If you’ve been studying audio and sound, you’ve likely come across the magical term “transducer” which refers to devices that convert one form of energy into another form of energy. Speakers and headphones are common transducers and knowing how they work will help us to better understand the world of audio.
How do speakers and headphones work as transducers? Headphones and loudspeakers are designed to convert electrical energy (in the form of audio signals) into mechanical wave energy (in the form of sound waves). They have drivers that use various working principles to turn the alternating current of audio into diaphragm movement and sound production.
In this article, we’ll discuss headphone and speaker transducers in greater detail and describe the driver designs that allow these audio devices to produce sound for our listening pleasure (or displeasure).
What Is A Transducer?
A transducer is a device that converts energy from one form to another.
In the case of headphone and loudspeaker transducers, electrical energy is converted into mechanical wave energy.
Note that microphones are also transducers that perform the opposite conversions, turning sound waves into audio signals.
For more information on microphone transducers, check out my article How Do Microphones Work? (The Ultimate Illustrated Guide).
The electrical energy is found in the audio signal. Analog audio signals are continuously variable alternating currents. Their amplitudes are defined by an AC voltage that oscillates between positive and negative values.
Below is a simple illustration of a 1 kHz single-frequency analog audio signal. We see the sinusoidal waveform with a maximum voltage (peak amplitude) above the zero line (horizontal dotted line) and a maximum negative voltage below the zero line.
Note that digital audio signals are very close at approximating analog audio signals but they are discrete rather than continuous. These digital representations of audio (regardless of the codec or file format) are not capable of driving headphone or loudspeaker transducers because they are not continuous.
In the simple illustration of a 1 kHz single-frequency digital audio signal show below, we see the individual amplitudes of the individual samples that represent a 1 kHz sine wave in digital audio form.
The “blocky” nature of the digital audio signal shows that, no matter how accurately the digital signal represents the analog signal, it is not a continuous signal.
Related article: Are Headphones Analog Or Digital Audio Devices?
The mechanical waves energy is found in the sound waves. Sound waves cause localized differences in pressure within a medium (solid, liquid or gas). As the waves propagate through the medium, they cause instances of maximum compression (max pressure) and maximum rarefaction (min pressure) in the molecules they pass through.
Let’s have a look at a simple illustration of a sound wave with a single frequency of 1 kHz. We’ll notice that it looks nearly identical to the 1 kHz analog audio signal drawn above except that its amplitude is measured in pressure rather than voltage.
Again, the role of the loudspeaker and headphone is to convert analog audio signals into sound waves.
Ideally, the loudspeakers and headphones will reproduce the audio signal as precisely as possible though this is not always feasible for various reasons due, in large part, to discrepancies in the frequency response of the transducers.
For more information on headphone frequency response, check out my article What Is Headphone Frequency Response & What Is A Good Range?
So we know that headphone and loudspeaker transducers turn audio signals into sound but how do they do so, exactly? How do headphones and loudspeakers work as transducers?
The answer can be found by learning about their drivers.
Drivers: The Transducer Elements Of Headphones & Speakers
The transducer elements of a headphone or a loudspeaker are known as drivers.
Without a driver, a headphone or loudspeaker would not be able to produce sound.
Headphones nearly always have two drivers (hence the plural nature of the name “headphone(s)”). Each driver typically accepts one channel of a stereo audio signal. The left headphone driver will reproduce left channel audio as sound while the right headphone driver will reproduce right channel audio as sound.
The Sennheiser HD 280 Pro (link to compare the prices on Amazon and B&H Photo/Video) is a headphone with two drivers.
The Sennheiser HD 280 Pro is featured in the following My New Microphone articles:
• Top Best Moving-Coil/Dynamic Headphones Under $100
• Top Best Closed-Back Headphones Under $100
• Top Best Circumaural (Over-Ear) Headphones Under $100
Sennheiser is featured in the following My New Microphone articles:
• Top Best Headphone Brands In The World
• Top Best Earphone/Earbud Brands In The World
• Top Best Microphone Brands You Should Know And Use
Of course, some headphones and headsets will have a single driver. Some double-driver headphones will also be tasked with producing mono audio, where the same signal will be sent to both drivers.
