What Is A Microphone Capsule And How Does It Work?

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Microphone capsules are what convert sound waves into microphone signals.

Choosing the right microphone is one of the most critical decisions on the road to a quality recording.

The choice of the microphone you use, the quality of the musical instrument, the abilities of the musician as well as the acoustic characteristics of the room in which the recording is made will affect the quality of the recording.

The microphone is the link between the acoustic sound created by an instrument or voice and how that sound will be immortalized.

A detailed understanding of the principle of operation and characteristics of the microphone, and all the processes, will help you make the right choice, and not be guided by the habit of working in a standard way.

When it comes to the principle of operation, that’s where you get to see that microphone capsules have a lot to play in the final sound output you get when you make use of a mic.

What Is A Microphone Capsule And How Does It Work?

In this article, before proceeding directly to the discussion of such an important topic, it’s worth giving a clear definition of some of the most important concepts.

What’s a Microphone?

A microphone is a device that converts changes in sound pressure into electrical current.

Microphones are classified according to many criteria, one of which is the principle of operation and purpose.

There’re many different principles of operation of microphones and the resulting areas of application.

All of them have advantages and disadvantages that we can use to our advantage if we are familiar with them.

Some of the most common techniques in the audio industry are:

Sensitivity (mV / Pa)

This characterizes the ability (efficiency) of a microphone to convert changes in sound pressure into electrical current.

In other words, it gives us an idea of ​​what voltage will be at the output of the microphone at a certain sound pressure.

Thus, the higher the sensitivity, the stronger the output signal at the same sound pressure.

Directivity

This is the ability of a microphone to respond to changes in sound pressure relative to the location of the sound source in space.

This ability is determined by the design of the capsule, which is the heart of any microphone.

Device and Principle of Operation

The device and the principle of operation are understood as a set of processes and logical relationships which lead to the desired result, in this case, an alternating current, the waveform of which is similar to the form of an acoustic wave.

The well-known concept of analog sound originates from all.

The vast majority of microphones used in the audio industry today belong to one of the two most widespread technologies – dynamic and condenser.

Dynamic Microphones

The principle of operation of a dynamic microphone is based on a physical law, which states that the movement of a conductor in a magnetic field creates an electric current.

This phenomenon is called induction.

The conductor to which the membrane is attached is placed in a constant magnetic field.

Changes in air pressure, as a result of the propagation of a sound wave, cause the membrane to move following the amplitude, phase, and frequency of this very sound wave.

The membrane, in turn, transmits this movement to the conductor.

The movement of a conductor in a constant magnetic field creates an electrical signal that exactly describes the sound wave that created this movement.

Hence the name – analog, since the resulting signal is an analog of a sound wave.

To facilitate the mechanism, and therefore to increase mobility, the conductor is made of thin wire, which is wrapped around a plastic hollow core.

This increases the amount of conductive material in the magnetic field, which in turn increases the induction and sensitivity of the microphone.

Condenser Microphones

The principle of operation of a condenser microphone is based on the property of a capacitor to change its electrical capacitance depending on the distance between its plates.

In a condenser microphone, one of the plates is movable and is a membrane, it’s made of the finest material to make it as light as possible.

Typically, a plastic film is used to which a thin layer of gold or nickel is applied.

The second plate is motionless.

Sound pressure, acting on the membrane, makes it move in the direction of the second plate, which shortens the distance between them and, as a consequence, causes a change in the capacitance of the capacitor.

The electric current resulting from this is the signal that describes the sound wave.

To create an electric field between two plates, which is necessary for the operation of a capacitor, two methods can be used: an external source (battery or phantom power) or coating one of the plates with a polarized material (such microphones are called electret microphones).

What are Microphone Capsules?

The capsule is the heart of any microphone, its direct task is to convert acoustic energy into electrical energy.

To do this, she must have several qualities, which in turn will determine some of her characteristics.

Types of Capsules

Microphone capsules by their design can be divided into closed and open types.

Let’s consider each of these types separately.

Closed Capsules

To simplify, the capsule is a metal cylinder, one side of which is a membrane.

Constant pressure is maintained inside the capsule since its inner world is completely isolated from the outer one and comes into contact with it only through a thin membrane.

