How Headphone Dynamic Drivers Work

The most common transducer type in headphones is the dynamic driver. It's basically a miniature version of the large driver you'd see in a typical speaker in the home. There are other headphone driver types, but for this article we'll focus on the dynamic driver.

Headphone101_DynamicDriver_DiaphragmAnimationThe dynamic driver is a transducer that converts an electrical signal into an acoustic signal. The image above shows a partially disassembled headphone capsule with the earpad and driver grill removed. The shiny circular transparent object is the driver diaphragm—it's like the speaker cone in a regular speaker. The electrical signal to the headphones causes it to move in and out (as show at right) and creates the sound you hear in the headphones.

To understand exactly how wire, electrical signal, and magnets cause the diaphragm to move, we'll need to start with electromagnetic principles...and I'll have to break out the Dremel tool and distroy some perfectly good headphones. RIP V-Moda Crossfade LP, you died for a good cause.

Electro-Motive Force
A dynamic driver converts an electrical signal that represents the music into a mechanical movement of the driver diaphragm that compresses and rarifies the air in front of it to create a sound wave representative of the original electrical signal. The basic principle at work rests on the fact that when you put current through a wire it creates a magnetic field around the wire proportional to the amount and direction of the current flow. If you immerse the wire within another magnetic field, the magnetic field around the wire will react with the larger field and force the conductor to move. This is the basic principle behind all electric fact, the assembly of wires and magnets within a headphone driver is called its motor.


At the bottom of the image above is a crudely drawn illustration of a wire within the field of a permanent magnet. A permanent magnet is a chunk of steel or other material that has been permanently magnetized. The wire is attached to the terminals of a battery and has current flowing though it. The interaction between the magnetic fields of the permanent magnet and conductor will cause the conductor to move. (Caution: If you wire up a system like the one above you'll blow up the battery. It will work for short period of time, but since it's effectively shorting out the battery the current will be very high and things will start to go sideways in short order. Wheeee!)

The top left illustration above shows a close-up of the magnetic poles (in green) with the magnetic flux lines between them (in red). It also shows the electrical conductor cross-section in blue with the current flowing in a direction coming out of the screen towards you, and the resulting magnetic flux lines around the conductor (in red). (There is a right hand rule to determine the direction of rotating magnetic flux around a current carrying conductor.)

Magnetic flux lines (the lines of magnetic force shown in red in the illustration) have a couple of rules you'll need to know:

  1. Magnetic flux lines can never cross.
  2. Magnetic flux lines always take the path of least resistance.
  3. If magnetic flux lines get bent away from the path of least resistance due to #1, they'll impart a mechanical force in the direction of returning to the path of least resistance.

As a result of the magnetic field circling the conductor, the flux lines from the larger magnetic field tend to want to go around the wire in the same direction. This causes more flux lines to go around the right side of the wire than the left. You can think of these flux lines as little rubber bands bent out of shape and wanting to get back to their straight line path of least resistance. So, it's like all those rubber band flux lines are pushing against the conductor's magnetic field and forcing it to move left in the illustration. Of course, if you increase or decrease the current flow in the wire, the force and movement of the wire will be more or less, and if you reverse the flow of current in the wire it will move in the reverse direction. Put an audio signal on the wire and it will move back and forth in a way directly related to that electrical audio signal.

The image of the hand at top right shows Flemings "Left Hand Rule" for electro-motive force. The index finger points in the direction of magnetic flux (North to South); the middle finger points in the direction of current flow in the conductor (positive to negative); and the thumb points in the direction of resulting force vector of movement on the conductor.

Now that we know we can use a magnet to get a wire to move along with the music, all we have to do is figure out how to construct a motor in such a way that we can attach a speaker cone, or in this case a headphone driver diaphragm, to it so we can make some sound.

Headphone Dynamic Driver Anatomy
First, we can take that "C" shaped magnet above and extrude it around in a circle so that the space between the north and south poles becomes a circular gap into which we can put the circular conductor that carries the audio signal—which from now on we'll call the voice coil.


The top illustration above is the "C" shaped magnet swept into a circle with part of it cut away so you can see what's going on inside. here you see the voice coil illustrated by a single blue conductor, but in a real headphone driver the voice coil has many dozens or even hundreds of windings. The bottom photo is of an actual driver of a V-Moda Crossfade LP with the diaphragm removed so you can see the top of the magnet structure and the magnetic gap in which the voice coil is inserted.

Headphone101_DynamicDriver_DiaphragmAnimationWhen current flows through the conductor it will move outward away from the magnet structure or inward depending on which direction the current is flowing. In the animated .gif to the right, you can see the copper color of the voice coil windings attached to the back of the diaphragm as it moves in and out of the gap. Let's take a closer look at the voice coil and diaphragm.


Generally, headphone diaphragms are made of one or more thin sheets of plastic formed into shape by pressure stamping with dies under heat. The central part is called the dome, the outer part is the flexure. The flexure area is fixed to the driver housing at its outer edge, and to the voice coil where the flexure meets the dome. The small creases in the flexure act to allow it to slightly change shape more easily as it moves in and out, and aid in stiffening the flexure to prevent modal break-up. (Non-pistonic vibrational modes when driven at high frequencies.)

