A range of compression techniques can be used to reduce the size of sound or audio files, making them easier to transmit over the Internet and reducing the amount of disk space needed to store them. This has led to the development of hand-held audio players, such as the iPod.
One of the best known audio compression formats is MP3, which allows a CD-quality recording to be compressed by a factor of 10 – 14 without noticeable deterioration of the audio quality. MP3 compression makes use of a technique known as perceptual noise shaping which takes advantage of the characteristics of the human ear. For example:
- there are some sounds that the human ear cannot hear at all because they are too high or too low
- the human ear hears some sounds better than others – high notes seem louder than low notes
- if two sounds are playing simultaneously we will only hear the louder one
MP3 is a lossy compression technique. Some of the original audio information is removed during compression and cannot he recreated exactly. However, it is possible to choose the amount of information that an MP3 file will retain or lose during compression. The most important factor is the bit rate, the number of bits per second encoded in the MP3 file. MP3 bit rates normally range from 96 to 320 kilobytes per second (Kbps). The lower the bit rate, the more information will be discarded. A bit rate of 128 Kbps is often described as FM radio quality. Many music download sites use a bit rate of 160 Kbps or higher, often described as CD quality.
Despite the fact that more recent audio compression formats, such as AAC (Advanced Audio Coding) offer better audio quality and smaller file sizes, MP3 remains the market-leading format. However, many musicians (Neil Young is one of the most notable) and audio fans are critical of the quality of the MP3 format and believe that it is better to use a lossless type of compression such as FLAC (Free Lossless Audio Codec). Ivor Tiefenbrun, chairman of Glasgow-based Linn Records, believes that MP3 was a necessary evil when download speeds were slow and disk storage expensive, but this is no longer the case. Linn manufactures FLAC players and sells many of its recordings as high-quality FLAC files which are superior to CD audio in quality.