The Complete Guide To Electret Condenser Microphones

Have you ever used a microphone in an electronic device, in the studio, or in film? There's a good chance it was an electret condenser microphone. These microphones are common in our day-to-day lives and are worth knowing about.

What is an electret condenser microphone? An ECM is a type of condenser microphone transducer which means its works on electrostatic principles. Condenser mic capsules essentially work as capacitors and require a charge supplied quasi-permanently by electret material (a portmanteau of electric and magnet) in the capsule.

In this complete guide, we'll further define electret condenser microphones (ECMs), looking at their design characteristics, how they work, their applications, and, of course, a few electret microphone examples.

Table Of Contents

• What Is An Electret Condenser Microphone?
• What Is Electret Material?
• A Bit Of History On Electret Condensers
• Types Of Electret Microphones
• How Do Electret Condenser Microphones Work?
• Applications Of Electret Condenser Microphones
• Electret Condenser Microphone Examples
• Related Questions

What Is An Electret Condenser Microphone?

An electret condenser microphone, as the name suggests, is a type of condenser microphone.

The main difference between an ECM and a “regular” condenser microphone is the method in which the condenser capsule of the microphone is polarized (charged).

As we'll get into in the next section, How Do Electret Condenser Microphones Work? Condenser capsules act as parallel-plate capacitors and require a fixed charge across their plates to function properly.

The polarizing voltage that causes a fixed charge across the plates is generally supplied by external means (typically via phantom power or an external power supply with FET and tube condensers, respectively).

However, there is another method of supplying the plates with a fixed charge, and that is, as you've probably guessed, the addition of electret material in the capsule design.

So electret condenser microphones utilize electret material in their diaphragms to maintain a “quasi-permanent” charge across their plates. This frees up resources so that the powering methods can be used more effectively to power the impedance converters, printed circuit boards and other active components within the microphone.

What Is Electret Material?

So what is this magical, permanently charged, electret material we've been discussing?

Electret material is any dielectric material that has a quasi-permanent electric charge or dipole polarization. These materials generate permanent internal and external electric fields and can effectively charge other electrical components, such as capacitors.

As mentioned, the term “electret” comes from electrostatic and magnet. Electrets are essentially the electrostatic equivalent of a permanent magnet.

Electret materials typically have high electrical resistance and chemical stability and will maintain their electrical charge for long periods of time (in the hundreds of years).

Electrets are typically made by melting down dielectric material and allowing this material to solidify whilst in a strong electrostatic field. Polar molecules naturally align themselves within this electrostatic field while the material is melted and remains in this position as the material hardens, producing a permanent electrostatic bias.

In microphones, the electret material is typically Polytetrafluoroethylene (PTFE) plastic in film form or solute form.

A Bit Of History On Electret Condensers

The first electret condenser microphones were crude, indeed. The first electret microphone was designed in 1920 (by Yoguchi of Japan), but it wasn't until 1961 that electret microphones could effectively be used in market-ready products.

More specifically, it was the foil electret microphone invented in 1961 by James West and Gerhard Sessler at Bell Laboratories.

The first early electret microphone was produced in 1938 by the Bogen Company and was known as the No-Voltage Velotron. Unfortunately, at this time, electret technology was crude (to say the least), and although these mics worked, it was not long that the electret material would begin to lose its charge and render the mic ineffective.

The first successful electret condenser microphone to hit the market was the Sony ECM-22P In 1968.

Even in the early days of commercially available electret mics, the technology was spotty at best. In fact, in these times, the term “true condenser” came about to differentiate the superior externally-polarized condenser microphones from their electret counterparts.

Electret technology has come a long way since then, and today it is found in many professional-grade studio condenser microphones and even in measurement microphones.

To learn a great deal more about microphone history, please consider reading through my article Mic History: Who Invented Each Type Of Microphone And When?

Types Of Electret Microphones

There are 3 main ways in which electret material is added to an ECM to provide the permanent charge. These 3 electret types are:

1. Foil electret
2. Back electret
3. Front electret

What is a foil electret condenser microphone? A foil electret condenser mic employs a film of electret material as its diaphragm rather than having a distinct diaphragm plate coated in electret material (like a front electret). Foil electrets are the most common, but the lowest-quality electret mics since electret films perform poorly as diaphragms.

What is a back electret condenser microphone? A back electret microphone is a condenser mic with a permanently charged capsule due to electret material being fixed to its stationary backplate. Not having electret material fixed on the front plate (diaphragm) increases diaphragm accuracy, and the electret is more durable since it’s stationary.

What is a front electret condenser microphone? A front electret condenser mic is an electret mic with no backplate. Rather, the capacitor is formed by the diaphragm and the inside surface of the mic capsule. An electret film is fixed to the inside front cover of the mic and the diaphragm is connected to the input of the FET.

Consumer electronics and project-grade ECMs also come with different output connector types. These include:

• Pin-type
• Terminal-type
• Wire-type

Pin-type ECMs have conductor pins to carry the unbalanced signal out of the mic.

Terminal-type ECMs are a bit more flexible and have their terminals available to connect to various circuits.

