Natl. Inst. of Stds. and Technol., Sound Building 233, Rm. A147, Gaithersburg, MD 20899-0001
Significant discrepancies occurred in results from the recent European intercomparison of free-field calibration of IEC type LS2aP (13.2 mm nominal diameter) laboratory standard microphones [K. Rasmussen and E. Sandermann Olsen, The Acoustics Laboratory, Technical University of Denmark (DTH) Report PL-07 (1993)]. Consequently, Dr. Richard Barham of the National Physical Laboratory, U.K. (NPL) visited NIST to resolve one such discrepancy between NPL and DTH by obtaining a NIST calibration of a microphone that also had been calibrated by NPL and DTH in the European comparison. All laboratories used the reciprocity method, with independently implemented apparatus at each laboratory. The NIST calibration agreed relatively well with that of DTH, but not NPL, if all laboratories used the same values for the frequency-dependent acoustic center positions of the microphones. The problematical DTH and NPL determinations of acoustic center positions for the type LS2aP microphone, which disagree with the values obtained by scaling the positions standardized for another microphone type, are considered. Various implications for the current draft IEC standard on primary free-field calibration are discussed. Selected NIST free-field comparison calibration methods and transducer characterization procedures, recently applied to relatively novel devices such as micro-machined silicon microphones, are described.