When musicians and audio engineers say “dynamic mic,” they are typically referring to a moving-coil dynamic mic. These popular microphones are used in broadcasts, recording studios, and live stages around the world.
What is a moving-coil dynamic microphone? A moving-coil dynamic mic is a transducer that converts sound waves into mic signals via electromagnetic induction. As the diaphragm moves, an attached conductive coil oscillates within a magnetic field. This induces an AC voltage across the coil, which is then outputted as the mic signal.
That’s the quick answer, but we’ll go into greater detail about moving-coil microphones in this article. Before we get into it, I’d like to present a short table of contents for easier scrolling.
Table Of Contents
- The Reason Behind The Name
- What Is Electromagnetic Induction?
- Anatomy Of A Dynamic Microphone
- The Energy Chain: From Sound Source To Microphone Output
- Optimization Of The Moving-Coil Dynamic Mic
- General Characteristics Of Moving-Coil Dynamic Microphones
- 5 Common Moving-Coil Dynamic Microphones
- Related Questions
The Reason Behind The Name
When I first started learning about the differences between microphone transducers, I thought that dynamic mics were named so because of their greater dynamic range over condenser microphones.
It’s true that generally speaking, a dynamic mic will have a greater dynamic range than its condenser counterpart. This is because dynamic mics having no self-noise and extremely high maximum sound pressure levels.
However, the dynamic range is not the reason why dynamic microphones carry their names.
The dynamic microphone is actually named after one of its important predecessors: the dynamo!
What is a dynamo? An electric dynamo is a transducer that, like microphones, converts mechanical energy into electrical energy via electromagnetic induction.
The dynamo works by rotating coils of wire through a magnetic field. The magnetic field is supplied by a permanent magnet (or magnets) in smaller dynamos or by field coils in larger dynamos.
According to Faraday’s law of induction, the motion of a coil of conductive wire through a magnetic field creates an electric current in the wire. By rotating its coils in one direction in a magnetic field, the Dynamo creates a pulsating direct current.
As a side note, “Dynamo” was first coined by the famous Michael Faraday in 1831 (he also discovered the law of induction that is named after him)!
To learn more about the history of microphone technology, check out my article Mic History: Who Invented Each Type Of Microphone And When?
From Dynamo To Dynamic Mic
Dynamos create DC (direct current) signals, but audio signals are AC (alternating current).
Though the dynamic mic is based on the dynamo principle, it’s certainly not a dynamo. The coil of wire in a dynamic microphone translates in two directions (oscillates back and forth) through a magnetic field. This type of motion induces an alternating current across the coil of wire rather than direct current (like the dynamo).
The small, “AC dynamos” found in dynamic microphones are technically called magnetos!
The Term “Moving-Coil”
All moving-coil microphones are dynamic (they are transducers that work of the principle of electromagnetic induction). However, not all dynamic microphones have a moving coil!
The popular ribbon microphone is also a type of dynamic microphone. And so, the term “moving-coil” is a differentiator between the two microphone types when necessary.
Moving-coil dynamic mics are much more popular than their ribbon counterparts. For that reason, the simple term “dynamic microphone” is reserved for them (I’ll be using the terms interchangeably throughout this article). Dynamic ribbon microphones are typically distinguished and referred to as “ribbon microphones.”
For an in-depth view of dynamic ribbon microphones, please check out my article Dynamic Ribbon Microphones: The In-Depth Guide.
For a fun article on the naming of microphones in general, check out Why Are Microphones Called Microphones?
Some headphones and loudspeakers are also designed one the same moving-coil/dynamic principles. To learn more, check out the following My New Microphone articles, respectively:
• Complete Illustrated Guide To Moving-Coil Dynamic Headphones
• How Do Speakers Produce Sound? (A Helpful Beginner’s Guide)
What Is Electromagnetic Induction?
So what is electromagnetic induction? Electromagnetic induction is the creation of a voltage across an electrical conductor in a closed circuit as it experiences a changing magnetic field. It is the working principle of dynamic microphone transducers.
In a moving-coil dynamic microphone, the coil (electrical conductor) moves through a permanent magnetic field supplied by a mic’s permanent magnet(s). The magnetic field relative to the moving-coil is changing, and so once we close a circuit with the moving-coil (and the coil moves), we will have an electromagnetically induced current (and mic signal)!
There is a physical law that is important to our understanding of electromagnetic induction. This is Faraday’s Law of Induction.
Faraday’s Law Of Induction
What is Faraday’s law of induction? Faraday’s law of induction states that the electromotive force (induced voltage) in a closed circuit is proportional to the rate of change over time of the magnetic flux through that circuit.
Let’s break this down into smaller definitions to better understand in the context of a moving-coil dynamic microphone:
- Electromotive force (emf) or “induced voltage”: the voltage created across the moving-coil as a result of electromagnetic induction
- Closed circuit: a complete electrical connection in which current (in this case alternating current) can flow.
