Frederiksen E.

During the later years National Metrology Institutes (NMIs) have shown an increasing interest in the primary calibration of Laboratory Standard Microphones at frequencies down to a fraction of a Hertz. Therefore, the Consultative Committee for Acoustics, Ultrasound and Vibration (CCAUV) of BIPM has initiated a key comparison calibration project (CCAUV.A-K2) on low-frequency pressure response calibration. Ten NMIs from all over the world, including DPLA, participate in this presently running project. Its idea is to apply the standardized pressure reciprocity calibration method (IEC61094-2) by extending its operation range down to 2 Hz or lower. As a supplement to this project, DPLA has started an internal low-frequency calibration project. Its purpose is partly to contribute to the determination of the low-frequency response of Laboratory Standard Microphones and partly to develop a secondary method for low-frequency calibration of general types of measurement microphone. Low-frequency reciprocity calibration is associated with major sources of uncertainty, such as the effect of heat exchange between the air and the couplers and also of the frequency dependent compliance of the microphone diaphragms. The secondary method is a comparison method that applies a reference microphone, which is designed for the purpose and has a very low fraction (about 0.7 %) of air-stiffness. This means that its low-frequency pressure response is designed to be flat within a band of 0.06 dB and, therefore, hardly needs any frequency response calibration. The comparison couplers are designed so that only the diaphragm – not the static pressure equalization vent - of the reference microphone is exposed to the sound. While for the unknown microphone also its vent may be exposed. This means that not only low-frequency pressure responses, but also free-field and diffuse-field responses may be determined. Method and results from 250 Hz down to 0.1 Hz are described.