JamMixLab

JamMixLab is an online music composition and mixing lab. You place repeating musical parts (loops) on a timeline, combine them with instruments, and decide when each part will play. As you change their arrangement, you can hear how the track takes shape and save your creation. You can also adjust the tempo and export your mix as a WAV file.

Try starting with a single rhythmic part and bringing in the melody later. Then have both parts play from the beginning and compare the two versions. How does the introduction change, and what happens to the overall structure of the track? Using the same sound material, you can experiment with different arrangements and decide where you want more or less musical activity.