White paper, ok it may be a blue paper if you've messed up your colour temp on the monitor ,  but the colour doesn't matter  this post is  within the realm of audio.  Located over at Telos , makers of fine broadcast studio equipment , this paper on  AAC ( Advanced audio codec )  takes a look at perceptual encoding and why AAC beats MP3 into the ground in terms of quality at lower rates.

Based on how we perceived sound the codec doesn't just  efficiently store the data by recognising patterns and then  knocking out the repetitious info like a jpg or zip file. This one's a little smarter. When we hear a sound at a certain volume it masks other sounds that we can perceive for a brief period of time.  Accurately modeling the ear in this manner allows the encoder to ignore  any data you wouldn't have heard anyway thus making more room available  for storing that which you can hear  at the same bit rate.