Recent work at the National Physical Laboratory on the free-field calibration of standard microphones

Recent developments at the National Physical Laboratory on free-field standards of sound pressure and microphone calibration are described. In the low-frequency region free-field plane-wave conditions are difficult to achieve in conventional absorbent rooms, and this has been met by the installation of a large duct in which truly plane progressive waves may be generated down to 25 c/s. At the higher frequencies the Rayleigh disc and reciprocity free-field standards have been compared and show excellent agreement.