If you have used several music sources to build up your iTunes library (e.g. online music services, ripping your audio CDs, recording live performances, etc.), then there is a high probability that the volume levels between each file that you play will vary -- sometimes enormously! This can spoil an otherwise great listening experience when you have to continually play around with the volume controls in iTunes or on your speaker system.
To try and resolve the issue of volume differences between tracks, Apple has included a feature called, Sound Check. Without going into too much detail, this option (which is disabled by default), scans the contents of your music library to measure the differences in loudness between all songs. It then computes a playback volume level for each file (a normalization value) which is usually stored in the digital audio file's metadata -- this is similar to how ReplayGain works in fact. The great thing about this method is that the sound data isn't directly changed and therefore it is a non-destructive alteration to the files in your iTunes library.
To configure Sound Check in iTunes, follow these steps:
For the PC version of iTunes
- Click the Edit menu tab on the main iTunes screen.
- On the pop-up menu that appears, choose the Preferences option.
- Click the Playback menu tab.
- Enable the Sound Check option by clicking the check box next to it. Click OK to save the settings.
For the Mac version of iTunes
- Click the iTunes menu tab on the main screen.
- Choose the Preferences menu option on the pop-up screen that appears.
- On the main preferences screen, click the Playback menu tab.
- Click the check box next to the Sound Check option to enable it. Click OK to save settings.
If you want to see the volume adjustment Sound Check has applied for a particular song, right-click one of them in your iTunes music library list and choose the Get Info option. You will see the normalization amount (measured in dB) next to Volume in the summary section.