MP3Gain analyzes mp3 files to determine how loud they sound to the human ear. It can then adjust the mp3 files so that they all have the same loudness without any quality loss. This way, you don't have to keep reaching for the volume dial on your mp3 player every time it switches to a new song.
MP3Gain operates in two modes, Radio and Album.
Radio mode allows a mix of unrelated songs to be volume-corrected to a selected level. MP3Gain calculates the volume level for each song individually. It then corrects each song to make its volume level match the Target Volume. For example, if you have 3 songs that have volume levels of 86, 91 and 89 dB and you use Radio Gain to convert them to the Target Volume of 98 dB, they will all be at approximately 98 dB.
Album mode allows a selection of related songs (as they would appear on an album) to be volume-corrected relative to each other. MP3Gain first calculates the album volume level by finding the volume level of each song and then calculating a volume level for the entire album. It then finds the ratio between the album volume level and the Target Volume. It then corrects the volume for each song based on the ratio. For example, if you have 3 songs that have volume levels of 86, 91 and 89 dB and an album volume level of 90 and you use Album Gain to convert them to the Target Volume of 98 dB, then the 3 songs will be corrected to 93.6, 99 and 96.9 dB respectively:
Album volume level 90 / Target Volume 98 = 1.088888
Song #1 volume level 86 dB * 1.088888 = 93.6 dB
Song #2 volume level 91 dB * 1.088888 = 99.0 dB
Song #3 volume level 89 dB * 1.088888 = 96.9 dB
With Album mode, you want some songs to be noticeably quieter than other songs, just like they are on an album. If you're playing a classical CD, you expect the track with the flute solo to be quieter than the track with the big full-orchestra finale. Album mode allows you to correct an entire album while keeping each song's volume level relative to the other songs.
MP3Gain is completely freeware. No registration, no time limits, no disabled features, no annoying pop-up messages, or anything like that.
There will always only be one current version of MP3Gain, and that version will always be freely available to everyone.
With that said, if you really like this program and feel like making a donation to the author, you can find the appropriate information by clicking Help -> About in the menu.