The BlueParrott B450-XT (link to check the price on Amazon) is a headband-worn single-driver Bluetooth headset.
To learn more about headphone drivers, check out my in-depth article titled What Is A Headphone Driver? (How All 5 Driver Types Work).
Some speakers have a single driver, though many are set up with crossover networks and multiple drivers.
Common speaker drivers include subwoofers, mid-range woofers and tweeters.
To reproduce the full frequency range of an audio signal with greater precision, many high-end loudspeaker systems will utilize multiple drivers. This is why you’ll often see two or more speakers in a single speaker enclosure.
The Adam Audio S3V (link to compare prices on Amazon and B&H Photo/Video) is a professional studio monitor with 3 distinct drivers:
- Low-frequency driver: 9″ HexaCone Subwoofer
- Mid-frequency driver: 4″ composite dome/cone hybrid woofer
- High-frequency driver: 2″ ribbon-type tweeter
Adam Audio is featured in My New Microphone’s Top Best Studio Monitor Brands You Should Know And Use.
We’ll discuss ribbon-type tweeters in greater detail in the section titled Other Driver Types.
The Creative Labs Pebbles (link to compare prices on Amazon and B&H Photo/Video) are USB computer speakers that have a single 2″ driver.
Note that these speakers are a stereo pair. Like stereo headphones, there is a single driver for each audio channel.
There are several types of headphone and loudspeaker drivers that utilize different design and working principles to convert audio into sound.
The most common driver type, by far, is the moving-coil dynamic driver.
The Moving-Coil Driver
The moving-coil driver converts audio signal into sound waves via principles of electromagnetism.
Let’s begin our discussion of the moving-coil driver with a simplified cross-sectional diagram. Note that the following diagram can be used to represent headphone and loudspeaker drivers:
As we see above, there are 4 key components to the moving-coil headphone/speaker driver:
- Voice coil (moving-coil)
- Magnet (magnet plus pole pieces)
We also notice that this over-simplified diagram shows opposite magnetic poles to the immediate interior and exterior of the voice coil. This concentrates the magnetic field around the coil which improves the efficiency of the driver as a transducer. We’ll get to that in a second.
First, let’s briefly go over the design of the moving-coil driver.
The voice coil is made of tightly-wound conductive wire (typically copper) wound in a cylindrical form. This voice coil is attached to a thin (typically circular) diaphragm.
The direct coupling of the coil and diaphragm means that as the coil moves, the diaphragm does as well.
The oddly-shaped magnet is achieved by the use of pole pieces to extend the magnetic poles in specific places. This magnetic structure has a cylindrical cutaway just big enough to allow the conductive coil to be suspended within it without the two components ever touching eachother.
To learn more about the role of magnets in headphone design, check out my article Why & How Do Headphones Use Magnets?
The housing holds it all together. In the case of headphones, the driver is held within the ear cup or ear bud (headphones vs. earphones). In the case of speakers, the driver is held within an enclosure.
So how does the moving-coil dynamic transducer work?
The audio signal (alternating current) is passed through the conductive voice coil. Electromagnetic induction takes place so that a forward-flowing current produces a positive magnetic field in the coil and a backward-flowing current produces a negative magnetic field in the coil.
The varying magnetic field of the coil interacts with the static magnetic field of the permanent magnetic structure. This causes the coil the move forward and backward depending on its own magnetic field (which happens as a result of the audio signal’s current flow).
As mentioned, any coil movement means diaphragm movement so the diaphragm oscillates according to the audio signal.
The diaphragm movement produces sound waves in the medium around it.
This fairly basic electromagnetic design is how most headphone and loudspeaker transducers work.
The Beyerdynamic DT 770 (link to check the price on Amazon) is an example of a moving-coil dynamic headphone.
