ITunes is quieter than Quick Look
I have some MP3 tracks that are nice and loud when opened with Quick Look, but they’re noticeably quieter in iTunes. I have turned up the preamp but it’s still not as loud as Quick Look. I use an iMac with Yosemite and have the latest version of iTunes. Derek Champley The preamp isn’t usually the best way to boost the volume because it will add distortion. The reason iTunes is quieter for most tracks is Sound Check is turned on in its preferences. This feature attempts to normalise the loudness of songs by calculating the average loudness of a track by itself and then adding an adjustment factor to that track’s ID3 tags. This boosts the volume of a few very quiet tracks, but most tracks end up slightly quieter to compensate – about 10% is the figure I’ve heard. Sound Check allows you to play music from lots of different artists, recorded at different gain levels, without having to continually tweak the volume level up and down yourself. Sometime in the last couple of versions, Apple quietly tweaked Sound Check so that it preserves the intrinsic loudness of a track when you are listening in album mode. This allows artists to have intentionally quiet tracks, without iTunes dragging them up to match everything else. If you listen in shuffle mode, those quiet tracks will sound much louder, because Sound Check is boosting them.
The important thing to realise is that Sound Check doesn’t change the audio data in a file at all. It just adds an extra field in the ID3 metadata of a file. When you listen to MP3s through Quick Look, you are using the QuickTime audio engine, which ignores this metadata and plays the track at the gain level that was set when the track was originally mastered. For almost all tracks this will be louder than the level with Sound Check enabled. To get this experience in iTunes as well, just turn off Sound Check under Playback in the app’s preferences.