Normalising

Normalisation is a process that changes the volume of a piece of audio. It does this by first analysing the audio, looking for the highest peak. Then an amount of gain is applied to the entire section of audio, so that the highest peak is at 0dBfs. Because of the need to analyse the audio before applying gain, normalisation is an offline process – meaning it can’t be applied in realtime (as a plugin, for example). Also, because static gain is applied, the dynamics of the audio do not change. It’s exactly the same as adjusting the fader on an audio channel, except that there is a pre-calculation to determine how much to adjust it.

There are two problems with normalising:

  1. You don’t know or control how much gain is being applied. That’s because the amount of gain is determined by analysing the audio.
  2. The amount of gain being applied has nothing to do with how loud the audio is (as we perceieve it). That’s because the amount of gain is calculated from the peak level of the audio – not the RMS or average level (see here for more details about peak vs RMS).

Normalising audio ONLY makes sense if:

  1. Your audio started higher than 16 bits; AND
  2. You’re about to quantise to 16 bits (or similar) directly after normalisation; AND
  3. You don’t care what the average (RMS) level of the audio is after quantisation.

In other words, this is a process that makes sense where there are a series of offline gain stages, and somewhere in the MIDDLE the audio is being quantised to 16 bits (but subsequent processing is at a higher bit depth). The uncontrolled amount of gain is not a problem if later gain stages will also be applied.

In these situations, normalising is a useful way to maximise the dynamic range of a low-resolution digital system. This is because the audio is made as loud as possible before quantising so that the higher noise floor (caused by the low resolution) is as low as possible relative to the audio. An even lower relative noise floor is possible by using dynamic processing (such as compression or limiting), but normalisation is the best solution that doesn’t affect the original dynamics of the audio.

On the other hand, if your task doesn’t meet all three criteria, then there are more appropriate processes than normalisation.

-Kim.

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