Acoustic sound sensor

　　The sound sensor has a capacitive electret microphone sensitive to sound. The sound wave vibrates the electret film in the microphone, resulting in the change of capacitance, and the corresponding change of tiny voltage, so as to realize the conversion of optical signal to electrical signal.

　　Sound sensor built-in a sound sensitive capacitive electret microphone, electret face and back electrode opposite, there is a small air gap in the middle, forming an air gap and electret as insulation medium, with the back electrode and the electret on the metal layer as two electrodes to form a flat capacitor. There are output electrodes between the two poles of the capacitor. Because of the free charge distributed on the electret film. When acoustic wave causes the displacement of electret film due to vibration; The distance between the two polar plates of the capacitor is changed, thus causing the capacity of the capacitor to change. Since the charge number on the electret is always constant, according to the formula:

　　Q =CU

　　Therefore, when C changes, it will inevitably cause the change of the voltage U at both ends of the capacitor, so as to output electrical signals and realize the transformation from sound signals to electrical signals.

　　Specifically, the total charge of the electret is unchanged. When the electrode recede under acoustic pressure, the capacitance decreases, and the voltage between the capacitor poles will increase inversely; otherwise, when the capacitance increases, the voltage between the capacitor poles will decrease inversely. Finally, the voltage at both ends of the capacitor is removed by a very high impedance field effect, and the voltage corresponding to the sound can be amplified at the same time. Because THE FIELD EFFECT tube (FET) is an active device, it needs a certain bias and current to work in the amplified state, so the electret microphone needs a DC bias to work.

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