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Audio 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.
Normalizers
- Audacity has a normalizer plugin which adjusts the gain of each individual channel based on each channel's peak.
- Audiograbber can normalize audio files in CD format.
- Goldwave can normalize audio files.
- iTunes has a nondestructive normalization feature introduced in version 3, called Sound Check, which embeds metadata about a file's loudness either within the file or in a separate database
- Levelator is a free multiplatform audio normalizer, compressor and more.
- Media Player Classic includes a built in audio switcher which can both normalize audio and also boost the audio levels by performing a gain.
- Mp3Doctor PRO can also normalize audio files.
- MP3Gain analyzes and losslessly normalizes mp3 audio files.
- Traverso can normalize audio files.
- Apple's SoundTrack Pro has a normalizer built in.
- Apple's GarageBand 4.1 applies normalization as a preference.
