Chroma Doze (white noise)

Now you can sleep soundly with the soothing algorithms of Fourier analysis. Just sketch your favorite spectrum, and listen to a generated stream of white noise (or more precisely, colored noise). If sleep's not your thing, you can also use it with headphones to drown out neighbors, or tweak the settings to erode eardrums and frighten small children.

Notable features:
- No ads or weird permissions
- Doesn't just play a looped sample
- Relatively power-efficient
- Runs in the background
- Save your favorite sounds in the Memory list
- Contains math
- Open Source / Free Software (GPLv3 license)

If you'd like a sleep timer, this apps is compatible, and (the last time I checked) free with minimal permissions:
- https://play.google.com/store/apps/details?id=robj.simple.sleeptimer