Psychoacoustics studies the brain's sound elaboration mechanisms. A knowledge of such mechanisms is indispensable for the practice of sound engineering, in that it allows, through appropriate manipulation, very sophisticated sound-effects to be obtained. One of the most important factors in the elaboration of sound, is that the brain elaborates two channels of information simultaneously: those that enter through the left ear, an those through the right ear. It is the (sometimes subtle) differences between these two signals that determine the coming-into-being of a new piece of information associated to the composition of the two sound waves. In this case we're talking about stereophonic sound. When, on the other-hand, two signals that reach to the ear are exactly the same we call this monophonic sound.
We have seen how the execution of a note excites the fundamental frequency and all its harmonics, the latter being multiple frequencies of the fundamental [Harmonic content of a wave form ] . The brain is capable of reconstructing, even if imperfectly, the fundamental frequency by basing itself on the higher harmonics. To contextualise this characteristic lets imagine a little beach radio with its tiny, super-rigid loudspeaker, playing the latest summer hit. The latter will be reproduced with a highly reduced frequency band but nevertheless we will be able to hear the bass line thanks to the brain's capacity to reconstruct the fundamental from the information received from its harmonics.
Related topics on Wikipedia
