ECM Microphone Operation

An electret condenser microphone (ECM) converts sound pressure into voltage by using diaphragm motion to vary a capacitor, while a permanently charged electret supplies the polarization field so no external high-voltage bias is required; the capsule’s internal JFET then buffers the resulting high-impedance signal into something a real circuit can load.

What an ECM physically is

At the capsule level, an ECM is a condenser mic with its polarization “battery” built into the structure:

This architecture exists for one practical reason: the raw capsule node behaves like a small capacitor with an extremely high source impedance, so you cannot connect it directly to downstream circuitry without collapsing the signal.

The transduction mechanism: variable capacitance under (nearly) constant charge

Capacitance varies with diaphragm displacement

Model the diaphragm and backplate as a parallel-plate capacitor:

𝐶(𝑡)=𝜀0𝜀𝑟𝐴𝑑(𝑡)

where 𝜀0=8.854×10−12 F/mε, εr​ is dielectric relative permittivity, 𝐴 is diaphragm area, and 𝑑(𝑡) is the instantaneous gap.

Sound pressure drives a small diaphragm displacement 𝑥(𝑡), modulating the gap:

𝑑(𝑡)=𝑑0−𝑥(𝑡)

and therefore modulating 𝐶(𝑡).

Charge–voltage relationship drives the output

For a capacitor:

𝑄=𝐶𝑉

In an ECM, the electret maintains an effectively constant surface charge density, and because the JFET gate impedance is treated as “infinite,” charge is approximately constant at audio frequencies.

With 𝑄≈constant voltage follows the inverse of capacitance:

𝑉(𝑡)=𝑄𝐶(𝑡)

So when diaphragm motion increases capacitance, voltage decreases, and vice versa. The key engineering point: small capacitance modulation produces proportional voltage modulation after linearization for small displacements.

Equivalent circuit model (what you actually design against)

A useful small-signal electrical abstraction is:

• A time-varying capacitor 𝐶(𝑡) for the diaphragm/backplate
• The electret represented as a DC source in series with that capacitance
• A JFET gate that is assumed to draw negligible current (minimizing charge leakage)

That abstraction is why ECMs behave like “charge-biased variable capacitors” rather than like low-impedance dynamic sources.

A practical system-level implication called out in the source material: sensitivity scales up with diaphragm area and scales down aggressively with diaphragm spacing (inverse square with spacing in the linearized form shown).

The electret: built-in polarization and its limits

An electret is a dielectric with quasi-permanent electric polarization (the microphone analogy to a permanent magnet). Typical electret materials include PTFE (Teflon), FEP, Mylar, and polypropylene.

Charging and equivalent “bias”

During fabrication, charge is embedded (e.g., corona discharge or electron-beam methods), producing an internal electric field equivalent to an external bias on the order of 100–500 V.

Charge stability (why humidity and heat matter)

Charge decay is modeled as an exponential relaxation with a time constant that is typically years to decades, but is accelerated by humidity, temperature, and surface contamination.

For product design, the takeaway is not philosophical: electret stability is part materials science, part packaging, part operating environment.

The impedance conversion stage: JFET source follower

The capsule’s variable capacitor is described as ~100 MΩ class, which drives the need for an internal JFET buffer.

Canonical biasing topology

• A standard ECM output stage is a source follower with:
• Supply (often labeled 𝑉​BIAS)
• Bias resistor 𝑅BIAS setting operating current
• AC coupling capacitor 𝐶COUPLING to block DC into the next stage

Small-signal behavior

The source follower relationship is governed by the device transconductance 𝑔𝑚gm​, with typical ECM JFET values stated as 1–3 mS.

The practical meaning: the JFET is not an optional “convenience.” It is the boundary between an electrostatic sensor node and a circuit node that can drive cable capacitance, ADC input networks, and EMI filters without being crushed.

Sensitivity and frequency response (what shows up in datasheets)

Sensitivity

Sensitivity is commonly expressed in dBV/Pa referenced to 94 dB SPL (1 Pa). Typical ECM values in the source material are −35 to −46 dBV/Pa.

Frequency response shape

Below the diaphragm resonance 𝑓0​, response is approximately flat; above 𝑓0, sensitivity rises and then rolls off.

Low-frequency response is also affected by acoustic venting behind the diaphragm, introducing a low-frequency corner determined by the vent’s acoustic resistance and compliance.

Noise and linearity (failure modes you can predict)

Dominant self-noise contributors

The stated contributors are:

Distortion mechanisms

Nonlinearity is driven by:

• Large diaphragm excursions where the “small displacement” linearization breaks down
• JFET biasing that drives the device toward cutoff or saturation

This is why ECM “distortion” is often a combination of acoustic overload and electrical headroom rather than just one or the other.

ECM vs MEMS microphones (use-case boundary conditions)

The source material closes with a direct comparison:

The engineering interpretation is straightforward: ECMs remain attractive when you want analog simplicity and stable long-life performance, while MEMS becomes attractive when you want integration and digital interfaces without external analog chain sensitivity.

Implementation checklist (what to verify before layout)

1. Bias network: Confirm 𝑅BIAS and supply voltage set a stable operating current for the capsule JFET.
2. AC coupling: Ensure 𝐶COUPLING and the next-stage input impedance do not introduce an unintended high-pass corner.
3. Noise budgeting: Treat 𝑅BIAS thermal noise and JFET channel noise as first-order contributors.
4. Linearity: Keep diaphragm excursions within the small-signal region and keep the JFET out of cutoff/saturation across temperature and supply tolerance.
5. Acoustics: If low-frequency response matters, treat venting resistance/compliance as a design parameter, not a footnote.

Summary

An ECM is a charge-biased variable capacitor where diaphragm motion modulates capacitance and therefore capsule voltage; an electret provides built-in polarization equivalent to a large bias field, and an internal JFET source follower is used to make the output electrically usable.

For more information on the science behind ECM microphones, read our whitepaper: The Operating Principles and Design of the Electret Condenser Microphone.