The sensitivity of the microphone is determined absolutely by primary calibration technique based on the reciprocity principle. The free-field calibration has the inherent problem due to the weak signal received in the microphone. Multi-path noises such as cross-talk and reflections corrupt the received signal, and thus the sensitivity evaluation is poor. The time selective method has been widely used as the useful tool to get rid of such noises. However FFT based signal processing invokes unwanted processing error such as leakage. A well designed frequency window which would be a kind of low-pass filter would reduce the leakage problem, and it helps to separate multi-path noises from the directly propagated signal well. The result well shows the advantages in separating the noises and the availability in the free-field calibration with the enhanced sensitivity evaluation.