The reason that we can localize the source of a sound accurately is that we have two ears. At each ear, a slightly different signal will be perceived and by analyzing these differences, the brain can determine where the sound originated. The two most important localization cues are the Interaural Time Difference, or ITD, and the Interaural Intensity Difference or IID. The IID arises from the fact that, due to the shadowing of the sound wave by the head (head shadow), a sound coming from a source located to one side of the head will have a higher intensity, or be louder, at the ear nearest the sound source. (The smallest IID most people can detect reliably is about 1dB.) One can therefore create the illusion of a sound source emanating from one side of the head merely by adjusting the relative level of the sounds that are fed to two separated speakers or headphones. This is the basis of the commonly used pan control.