Via LifeHacker is a tool called BPM Analyzer. Feed it some music files, and it will analyse them to calculate the number of beats per minute (BPM) in the songs. If the file is an MP3, it will also store the BPM value in the file’s ID3 tag, but it will also analyse other formats without saving anything. There are both Windows and Mac versions to download.
Why would you do this? Well, you can use it to create playlists of music of different tempos – a low BPM playlist for one you want to chill out, or a high BPM playlist if you’re working out. And if you use iTunes DJ and its crossfade ability, you can ensure that the music you’re playing is at similar tempos for a smoother mix.
One of iTunes cool but under-appreciated features is its Smart Playlists ability. Rather that dragging and dropping music into a playlist every time you add a new song, you can set a series of criteria and iTunes will automatically add matching songs to the playlist. By default iTunes come with playlists such as ’90s Music’, so all files with a released date between 1990-1999 are added to it. It will also dynamically update, so if one of Bon Jovi’s greatest hits is accidentally marked as being from 1992 but corrected back to 1987, it will be removed from the playlist.
One of the criteria you can set is ‘BPM’, so we can make a low BPM chillout smart playlist.
First of all, download and install BPM Analyzer and set it to analyse your iTunes library. You’ll probably have to do each artist individually as it seems to crash when presented with lots of files (on OS X at least). Note that while it will update the tags for MP3 files, for any AAC files you’ll have to note that BPM value and manually enter it in iTunes by right-clicking the file, selecting ‘Get Info’, choosing the ‘Info’ tab and typing the number in the ‘BPM’ box. You may also find that iTunes doesn’t pick up the BPM from MP3 files unless you open ‘Get Info’ for each one.
Once you have scanned your library, in iTunes, go to ‘File’ and ‘New Smart Playlist’. Under ‘Match the following rule’, change ‘Artist’ to ‘BPM’. Change ‘is’ to ‘is less than’ and type in ’90′. Make sure ‘Live updating’ is ticked. You can also set a maximum song limit if you wish. Click OK, and then give your new playlist a name. Et voilà.