1. The file extension for the audio layer (layer 3) of a MPEG file is MP3. This layer uses perceptual (what can be heard) audio coding and psychoacoustic compression to remove redundant or irrelevant sound signals. It then adds a filter bank known as a Modified Discrete Cosine Transform (MDCT), which increases the resolution of the frequency. The resulting file is shrunk by a factor of 12 (from 1411.2 kbps to 112-128 kbps) without sacrificing sound quality. MP3 files are small enough to easily transfer across the Internet.