Extension of reciprocity calibration of microphones to frequencies below 1 Hz

For many years, primary measurement standards of sound pressure have been realized by calibration of laboratory standard, LS, microphones with the reciprocity calibration technique specified in international standard IEC 61094-2. Recent BIPM key comparisons, CCAUV.A-K5 and CCAUV.A-K6, have extended the lower frequency limit of the method from 20 Hz to 2 Hz, but with known uncertainty on the standard’s models of thermos-viscous effects and acoustic impedance of the microphones, which in practice prevented further excursion to lower frequencies. This uncertainty has now been minimized with improved calculation methods that have been included in an amendment to the standard, IEC 61094-2 AMD1. These developments have opened the possibility of reducing significantly the lower frequency limit of the method, subject to practical considerations.

A significant advantage of extending the frequency range of the reciprocity technique to the low frequencies targeted in the Infra-AUV project is that it will enable one primary method to cover the entire frequency range of pressure sensitivity from quasi-static pressure variations at 40 mHz, or lower, to 10 kHz for LS1 (one-inch LS) microphones.

With the reliable calculations of the acoustic properties of air in cylindrical cavities at low frequencies now available, the dominating challenge of extending the reciprocity technique to frequencies below 2 Hz has been to ensure a very low, and at the same time highly reproducible, cut-off frequency of the necessary static pressure equalization of the cylindrical cavities, couplers, used in the calibrations. Prototype couplers have been developed in the project that solves this challenge. The achieved reproducibility of reciprocity calibrations at frequencies down to 25 mHz and some independent verification of the results will be presented.