- “Proportional to the rate of change over time”: simply means that changing the magnetic flux results in an induced voltage.
- Magnetic flux: the total magnetic field which passes through a given area.
In the moving-coil microphone design, we have a permanent magnet. This magnet is complex in its design and has a complex magnetic field that is concentrated around the moving coil.
The magnetic field’s strength can be measured with field lines. These are vectors that show both the strength and direction of the magnetic field at any given point.
The magnetic flux is the strength of the magnetic field over a given area:
- We can imagine a strong magnetic flux as having many strong field lines going through a big area.
- Imagine a weak magnetic flux as having fewer field lines going through a given area.
- If no field lines go through the area (like if the area is parallel to the direction of the field lines), there is no magnetic flux!
The movement of the moving-coil in a permanent magnetic field causes a change in magnetic flux in the moving-coil. This change in magnetic flux in the moving-coil causes an AC voltage to be created across it via electromagnetic induction. This AC voltage is then outputted as the mic signal.
Depending on the direction of relative displacement between the conductive wire and the magnetic field, a positive or negative voltage will be applied across the conductor. This means we’re dealing with alternating current, which is also how audio signals work (we’re getting there!).
There are 3 factors that determine the amount of voltage that will be applied across the moving coil of a dynamic microphone. They are:
- The number of loops in the moving-coil: by increasing the number of loops in the conductive coil, we essentially increase the number of conductors cutting through the magnetic field. The amount of induced voltage across the entire moving-coil is the sum of all the voltage in each individual loop of the coil.
- The velocity of the moving-coil: by increasing the velocity of the moving-coil, we move through the magnetic field faster and therefore have a faster rate of change of the magnetic flux.
- The strength of the magnetic field: by increasing the strength of the magnetic field, we have a greater magnetic flux when field lines are perpendicular to a given area. The potential change in magnetic flux is therefore greater.
In a microphone, the number of loops in the moving coil and the strength of the magnetic field are constants. It is, therefore, the velocity of the moving coil that determines the changing of the voltage across that moving-coil.
The diaphragm of the microphone is attached to the moving-coil. And so it’s the movement of the diaphragm that results in an audio signal.
Anatomy Of A Dynamic Microphone
Dynamic microphones come in all shapes and sizes. As different as they may be, though, their operating principle remains the same.
There are many parts of microphone anatomy that are common among dynamic mics: the diaphragm, moving-coil, magnets, etc. Without getting into every single part of a microphone, let’s talk about the essential elements that make up moving-coil dynamic microphones.
The four determining parts of a moving coil dynamic microphone:
- The Dynamic Diaphragm
- The Conductor (Moving-Coil)
- The Magnet And Its Pole Pieces
- The Passive Circuitry/Step-Up Transformer
Related article: How Are Microphones Made? (Designs, Materials, Production)
The Dynamic Diaphragm
What is a microphone diaphragm? A mic diaphragm is a thin membrane suspended at its edges in a mic capsule. The purpose of a diaphragm is to move when subjected to sound pressure and, in turn, begin the microphone/transducer’s process of converting sound waves to mic signals.
The dynamic mic’s diaphragm is attached to the moving-coil and so when sound pressure causes the diaphragm to vibrate, the coil vibrates with it. Now that we understand electromagnetic induction, we can really understand the central role that the diaphragm plays in a dynamic mic.
What Is The Diaphragm Made Of?
Professional dynamic microphone diaphragms are typically made of polyester film (also known as “plastic sheet” or by the common brand name “Mylar”). The exact type, grade, and thicknesses of the polyester film will vary from mic to mic and from manufacturer to manufacturer.
Related article: What Are Microphone Diaphragms Made Of? (All Diaphragm Types).
How Does The Moving-Coil Affect The Diaphragm?
The moving-coil is attached to a circular groove of the diaphragm. If we look at a dynamic mic diaphragm, we’ll see a smaller circle within the diaphragm that indicates the position of the coil.
This groove means the diaphragm is not one smooth piece and therefore it is difficult to express exactly how it reacts to sound frequencies across the audible spectrum.
Because the moving-coil is attached, it adds a relatively large mass to the diaphragm. This added weight lowers the resonant frequencies of the diaphragm and makes it more difficult for higher frequencies (shorter wavelengths) to move the diaphragm. Both of which affect the overall frequency response of the microphone.
The inner circles of the cloth/plastic coincide with the groove of the diaphragm necessary for the attachment of the moving-coil.
Removing the cloth would have exposed the diaphragm for a better picture, but I didn’t want to remove the dampening cloth on my microphones.
How Does The Diaphragm React With Sound?
It’s the difference in sound pressure between the front and back of the diaphragm that causes the diaphragm to move back and forth about its resting position. Note that the diaphragm displacement is very small when reacting to varying sound pressures. But it doesn’t take much movement to get what we need!
