Naturespace, one of the top-grossing health & fitness apps, helps you relax by immersing you in the sounds of a natural environment. I have this app and like it a lot. There are over 80 tracks, all recorded by Naturespace team in the wild. And all use a 3-D recording system that gives you the experience of being there. You can read more about the Naturespace Holographic Audio technology on the Naturespace website, as well as listen to some stunning nature sounds. The app is free, and comes with 6 tracks. An in-app catalog lets you purchase additional tracks. Version 3.0 was just released and, for an in-app purchase of a Visitors Pass for $1.99, you get access to 30-second examples of all of the tracks. Also, currently they have a sale that lets you purchase the entire catalog at one-third of what it would normally cost.