# Electret microphone: convert sound to current or resistance?

See this picture (from Wikipedia):

The electret microphone consists of a capacitor with a static charge and a JFET. I wonder how the electrical signal relates to the sound signal.

I understand that with only a capacitor capsule the sound, with a change in distance between the two plates and capacitance, changes accordingly.

But what happens when we add a JFET to the system? Why does it require 2 V to operate?

In the final circuit we have a voltage signal but for the microphone itself, does it convert sound into current or resistance?

I want to understand how power supply ripple or noise affects the SNR of the microphone circuit.

• ti.com/lit/ug/tidu765/tidu765.pdf, section 2, Theory of Operation, under equation (1) '...the microphone capacitor acts as an ac-coupled voltage source.' Next page, see Figure 5, and right above equation (2) you read 'Op amp U1 acts as a transimpedance amplifier, and attempts to hold its inverting input at a constant voltage (VB) by varying its output.' The mic capacitor acts as an ac-coupled voltage source, JFET CS amp generates a current $I_{ac}$ (it flows through C3 in Fig 5). JFET CS amp requires an external power to operate. Commented Nov 29, 2021 at 10:04
• “does it convert sound into current or resistance” - I tend to say voltage/E-field. Once produced by changing H-fields/current in a speaker. Commented Oct 5, 2022 at 20:07