Temperature characteristics of electret condenser microphones

Electret condenser microphones for mobile system terminals should be more robust than those for ordinary consumer equipment. Generally, the diaphragm electret can achieve flat temperature characteristics with respect to sensitivity because the temperature dependence of the diaphragm stiffness and gain characteristics of the FET offset each other. However, the fixed electrode electret using a PET diaphragm is not sufficiently robust with respect to temperature because the PET membrane has the same temperature coefficient as FETs, so there is no offsetting and it is difficult to compensate the temperature characteristics. In order to improve robustness, this paper proposes a temperature dependence evaluation method for acoustic characteristics of the electret condenser microphone. It then reports the temperature dependence of the dominant parameter that affects the sensitivity of the microphone and describes a method to design a microphone with stable temperature characteristics. In a new trial, silicon, an inorganic material, is applied to the diaphragm. It is subsequently demonstrated that an impedance converter composed of a source-follower circuit is effective for a high temperature environment of up to 80°C. Finally, the possibility of an electret condenser microphone with flat temperature characteristics is proposed.