The Beyerdynamic DT 770 is featured in the following My New Microphone articles:
• Top Best Headphones For Podcasting Under $200
• Top Best Closed-Back Headphones Under $200
• Top Best Circumaural (Over-Ear) Headphones Under $200
• Top Best Moving-Coil/Dynamic Headphones Under $200
Beyerdynamic is featured in My New Microphone’s Top Best Headphone Brands In The World.
The Apple EarPods (link to check the price on Amazon) also utilize moving-coil dynamic drivers.
Apple is featured in My New Microphone’s Top Best Earphone/Earbud Brands In The World.
To learn more about moving-coil dynamic headphones, check out My New Microphone’s Complete Illustrated Guide To Moving-Coil Dynamic Headphones.
The Klipsch THX-1200-SW (link to check the prince at B&H Photo/Video) has a 12″ moving-coil dynamic subwoofer.
Klipsch is featured in the following My New Microphone articles:
• Top Best Home Speaker Brands You Should Know And Use
• Top Best Subwoofer Brands (Car, PA, Home & Studio)
• Top Best Loudspeaker Brands (Overall) On The Market Today
The DS18 TX1S (link to check the price on Amazon) is a 1.38-inch moving-coil dynamic tweeter speaker.
The KRK Rokit 10 G4 studio monitor (link to compare prices on Amazon and B&H Photo/Video) utilizes 3 different moving-coil dynamic drivers:
- LF Driver: 10″ (25.4 cm) moving-coil Kevlar Woofer
- MF Driver: 4.5″ (11.43 cm) moving-coil Kevlar Woofer
- HF Driver: 1″ (2.54 cm) moving-coil Kevlar Tweeter
There are also moving-coil microphones that work on the same design and electromagnetic principles, only in reverse.
To learn more about moving-coil dynamic microphones, check out my articles The Complete Guide To Moving-Coil Dynamic Microphones and How Do Microphones Work? (The Ultimate Illustrated Guide).
Other Driver Types
As mentioned, the section on moving-coil drivers sums up how the vast majority of headphone and speaker transducers work.
However, there are other driver types we should be aware of in headphones and loudspeakers that will help up to better understand how all headphones and speakers work as transducers.
Note that each of the following headphone and loudspeaker driver types is a transducer the converts audio signals (electrical energy) into sound waves (mechanical wave energy). The differences between the types has to do with either the driver design and/or the working principle used to convert the audio into sound.
Headphone Driver Types
There are 5 headphone driver/transducer types worth noting in this article:
Let’s skip over the already-discussed moving-coil dynamic driver design and briefly go over the other 4 headphone driver types.
Planar Magnetic Headphone Transducer
Planar magnetic headphone transducers work similarly to the magnetostatic/planar magnetic loudspeaker transducers, only on a smaller scale.
These transducers work on electromagnetic principles.
The thin diaphragm has a printed or embedded conductive wire (rather than a conductive coil like the moving-coil dynamic driver). This diaphragm is sandwiched between two magnet arrays.
As the audio signal is passed through the diaphragm, the diaphragm will oscillate back and forth between the magnetic arrays according to the magnetic attraction/repulsion between the diaphragm and magnets.
This diaphragm movement causes coinciding sound waves to be produces.
The Audeze LCD-X (link to compare prices on Amazon and B&H Photo/Video) is a pair of higher-end planar magnetic headphones.
The Audeze LCD-X is featured in My New Microphone’s Top Best Planar Magnetic Headphones.
Audeze is featured in My New Microphone’s Top Best Headphone Brands In The World.
To learn more about planar magnetic headphone transducers, check out my article Complete Guide To Planar Magnetic Headphones (With Examples).
Electrostatic Headphone Transducer
Electrostatic headphones transducers work similarly to electrostatic loudspeaker transducers, only on a smaller scale.
As the name suggests, these transducers work on electrostatic principles.
