Frequencies

As encoding audio, it is important to understand how sound operates and at what frequencies the human brain processes audio data. The human ear is tuned to a very narrow audio vibratory band. Frequencies are usually described in units called Hertz (Hz), which simply translates as "cycles per second."

Typically, humans cannot hear frequencies below 20Hz or 20 cycles per second, nor above 20kHz or 20,000 cycles per second. While hearing abilities vary from one individual to the next, it's generally true humans perceive midrange frequencies more strongly than high and low frequencies, and that sensitivity to higher frequencies diminishes with age and prolonged exposure to loud volumes. Past the age of 35, most adults cannot hear much of anything above 16kHz, although women tend to preserve the ability to hear higher frequencies later into life than do men. The most sensitive range of hearing for the average listening audience hovers between 2kHz to 4kHz, a level probably evolutionarily related to the normal range of the human voice, which runs roughly from 500Hz to 2kHz.

An underlying tenet behind any audio codec is that there is little point in storing data that cannot be heard. Contrary to this principle, most commercial recordings store a tremendous amount of audio data that is never heard on account of the sensitivity of recording equipment and the broader range of sounds and audio resolutions undetectable by the human ear.

The term codec, used here frequently, is a foreshortening of the words "compress" and "decompress". It refers to any of a class of processes that allow for the systematic compression and decompression of data. While there are various codecs for the many file formats and transmission methods in existence (image and video compression formats having their own codecs) this book's focus lies with the different audio codecs.