Audio normalization is the application of a constant amount of gain to an audio recording in order to bring the average or peak amplitude to a target level (the norm).
Because the same amount of gain is applied across the given range, the signal-to-noise ratio and relative dynamics are generally unchanged. Normalization differs from dynamic range compression, which applies varying levels of gain over a recording to fit the level within a minimum and maximum range.
Normalization is commonly amongst the functions provided by a digital audio workstation.
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One type of normalization is peak normalization, wherein the gain is changed in order to bring the highest PCM sample value or analog signal peak to a given level.
Since it only searches for the highest level, it does not account for the apparent loudness of the content. As such, peak normalization is generally used to change the volume in such a way to ensure optimal use of the distribution medium in the mastering stage of a recording.
Another type of normalization is based on a measure of loudness, wherein the gain is changed in order to bring the average amplitude to a target level. This average can be a simple measurement of average power, such as the RMS value, or it can be a measure of human-perceived loudness, such as that offered by ReplayGain and EBU R128.
Depending on the dynamic range of the content and the target level, loudness normalization can result in peaks that exceed the recording medium's limits. Software offering such normalization typically provides the option of using dynamic range compression to prevent clipping when this happens. In this situation, signal-to-noise ratio and relative dynamics are altered.