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MP3, formally known as MPEG-1 Audio Layer III or MPEG-2 Audio Layer III, is a coding format for digital audio. It was developed largely by the Fraunhofer Society in Germany under the lead of Karlheinz Brandenburg, with support from other digital scientists in other countries.
MP3 is defined in two ISO/IEC specification families: MPEG-1: 11172-3 and MPEG-2: 13818-32. It uses lossy compression, which often allows for large reductions in file size compared to uncompressed audio.
Lossy MP3 compression works by attempting to reduce (or approximate) the accuracy of certain components of sound that could be considered (by some psychoacoustic analysis) to be beyond the hearing capabilities of most humans and storing the coefficients corresponding to these more salient frequency bands.
Compared to CD-quality digital audio, MP3 compression can commonly achieve a 75 to 95% reduction in size. For example, an MP3 encoded at a constant bit rate of 128 kbit/s would result in a file approximately 9% of the size of the original CD audio. MP3 audio is considered transparent at 320kb/s.
It is still very common to see MP3 files in the wild today, despite the fact that the format was finalized in 1993 (with modifications in 1995 to support lower sample rates and bit rates). This is due to the fact that MP3 could be considered the first widely adopted audio format that allowed for high quality audio to be compressed to a relatively small file size. Compared to more modern formats like Opus and AAC, MP3 may not seem as impressive, but it is still widely supported by many devices and pieces software and has certainly left a powerful legacy to live up to for modern codecs.