What is A Dynamic Microphones and How Do They Work?

Dynamic microphones are among the most widely used audio recording devices in the professional and consumer audio industries. From stage performances to radio broadcasting and home recording, these reliable workhorses have become indispensable tools for capturing sound. In this article, we will explore the science, construction, applications, and advantages of dynamic microphones, unraveling the technology that has made them a staple in audio recording for decades.

The Basic Principle: Electromagnetic Induction

At their core, dynamic microphones operate on a fundamental principle of physics discovered by Michael Faraday in 1831: electromagnetic induction. This principle states that when a conductor moves through a magnetic field (or when a magnetic field moves around a conductor), an electrical current is induced in the conductor. This elegant and straightforward concept forms the foundation of how dynamic microphones convert sound waves into electrical signals.

From Sound Waves to Electrical Signals

Sound, in its physical form, consists of pressure waves traveling through air. When you speak, sing, or play an instrument, you create vibrations that compress and decompress air molecules, forming waves that radiate outward from the source. The journey from these acoustic waves to an electrical signal in a dynamic microphone follows a fascinating chain of energy transformations:

Sound waves strike the microphone’s diaphragm
The diaphragm vibrates in response to these waves
A voice coil attached to the diaphragm moves within a magnetic field
This movement generates an electrical current that corresponds to the original sound

This process effectively transforms acoustic energy into mechanical energy (diaphragm movement), and finally into electrical energy (the audio signal).

Anatomy of a Dynamic Microphone

Understanding how dynamic microphones work requires examining their internal components. While designs may vary between manufacturers and models, most dynamic microphones share several key structural elements:

The Diaphragm

The diaphragm is a thin, lightweight membrane typically made of polyester, mylar, or similar materials. It’s designed to respond to sound pressure waves, vibrating in sympathy with the incoming acoustic energy. In most dynamic microphones, the diaphragm is circular and mounted at its edges, allowing it to move freely in response to sound waves.

The Voice Coil

Attached directly to the rear of the diaphragm is a small coil of wire known as the voice coil. This coil is typically made of copper or aluminum wire wrapped around a former (a small cylinder that provides structural support). The voice coil moves in tandem with the diaphragm, and it’s this movement within the magnetic field that generates the electrical signal.

The Magnet

A permanent magnet creates a static magnetic field around the voice coil. Most modern dynamic microphones use strong neodymium or ferrite magnets to create powerful magnetic fields. The magnet is typically arranged in a ring shape, with the voice coil positioned within the gap where the magnetic field is strongest.

The Pole Piece

The pole piece is a structure made of magnetically permeable material (usually soft iron) that helps direct and concentrate the magnetic field around the voice coil. This component ensures that the magnetic field is properly focused to maximize the efficiency of the electromagnetic induction process.

Transformer (in some models)

Some dynamic microphones include a small transformer that helps match the impedance of the voice coil to the impedance of the microphone preamplifier, ensuring efficient signal transfer and minimizing noise.

Protective Grille

The external grille serves multiple purposes: it protects the internal components from physical damage, reduces the impact of wind and plosive sounds (like “p” and “b” sounds), and in some cases, can even help shape the microphone’s frequency response.

Technical Operation in Detail

The actual operation of a dynamic microphone involves several physical processes working in harmony:

When sound waves strike the diaphragm, they cause it to move back and forth in proportion to the pressure variations in the air.
Since the voice coil is attached to the diaphragm, it moves within the magnetic field created by the permanent magnet.
As the voice coil moves through this magnetic field, it cuts through magnetic flux lines, inducing an alternating current in the wire.
This induced current forms an electrical signal that precisely mirrors the acoustic waveform of the original sound, with voltage levels proportional to the amplitude of the sound waves and frequencies matching those of the original acoustic signal.
This electrical signal then travels through the microphone’s output connections to a preamplifier, mixer, or recording device.

The beauty of this system lies in its analog nature – there’s a direct, continuous relationship between the physical movement of air molecules and the electrical signal produced. No sampling or digital conversion is happening at this stage.

Frequency Response Characteristics

The frequency response of a dynamic microphone – how it responds to different frequencies in the audible spectrum – is determined by several factors:

Mass of the diaphragm and voice coil: Heavier moving parts tend to respond less efficiently to high frequencies, giving many dynamic microphones a natural roll-off in the higher frequency ranges.
Compliance of the diaphragm: How easily the diaphragm moves affects its ability to reproduce low frequencies.
Acoustic chambers and ports: Many dynamic microphones incorporate carefully designed chambers and ports that modify the frequency response, often to enhance vocal presence or control proximity effect.
Mechanical damping: Materials used to dampen resonances can shape the microphone’s response across the frequency spectrum.

Most dynamic microphones have a frequency response that extends from around 50Hz to 15kHz, though specialty models may have broader or more tailored responses. This range covers most musical instruments and voices, making dynamic microphones versatile tools for various recording applications.

Types of Dynamic Microphones

Within the category of dynamic microphones, there are several important subtypes:

Moving-Coil Dynamic Microphones

This is the most common type, using the voice coil and diaphragm arrangement described above. Famous examples include the Shure SM58 (a staple for vocalists) and the SM57 (frequently used for instruments and guitar amplifiers).