Manipulating the differences in air pressure between the front and back of a diaphragm is how manufacturers achieve various polar patterns.
Since the coil moves along with the diaphragm, the diaphragm displacement is crucial to getting a strong signal out of a moving-coil dynamic microphone.
For more information on microphone diaphragms, check out my article What Is A Microphone Diaphragm?
The Conductor (Moving-Coil)
What is an electrical conductor? An electrical conductor is any object or material that allows electricity to flow in one or more directions.
The moving-coil of a dynamic microphone is a tightly wound coil of small-diameter conductive wire. It looks almost like a small ring.
The coil moves with the diaphragm through a magnetic field supplied by the mic’s permanent magnet. It is part of a closed circuit. Electromagnetic induction states that as the conductive wire moves through the field lines of the magnetic field, a voltage is induced across it.
The moving-coil is one of the two key transducer components in a moving coil dynamic microphone. The moving-coil, in conjunction with the magnet, changes mechanical wave energy (sound) into electrical energy (mic signals).
What Is The Moving-Coil Made Of?
The moving-coil is made of conductive, but flexible material. This usually means copper. More specifically, a typical moving-coil is made of very small-diameter insulated copper wire that is wound many times around.
Copper is most common not only because of its price, but because it helps to maximize the electromagnetic induction and produce a strong audio signal.
- Copper is very conductive (5.96×107 Siemens per meter).
- Copper is light (8.96 g/cm3) and allows the diaphragm/coil combo to be more reactive than heavier materials.
How Is The Moving-Coil Positioned In The Dynamic Microphone?
The moving-coil, as we mentioned, is physically attached to a groove in the diaphragm.
Other than the diaphragm, the moving-coil does not touch anything! The coil is suspended inside the microphone capsule in what is known as the “gap” of the specially designed magnet.
The gap is basically a ring of empty space within the magnetic structure that is just wide enough for the moving-coil to reside without touching the magnet. The magnetic north pole piece is typically on the interior of the coil while the magnetic south pole piece if on the exterior of the coil.
Note that the wire is has a very small diameter, but is wound many times in the overall coil. This yields three main advantages:
- The magnet gap can be smaller, concentrating (strengthening) the magnetic field around the coil.
- A longer length of wire is possible in the same amount of coil. This increases conductivity.
- More loops in the coil essentially mean an increase in the number of conductors cutting through the magnetic field.
For more on the moving-coil of dynamic microphones, check out my article What Is A Microphone Voice Coil?
The Magnet And Its Pole Pieces
What is a magnet and what pole pieces? A magnet is any material or object that produces a magnetic field. Pole pieces are masses of iron 9or other magnetic material that form the end of an electromagnet and work to concentrate/direct the lines of magnetic force.
The magnet, complete with its pole pieces, typically has a complex (somewhat awkward-looking) shape inside the dynamic microphone.
The “typical design” I’ll discuss here utilizes the following:
- Main magnetic ring.
- Ring-like top pole plate.
- Disc-like bottom pole plate.
- Cylindrical pole piece.
The permanent magnet inside a dynamic microphone provides the magnetic field necessary for the conversion of mechanical wave energy into electrical energy. Without the magnet, no electromagnetic induction would occur and no amount of diaphragm displacement or coil movement would result in any audio signal.
What Are The Magnet And Pole Pieces Made Of?
The main magnetic ring needs to be powerful for its small size and is typically made of ferrite or powerful neodymium.
The pole pieces needed to properly “extend” the magnetic poles of the magnet and are typically made of soft iron.
How Are The Magnet And Pole Pieces Assembled?
The magnet needs to create a powerful and concentrated magnetic field around our tiny moving coil of conductive wire.
We need a specific and magnetically complex design with a circular gap for the moving coil to reside. On top of that, we typically need the interior of the gap to be the north pole of the magnet and the exterior to be the south pole.
This is not practical with a single magnet. Therefore, pole pieces are incorporated into the design!
The main magnetic neodymium ring provides the bulk of the strength of the magnetic field. The ring is positioned parallel with the diaphragm with its south pole closest to the diaphragm.
A top pole ring is put on top of the magnet (diaphragm side) to extend the south pole.
The bottom pole plate is put on the bottom of the diaphragm and extends the north pole.
A cylindrical pole piece is then extended from the center of the bottom pole plate up toward the diaphragm, further extending the north pole of the magnet.
To recap, the moving-coil dynamic mic capsule/cartridge design involves the following:
- A magnetic ring (south pole closest to diaphragm): This is a cylindrical magnet with a circular hole in its center.
- Top pole ring (extends the south pole of the magnet toward the diaphragm): The top ring has an interior hole slightly bigger than the diaphragm of the moving-coil.
