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The paper presents a brief description of the principles and the uncertainty of the microphone free-field calibration methods used in recent times by National Metrology Institutes across the globe. Microphone comparison calibrations serve as cost and time effective methodology for calibrating the microphone sensitivity in audio frequency range. Also, the free-field correction is widely used for estimating a free-field sensitivity, which can only be obtained by applying absolute calibration techniques. The difference between the insert voltage method and reciprocity results at 250 Hz and 1 kHz is analyzed for reference standard microphone. The study also shows an example of a transportable anechoic chamber (Make: SPEKTRA, Germany) of internal volume 2 m3 utilized by some laboratories for free-field calibrations in frequency range of 125 Hz to 20 kHz. The present study discusses the accuracy and precision issues in free-field absolute and comparison calibrations and implications associated with free-field corrections for field measurements.
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