" the voltage across the cap should increase (greater charge density), which causes the FET to conduct more,¡±
But the + terminal of the capsule capacitor (the diaphragm) is connected to the shell of the electret capsule. Ground. Signal comes off the negative terminal (backplate), driving the gate of the FET negative. Fet conducts less, so drain voltage goes up. Positive air pressure, negative voltage at backplate, signal inverted by FET amplifier.
Electrets behave differently than ¡°true condenser¡± capsules. The electret¡¯s polarizing field is built in, so reversing the capsule connections reverses the polarity of the output. This is true whether the FET is built into the capsule or external.
The ¡°true condenser¡¯s¡± polarizing field comes from the mic¡¯s electronic circuitry, so reversing capsule connections as seen by the capsule reverses both signal polarity and polarizing voltage polarity, and signal polarity at the FET gate does not change. What determines signal polarity is polarity of the bias or polarizing voltage relative to the signal terminal of the capsule.
For examples of electret mics with multiple patterns, see:
and:
If one were to substitute a pair of ¡°true¡± condenser capsules for the electrets, and supply polarizing voltage from the electronics, the circuit won¡¯t work. The easy way to switch signal polarity of a ¡°true¡± condenser capsule is to switch the polarizing supply from + to -. But that¡¯s a different topic.
