Microphone Capsules Explained: Types, Function, and Sound

The capsule is the part of a microphone that actually turns sound waves into an electrical signal. Everything else is support hardware.

Picking the right mic for a recording is one of the most consequential decisions in the entire chain. The mic choice, the instrument, the player, and the room acoustics all stack together to shape the final sound, and the mic is the bridge between the acoustic event and whatever permanent record you end up with.

Knowing how the capsule actually works takes the guesswork out of that decision. Once you understand the physics, you stop defaulting to the same mic out of habit and start matching the capsule to the job.

What Is A Microphone Capsule And How Does It Work?

Before digging into capsules specifically, it helps to nail down a couple of basic concepts.

What’s a Microphone?

A microphone is a device that turns changes in sound pressure into an electrical current. Simple definition, wildly different implementations.

Mics get categorized by how they work and what they’re built for, and there are more principles of operation than most people realize. Each one has strengths and weaknesses that only really make sense once you know what’s happening inside the housing.

Here are the most important specs to understand.

Sensitivity (mV / Pa)

Sensitivity measures how efficiently a mic converts sound pressure into electrical current. We break this down further in our guide on what microphone sensitivity is if you want the full picture.

Put simply, it tells you how much voltage you’ll get at the output for a given sound pressure hitting the capsule. Higher sensitivity means more output signal from the same amount of sound, which matters when you’re recording quiet sources.

Directivity

Directivity is the mic’s ability to react differently to sounds based on where they’re coming from. It’s set by the physical design of the capsule — the heart of every microphone.

Device and Principle of Operation

“Principle of operation” is just fancy language for the sequence of physical events that turns an acoustic wave into an alternating current whose shape mirrors the shape of the sound. That’s where the whole idea of analog sound originates.

Almost every mic used in the audio industry today falls into one of two families: dynamic or condenser.

Dynamic Microphones

Dynamic mics work on a piece of physics called electromagnetic induction: move a conductor through a magnetic field and you generate current. That’s it.

Inside a dynamic mic, a small diaphragm is attached to a coil of thin wire that sits inside a permanent magnet. When sound pressure hits the diaphragm, it pushes the coil back and forth through the magnetic field, and the resulting current mirrors the original sound wave — amplitude, phase, frequency, the lot.

The signal is literally analogous to the sound that produced it, which is where the term “analog” comes from.

To keep the moving mass as low as possible, the coil is wound from extremely thin wire around a hollow plastic former. More wire in the magnetic field means more induction, and more induction means higher sensitivity.

Condenser Microphones

Condenser mics run on a totally different physical trick. A capacitor’s electrical capacitance changes based on the distance between its two plates, and a condenser capsule turns that property into a signal.

One plate is the diaphragm itself, built as light as physically possible — usually a plastic film with a micro-thin layer of gold or nickel evaporated onto it. The other plate is fixed.

When sound hits the diaphragm, it flexes toward and away from the back plate, the distance between the two changes, and the capacitance changes along with it. That moving capacitance produces the electrical signal.

For any of this to work, the capsule needs a charge across the two plates. You get that charge either from an external source like phantom power or a battery, or by coating one plate with a permanently polarized material.

Mics that use the second approach are called electret condensers.

What are Microphone Capsules?

The capsule is the heart of any microphone — the one component that has to physically convert acoustic energy into electrical energy. Everything it does well (and everything it does poorly) comes down to a handful of design qualities baked into its construction.

Types of Capsules

At the most basic level, capsules come in two shapes: closed and open. Let’s take them one at a time.

Closed Capsules

Picture a metal cylinder with a thin membrane sealing off one end. That’s the rough mechanical shape of a closed capsule.

The inside of the cylinder is isolated from the outside world, which means the air pressure inside stays constant. The only way the outside air communicates with the inside is by flexing the diaphragm.

When a sound wave arrives, that changing pressure outside makes the membrane move:

• inside the capsule when the sound wave is in its positive phase (pressure increase)
• outside the capsule when the sound wave is in its negative phase (pressure drop)

The bigger the sound wave, the bigger the pressure swing inside the capsule, and the bigger the voltage out. Simple relationship.

Here’s the key insight: because a closed capsule only cares about whether the pressure is rising or falling, it doesn’t care where the sound is coming from. It reacts to pressure changes from every direction equally.

Direction of travel is irrelevant because only magnitude matters.

Worth noting too: directivity isn’t a single fixed number. It varies with frequency, which is why manufacturers publish polar pattern graphs that show the pickup shape at different frequencies.

Closed capsules are always omnidirectional by nature. That makes them ideal for situations where the source might move — a presenter turning their head mid-sentence, for example — or where you want to capture the whole atmosphere of a room.