- Bottom pole plate (extends the north pole away from the diaphragm): The bottom plate is actually a plate there is no hole in its design. In the center of the bottom plate, there is a pole piece that extends out of it toward the diaphragm.
- Pole piece (extends the north pole from the bottom plate toward the diaphragm on the interior of the moving-coil): The pole piece is slightly smaller in diameter than the interior of the moving-coil and extends the south pole of the magnet to be flush with the top pole plate.
Here is a cross-sectional diagram I made to better visually represent the dynamic microphone capsule.
- The diaphragm is drawn in orange.
- The moving coil is drawn in purple.
- The main magnet is drawn in red.
- The pole pieces are drawn in green.
- The sound waves are represented in black.
- The poles of the overall magnetic structure are labelled N (north pole) and S (south pole).
- The signal wires from the ends of the moving coil are drawn in blue, which completes an electrical circuit with the transformer.
For more on magnets and microphones, check out my article Do Microphones Need Magnetism To Work Properly?
The Passive Circuitry/Step-Up Transformer
I’ve lumped the passive circuitry and transformer together here. Together, they are made up of:
- 2 signal wires (1 taken from an end of the moving coil).
- The step-up transformer.
- Balanced audio signal wires.
- The microphone output pins.
Why Are 2 Signal Wires Taken From The Moving-Coil?
In order for electromagnetic induction to properly occur, we need more than just a magnetic field and conductive coil of wire. We need a closed circuit that includes the coil of wire!
In a moving-coil dynamic microphone, two signal wires are attached to the moving-coil: one at each end of the tightly wound copper wire. These two signal wires connect to a transformer, thereby closing an electrical circuit and allowing electromagnetic induction to occur.
What Is A Step-Up Transformer?
What is a step-up transformer? A step-up transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another circuit (or multiple circuits). It has a primary winding with fewer turns than the secondary in order to increase the voltage from the primary to the secondary (while reducing current and impedance).
A step-up transformer “steps-up” or increases the voltage of the signal received from the moving coil circuit. It is an electrical device that receives an AC input audio signal (from the moving coil) and produces a related AC output audio signal (to the microphone output) without any physical connection between input and output.
The step-up transformer is designed with two separate coils of insulated wire, both of which are wound around the same magnetic core. These two coils never make a physical connection with one another and are therefore isolated from each other. These coils are referred to as “windings.”
Here’s a simple drawing I made to represent the step-up transformer of a moving-coil dynamic mic:
- The signal wires are drawn in blue.
- The primary winding (in a circuit with the capsule/cartridge) is drawn in orange.
- The magnetic core is drawn in red.
- The secondary winding (in a circuit with the mic output connection) is drawn in green.
- The centre tap is drawn in purple.
Let’s discuss each of the windings and their circuits:
- The primary winding completes an AC circuit with the moving coil of the dynamic capsule (this is the microphone transformer “input”).
- The secondary winding completes an AC circuit (a balanced audio signal) with the microphone output.
The windings are typically made of conductive copper wire and the magnetic core is made from materials like iron and ferrite.
The alternating current from the primary winding induces a changing magnetic field in the magnetic core of the transformer, which, in turn, induces an alternating current of the secondary winding.
This is due to a phenomenon called inductive coupling, which states that whenever an AC signal passes through the primary winding, a related AC signal appears on the secondary winding. Inductive coupling, like the electromagnetic induction that happens in the dynamic mic capsule, is based on the principle of electromagnetism.
Let’s recall the 3 factors that determine the amount of voltage that can electromagnetically be induced on a conductive coil:
- The number of loops in the coil.
- The velocity of the coil through a magnetic field.
- The strength of the magnetic field.
There’s no relative movement or relative change in the magnetic field strength between the primary (input) and secondary (output) windings of the transformer. So the number of loops in the secondary winding must be greater than the number of loops in the primary in order for the signal to be effectively “stepped-up.”
The ratio of turns between primary and secondary windings is theoretically equal to:
- The ratio of voltage between primary and secondary windings.
- The ratio of current between secondary and primary windings.
So a step-up transformer increases the voltage of the audio signal while decreasing the current of the audio signal.
What Is The Purpose Of A Transformer In A Moving-Coil Dynamic Microphone?
Dynamic microphones are designed with step-up transformers in order to:
- Increase or “step-up” the voltage of the induced signal.
- Increase the impedance of the induced signal voltage.
- Change the induced signal into a balanced audio signal.
- Protect the microphone from DC voltage like phantom power.
- Aid in isolating the microphone from other electronic devices and RFI.
Increase Or “Step-Up” The Voltage Of The Induced Signal
A transformer with more turns in its secondary winding than its primary winding will step-up the voltage of the signal.
Increase The Impedance Of The Induced Signal Voltage
With all other factors being equal, a greater number of turns in a coil equals a greater impedance. Since the secondary winding has more turns in coil than the primary winding, we effectively increase the impedance and strength of the audio signal
This is important since the signal induced on the moving-coil is weak and has a very low impedance.
