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Newbie question. I originally asked this here in the comments but I was advised to post this as a separate topic.

If I understand correctly, compression reduces the volume range of a sound so that it can be made to sound louder without going over 0 dB. When I enable normalization in iTunes, it tries to ensure that all my music is played at approximately the same volume.

How are these two things different?

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2 Answers

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Normalization is the process of both making the loudest peak 0 dB and making all the tracks the same volume.

Compression means that you lower the peaks to get a more consistant volume so you can make it louder to get the highest peak at 0 dB.

Well, you can't really go over 0 dB. At infinity dB the speaker is in the middle. At 0 dB the speaker is at the far end. So if you push the soundwaves above 0 dB the speaker will not go further out than the 0 dB point, and thus stop vibrating at these peaks leading to a eardefening screech.

In iTunes it means that it tries to get the overall volume to be the same in all your tracks.

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"making all the tracks the same volume" But what if you have a file at a constant volume of 0 dB and another file with a constant volume of -20 dB except for a short peak at 0 dB? Then you can't bring them at the same volume unless you lower the volume of file 1 or apply compression to file 2, can you? –  Pieter Feb 15 '12 at 15:18
    
@Pieter: true, that's why peak normalization does not make all tracks the same volume. But that's not the only possible choice of normalization, better software usually offers also RMS normalization, which actually makes the overall average volume equal. The problem is that you then always have to keep some "safety headroom". — While that "speaker membrane at the far end" description makes the right point, one should not take it too literally: the actual amplitude of the speakers depends a lot on frequencies, power amp setup etc.. The 0 dB limit is due to electronic / digital restrictions. –  leftaroundabout Feb 15 '12 at 15:37
    
"The 0 dB limit is due to electronic / digital restrictions." Yes! If you look at the waveform, infinity is kind of no power to the speakers, and 0 dB is full power. But in reality the electrons in the cable goes back and forth thus making the speaker membrane go back and forth, thus making sound. If at 0 dB the membrane has more to go, you can AMPLIFY the signal (increase the power of the electricity) to make it louder, but that doesn't give more room for the waveform, it just gets played louder.. –  Friend of Kim Feb 15 '12 at 15:53
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Normalisation usually looks at the waveform over the entire track (or entire album) to identify the peaks and lowest points then alters the amplitude to make those consistent with all the other tracks.

Normalisation may be carried out by compression, or it may be just by simply increasing or decreasing volume.

Compression is more around making the loudest parts of a track quieter by compressing the dynamic audio range.

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