How MP3 formatting reduces audio file size

Have you ever wondered why or how an audio file is converted into an MP3 format and seems to magically go from a rather large amount of megabytes to a seemingly impossible smaller amount usually under 5 megabytes?

Obviously the main reason for converting audio files to the smaller and more efficient MP3 format is to save space on music players and hard drives. More space means more room for even more files.

The abbreviation MP3 actually stands for MPEG Layer 3 and is one of the 3 kinds of MPEG audio formats available. Layer 3 has become the standard for most audio conversions due to it's ability to not only compress files in size but minimize the amount of audio quality loss due to compression.

So what happens to an audio file when it is changed or compressed to the MP3 format?

In technical terms, when a file is converted to MP3 format the coding in MPEG Layer 3 is using perceptual audio coding and psycho-acoustic compression to remove all the superfluous information that isn't going to be needed when utilizing the file later on. In layman terms Layer 3 coding takes out of the the audio in the file that is either redundant or unable to be heard by the human ear at all.

Some people may falsely believe that compressing into the MP3 format means the end product is reduced in the amount of audio range one is hearing. While that is technically true since some audio is removed during compression our human ears wouldn't have been able to hear audio on those frequencies anyways. Since any of the information in the file takes up space, so does this audio that we can't hear, removing those bits of information only reduces the size of the file.

Usually when a file is compressed some aspect of the quality is reduced. With other MPEG layers and some other formats this may be the case, but with MP3 files there is coding built in that in essence fixes any quality reduction so that it sounds the same as the original file. Sound data is created and stored in various bit rates, on a CD the average bit rate is around 1411 kilobits(kbps) per one second of stereo music. The MP3 format reduces this amount to about 114 kilobits per one second of stereo music.

Now that is a large reduction in file size, so how does this MP3 format maintain the original audio quality as accurately as possible?

Besides only removing the audio our ears can't detect to reduce size, the format coding also adds a MDCT (Modified Discrete Cosine Transform) which puts a filter into place that restores the frequency resolution to about 18 times the resolution of a Layer 2 format. In plain terms this means that the MP3 format successfully reduces the file size greatly and then restores most of the original frequency quality so that the new file sounds just like the other to human ears.

Some individuals may argue that the MP3 format is not accurate in recreating the original uncompressed piece of audio. Some even state that they can hear the difference which for most people is only possible through using equipment or software that can detect and show the differences.

A lot of companies that sell MP3 versions of CDs and singles have noticed this kind of 'higher kilobyte audiophile' in the consumer market and offer 'higher quality' formats that have a bit-rate that is on average double the size of an MP3 file. Generally, and as good business sense would support, these formats cost more. However for most music listeners, the audio format known as MP3 blends space saving file conversion with relatively high quality audio representations of the original files.