Oscillations of pressure outside, created by sound waves, cause the membrane to 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 greater the amplitude of the sound wave oscillations, the greater the amplitude of deviations from constant pressure inside the capsule, and, accordingly, the higher the voltage at the exit from the capsule.

We can conclude that the location of the sound source and the direction of propagation of the vibrations created by it in this structure does not matter since the only factor necessary for its operation is a pressure change, and the side it happens has no part to play.

The directionality of the microphone capsules is a frequency-dependent parameter.

This means that it changes with the frequency of the acoustic wave.

For greater clarity of the characteristics of the capsule’s sensitivity to direction and frequency, special graphs were introduced.

They exhibit all of the above characteristics.

Closed capsules, by their design, will always have an omnidirectional characteristic (Omnidirectional) of the capsule, that is, they will not be sensitive to the location and direction of propagation of the sound wave.

In certain situations, this can have an advantage and become a decisive factor when choosing a microphone.

For example, the signal must remain stable, even when, the presenter turns his head to the side or, for example, if it’s necessary to record the sound of an instrument in a certain room as an atmospheric track.

Open Capsules

Open-type capsules have a design different from the one described above and, accordingly, the principle of operation.

As the name suggests, in this case, the membrane is open on both sides – front and rear, and is sensitive to the direction of the sound source relative to the membrane.

To understand how this works, consider three different situations.

1. In the first case, the sound wave comes from the front of the membrane.

Pressure changes resulting from the propagation of the sound wave cause the membrane to move.

Depending on the phase of the wave at a particular moment in time, the membrane will either bend when the phase is positive, i.e., an increase in pressure, or move in the opposite direction, with a negative phase, i.e., a decrease in pressure.

2. In the second case, when the sound wave comes from the back of the membrane, everything will be repeated, but exactly the opposite.

3. In this case, the sound wave comes from the side, that is, the membrane is located on its side.

This means that pressure changes will occur on both sides of the membrane.

On both sides of the membrane, there will be an increase in pressure in the positive phase of the wave or a decrease in pressure in its negative phase.

If we display the sensitivity of this structure on the graph, we are already familiar with, it’s not difficult to notice that the highest sensitivity will be from the front 0 degrees and the back 180 degrees.

As you approach the sides, the sensitivity drops and disappears at the point of 90 degrees.

As in the case of the omnidirectional characteristic, figure-eight also has several features that can become advantages in certain situations.

But there may be disadvantages.

Therefore, the choice of microphone directivity should be given due attention.

Also, there are several other common directivity characteristics of microphones.

They are the result of the summation of the above two characteristics.

Depending on the proportion taken, various new characteristics can be obtained.

Classification of Microphones By Directivity

Cardioid Microphone

This has a high degree of sensitivity on the front side and very low on the back.

This characteristic got its name due to the similarity of the graph with the heart symbol.

In practice, microphones with this directivity characteristic are often used when working with live sound.

Due to their reduced sensitivity on the back, which greatly reduces the portion of unwanted acoustic signals entering the microphone from the stage, they provide a cleaner, or, if you prefer, more sterile sound capture from the desired source.

Also, it increases the degree to which the signal can be amplified to the point of danger of feedback, thereby reducing this likelihood.

Dedicated Microphones Capsule (Pressure Zone Microphone)

A feature of these microphones is their ability to reduce phase conflicts between direct and reflected signals, as well as a higher signal-to-noise ratio.

In the vast majority of cases, we are talking about a condenser microphone.

In this case, the characteristic of the direction of the capsule can be both Omnidirectional and directional (cardio and its varieties).

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).

Final Thoughts

As stated in the beginning, the choice of microphone is a very important decision.

Listen carefully to all the features of the sound of a given instrument or vocals, and think about which microphone you can use to mask flaws and emphasize the advantages.

To do this, you need to be familiar with all the features of the microphones at your disposal.

Which of them suffer from the effect of proximity, which does not have very good sensitivity at high frequencies, which can cope well with high sound pressure levels?

Try to get close to the result even at the recording stage and then, you will not need to rack your brains when mixing and try to emphasize what is not in the recorded signal, because the wrong microphone was selected.

If perhaps the microphone doesn’t work well with the equipment, it may be that both are not suitable for each other.

All of these arise from the type of microphone capsules used in different mics and using the right mic for the right purposes.