The ratio of the primary impedance to secondary impedance is the square of the turns ratio, and so there is a considerable increase in impedance between the step-up transformer input and output.
Change The Induced Signal Into A Balanced Audio Signal
The transducer changes our unbalanced signal from the microphone capsule into a balanced signal at the microphone output. This is done is through a center-tap on the secondary winding.
A center-tap is a contact point made at the halfway point of a conductor (in this case the secondary winding). The center-tap effectively breaks the overall voltage across the secondary winding into two halves and separates it into two signals.
These two signals are in opposite polarity to one another, which is exactly what we need for a balanced audio signal.
- Pin 2 lead wire takes the “positive polarity” half of the winding.
- Pin 3 lead wire takes the “negative polarity” half of the winding.
Pin 1 or ground completes the balanced output of a professional moving-coil dynamic mic.
A really cool side note here is that because of the transformer’s bi-directional nature, its output (secondary winding) can become its input (primary winding).
This means that if we send an audio signal to the microphone (into its output), we can effectively “step-down” and send it to the moving coil. The moving-coil will then start moving due to its own magnetic field, effectively turning our dynamic microphone into a very tiny speaker!
The dynamic capsule design is very similar to that of a loudspeaker, and the dynamic mic is often referred to as “a loudspeaker in reverse.”
Protect The Microphone From DC Voltage Like Phantom Power
DC voltage doesn’t cause an alternating magnetic field. Therefore, transformers do not allow DC voltage to pass through them, effectively “protecting” any dynamic microphone from 48 volts DC phantom power or any other amount of DC voltage for that matter.
To learn more about phantom power, check out my article What Is Phantom Power And How Does It Work With Microphones?
Aid In Isolating The Microphone From Other Electronic Devices And RFI
Transformers also isolate their microphones from other electronic devices and block RFI (radio frequency interference). This is because the primary and secondary do not physically touch one another. We can solve hum problems by isolating (“lifting”) the grounds of different devices.
Note that some moving-coil dynamic microphones do not have a transformer and rely on built-in preamplifier circuitry to amplify and balance the signal while rejecting RFI and AC hum. Some microphones also include a type of humbucking coil (like in guitar pickups) to help reduce hum in the audio signal.
Also, note that not all transformers are built the same and that the quality of the transformer will impact frequency response and maximum voltage input before distortion. Cheap transformers will oftentimes degrade the signal. More on this in the optimization of the moving-coil dynamic mic section.
To learn more about microphone transformers, check out my articles What Are Microphone Transformers And What Is Their Role? and Do All Microphones Have Transformers And Transistors? (+ Mic Examples).
We’d discussed voltage, current and impedance in this section. For more information on these units of measurement and many others, check out my article Units Of Measurement & Prefixes In Sound & Audio Electronics.
The Energy Chain: From Sound Source To Microphone Output
I thought it would be cool to describe the energy path of vocals through a dynamic microphone. I’ll refer to the sound/audio as energy for the sake of furthering our understanding of the microphone as a transducer (a device that converts one form of energy into another form of energy).
Here’s a list of the forms of energy that will be involved in this section:
- Mechanical wave energy: the energy associated with the motion and position of a physical object.
- Acoustical energy: the energy associated with the vibration of matter in a fluid (air) along a mechanical wave (sound wave).
- Electrical energy: the energy associated with voltage and current through a circuit.
Note that these are not perfect descriptions, but simply brief explanations to help avoid confusion.
Let’s start with the first interaction the microphone has with the vocal sound waves.
- Sound vibrates around the diaphragm.
- The difference in sound pressure between the front and back of the diaphragm causes it to vibrate back and forth about its resting position.
Transduction of Acoustical to Mechanical Wave Energy.
- The moving coil is attached to the moving coil and moves with it.
- The movement of the coil in the magnetic field causes an AC voltage to be induced across it.
Transduction of Mechanical Wave to Electrical Energy.
- A signal wire from each end of the moving coil creates a circuit with the primary winding of the step-up transformer.
- The AC voltage across the primary winding induces a changing magnetic field in the transformer’s magnetic core.
- The changing magnetic field in the step-up transformer induces a greater AC voltage in the secondary winding.
- The secondary coil is center-tapped, creating reverse polarity on pins 2 and 3 (balanced audio).
- Pin 1 is grounded in the microphone, and together with pins 2 and 3, the audio signal is sent through the microphone output.
Where we send this output audio signal is beyond the scope of this article, but it could be to a microphone preamplifier, an audio-interface, directly to a mixer or loudspeaker, etc. There are plenty of options!
Optimization Of The Moving-Coil Dynamic Mic
So we now have a solid idea of how the moving coil dynamic microphone converts sound into electrical signals. However, that doesn’t necessarily mean that the audio signal sounds very good!