Wire-type ECMs have their signals carried out by a wire and can be positioned further away from their intended PCBs.

How Do Electret Condenser Microphones Work?

Now that we have an understanding of what electret microphones are let's take a deeper look into how they function.

We'll start with a simple and basic diagram of a back electret condenser microphone capsule and impedance converter for reference throughout this section:

As with all microphones, ECMs have diaphragms that react with external sound waves (variations in sound pressure). This diaphragm movement is converted into a coinciding mic signal that is then outputted by the microphone.

But there's a lot more to know than these basics!

The Electrostatic Principles Behind The ECM Transducer

First, let's discuss the electrostatic principles that are the basis of ECM functionality. Note that these principles are the same for all condenser microphones.

We'll start with the fact that the condenser microphone capsule is essentially a parallel-plate capacitor.

This capacitor is made of a movable front plate (the capsule diaphragm/membrane) and a stationary backplate (known simply as the backplate).

This capacitor must hold a constant charge between the diaphragm and backplate to function properly. With ECMs, this charge is supplied by the electret material (either on the diaphragm, backplate, or elsewhere in the capsule design).

Once the capsule has its fixed charge, we can use the following electrical formula to understand how the capsule works:

V=Q•C

• V = voltage across the plates.
• Q = electrical charge between the plates.
• C = capacitance of the parallel-plate capacitor.

The microphone's audio signal begins as the variation in voltage across the capsule's capacitor plates. Analog audio signals are, after all, AC voltages with frequencies between 20 Hz – 20,000 Hz.

The AC voltage from the capacitor must be altered before it can effectively be outputted from the microphone, but the capsule transducer is the beginning of the microphone signal.

So, by looking at the above formula, we see that, with a fixed charge, any change in capacitance will produce an inversely proportionate change in voltage. This means that, in order to create an AC voltage mic signal, we need the capacitor's capacitance to vary up and down (oscillate about its setpoint).

How can the capacitance be altered within a condenser microphone capsule? Let's take a look at another capacitance formula to find out:

C = ε_0(\frac{A}{d})

• C = capacitance of the parallel-plate capacitor.
• A = area of the plates.
• ε₀ = dielectric constant.
• d = distance between the plates.

In the above formula, we have two constants: the dielectric constant and the area of the plates (the movable diaphragm and stationary backplate). The movable diaphragm, which reacts to variations in sound pressure level, allows for changes in the distance between the plates (d in the above equation).

So by moving the diaphragm, we alter the distance between the plates of the capacitor.

According to our second equation, any change in distance between the capacitor plates causes a proportionate change in the capacitance of the capacitor/capsule.

According to our first equation, any change in capacitance causes an inversely proportionate change in voltage across the plates.

As we've discussed, an AC voltage across the plates is essentially our mic signal. Via the electrostatic principles mentioned above, any sound waves at the condenser mic diaphragm cause a coinciding mic signal!

The Electret Material

What really separates ECMs from regular condenser microphones is the electret material. As previously mentioned, the electret material allows for a fixed electric charge across the parallel-plate capacitor. This fixed charge, again, is necessary from ECM capsules to function properly.

The Impedance Converting Transistor

The transducer element and the electrostatic principles that govern it are pretty clever. However, there's one big issue with ECM capsules (and condenser microphone capsules in general, for that matter).

This issue is the extremely high impedance at the capsule output.

The condenser capsule must maintain a very high impedance in order to keep the stored charge across the plates from leaking away.

Similarly, it is essential to have an impedance converter immediately after the capsule to take the audio signal from the ECM capsule effectively. With ECMs, this impedance converter is typically in the form of a JFET (junction-gate field-effect transistor).

A JFET is an active electronic device with three terminals. Let's have a look at a simple diagram of a JFET following by a list of its terminals:

• S = source
• D = drain
• G = gate

The capsule's high-impedance output signal is sent to the gate of the JFET where it creates a circuit with the source-gate terminals.

The gate can be thought of as a high-impedance input, capable of receiving the capsule's output signal without significant degradation (which would be the case if the input was low-impedance).

The JFET is supplied power via an external source (typically phantom power or DC biasing). This effectively sets up the source-drain terminals to have electrical current flow through them. This current is relatively low-impedance and can be sent through the rest of the microphone and the final mic output.

The current between the source-drain can be thought of as the JFET (impedance converter) output. The “outputted” AC voltage, as we could guess, has a much lower impedance than the “inputted signal.”

The “input signal” essentially modulates the current of the “output signal.” Therefore, a high-impedance signal going to the gate-source terminals of the JFET can effectively modulate the low-impedance signal at the source-drain. This is where the impedance comes in.

Note that JFETs may also provide a sort of pseudo-amplification between their input and output.

To learn more about transistors in microphones, check out my related article, titled Do All Microphones Have Transformers And Transistors? (+ Mic Examples).

Further Circuitry And The Microphone Output

Depending on the particular electret condenser microphone, there could be additional circuits for the mic signal to pass through before being outputted from the microphone.

These circuits may include (but are not limited to) the following components:

• High-pass filters
• Passive attenuation devices (PADs)
• Amplifiers
• Analog-to-digital converters

Powering The Active Components Of An Electret Condenser Microphones

ECMs have [practically] permanently charged capsules and do not require an external polarizing voltage to apply a fixed charge across their plates.

