Practically all reference microphone calibrations that are performed by national metrology institutes are pressure response calibrations. This is the case even if most practical measurements are carried out under free- or diffuse-field conditions.
Fortunately, for measurement microphones, there are essentially fixed ratios between the pressure-, free- and diffuse-field responses. Therefore, the free- and diffuse-field responses may be determined by adding corrections to the pressure response. However, some institutes must also develop the free-field calibration technique, calibrate microphones and determine the necessary corrections.
Until now, no commercial free-field calibration systems have ever been available, some institutes have built systems themselves, but only a few have succeeded in making free-field calibration a routine task and even fewer have established a calibration service.
Looking at the calibration principles only, free-field reciprocity calibration is simpler than pressure calibration, but it is technically more difficult, especially due to the very low sound pressure that is obtainable. However, after several years of thorough research, the Danish Technical University (DTU) has developed an elaborate system and obtained international respect for their free-field reciprocity calibration. Brüel & Kjær has supported this work with instruments and technical modifications.
A paper describing the technical aspects of this system, which is now offered by Brüel & Kjær, has been published (see link right).
The Chinese National Institute of Measurement and Testing Technology (NIMTT) in Chengdu took delivery of a Primary Free-field Reciprocity Calibration System in the Autumn of 2009. With the help of Professor Knud Rasmussen from Danish Fundamental Metrology and Erling Frederiksen of Brüel & Kjær, Mr. Yuchuan Hao and Mr. Zhiqiang Pu, together with their team, were very quickly making consistent calibrations.
Professor Rasmussen commented on trials of the calibration system made with known microphones, 'The repeatability of the Chinese setup is very good and the results look fine as well. At DFM we also performed a pressure calibration of the two Type 4180 microphones and I can inform that the deviations with respect to the Chinese results are typically less than 0.03 dB'.
Mr Zhigiang Pu adds, 'We believe we can get even better results after we improve the (room) ventilation device by decreasing the noise of the fan'.