Audio files contain complex signal information that is spread over a wide range of frequency bands. At a particular time in a song some of these frequency bands will have louder sounds in them while other bands will have softer sounds.

The MP3 and AAC methods reduce the total amount of audio data by using less accuracy to encode the softer sounds, making use of the fact that loss of accuracy of the softer sounds is not as overtly apparent to your ears as loss of accuracy of the louder sounds.

However, we have found that this loss of accuracy of the softer sounds of a song does cause a recording to lose a significant amount of the richness and fullness of the original CD recording, leading to a somewhat muffled sound quality. For example, the upper harmonics of an instrument note or vocal can be at a soft level but they are very important to conveying the full rich sound of the note, and they are typically highly diminished in an MP3 or AAC song file.

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