That being said, electret condenser microphones are still active microphones. Their impedance converters require power (provided by external means) to function properly, as do the components mentioned in the section above.

In studio-grade and measurement ECMs, the preferred powering method is phantom power, which applies +48 V DC on pins 2 and 3 (relative to pin 1) of the balanced audio cable connected to the microphone.

With lavalier and other miniature ECMs, the preferred powering method is often DC biasing. This method entails sending +5 V DC along the audio conductor of an unbalanced line and is usually supplied by the wireless transmitter bodypack the lav mic connects to.

In consumer devices, which commonly use electret microphones, the mics are powered by the same battery, power supply, or power mains that power the rest of the device.

With all that information in mind, the following simplified diagram of an ECM should make sense:

Applications Of Electret Condenser Microphones

At the beginning of this article, I mentioned that electret condenser microphones are one, if not the most, commonly used microphones on Earth.

With that, let's takes a look at some typical ECMs:

• Measurement microphones
• Studio condenser microphones
• Film microphones (shotgun mics, lavalier mics, etc.)
• Consumer electronics (laptops, cellphones, etc)
• Professional medical devices (such as hearing aids)

The applications of the above microphone types range from telephone calls to blockbuster movie audio; hit records to helping those of us with hearing impairments.

Electret Condenser Microphone Examples

To really learn about electret condenser microphones, we should take a look at some examples:

• Earthworks M50
• DPA 4006A
• Rode NT1-A
• Sanken COS-11D
• Challenge Electronics CEM
• iPhone

Earthworks M50

The Earthworks M50 is an excellent measurement microphone with an electret condenser capsule. It has a wide frequency response ranging all the way from 5 Hz to 50,000 Hz (the human range of hearing, which most EMCs aim to reproduce, is only 20 Hz – 20,000 Hz).

This microphone tells us that electret microphones, with the right design, can be top performers and incredibly accurate transducers.

DPA 4006A

The DPA 4006A is a top-of-the-line microphone in general (not only compared to other electret condenser microphones).

The DPA 4006 is featured in the following My New Microphone articles:
• Top 50 Best Microphones Of All Time
• Top 11 Best Solid-State/FET Condenser Microphones

This pencil microphone performs nearly as precisely as a measurement microphone but is not marketed that way. Rather, the 4006A is produced to be a go-to microphone for accurate and detailed sound production in the studio.

To learn more about pencil microphones, check out my article What Are Pencil Microphones And What Are They Used For?

Rode NT1-A

The Rode NT1-A is one of my personal favourite microphones. I would consider this microphone a “prosumer” product, being somewhere between top-of-the-line professional and full-out consumer-grade. I've personally used this microphone on many professional projects in my career as an audio engineer.

The Rode NT1-A is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• 12 Best Large-Diaphragm Condenser Microphones Under $500
• Top 12 Best Microphones Under $1,000 for Recording Vocals
• Top 10 Best Microphones Under $500 for Recording Vocals
• Top 20 Best Microphones For Podcasting (All Budgets)

Sanken COS-11D

The Sanken COS-11D is an industry-standard lavalier microphone for film, television and other screened media.

This mic has a miniature electret condenser capsule and works on DC bias voltage.

To learn more about miniature lavalier microphones, check out the following My New Microphone articles:
• How And Where To Attach A Lavalier/Lapel Microphone
• Best Lavalier Microphones For Interviews/News/Presentations
• Best Lavalier Microphones For Actors

Challenge Electronics CEM-C9745JAD462P2.54R

The Challenge Electronics CEM-C9745JAD462P2.54R is one of the many examples of inexpensive ECMs on the market today (it's also quite a mouthful). These small pin-type project ECMs would be very similar to what we would expect to find in consumer-grade electronics.

iPhone (2008)

The original iPhone utilized an ECM with an immediate ADC to record and transmit audio. subsequent models have been designed with MEMS microphones.

With the rising popularity and development of MEMS microphones, many consumer device manufacturers are opting for MEMS mics over ECMs. MEMS mics are much smaller and becoming cheaper to build and, in some ways, outperform ECMs (particularly in consumer devices like cellphones).

To learn more about MEMS microphones, check out my article What Is A MEMS (Micro-Electro-Mechanical Systems) Microphone?

Related Questions

What is a FET microphone? A FET microphone is a solid-state active microphone that utilizes a FET (field-effect transistor), as opposed to a vacuum tube, as its impedance converter. FET mics are typically condenser microphones but can have dynamic transducers as well.

To learn more about FET microphones, check out my articles What Are FETs & What Is Their Role In Microphone Design? and What Are The Differences Between Tube & FET Microphones?

What is an active microphone? An active microphone is a mic that requires power to function properly. All condenser mics are active, and some ribbon dynamics are active. Active components within mic designs include impedance converters (tubes or FET), externally-polarized capsules, and some components within a mic's printed circuit boards.

To learn more about active (and passive) microphones, check out my article Do Microphones Need Power To Function Properly?

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