To calibrate the FFT analyzer in the general sound measurement, the calibration signal (AC signal) of the sound calibrator is used because of the following reasons;

-Generation of the microphone sensitivity value
-Generation of the preamplifier and microphone gain (insertion loss)
-Reference value of sound pressure is not 1 EU (reference value is 20 µ Pa)

Therefore, read voltage level of the AC signal (normal range sine wave) which corresponds to the constant sound voltage level which is generated by the sound calibrator by the FFT Analyzer. And convert this value into the sound pressure level of the sound calibrator to allow direct reading of the peak value or overall value of the power spectrum in sound pressure level (unit: dB).