Car Audio Toolbox provides various tools every car audio installer needs into one package, it includes:

-A decibel meter

-An offset to calibrate the decibel meter

-Various test tones for a subwoofer

-Two resistor calculators for four and five stripe resistors

note: the offset is included so that you can calibrate the decibel meter, also because different cases for the iphone affect the sampling quality

note: ipod touch users must have a microphone, the apple earbuds with the integrated mic work great but you may have to adjust the offset to compensate