Realizing the primary standard for sound pressure: The trouble with IEC 61094-2

摘要

The primary standard for sound pressure is defined through the reciprocity calibration procedure specified in IEC 61094-2, placing a unique reliance on the accuracy of the published standard. The technique involves forming configurations of pairs of microphones together with an air-filled coupler. Over most of the frequency range, adiabatic conditions exist within the coupler, but at low frequencies (< 500 Hz) there is a non-negligible transfer of heat between gas and bounding surfaces, and a ‘heat conduction correction’ must be employed. IEC 61094 2:2009 contains two standardized solutions for the correction, and it is shown that these solutions are not equivalent, leading to a large, but hidden, source of uncertainty in the primary standard at low frequencies. The origin and nature of each correction is examined, and an error in the theory of one correction identified. Evidence of the relative performance of the two solutions is provided, in the form of theoretical analysis and measurements.