Brain Waves is an app that lets you listen to sound frequencies that in theory have different properties. You can customize the sound frequency you want to hear, although the app comes with certain presets: concentration, relaxation, healing, etc. It's important to keep in mind that to properly use Brain Waves you don't need to have the sound too high. In fact, although you can set the frequencies at quite a high volume, it's not advisable to do so. Especially since it's recommended to use the app with headphones. Brain Waves is an app whose benefits aren't fully proven but that's sure to interest a lot of people. In the end, whether these kinds of sounds work or not, there are lots of users who find them relaxing or stimulating.
Requires Android 4.1 or higher.