This app allows you to choose 5 Ancient Solfeggio Harmonics: 174 hz, 396 hz, 528 hz (most popular), 741 hz and 963 hz. You can also select 1 or more of the 5 background tracks: Alpha, Beta, Delta, Theta and Water. The app is very easy to use and you can do research on the web by looking up the Solfeggio Frequencies.