Study on the linearity of laboratory standard condenser microphone

Because of the many features such as wide frequency range, high stability, good transient response, etc., the condenser microphone has been used as a laboratory standard microphone and a measurement microphone. One problem is that the condenser microphone has essentially nonlinear characteristics on the electrical output/acoustical input function. As sound-pressure level becomes higher, nonlinearity becomes larger. In this study, linearity of the laboratory standard condenser microphone at high sound-pressure level was investigated. In the case of the 12-in. condenser microphone MR-112, linearity was ensured up to 120 dB SPL. Over 120 dB SPL, output/input function becomes larger and distortion becomes detectable. By experiment, measured distortion factor was 2% at 151 dB SPL and 5% at 154 dB SPL. The nonlinearity at high sound-pressure level was studied analytically, and the effect of some parameters such as shape of back electrode, biasing voltage, and stray capacitance was clarified. Based on the analytical study, the possibility of lowering the nonlinearity was investigated. A method to realize the condenser microphone having low distortion and high sensitivity was suggested.