How to judge the polarity of electret microphone
The electret microphone is small in size, simple in structure, good in electroacoustic performance, low in price and widely used. The internal structure of the electret microphone is shown in the figure. It is composed of two parts: acoustic-electric conversion system and field effect tube. There are two ways to connect its circuit: source output and drain output. The source output has three lead wires, the drain D is connected to the positive power supply, the source S is grounded through a resistor, and then a capacitor is used for signal output; the drain output has two lead wires, the drain D is connected to the positive power supply through a resistor, and then A capacitor is used for signal output, and the source S is directly grounded. Therefore, before using an electret microphone, it is necessary to determine its polarity. 
It can be seen from the basic structure shown in the figure that a diode is connected between the gate and the source of the field effect tube, so the forward and reverse resistance characteristics of the diode can be used to distinguish the drain D and source S of the electret microphone.
Dial the multimeter to the R × 1kΩ position, the black test lead is connected to one pole, and the red test lead is connected to the other pole. Then swap the two test leads and compare the two measurement results. When the resistance is small, the black test lead is connected to the source and the red test lead is connected to the drain.
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