SoundMeter X for iOS turns your device into a handheld data-logging sound level meter.
Input signals can be acquired from the current audio input path (iOS automatically selects the audio input path according to which external audio devices, if any, are connected to your device). Sensitivity calibration can also be performed for any audio input device or input channel.
One reason for including separate units and sensitivities, for the input device and the input channels, is to enable a voltage calibration at the device level, which then allows you to enter a standard sensitivity for a transducer connected to a particular input channel (e.g. 50 mV/Pa for a 1/2-inch measurement microphone).