There are numerous other materials and geometries used for headphone driver diaphragms, but the basic principle described here remain true for all types of dynamic drivers. With very few exceptions, all dynamic drivers have:

  • Voice Coils on Diaphragms/Cones
  • Circular Magnetic Gaps
  • Pistonic (straight in-and-out) Motion

Now let's take a much closer look at the magnetic circuit of a real headphone driver.


You probably remember pictures from your grade school text book showing you how to magnetize a nail by wrapping a coil of wire around it and attaching the ends to a battery. Well, most all commercial magnets are made in a similar, but much more complicated way. Magnets are made by putting various materials in a linear magnetic field within a large coil of wire. The problem with that "C" shaped in cross-section circular magnet is that you can't actually magnetize it in such a field—the geometry just doesn't work out.

Instead, a headphone driver magnet system is made of washer-shaped magnet sandwiched between to permeable metal pole pieces. The washer-shaped magnet has it's poles on the top and bottom. Once squeezed between the pole pieces, the magnetic flux lines will travel easily through the permeable metal of the pole pieces and jump the gap as it's the easiest place to complete the magnetic circuit.

This washer-shaped magnet is made by magnetizing a long tube of the appropriate dimensions first, and then slicing it into the final washer thickness. Most magnets in today's headphones are made from rare-earth ceramic materials—most commonly samarium-cobalt and neodymium-iron-boron—and are very strong.

Okay, so now you know how the voice coil in the magnetic gap moves the diaphragm back and forth to the music, and how sound radiates from the front of the diaphragm as it moves resulting in sound that propagates to your ears. But there's a problem, the sound also radiates off the back of the diaphragm right into the motor structure that can create acoustic resonance problems, which in turn can cause problems with diaphragm movement and add distortion to the sound heard.

Vents and Acoustic Control
To combat problems from the sound being trapped behind the diaphragm, acoustic vents and damping are included in the driver itself. There are two main areas where sound is trapped behind the diaphragm: behind the dome and bounded by the voice coil; and behind the flexure between the voice coil and the attached outer edge of the diaphragm.


The air volume behind the dome is controlled using a central vent sometimes called a bass port. Though not shown in the image above, this vent is very often stuffed with a small piece of open cell foam to damp the vent.

The air volume behind the flexure is vented by a series of holes in a ring below the flexure. Damping is accomplished with porous paper covering the holes, and sometimes holes in the paper itself.

The exact amount of damping in each of these areas is critical to provide the appropriate resonance control in each area, and the balance of acoustic impedance between the two volumes aids in overall diaphragm performance.

Because the air volumes we're talking about are very small, poor driver acoustic design will result in problems at relatively high frequencies. Interestingly, acoustic problems will resist the desired voice coil movement and show up in changes in impedance at problematic frequencies.


Resonances behind the driver result in acoustic impedances at the driver at the resonant frequency. This change in acoustic impedance results in a change to mechanical impedance to diaphragm movement, and the change in mechanical impedance to the diaphragm movement can be sensed as a change of the electrical impedance to the drive signal at the resonant frequencies. For this reason it is sometimes possible to detect driver resonance problems in the headphone impedance measurements as somewhat erratic changes in the area between 2kHz and 8kHz.

Above are two sets of headphone harmonic distortion (bottom graphs) and electrical impedance and phase response (top graphs) measurements. The set at the left show a well designed driver as indicated by very little impedance or phase change and very low distortion in the 2kHZ to 8kHz region. The set of measurements to the right show a poor driver design as indicated by erratic bumps in the impedance and phase plot, and dramatic peaks in distortion at related frequencies.

A headphone dynamic driver has a small motor that uses a permanent magnet with a ring gap and a voice coil suspended in the gap to provide linear motion back and forth when driven by an electrical audio signal. The diaphragm is attached to the voice coil and pressurizes and rarifies the air with the drive motion creating the sound you hear.

Sound comes off the back of the diaphragm as well, which is trapped in various volumes by the motor structure and causes potential problems with acoustic resonances. Headphone dynamic driver assemblies are designed with numerous small vents that often have porous paper and foam materials in them to tune and damp the acoustics of the driver's trapped air volumes.

Lawk's picture

Is there a Master & Dynamic MH40 and/or Philips A5 Pro review in the pipeline?

BarbecueGamer's picture


I was hoping to get your help Tyll. I wanted to know if the NAD Viso HP50 is truly better sounding than the Momentum. You described it as being "somewhat" better sound. Could you please go into a little more details.

bogdanb's picture

yes, it really is!
Also comfort! in the sense that from the 2 HP50 are the only over ear. great in airplane also!
I own the NAD's for almost a year. OMG, you really don't feel them sealed, they feel so open!
The only fault that I found is imaging/ space description let's say but only when carefully compared to another headphone (mdr zx700). Something that I don't really observe in normal listening.