Ribbon Microphones

Though technically a different category, ribbon microphones operate on the same electromagnetic induction principle as moving-coil dynamics. Instead of a diaphragm and voice coil arrangement, they use a thin metal ribbon suspended in a magnetic field. When sound waves cause the ribbon to vibrate, a current is induced directly in the ribbon itself. Ribbon microphones typically offer excellent transient response and a natural sound, though they tend to be more fragile than moving-coil dynamics.

Advantages of Dynamic Microphones

Dynamic microphones have remained popular for decades due to several key advantages:

Durability and Reliability

The relatively simple construction of dynamic microphones, with few moving parts, makes them highly durable. They can withstand rough handling, high sound pressure levels, and even exposure to moisture – qualities that make them ideal for live performance settings.

No External Power Required

Unlike condenser microphones, dynamic microphones operate without the need for phantom power, batteries, or external power sources. This passive operation simplifies setup and eliminates potential points of failure.

Handling Noise Rejection

Dynamic microphones are less sensitive to handling noise compared to condenser microphones. This makes them practical for handheld use in live performances.

High SPL Handling

Dynamic microphones can handle extremely high sound pressure levels without distortion, making them suitable for recording loud sources like drum kits, guitar amplifiers, and brass instruments.

Cost Effectiveness

The relatively straightforward construction of dynamic microphones allows manufacturers to produce high-quality units at more accessible price points than many condenser models.

Common Applications

The characteristics of dynamic microphones make them particularly suitable for certain applications:

Live Performance

Their durability, handling noise rejection, and ability to handle high SPLs make dynamic microphones the standard choice for live vocal and instrumental performances. The Shure SM58, with its characteristic grille and reliable performance, has become an icon in this space.

Broadcasting

Many radio stations and podcasters use dynamic microphones for their warm sound characteristics and ability to reject background noise. The ElectroVoice RE20 and Shure SM7B have become standards in broadcast applications.

Instrument Recording

Dynamic microphones excel at recording many instruments, particularly those with high sound pressure levels:

Drums: Dynamic microphones capture the attack and punch of kick drums and snares effectively
Guitar amplifiers: The focused pickup pattern and ability to handle high SPLs make dynamics ideal for electric guitar cabinets
Brass and woodwind instruments: The natural compression effect of dynamic microphones helps control the dynamics of these instruments

Limitations

While dynamic microphones excel in many situations, they do have some limitations:

Sensitivity

Dynamic microphones are generally less sensitive than condenser microphones, often requiring more gain from preamplifiers. This can be a benefit when recording loud sources but may be a limitation for capturing quiet or distant sounds.

Frequency Response

Many dynamic microphones have a more limited frequency response compared to condenser microphones, particularly in the higher frequencies. This can result in a less detailed sound for some applications, though it can also be advantageous for taming harsh sources.

Transient Response

The mass of the moving elements in dynamic microphones means they typically respond more slowly to transient sounds than condenser or ribbon microphones. This can result in a slightly less detailed capture of percussive sounds.

Design Variations and Special Features

Manufacturers have developed various design modifications to enhance the performance of dynamic microphones for specific applications:

Directional Patterns

Most dynamic microphones feature a cardioid polar pattern (heart-shaped sensitivity pattern), which picks up sound primarily from the front while rejecting sound from the rear. This helps isolate the desired sound source and reduce feedback in live settings. Some dynamic microphones offer other patterns:

Supercardioid and hypercardioid: Even more directional than cardioid, with some sensitivity at the rear
Omnidirectional: Equal sensitivity in all directions
Figure-8: Equal sensitivity at front and rear, with rejection at the sides (common in ribbon microphones)

Proximity Effect

Many directional dynamic microphones exhibit proximity effect – a bass boost that occurs when the microphone is used close to the sound source. Some microphones are designed to mitigate this effect, while others embrace it as a feature that can add warmth to vocals.

Off-Axis Coloration

The way a microphone responds to sounds arriving from different angles (off-axis) affects its utility in various recording scenarios. Well-designed dynamic microphones maintain consistent tonal quality even for off-axis sounds.

The Future of Dynamic Microphones

While condenser and digital microphone technologies continue to evolve rapidly, the fundamental design of dynamic microphones has remained relatively stable. This speaks to the elegance and effectiveness of the basic design. However, innovations continue in several areas:

Materials science: New magnet materials and lighter, stronger diaphragm materials enhance performance
Manufacturing precision: Computer-controlled manufacturing allows for more consistent quality
Hybrid designs: Some manufacturers explore combinations of dynamic and condenser elements in single microphones

Conclusion

Dynamic microphones represent a perfect harmony of physics, engineering, and practical design. Their ability to convert acoustic energy into electrical signals using nothing more than a diaphragm, a coil of wire, and a magnet remains one of the most elegant solutions in audio technology. Despite being based on principles discovered nearly two centuries ago, dynamic microphones continue to be essential tools in modern audio recording and reinforcement.

Their durability, reliability, and distinctive sound characteristics ensure that dynamic microphones will remain studio and stage staples for generations to come. Whether capturing the roar of a guitar amplifier, the intimacy of a radio voice, or the power of a live vocal performance, dynamic microphones continue to prove that sometimes the simplest technologies are the most enduring.