From the "Because You Can, That's Why" department comes a web app from Music Hack Day Montreal that lets you draw a line by dragging dots around on a nice-looking graph, indicating the loudness (basically, volume level) of the song you want to hear. By matching your drawing against the loudness profiles of one million songs, the app find the one that best fits what you drew, then lets you hear that song on Spotify.
Maybe you want a song that starts out loud and gets quiet, or vice-versa -- or maybe you're wondering which song's loudness profile most resembles that of a camel's back.
Regardless, you can now find the answer, thanks to the aptly-named web app, Search for music by drawing a picture of it, developed by Paul Lamere, director of developer platform for The Echo Nest (publisher of Evolver.fm). All of the songs come from the Million Song Dataset, an open-source resource that helps developers build stuff like this without compiling their own musical data.
The app is live, and works even if you don't use Spotify (maybe you don't like Facebook). But if you do use Spotify, you'll be able to hear full-length versions of these songs by clicking them in the results list.
For the above drawing, it turns out the most-similar song was Circle Researcher's "After the Laughter," (Spotify link), which is a remix of Wendy Rene's "After Laughter" (Spotify link). Out of all of those million songs, its loudness profile fit our drawing the most accurately. Give it a try.
Adjust the shape of the curve and click Search to find tracks [on Spotify] that match the drawn loudness profile. More information here.