Here is another iOS app that helps you experiment with sound frequencies. iFunctionGenerator lets you create up to 20 generators at different frequencies. It supports square, triangle, sine waves as well as white and pink noise. You can customize it by entering a frequency from 1 to 20,000Hz.
For each generator, you can change left/right phases, volume, and other settings. You also get a convenient mute button to silence the generators. The app is available as a download for $2.99.[where to get it]