Marshal Day Acoustics released a free app for iPhone and iPad to add and subtract decibel levels ±DBCalc. ±DBCalc is also available on the Google Play store.
A second free app, dBDelta calculates the change in decibel level over a change in distance, i.e. distance attenuation. The distance attenuation can be calculated for both point and line sources. dBDelta for iPad and iPhone is also available on the Google Play Store.