Without any sort
of compression, it would take 1.4 megabytes to represent just one second of CD quality
music in WAV format, the de facto standard for sound on PCs. But by using MPEG audio
coding, the size can be shrunk down by a factor of twelve without losing any of the sound
quality.
Though MP3 compression is considered lossy because some data cannot be recovered after
compression, the MPEG algorithm can achieve transparent, or perceptually lossless,
compression. After testing, it was concluded that expert listeners could not distinguish
between coded and original audio clips even with a six to one compression ratio (Pan 3).
MP3 compression is so successful for imitating CD quality music because it utilizes the
concept of auditory masking. Basically, this type of masking occurs when the presence of a
strong audio signal makes weaker audio signals in the proximity imperceptible. This
non-linear and adaptive threshold of hearing (the level below which a sound is not heard)
varies with frequency and between individuals. Whether a person hears a sound or not
depends on the frequency of the sound and whether the amplitude is above or below that
persons hearing threshold at that frequency. For example, in the vicinity of a loud noise
such as an airplane passing overhead, it is impossible to hear ordinary conversations due
to the distortions present at the hearing thresholds of the individuals. Sounds that are
inaudible due to dynamic adaptation of the hearing threshold are said to be masked.
This effect is particularly relevant to music. A loud orchestra easily masks the sounds
of some individual instruments playing softly. Similarly, when this orchestra is recorded,
the masked instruments will not be audible to the listener. A recording, in order to
achieve efficiency, should discard the inaudible data and instead use that space to store
other audible data. Drawing a parallel to digital audio compression, in order to achieve
efficiency, an MP3 should drop the inaudible areas of data and, in that form of
compression, save space.
When sounds are digitized, the computer grabs all the sound and stores it as binary
data. Even if parts of the sound are beyond the range of human hearing, they're digitized
anyway. MP3 compresses the file by removing any numbers representing sounds beyond the
range of human hearing.
