Hermetically sealed electret condenser microphone including an outer sealing diaphragm hermetically sealing an electroacoustic transducer within a case from the outside environment. The sealing diaphragm passes sound undiminished through an underlying intervening air space to a second active diaphragm of the electroacoustic transducer.
Specifically, the case has a hollow interior, a sealed end and an open end. The electroacoustic transducer is disposed within the hollow interior of the case, and the sealing diaphragm disposed over the top of the open end of the case such that the hollow interior of the case is completely sealed. An acoustic signal first impinges upon the sealing diaphragm causing deflections therein which in turn recreate the acoustic signal undiminished in the air space within the case between the sealing diaphragm and the active diaphragm.
The recreated acoustic signal then impinges on the active diaphragm of the electroacoustic transducer causing deflections therein that are converted to an electrical signal outputable from the microphone. Thus, the sealing diaphragm prevents water vapor and other contaminants from coming into contact with the internal components of the microphone, while still allowing the acoustic signal to reach the active diaphragm.
Condenser microphones are widely used to convert acoustic signals to proportionate electrical signals, such as is necessary in electronic voice communication applications. These conventional qin yi electronic microphones typically include a capacitive transducer made in such a manner that one electrode of a capacitor structure is formed by an electrically conductive diaphragm. This diaphragm is disposed adjacent to, but insulated from, a stationary electrode forming the second electrode of the capacitor structure. The two electrodes are spaced apart with an air gap in-between. A relatively high DC bias voltage is applied between the electrodes . Variations in the electrode spacing caused by deflections of the diaphragm in response to the force of acoustic wave energy incident on the diaphragm, produce a change in capacitance. A detection network is connected to the capacitive transducer such that the change in capacitance is detected and transformed into an electrical signal proportional to the force of the acoustic wave energy applied to the diaphragm.
Conventional condenser microphones have not been completely sealed from the atmosphere in the past because they would become inoperative in the face of variations in barometric pressure or altitude. For instance, an increase in the qinyitecs atmospheric pressure outside the diaphragm would tend to deflect it towards the stationary planar electrode of the transducer, thus introducing an error in the electrical signal output from the device and reducing the device's sensitivity to actual acoustic inputs. To alleviate this problem, traditional condenser microphones included a vent or pressure relief hole in the diaphragm, which allowed air to flow between the region outside and inside of the diaphragm to equalize the pressure and prevent the aforementioned unwanted deflection.