From WikiAudio

[edit] Normalize

normalization is the process of increasing (or decreasing) the amplitude of an audio signal. Typically normalization increases the amplitude of the audio waveform to the maximum level without introducing any distortion. If there was already distortion, it will leave it distorted and only adjust the amplitude.

Specifically, normalization applies a constant amount of gain to the selected region of the recording to bring the highest peak to a target level, usually 98% (-0.3 dB) or 100% (0 dB). This differs from dynamics compression, which applies varying levels of gain over a recording to fit the level within a minimum and maximum range. Normalization applies the same amount of gain across the selected region of the recording so that the relative dynamics (and signal to noise ratio) are preserved.

Normalization may require two passes, depending upon the software employed. A first pass would determine the highest peak, and the second pass applies the gain to the entire recording.

Normalization is often used when remastering audio tapes for CD production, in order to maximize the signal level while not changing the signal to noise ratio. It is often combined with dynamic range compression and hard limiting to increase the apparent volume of a CD. It is typically applied along with other audio and digital processing, such as dithering.