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I have found that many do not like the Sound Check feature in iTunes.

Will enabling Sound Check lower sound quality?

Why does/does it not reduce sound quality?

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No, it is not bad to have sound check enabled in iTunes.

Having it enabled does not make the sound quality of songs worse.

Sound Check works simply by dynamically raising or lowering the playback volume, just as you yourself can raise and lower the playback volume in iTunes without affecting sound quality.

iTunes raises/lowers playback volume according to the audio normalisation ID3-tag within the audio file. These tags are created automatically when you rip a CD with iTunes, and automatically added when you import files lackings these tags into iTunes.

The normalisation tag simply tells what the average volume of the song is. Note that iTunes recognizes albums, so that when playing back and album, it will adjust according to the average track volume of the album as a whole, instead of changing the playback volume for each song individually. Therefore your album listening experience is not harmed.

iTunes automatically ensures that the volume is not raised so high that neither clipping occurs or compression effects are introduced. Therefore your sound quality is not harmed.

All in all this means that enabling Sound Check is safe, does not change the audio content of your songs, thus not reducing their quality. It simply change the playback volume slightly so that you get an pleasurable listening experience without having to tweak your playback volume every time you select a new album.

  • When you enable sound check on and iDevice, does it do the exact same thing as I would do in iTunes on the Mac? Even the part about if you were only the play an album, it would adjust to the whole album and not every song that you have on your device? – StrangeRanger Aug 8 '17 at 1:18
  • Note that whilst this answer is basically correct, the explanation of how it works is in error. There is no normalisation or compression applied, it is done using a 'last before the output' volume control called ReplayGain – Tetsujin Aug 8 '17 at 7:00
  • @Tetsujin This is exactly how I wrote that it works - iTunes ensures that no compression occurs, and the audio content is unchanged. The name of the tag is "normalisation", so even though no normalisation is applied to the audio content, the tag is still called that. – jksoegaard Aug 8 '17 at 9:26
  • "ensures that no compression occurs" is simply an incorrect statement. If you increase the signal on digital audio far enough you do not get compression, you get clipping. Secondly, changing the volume even at the last stage before converting to analog always introduces change, however minimal [this would not be noticeable by anything other than a direct digital comparison so can be considered irrelevant] [cont'd...] – Tetsujin Aug 8 '17 at 9:32
  • [cont'd...] Thirdly, whatever the tag is called, normalisation is not an average volume, it is a peak measurement. Almost all commercial audio has already been 'normalised' [if not also then re-balanced at album level] – Tetsujin Aug 8 '17 at 9:32
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I was experiencing distortion on both headphones and via airplay (when playing music from my iPhone). When investigating, I discovered that sound check was on. I turned it off and the distortion went away. The tracks where I noticed this were:

Field of Dreams, opening credits (track 1). Star Trek V, opening credits (track 1).

Note: Both albums were ripped from CD using iTunes, 256 kbps AAC and played via iTunes Match.

  • I had exactly the same issue. – pkolodziej Dec 22 '18 at 11:24

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