Q: I’m curious about the Sound Check options in iTunes and on the iPod. Do they only adjust the volume during playback, or do they actually adjust the volume of the actual MP3/AAC files? I’ve heard some people complain about it distorting songs. Can you give me a better description of what the Sound Check option really does and how effective it is?
A: When you turn on the Sound Check option in iTunes (Preferences > Audio), iTunes analyzes your music to determine each track’s “average” volume. It determines how much of an adjustment it should make when playing back music to ensure that the overall volume level is more or less constant. You can see the amount of adjustment that iTunes makes as a number of decibels (dB) when looking at info about a track. Select a track, then select File > Get Info and click the Summary tab. The Volume shown (with either a plus or minus amount) is the adjustment made during playback.
If you have turned this option on in iTunes, and then activate it on your iPod (Settings > Sound Check), the iPod will take this volume change into account and increase or decrease the volume accordingly.
Sound Check is only really useful if you listen to many types of music. Say you listen to Metallica and Bach; the Metallica will get lowered a few dB, and the Bach will be played back a bit louder. It can save you changing the volume manually as tracks change. But note that you must turn this on in iTunes to be able to use it with your iPod; the iPod can only make this adjustment if iTunes has set a volume level change.
As for distortion, it is entirely possible that very soft or very loud music could be distorted if the adjustment is large enough. However, we looked through one of our ambient music collections and found tracks where the volume was adjusted as much as +17 dB without any noticeable distortion. The highest negative adjustment we found in that iTunes library was about -11 dB, and again, we didn’t hear a noticeable difference with that song. Noticeable distortion will depend as much upon the quality of your ears and source material as on iTunes and the iPod’s ability to change specific levels.