Measurement Uncertainty in Microphone Free-Field Comparison Calibrations
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The present study demonstrates the use of a transportable anechoic chamber for conducting microphone free-field calibrations with lowest levels of uncertainty. A dedicated transportable anechoic chamber (make SPEKTRA, Germany) of internal volume of 2 m3 completely lined with wedge-shaped absorbers is utilized for free-field calibrations in the frequency range of 125 Hz–20 kHz using the substitution method as per the IEC 61094-8 standard. The study has identified the usable working space inside a free-field chamber using the inverse-square pressure law, and the deviations from the inverse-square law of the free-field chamber are quantified. The measurement uncertainty of ± 0.36–0.52 dB (k = 2, 95% confidence level) is evaluated in the frequency range of 125 Hz–20 kHz and is validated in a bilateral comparison. The application of Monte Carlo simulation approach in the evaluation of measurement uncertainty in microphone free-field calibrations is also demonstrated.
KeywordsMicrophone free-field calibration Monte Carlo simulation (MCS) Law of propagation of uncertainty (LPU)
Authors thank Dr D. K. Aswal, Director, CSIR-National Physical Laboratory, India, and Dr. Sanjay Yadav, Head, Physico-Mechanical Metrology Division, for providing the necessary infrastructural support for the study and granting permission to publish the paper. The authors also wish to place on record their gratitude and thanks to Dr. Jacob Chandapillai, Director, FCRI, Palakkad, and Dr. S Rammohan, Head, Noise and Vibration facility, for allowing to do this study and publish the paper. Authors would also recognize the efforts, innovation and support of M/s SPEKTRA Schwingungstechnik und Akustik GmbH Dresden, Germany, in developing a small anechoic chamber at M/s FCRI, Palakkad. Authors also acknowledge the SPEKTRA Manual (published by Bühn and Begoff) of MUB estimation of CS18 FF system used for uncertainty budget calculations and for identification of the usable working space inside a free-field chamber using the inverse-square pressure law.
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