Octave RTA and FFT modules support real-time microphone frequency response compensation. Measurement microphones are provided by the manufacturer with a unique calibration file which can be downloaded based on the serial number.
A plain text file, with frequency response entries listed one per line. Each entry includes frequency and dB difference, separated by tab or space. The app will use linear interpolation to find the correction value for a particular frequency. Lines starting with * or ; are considered comments.
Below is a sample file for the UMIK-1 microphone
For a quick reference get an inexpensive external mic such as Dayton Audio iMM-6 which is compatible with the iPhone and plugs directly into the headset jack.
Pro users may consider using a dedicated measurement mic such as miniDSP UMIK-1 Measurement Microphone. Each unit comes with individual calibration file, a tripod and USB cable. Requires Lightning to USB Camera Adapter.