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Environmentally robust electret condenser microphone

A new structure of front filter of ECMs has been introduced to protect in practical use from such common hazards as water or alcohol. This protection structure is composed of acoustically transparent mesh with a water-repellent coating on an electrically-conductive layer. This water-repellent coating has a contact angle of more than 150 degree for water on the filter having less than 50 /spl mu/m aperture which chokes water invasion but is enough for acoustical transmission. The new protection structure also has an electrically conductive layer inside which copes with the induction noise problem by the effect of electrostatic shielding. The reliability of the coating was confirmed by several accelerated tests such as long term immersion in water, heat cycling in the presence of both high and low moisture, heat shocks and surface scratch. These tests showed no damage to the coating. The fabricated electret condenser microphone has met the requirements for the IEC 60526 class 7 which is 30 minutes under water at a depth of 1 meter. The induction noise was reduced by approximately 10 dB at 2 GHz.