Both microphones and loudspeakers utilize voice coils to convert sound to audio or audio to sound, respectively. To be more specific, dynamic microphones have voice coils, and most loudspeaker designs do as well.
What is a microphone voice coil? A voice coil is a coil of conductive wire that is attached to a moving-coil dynamic mic’s diaphragm. The voice coil is surrounded by but not touched by magnets. As the diaphragm/voice coil element vibrates in the magnetic field, electromagnetic induction occurs, and a mic signal is created.
In this post, we'll discuss voice coils in great detail and how they benefit dynamic microphones and the world of audio in general.
What Is A Microphone Voice Coil?
The voice coil (otherwise known as the “moving-coil” or “conducive coil”) is a small coil of conductive wire typically made from copper and coated for electrical insulation. It takes on the shape of a tightly wound helix with a shallow cylindrical shape.
When referring to voice coils, we are typically talking about loudspeakers, but dynamic microphones also use voice coils. We'll discuss loudspeakers later in this article.
The voice coil is a necessary part of the moving-coil dynamic microphone capsule/cartridge design. Let's take a look at a simple diagram of a dynamic mic cartridge:
Note that the voice coil is actually attached to the diaphragm. I left space to help differentiate between the two, as they are separate components. However, the voice coil needs to be physically attached to the diaphragm for the mic to work.
Let's talk about how the voice coil interacts with each of the other components in a moving-coil dynamic microphone cartridge.
Microphone Voice Coil And The Diaphragm
As mentioned above, the voice coil of a microphone is attached to the diaphragm.
As the diaphragm moves in reaction to changing sound pressure, so too does the voice coil.
The relatively heavy voice coil typically means that the moving-coil microphone diaphragm is thicker than a condenser microphone diaphragm. Each diaphragm type is usually made of plastic BoPET film known as Mylar (a popular trademark name).
For more information on microphone diaphragms, check out the following My New Microphone articles:
• What Is A Microphone Diaphragm?
• What Are Microphone Diaphragms Made Of? (All Diaphragm Types)
Microphone Voice Coil And The Magnetic Structure
The voice coil sits within a cylindrical cut-out in a magnetic structure.
This magnetic structure is often made of a compound of rare earth neodymium, iron, and boron but can be made of other magnetic materials as well.
The magnetic structure is quite involved, being made from a main magnet and pole pieces. Pole pieces are attached to the magnet to extend the magnetic poles to certain locations.
Remember that the voice coil is shaped like a hollow cylinder. To the interior of the voice coil, the magnet provides a south pole and to the exterior, a north pole (or vice versa).
To learn about all the functions magnets have in microphones, check out my article Do Microphones Need Magnetism To Work Properly?
What is electromagnetic induction? Electromagnetic induction is the production of a voltage across an electric conductor in a changing magnetic field. It is the working principle of dynamic mics, where the voice coil (conductor) moves within a permanent magnetic field, and a voltage (mic signal) is induced.
Electromagnetic induction is a passive process, which means that moving-coil dynamic mics do not require power to function as transducers.
Let's quickly break down how electromagnetic induction works with the voice coil of dynamic microphones:
- Sound causes the diaphragm to move (alternating directions).
- The voice coil is attached to the diaphragm and moves with it.
- As the voice coil moves within a permanent magnetic structure, it experiences a changing magnetic field.
- Electromagnetic induction produces a proportionate voltage across the voice coil.
- Because the diaphragm moves in alternating directions, this induced voltage in an AC voltage (mic signal).
Microphone Voice Coil And The Electrical Leads
A voltage across the voice coil is effectively our microphone signal. To properly use this signal, we need electrical leads to move the signal to where it needs to go (to the microphone's output and ultimately to a speaker for playback).
To read more about microphone signals, check out the following My New Microphone articles:
• What Is A Microphone Audio Signal, Electrically Speaking?
• Do Microphones Output Mic, Line, Or Instrument Level Signals?
• What Is Microphone Gain And How Does It Affect Mic Signals?
Loudspeakers Also Have Voice Coils
The term voice coil generally refers to loudspeakers. I think it's appropriate to touch on loudspeaker voice coils as well as the similarities between moving-coil dynamic microphones and loudspeakers.
With loudspeakers, an audio signal (AC voltage) is applied across the voice coil.
The signal is much stronger than a mic level signal, and the voice coil is physically bigger. Like the microphone, a loudspeaker's voice coil attaches to a diaphragm suspended within a magnetic structure. The diaphragm is larger and thicker, and the magnetic structure is also bigger.
As the AC voltage is applied across the voice coil, it causes a change in the magnetic field, which moves the voice coil back and forth proportionately to the AC signal. This is due to electromagnetic induction.
The movement of the voice coil causes movement in the diaphragm because they are attached. The movement of the diaphragm pushes air and projects sound waves into the medium.
So we see that voice coils are used in the designs of both moving-coil dynamic microphones and loudspeakers. Loudspeakers are essentially dynamic mics wired in reverse.
To learn how to convert a loudspeaker into a microphone, check out my article How To Turn A Loudspeaker Into A Microphone In 2 Easy Steps.
Why is a microphone called a microphone? The term ‘microphone’ can be broken into ‘micro’ and ‘phone.’ Micro (from Greek mikros) means “small,” and phone (from Greek phone) means “sound” or “voice.” Microphone translates to “small sound,” which is accurate, as the microphone deals with small audio signals.
To read about the term “microphone” in greater detail, check out my article Why Are Microphones Called Microphones?
What do you plug a microphone into? Microphones typically plug into mic preamplifiers (which can be standalone devices or built into mixers, recorders, interfaces, etc.). However, mics can plug into any audio input, with limited functionally, given the proper connections/adapters.
To read more about microphone connections, please read my article What Do Microphones Plug Into? (Full List Of Mic Connections).
Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.