Current developments at the National Institute for Standards and Technology in pressure calibration of laboratory standard microphones

Abstract

Current research effort aims at improving the apparatus and methods for determining the pressure sensitivities of IEC types LS1Pn and LS2aP laboratory standard microphones. Among the improvements that are being systematically incorporated in an evolving test bed is the capability to operate at adjustable power line frequencies other than the usual 60 Hz. Suitable choices of line frequency relative to frequencies of calibration and adjustable bandpass filter characteristics can be used to improve the signal-to-noise ratios of measurements performed near the usual line frequency and its first few harmonics. This can enable the use of relatively large volume couplers for which uncertainties in microphone front cavity volume and equivalent volume, capillary tube effects, and heat conduction corrections have a lesser influence than they have for small-volume couplers. Another improvement aims to control and to stabilize the ambient static pressure during microphone calibrations, to reduce or eliminate the effects of barometric pressure fluctuations on these calibrations.