Click here to purchase the entire book in PDF format. Introduction Our ability to perceive things using any of our senses is limited by two things: physical limitations and the brain's ability to process the information. Physical limitations determine the absolute boundaries of range for things like hearing and sight. For example, there is some maximum frequency (which may or may not be something about 20 kHz, depending on who you ask and how old your are...) above which we cannot hear sound. This ceiling is set by the physical construction of the ear and its internal components. The brain's ability to process information is a little tougher to analyze. For example, we'll talk about a thing called ``psychoacoustic masking'' which basically says that if you are presented with a loud sound and a soft sound simultaneously, you won't ``hear'' the soft sound (for example, if I whisper something to you at a Motorhead concert, chances are you won't know that I exist...). Your ear is actually receiving both sounds, but your brain is ignoring one of them.