iTestMic works with all of the acoustic testing modules in AudioTools, including the SPL Meters, RTA, FFT, ETC, Impulse response, and now transfer function, using the internal signal generator. Every unit is calibrated at the factory with a NIST-traceable pressure-compensated SPL calibrator, and the calibration values for both low and high range are stored in the microphone itself, in non-volatile memory. Then, when you plug it into your iOS device, the calibration values are transmitted to the app automatically, giving you an instantly calibrated SPL meter on any iOS device, with no other calibration required.