There are some inherent problems with dynamic microphones that need some “fixing” to make their output signals usable in the real world.
Let’s talk about how manufacturers fix the following issues inherent in moving-coil dynamic microphones:
- Non-linear frequency response.
- Handling noise due to mechanical vibration.
Non-linear Inducted Frequency Response
The big, oversimplified, issue is the frequency response. The “bare bones” audio signal we get from a diaphragm, moving coil, magnet, and transformer has a nonlinear frequency response.
Why are the frequency responses of moving-coil dynamic mics inherently coloured?
- The resonant frequency of the diaphragm (dependent on shape, size, stiffness, mass of diaphragm and mass of the moving coil).
- Housing, grille, cloth, and other material (dampening higher frequencies/ have their own resonant frequencies).
- The mass of the diaphragm and moving coil creates inertia and makes the microphone less sensitive to short wavelengths/high frequencies.
- Transformers often roll off the low and high frequencies.
Acoustic Resonant Frequencies
Because the diaphragm of a dynamic microphone is attached to a moving coil (and therefore has more mass), it tends to have a low resonant frequency that is in the range of our hearing.
The diaphragm is subjected to the largest displacement at its resonant frequency. This largest displacement means that the microphone will output the most gain at this frequency, which makes a nonlinearity in the microphone frequency response.
In order to minimize or “dampen” this resonance frequency, manufacturers utilize tuned air cavities behind and around the diaphragm along with dampening cloth material with specific acoustic impedance.
The air cavities are sized so that the standing waves within them nullify the effects of the resonant frequency of the diaphragm/moving coil combination. Dampening cloth is placed inside the cavities of the microphone and has a specific acoustic impedance that is the strongest at the resonant frequency.
Tiny slots are often arranged into dynamic microphone diaphragms in order to “soften” the peaks in resonance frequencies, helping to smooth out the frequency response.
The microphone casing and capsule housing, too, have their own resonant frequencies which could potentially cause vibration in the diaphragm.
Decreased Sensitivity To High Frequencies
The diaphragm of a moving coil microphone is heavy. This is because the moving-coil is attached to the diaphragm.
High frequencies have short waveforms, which have difficulty moving heavy diaphragms. For this reason, moving-coil dynamic mics often have a sharp roll-off in upper-frequency response.
In order to extend the range of the frequency response, a type of resonator cap is often included in the dynamic microphone design.
This resonator cap is a volume of air on top of the front of the diaphragm that is tuned to a high resonance frequency. The smaller the cavity, the higher its resonant frequencies.
In reality, the resonator cap is not overly capable of extending a dynamic mic’s frequency response. However, it can be (and most often is) tuned at the point where the diaphragm’s frequency response starts rolling off, creating a sort of resonance peak before the high-end roll-off.
Transformer Or Transformerless?
The step-up transformers in dynamic mics have limitations. Higher turn ratios yield a greater output signal relative to the input but tend to have more limitations. There is a balancing act to design a transformer that’s just right for a microphone.
When designing or sourcing a microphone’s transformer, manufacturers must take the following into account:
- More turns on the secondary winding mean more resistance between windings and cause a lower high-frequency roll-off.
- Less turns in the primary winding means less primary inductance and therefore a higher roll-off of low frequencies.
There are many high-quality transformers designed for audio that work well with moving-coil dynamic mics.
That being said, to avoid the high cost of high-quality transformers, some dynamic microphones are designed with more complex circuitry that have virtually all the benefits of high-quality transformers.
Noise Due To Mechanical Vibration
Any vibration of the moving-coil will cause a coinciding signal at the microphone output.
To reduce mechanical vibration, insulating materials like rubber are often used in between adjacent parts of the microphone.
All dynamic microphones have some sort of internal shock mount that isolates their capsule/cartridge (diaphragm, moving coil, magnet, and housing) from their handle.
To learn more about microphone noise and how to reduce it, check out my article 15 Ways To Effectively Reduce Microphone Noise.
Everything Here Is Interrelated
The optimization of a moving-coil dynamic microphone is one big balancing game. Everything is related.
No volume of air, piece of dampening cloth, soundhole, insulator, or shock mount will solve one issue without changing other aspects of the microphone.
Special care is taken in each and every measurement in designing any professional microphone. Seemingly small pieces of dynamic microphone design will have big effects on the mic’s overall sound.
General Characteristics Of Moving-Coil Dynamic Microphones
Some generalizations can be made of dynamic microphones.
Although not always the case, moving-coil dynamic mics are usually assumed to have the following characteristics:
- Colouration Of Frequency Response.
- Simple Passive Circuitry.
- Low Sensitivity And High Maximum Sound Pressure Level.
- Excellent Durability.