Getting your microphone placement right still makes a huge difference in how well the capsule’s omni pattern serves you.

Open Capsules

Open capsules work on a completely different geometry. The membrane is exposed on both the front and the back, which means it responds to the direction the sound is coming from, not just the raw pressure change.

To see why, it helps to run through three different scenarios.

1. Sound arrives from the front. The pressure wave flexes the diaphragm inward on the positive phase and outward on the negative phase, producing a signal in the normal way.

2. Sound arrives from the back. The same thing happens, but the phase is reversed — the diaphragm moves the opposite direction for the same wave shape.

3. Sound arrives from the side. Now the pressure changes hit both faces of the membrane at the same time. Positive phase pushes equally on both sides, negative phase pulls equally on both sides, and the net movement cancels out to zero.

Plot that response on a polar graph and the pattern is obvious: maximum sensitivity at 0 and 180 degrees, complete rejection at 90 degrees. This is the classic figure-eight pattern.

Like omni, figure-eight has specific strengths and specific weaknesses. Neither pattern is “better” — they just solve different problems.

Most of the other common polar patterns (cardioid, hypercardioid, supercardioid) are created by mixing these two basic responses in different ratios inside the capsule.

Classification of Microphones By Directivity

Cardioid Microphone

Cardioid mics are highly sensitive on the front and barely respond to anything behind them. The name comes from the heart-shaped pickup pattern on a polar graph.

You see cardioids everywhere in live sound for good reason. A supercardioid microphone takes the same idea even further with a tighter pickup angle and a small rear lobe.

Rejecting sound from the back cuts down on stage bleed dramatically, giving you a cleaner, more focused capture of whatever’s in front of the mic. It also lets you push gain harder before feedback kicks in — understanding what gain does on a microphone helps you walk right up to that line without crossing it.

Dedicated Microphones Capsule (Pressure Zone Microphone)

PZMs are built to reduce phase conflict between direct and reflected signals while also boosting the signal-to-noise ratio. Almost all of them are condensers, and the capsule inside can be omnidirectional or a directional pattern like cardioid.

Operating Principle

You can run the risk of receiving a signal that suffers from the comb filter when phase conflicts are reduced.

This effect arises as a consequence of the interaction of direct and reflected, that is, somewhat delayed, signals.

Depending on the additional distance traveled (on the amount of delay), certain frequencies in the original signal will be in antiphase or vice versa in phase with the reflected ones, which in turn will cause positive or negative interference.

PZM microphones solve this problem by placing the capsule in a flat housing that sits directly above the reflective surface.

In this case, the distance of the capsule to it’s from several millimeters to a couple of centimeters

Thus, the delay between the direct and reflected signal becomes so small that it does not affect the frequency range, which is of interest to us.

The comb filter effect doesn’t miraculously stop happening.

Instead, by simply shortening the distance and, accordingly, the delay time between the direct and reflected signal, we shift it along the frequency axis beyond the limits of our hearing.

Higher Signal to Noise Ratio

This ability of PZM microphones is based on a phenomenon that is probably familiar to you from acoustics - the sound pressure near walls can be up to 6 dB higher than at any other point in the room.

Since the PZM microphone capsule is located near the reflective surface, that is, in the zone of maximum acoustic pressure of the sound wave, this provides a higher level of the useful signal, improving the signal-to-noise ratio.

Some microphones of this family are equipped with a plate, which acts as a reflective surface and allows such a microphone to be placed not only on walls or on the floor.

However, it should be borne in mind that the larger the reflective surface, the lower the frequency on which this effect has an influence.

In other words, if the reflecting surface is too small, then low frequencies will not be reflected from it and the above effect will not affect them.

This can lead to unnatural frequency responses.

To avoid this, these microphones should be placed on large surfaces such as floors or walls.

A bright example of the use of such microphones is when they are used to take the sound from a grand piano.

In this case, the PZM microphone is mounted on the inner side of the wing of the grand piano, which acts as a reflective surface.

Another example would be sounding a drum.

In this case, the PZM microphone is placed on the floor directly next to the drum, usually frontally.

Non-linearity of the Amplitude-Frequency

To the above characteristics, such as type, the principle of operation, and directivity, you can add another very important aspect that plays an equally important role in choosing a microphone for a specific task - amplitude-frequency characteristic (AFC) and the level of distortion.

Most often, it’s they who predetermine the tone of the sound characteristic of a particular microphone model.

Frequency response describes the deviation of the signal amplitude from the amplitude of the original at a particular frequency in a certain range.

These deviations occur due to various factors, among which are the design features of the membrane, its material and weight, as well as design solutions for the implementation of internal electrical circuits and microphone units.

