23.07.2013
A scaled reverberation room has been successfully used for the primary calibration of diffuse-field microphones at PTB.
The primary calibration of measurement microphones is performed by the reciprocity technique where microphones act as sound source and as receiver. The ratio between acoustic and electric quantities is identical in both directions. Therefore, the sensitivity of microphones can be determined by measuring the input current into the sending microphone and the output voltage of the receiving microphone. Additionally, the properties of the acoustic field must be known. The technique requires the use of three microphones where each microphone is used once as emitter and once as receiver. This method is standardised in pressure- and in free-fields. The application in diffuse sound fields is more difficult to implement because the source strength of a microphone is too small to excite a reverberation room with a volume of about 200 m³ with a measurable sound pressure. Therefore, a scaled model of a reverberation room with a volume of 0,23 m³ is used at PTB to implement the method.
Within a bachelor thesis, the reciprocity procedure was completely implemented and tested at PTB. It turned out that the measured diffuse-field sensitivities comply very well with the best estimates for this quantity which were derived from pressure calibration and appropriate corrections (Figure 1). A measurement in a second scaled reverberation room showed furthermore very consistent results.
Due to its outstanding quality, the bachelor thesis was awarded a grant of the „Deutsche Gesellschaft für Akustik“, and the results were presented at the conference AIA/DAGA 2013 in Merano (Italy).
Figure 1: Diffuse-field sensitivities measured by the reciprocity technique compared to the estimates derived from a pressure calibration and appropriate corrections for two different types of microphones.
References:
[1] Weihe, S.; Wittstock, V.; Blau, M.: Diffuse-field reciprocity calibration. Proceedings of AIA-DAGA 2013 on CD-ROM, Merano, Italy, March 2013
Contact person:
Volker Wittstock, Dept. 1.7, WG 1.72, volker.wittstock@ptb.de