If you enjoy listening to the music, best choice. If you listen to find defects you should go in a bigger price range and go for sennheiser hd 800 or something in that price range, or a planar magnetic...
But if you plan to use them with the phone and a home amp I think NAD HP50 is the better choice.

Sturdiness: go with the V-moda (for this reason only I consider them in my future acquisitions, but I might save more and by a E61 group espresso machine ;) )

johnjen's picture

This is a well crafted writeup on the basics of transducer design and implementation for those who are not familiar with such things.

And yes it is difficult to explain these technical details so they are easily understandable, and inviting at the same time.

Good job Tyll…

ps nice animation and illustrations :thumb

JRAudio's picture

I respect your work and I can imagine, that it takes a good amount of time, to bring all basics together. Continue your great writing. Thank you.
Best Regards

johthor's picture

A truly excellent description of how a dynamic driver works. Thank you Tyll from all of us in the hobby who are "non engineer" types.

Three Toes of Fury's picture

Might be one of my all time favorite postings on this site.

Love it.

keep the edumakation coming Tyll!

Peace .n. Living in Stereo


PoorAudiophile's picture

Great post tyll, will have to read later as I am at the library computer. QUite a coincidence I ordered the V Moda Crossfade LP for my kicking around pair of cans and i log on the same week to find this post.

Three Toes of Fury's picture

Just re-read the article....its exceptional. Thanks Tyll for taking the time to create a wonderfully detailed break down of some tricky concepts. Your use of visuals, and willingness to dig deeper (particularly with magnet construction) is wonderful.

A few random follow up questions:

* Regarding the sandwiching of the magnets...any idea why the Permanent Magnet would be "happy" between the Permeable Metal South and North pole? Based on the drawing it looks like the permanent magnet poles match those of the permable magnets on either side...i thought like-poles always repel?

* Not sure if this is a topic for a future article, but is the entire difference between open and closed headphones tied to the venting ports behind the dome and flexure? Namely that open headphones have these ports completely open to the rear of the headphone and closed, somehow, dissipate them internally? If so, then i would think that the volume of space behind the driver in closed headphones is an essential variable in its sound, yet alot of great closed cans have a small area behind the driver...hrmmmm.

* The inclusion of the measurements (impedance and distortion) are soooooo appreciated. Ive read other articles on your site about the readings but havent, until now, connected the dots on some finer details. By all means continue to pepper in these kind of graph-facts on future articles.

Thanks again dude...learning about the science behind the sound is fascinating. I can honestly say this article has permanently changed how i will view the headphones in my collection.

Peace .n. SCIENCE!


Tyll Hertsens's picture
The permeable metal of the pole pieces are not magnets. You can think of them like magnetic flux conductors. The magnetic field of the permanent magnet travels more easily through the pole pieces than the air. You can think of the permeable metal as ways to extend the shape of the permanent magnet.

Yes, I will be doing separate articles on open, sealed, and all the various iterations of acoustic designs in later articles. (Frankly, I'm studying up on the subject's quite complicated.)

Three Toes of Fury's picture

ok...last question for today...i promise...

* After reading your article i read a few on speaker design which conveyed similar information (thou not as clearly or detailed). It made me think of the following question: by and large, when it comes to speaker design, there are different shaped speakers for different sections of the frequency bandwidth...primarily woofers (big), midrange (medium), and tweeters (small) does a good single headphone dynamic driver accomplish the same job as those 3 to convey all 3 frequency bands?!?!

Peace .n. Knowledge is Power


Tyll Hertsens's picture
The big difference is that speakers radiate into free air, and as a result need larger and larger speakers to propigate a long wavelength acoustic signal.

Headphones, on the other hand, are couplers---meaning the driver doesn't radiate into open space. The earcup acts to enclose a fixed amount of air, and the driver is able to pressurize this air within the earcup chamber.

Three Toes of Fury's picture

So much more to learn!

Your earcup/pressurized air helps explain (i think) to me another situation i experienced...when i first found this site and started picking up some suggested Wall o Fame IEM headphones (Sure SE215), i was, initially, underwhelmed at the sound quality. It wasnt until later that i realized i had used the wrong headphone tips..the one i used did not create a good seal in my ear canal. Once i fixed that, the sound drastically improved. So..based on would appear that the improper tip selection was preventing the optimal pressurization of the air within my ear canal. Science!

Tyll Hertsens's picture
That's the way it works.
Cami's picture

What is responsible for the large impedance swings of dynamic headphones, compared the nearly flat impedance curves of planar-magnetic headphones? How does this factor and difference interact with headphone Amps, and is there any logic - and evidence - to the claim that current mode Amps would work better with flat impedance curves (ie planar-magnetic headphones)?

bronson's picture

Awesome detailed explanation of how a dynamic driver actually works.

Still hard to believe that that piece of transparent film is what actually makes audible sound - I always thought manufacturers sprinkled magic audio dust into each ear cup so can't feel like you've lost many a readers innocence, but Santa Claus is still real - right?