As a rule, the frequency response is presented in the form of a graph (see graph 1), on which you can see at what frequencies and how many decibel deviations from linearity occur.

However, it can be represented as follows: 60 Hz - 20 kHz (+/- 2 dB).

In this case, it’s impossible to know at which frequencies the deviation occurs.

Based on these data, we can only conclude that in the range from 60Hz to 20KHz the maximum deviation is 2dB.

Taking a look at the frequency response of the microphone in the accompanying documentation, one can draw preliminary conclusions about the “color” and “shade” of the microphone.

But the conclusions can only be drawn by carefully listening to the microphone on various sound sources, this is the best indicator.

When working with sound, rely on your ears, not your eyes.

In addition to the above characteristics, there are several more important ones:

Maximum sound pressure level (dBSPL)

This parameter should include the percentage of total harmonic distortion at the declared level.

Self-noise level (dB / dBA), which is typically less than 30 dBA, and output resistance (Ohm).

All professional microphones have a low output impedance (Lo-Z), no more than 600 Ω.

This is very important for the ability to transmit a signal over relatively long (about 100 meters) distances without loss of signal quality and level.

Classification by Purpose

There are many more types of microphones that have specific uses.

These include:

Zone pressure mic Pressure zone microphone

The capsule is mounted above a metal surface to prevent signals reflected from nearby surfaces from entering the diaphragm, which can cause phase distortion.

It’s often used for recording grand pianos as it can be attached to the lid and can also be used on stage in the theater.

Stereo Mic Stereo microphone

In one case, two capsules are mounted in such a way that each of them faces in the opposite direction.

This achieves a wider stereo image.

An MS or X / Y technique may also be implemented in the microphone.

Boom mi Shotgun

This condenser microphone is designed for use in open areas and is widely used in cinematography.

It has a very narrow directivity, which is achieved by phase shifts of audio information coming from the sides of the microphone.

For this, there are slots on the body through which third-party information enters the microphone.

But since the capsule is located at the very end of the body, what reaches it’s the sound that came from the front.

Everything else receives a phase shift and is thus drowned out.

These are perhaps the most basic types, although not all.

In the course of your professional activity, you will come across additional types of microphones, as well as various types of their application.

Additional Functions

Today there are many models of microphones that combine several types of directivity and make it possible to use any of them, as needed, simply by switching the position.

Almost all modern microphones have a function that allows you to reduce the output level (Pad / Trim).

Typically, the sound pressure is 6/12/18 dB (remember that increasing/decreasing 6.02 dB means twice) and this is a very useful feature when working with high SPL sound sources.

However, if the sound pressure exceeds the maximum possible amplitude of movement of the microphone membrane, this will no longer help since the signal distortion will be mechanical.

There are also built-in filters for cutting low frequencies (40,60,80 Hz).

It’s very applicable when working in echoing rooms and especially in cases with the live broadcast, when “then cut off” does not work.

On some models, you can even choose the steepness of the cut, while on others it’s constant and determined by the manufacturer (so it’s worth reading the technical documentation).

Frequently Asked Questions

What is the difference between a closed capsule and an open capsule in a microphone?

A closed capsule has its membrane sealed on one side, making it sensitive to pressure changes from any direction, which gives it an omnidirectional pickup pattern. An open capsule exposes the membrane on both sides, making it sensitive to the direction of the sound source and enabling directional patterns like cardioid and figure-eight.

Can you replace the capsule in a microphone to change how it sounds?

Some microphones, especially modular condenser models, allow you to swap capsules to change the directivity pattern or tonal character. However, most consumer and dynamic microphones have permanently installed capsules.

Replacing a capsule on these requires professional soldering and is not always cost-effective.

Why do condenser microphone capsules need phantom power but dynamic capsules do not?

Condenser capsules work by measuring changes in electrical capacitance between two plates, which requires an external voltage to charge the capacitor. Dynamic capsules generate their signal through electromagnetic induction when the diaphragm moves a coil in a magnetic field, so they produce a current on their own without any external power.

Final Thoughts

Picking a microphone is a bigger decision than most people realize. Listen carefully to whatever you’re recording and think about which mic will mask the weaknesses and emphasize the strengths, because no amount of mixing later will fix a bad pairing at the source.

That only works if you actually know the mics in your locker. Which ones suffer from proximity effect?

Which ones lose detail in the high end? Which ones can handle a blasting kick drum or a trumpet bell at close range without folding?

Get as close to the final sound as you can at the recording stage and your mix sessions get ten times easier. You won’t be trying to EQ in presence or air that was never captured to begin with.

When a mic and a source just don’t click, no processing in the world will save the recording — and all of that, ultimately, comes back to the capsule inside and whether it was the right one for the job.