The headset input device, including the Lightning headset adapter that comes with recent iPhone models, supports units of FS (full-scale, or maximum input before clipping), and V (volts). The single headset input channel supports various engineering units, in addition to FS. One reason for including separate units and sensitivities, for the headset input device and the headset input channel, is to enable a voltage calibration, which then allows you to enter a standard sensitivity for a transducer (e.g. 50 mV/Pa for a microphone or 100 mV/g for an accelerometer).
The default voltage sensitivity of the headset input devices is defined for each iOS device model and the default pressure sensitivity of the headset input channel is 5.0 mV/Pa.
It is possible to adjust the input gain of the headset input for some iOS devices. In this case, SoundMeter offers Low, Mid, and High gain settings with nominal sensitivity values for each setting. This means that the gain can be changed and the level measurements retain their accuracy. User calibration can be performed for each gain setting independent of the others. The channel sensitivity is independent of the device sensitivity and is therefore independent of the device input gain setting.
The Gain setting can be used to adjust the measurement range. To measure higher signal levels, choose a lower gain setting.