Perceptual coding: The miracle of acoustic masking 

All of the MPEG perceptual codecs rely upon the celebrated acoustic masking principle ? an amazing property of the human ear/brain aural perception system. When audio is present at a particular frequency, you cannot hear audio at nearby frequencies that are sufficiently low in volume. The inaudible components are masked owing to properties of the human ear that occur at a very low ?hardware? level ? researchers say the information is dropped straightaway within the ear and is not passed to the brain. This appears to be a kind of ?natural rate reduction? that helps to keep the brain from being overloaded with unnecessary information. There is a similar effect working in the time domain, with signals coming soon after the removal of another being also inaudible.