This spirit level app helps you to bring your work into level. The sound indicates when the app is running. When the sound changes the iPhone is in level, if it is calibrated correctly. You can turn the sound on / off. Use the calibrate buttons to calibrate your iPhone. Below the air bubble you can read the value of calibration for next time. The default button restores the values.
Use the app if you for example have to put up a shelf and have to mark where you have to drill hole number two.
The app will function optimally when the iPhone is laying down.