Colouration Of Frequency Response
A coloured frequency response basically means that a microphone is not equally sensitive to all audible frequencies. Coloured microphones have peaks, valleys, and/or roll-offs in their frequency responses.
As we’ve discussed, the frequency response of a moving-coil dynamic microphone is far from linear:
- The high-end frequencies are typically rolled off since the short wavelengths aren’t effective at vibrating the heavy diaphragm.
- There are typically audible resonance frequencies in the diaphragm/moving-coil piece of the microphone.
- The “balancing game” of solving inherent resonance frequencies may or may not cause other peaks and valleys in the audible frequency response of the dynamic microphone.
- The transformer will often have some effect on the roll-off of low and high frequencies.
Though these non-linearities are often thought of in a negative sense, the colouration of dynamic microphones has become one of their biggest selling points. For example:
- Dynamic mics often have natural boosts between 2-10 kHz, which are marketed as “presence boosts.”
- Their high-end roll-offs are often preferred for vocals and bass-heavy instruments.
For more information on microphone frequency response, check out my article Complete Guide To Microphone Frequency Response (With Mic Examples).
Simple Passive Circuitry
The typical moving-coil dynamic microphone is completely passive. The transducer principle of electromagnetic induction requires no power.
The circuitry of moving-coil dynamic microphones (especially those with transformers) is very simple.
The typical circuitry is made up of a closed circuit between the moving coil and the primary winding; the transformer; and the balanced open circuit from the secondary winding to the microphone output connector.
Things get a bit more complicated when replacing the transformer with a passive response shaping circuit, but compared to condenser and active ribbon dynamic mics, this is still a very simple design.
To learn more about passive mics, check out my article Do Microphones Need Power To Function Properly?
Low Sensitivity And High Maximum Sound Pressure Level
Because there is no amplifier or other active circuitry in passive moving-coil mics, these dynamic mics display the following traits:
- Low sensitivity ratings: the typical mic level outputs of moving-coil dynamic microphones are much less than those of active mics.
- High maximum sound pressure levels: it’s practically impossible to overload the diaphragm of a moving-coil mic. Similarly, it’s practically impossible to overload the simple passive circuitry.
Also, since the moving-coil adds a relatively large mass to the diaphragm, dynamic microphones are not very sensitive to subtle sounds.
If we are to think of sensitivity in terms of the diaphragm’s reactivity to sound pressure, we’d see moving-coil mics, again, as fairly insensitive. Their weight makes it relatively hard for sound waves to move them. This yields a relatively slow transient response and insensitivity to weaker sound waves.
For more information on microphone sensitivity and max SPL, check out the following My New Microphone articles, respectively:
• What Is Microphone Sensitivity And Why Does It Matter?
• What Does Maximum Sound Pressure Level Actually Mean?
Mentors of mine would joke about dynamic microphones being able to survive nuclear explosions or about hammering nails with them. The point is that moving-coil dynamic microphones are tough and durable.
The outer casing aside (all professional microphones have a case), dynamic microphones are very robust.
- Their passive circuitry and capsule/cartridge is humidity resistant.
- The diaphragm, moving-coil, and magnet are protected by the housing of the capsule inside a shock mount and are very resistant to physical trauma.
- The most sensitive part of the microphone is the diaphragm itself, which is typically very well protected within a grille.
5 Common Moving-Coil Dynamic Microphones
You may think I’m obsessed with the American microphone manufacturer Shure by looking at the following list. I assure you I’m not alone. The SM58, SM57, and SM7B are arguably the 3 best moving-coil microphones on the market. I’ve used all 5 of the following microphones professionally and so I’ll share my experience with each of them.
Rather than creating “mini-reviews” of each of the 5 common mics, I’ll instead share the specs that make them characteristically dynamic. I’ll also link to the spec sheets I’m referencing so you can take a better look at the microphone specs (particularly the frequency response charts).
I’ll add links to check the prices of these mics on Amazon. Not that this is a buyer’s guide, but if you would like to buy one of these mics and support My New Microphone, please consider using the links provided.
So here are 5 common (if not the most common) moving-coil dynamic microphones on the market:
Each of these 5 microphones holds a place in My New Microphone’s 50 Best Microphones Of All Time.
It’s worth noting that none of these microphones have explicitly stated a maximum sound pressure level on their spec sheets.
It’s also worth noting that all 5 of these mics have a cardioid polar pattern. This has nothing to do with dynamic microphones and all to do with the popularity of the cardioid directional pattern.
The Shure SM58 is what I’d call a quintessential microphone. I can’t think for other people, but if I had to guess what most people envision when they hear the word “microphone,” I’d guess it would be a mic that looks like the Shure SM58. It’s likely the most common microphone in live settings and countless “knock-offs” had been made in its image.
Shure SM58 characteristic moving coil dynamic specs:
- A frequency response of “50 Hz – 15,000 Hz” with a bass roll-off at 100 Hz and a steep high-frequency roll-off starting at about 10,000 Hz.
