On the acceptance limits for short-term level fluctuation of sound calibrators

To ensure noise measurement accuracy, sound calibrators are used to check the sound level meter accuracy before and after any such measurements, as required by most noise regulations. The sound calibrator itself needs to be calibrated routinely and traceable to national standards. The International Standard IEC 60942, Electroacoustics - Sound calibrators, specifies performance requirements for the sound pressure level, short-term level fluctuation, frequency, and distortion and associated test methods. However, the acceptance limits for short-term level fluctuation specified in IEC 60942 is problematic. The short-term level fluctuation acceptance limits specified in Table 2 of IEC 60942 for the level variations inherited by a stable sinusoid at low frequencies are either underestimated or overestimated. In this paper, a detailed theoretical analysis is presented addressing this particular issue. Closed-form solutions are obtained for typical sinusoidal sound signals. Two experimental setups and measurement results for the validation of the theoretical analysis are also presented. One uses a commercial sound level meter and the other uses a precision sound pressure measurement system developed recently at the National Research Council Canada. A revised table for the acceptance limits is then proposed for the future revision of Table 2 of IEC 60942.