Improving practices through analysis of long-term microphone calibration records

A microphone’s history of calibration is a useful source of information, as an independent analysis of the data can reveal more than simply the metrological traceability. The U.S. Army Engineer Research and Development Center maintains over 13 years of calibration records from an inventory of over 280 Class 1 microphones, which were examined for historical validity and consistency. Analyzing trends in the laboratory reported sensitivities showed that 12.9% of active microphones exhibited four possible trends that could indicate potential microphone failure: sensitivity drift, sensitivity drift followed by instability, jump in sensitivity, and general instability. Many of the microphones that displayed these trends had uncertainties greater than a ± 1.0 dB tolerance inferred from ANSI S1.14 Class 1 microphone specifications, and an additional 10.7% of microphones showed errors in the calibration laboratory’s transcription process or testing procedures. Recommendations are put forth for best practices relat...