- Sensitivity of –54.5 dBV/Pa (1.85 mV) 1 Pa = 94 dB SPL.
- Does have a step-up transformer.
- An output impedance of 300 ohms.
- Pneumatic shock-mount system.
- “Legendary Shure quality, ruggedness and reliability.”
The Shure SM57 is a great dynamic microphone and is extremely common in both live and studio settings.
Shure SM57 characteristic moving coil dynamic specs:
- A frequency response of “40 Hz – 15,000 Hz” with a bass roll-off at 200 Hz and a steep high-frequency roll-off starting at about 12,000 Hz.
- Sensitivity of -56.0 dBV/Pa (1.6 mV) (1 Pa = 94 dB SPL).
- Does have a step-up transformer.
- An output impedance of 310 ohms.
- Pneumatic shock-mount system.
- “Legendary Shure quality, ruggedness, and reliability.”
This is the microphone pictured in the featured image of this article!
The Shure SM7B is a personal favourite for voiceover recording and for loud “scream” type vocal performances.
Shure SM7B characteristic moving coil dynamic specs:
- A frequency response of “50 Hz – 20,000 Hz” with a pretty flat bass response but a steep high-frequency roll-off starting at about 12,000 Hz.
- Sensitivity of– 59.0 dB (1.12 mV) 0 dB = 1 volt per Pascal.
- Does have a step-up transformer.
- An output impedance of 150 ohms.
- Internal “air suspension” shock isolation.
- “Rugged construction and excellent cartridge protection for outstanding reliability.”
Interesting note: the Shure SM7B actually has the same capsule at the SM57 but a different transformer and an obviously different case and grille design.
To read more about the Shure SM7B, check out my article The Microphones Used In The Joe Rogan Podcast.
Shure is featured in the following My New Microphone articles:
• Top Best Microphone Brands You Should Know And Use
• Top Best Earphone/Earbud Brands In The World
• Top Best Headphone Brands In The World
I’ve used the RE20 exclusively in the studio on kick drums, bass cabinets, voiceover, and podcasts.
The RE20 is another personal favourite for voiceovers. Particularly if the speakers move a lot.
Electro-Voice RE20 characteristic moving coil dynamic specs:
- A frequency response of “45 Hz – 18,000 Hz” with a bass roll-off at 75 Hz and a steep high-frequency roll-off starting at about 12,500 Hz.
- Sensitivity of 1.5 mV/Pascal.
- Does have a step-up transformer.
- An output impedance of 150 ohms.
- Steel case and hum-bucking coil provide exceptional magnetic shielding.
- “Exceptionally rugged with superior handling noise rejection.”
The Electro-Voice RE20 is featured in the following My New Microphone articles:
• 50 Best Microphones Of All Time (With Alternate Versions & Clones)
• Top Best Dynamic Microphones On The Market
• Top Best Microphones For Podcasting (All Budgets)
Electro-Voice is featured in the following My New Microphone articles:
• Top Best Microphone Brands You Should Know And Use
• Top Best PA Loudspeaker Brands You Should Know And Use
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
Sennheiser MD 421 II
I’ve used the MD421 exclusively in the studio on tom drums, snare drums, bass cabinets, and guitar cabinets.
I really like the MD421 for recording toms and for close-miking guitar and bass cabinets.
Sennheiser MD421 II characteristic moving coil dynamic specs:
- A frequency response of “30 Hz – 17,000 Hz” with a bass roll-off at 80 Hz and a steep high-frequency roll-off starting at about 15,000 Hz.
- Sensitivity of2 mV/Pa +/- 3 dB.
- Does have a step-up transformer.
- An output impedance of 200 ohms.
- “Rugged professional microphone.”
Sennheiser is featured in the following My New Microphone articles:
• Top Best Microphone Brands You Should Know And Use
• Top Best Earphone/Earbud Brands In The World
• Top Best Headphone Brands In The World
Do dynamic mics need preamps? Yes, all microphones output mic level signals, which require preamp gain in order to become line level and work properly with professional audio equipment. Dynamic mics have lower sensitivity ratings/output levels than condenser mics and require more preamp gain.
To learn about microphone preamps, check out the following My New Microphones articles:
• What Is A Microphone Preamplifier & Why Does A Mic Need One?
• Complete Guide To Microphone Preamplifier Specifications
• Top Best Microphone Preamplifier Brands In The World
• Best Microphone Preamplifiers
What are the applications of moving-coil dynamic mics? Moving-coil dynamic mics have a wide range of applications. They are often used on loud instruments in the studio and on stage; on vocals (especially in broadcast and live situations); in humid, loud, or otherwise less-than-ideal scenarios; and many other situations.
This article has been approved in accordance with the My New Microphone Editorial Policy.