louder sound than the other). Furthermore, the outer ear, or pinna, modulates sound so that it sounds different depending on whether the sound is coming from in front of you or behind. This seems to be the dominant effect for sound sources that lie on the median plane, as pointed out by dmckee.

How can I hear if something is behind or in front of me?
If an NPN BJT is being used as a switch, why is the load usually connected between +Vcc and collector? How is the performance affected if the load is connected between emitter and ground?
