With the expanding market for low-cost microphones, raising the manufacturing yields (lowering the rejection) becomes a central technical issue. This report explores a case study of possible causes of electret microphone rejection. Because the polarization voltage developed on the microphone diaphragm has a direct effect on the microphone sensitivity, hence variations in this voltage were investigated. Initial studies were conducted with an arrangement of fixture plate (bearing holes for holding microphones) which was placed on two base (support) plates. The investigations considered 23 microphone positions across 11 readings. The acceptable and unacceptable polarization voltages were designated, and the corresponding failure percentage was determined. Furthermore, one base plate was removed to increase the distance between top electrode and diaphragm, and polarization voltage was measured. The results showed an appreciable reduction in the polarization voltage that indicated a promising reduction in the microphone failure rate. For a representative electret condenser microphone, a nonlinear variation between sensitivity and polarization voltage was established. Statistical analysis revealed that measurement data is symmetric and distributed normally. The proposed modification, when implemented on the shop floor, reduced rejection from 33% to 16%.
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