Acoustic studies are suitable for the analysis of various problems. To determine measurement results with the required precision, calibrated microphones are necessary. The reciprocity method is, in comparison to other calibration techniques, the most accurate one. It is based on the reciprocity of condenser microphones and does not require a reference–microphone. Reciprocal microphones are microphones that can be used as transmitter and receiver. Two of those and one, which is not necessarily reciprocial, are required for calibration.
This work describes design, construction and testing of a measurement setup for reciprocity calibration, which uses a commercially available sound card as signal source and measuring device. The automated measuring and estimation process is implemented with the software MATLAB.
The designed measurement setup uses three identical electret–microphones, which were originally not intended for a reciprocal operation. Any problems caused by this are accepted, because the microphones are given. Highly nonlinear distortions at low frequencies are caused by the amplifiers and therefore because of this, the evaluated amount of the frequency response is limited between 700 Hz and 20 kHz. The measurement method requires pairs of sound transceivers and receivers connected by a coupler, which is designed referring to IEC 61094-2:2009. Three independent combinations of microphone pairs, yield enough information to determine the sensitivity of each microphone.
An additional parameter which characterizes the sound field is required. The stiffness of the diaphragm influences the acoustic coupling of the two microphones. A measuring procedure with iterative evaluation is implemented in this work.
Keywords: microphone calibration, reciprocity calibration, coupler method, acoustic transfer impedance, microphone equivalent volume
August 09th, 2015