This article isn’t about mastering a CD so that the songs are all at the same perceived volume, it’s about getting your music collection to play back at reasonable volumes relative to each other. It’s also about providing this feature to your digital distribution customers.
I’m sure you’ve at some point put your music collection into shuffle play only to find that the transition from Rachmanninof’s Symphony #2 into Marilyn Manson’s Mechanical Animals is enough to make you piss your pants. You aren’t alone. The audio geekery community has come up with a solution to the problem of CDs having drastically different volumes: ReplayGain. Apple has their own solution they call Sound Check, although by most accounts it seems to be less sophisticated.
None-the-less, there is a wealth of software and hardware out there between iTunes/iPods and the various ReplayGain-capable audio players that support these features. All you have to do before making your music available for download is to run it through a program that supports writing of SoundCheck (such as iTunes) or ReplayGain tags (such as Foobar 2000) into your music format of choice. The best thing to do is to run your music through both iTunes and a program that writes ReplayGain tags so that your music will play back volume-corrected on both systems if the listener has them enabled. Otherwise, if a listener drops your untagged music into a player that is volume-correcting the rest of their collection, your tracks may blow them out of the water. It’s just good etiquette to provide those tags out-of-the-box.
Essentially what both of these algorithms do is scan the music for perceived volume by calculating the RMS volume of the material, then attenuates (or, if necessary increases) the volume to achieve a reference RMS volume. The cool thing about this is that the volume is adjusted by the player, not in the file, so your files are effected losslessly. Sound Check does this on a track-to-track basis, whereas ReplayGain supports both per-album and per-track volume correction (depending on how you like to listen to your music).