Basically, these drivers work with a positively-charge diaphragm (either via electret material or a biasing voltage) that is sandwiched between two perforated stator plates that act as a parallel-plate capacitor.
The impedance of the drive is very high and so the audio signals sent to an electrostatic driver must be amplified before they can properly driver the headphones.
Electrostatic headphones, therefore, require specialized headphone amps. Note that amp may also provide the DC bias voltage for the diaphragm.
As the audio signal is applied to the stator plates (capacitor-like component), the plates will have equal but opposite electric charges. These charges repel/attract the positively-charge diaphragm.
The diaphragm moves in a bipolar planar fashion (back and forth without too much bending in the diaphragm) between the perforated stator plates and produces sound waves that pass through the perforated plates and reach the ears of the headphone wearers.
The STAX SR-007A MK2 (link to check the price on Amazon) is an example of an electrostatic headphone with a circumaural open-back design.
The STAX SR-007A MK2 is featured in My New Microphone’s Top Best Electrostatic Headphones.
Stax is featured in My New Microphone’s Top Best Headphone Brands In The World.
To learn more about electrostatic headphone transducers, check out my article Complete Guide To Electrostatic Headphones (With Examples).
Balanced Armature Headphone Transducer
The balanced armature headphone transducer design is based on the same design and electromagnetic working principles as the moving-iron speaker transducer.
A balanced armature has a stationary conductive coil that passes the AC voltage of the audio signal. This coil experiences a varying magnetic field and extends this field to a conductive armature that is balanced between two magnets (hence the name).
This armature is mechanically coupled to a movable diaphragm via a drive pin. Therefore, as the audio signals are applied to the BA driver, the diaphragm moves and produces coinciding sound waves.
These rather complex drivers are notorious for their limited frequency responses. So just like many loudspeakers utilize multiple speaker drivers, balanced armature earphones use multiple drivers to produce the entire audible range of frequencies.
The Audiosense T800 (link to check the price on Amazon) is an example of a pair of balanced armature earphones that utilize 8 balanced armature drivers in each earpiece.
To learn more about balanced armature headphone transducers, check out my article The Complete Guide To Balanced Armature IEMs/Earphones.
Bone Conduction Headphone Transducer
Bone conduction headphones work with piezoelectricity and are, therefore, similar to piezoelectric loudspeaker transducers.
The diaphragm of the bone conduction “driver” is actually just a piezoelectric crystal.
As the audio signal is applied to the crystal, a voltage is produced across the crystal. This causes deformations in the crystal that cause it to vibrate relative to the amplitude and frequency of the audio signal.
This vibration is transmitted through the bones of the listener’s skull and to his or her inner ear (bypassing the eardrum completely) where the vibration is picked up and heard as sound.
The Aftershokz Aeropex (link to check the price on Amazon) is an example of a bone conduction headphone.
The Aftershokz Aeropex is featured in the following My New Microphone articles:
• Top Best Wireless Earphones For Running Under $200
• Top Best Bone Conduction Headphones
For more information on bone conduction headphone transducers, check out my article The Complete Guide To Bone Conduction Headphones (With Examples).
Loudspeaker Driver Types
There are 7 loudspeaker driver/transducer types worth noting in this article:
- Moving-coil dynamic
- Magnetostatic/planar magnetic
Again, we’ll skip the moving-coil dynamic driver design and focus on the other 6 loudspeaker transducer types.
There are a few experimental loudspeaker designs I’ll list here that are worth knowing about but overly specialized and perhaps overly theoretical for practical use. These other designs are:
- Bending wave: an electromagnetic design with a full-range single driver that produces a cylindrical sound field.
- Flat-panel: miniature speakers designed with flat diaphragms and “exciter” transducer coils. These transducers suffer from high resonances and poor sensitivity.
- Heil air motion: an electromagnetic design with a pleated diaphragm is mounted in a magnetic field that pushes and pulls air into/out of its pleats as it moves. An improvement upon the ribbon driver design.
- Transparent ionic conduction: uses high voltages and transparent ionic conduction to move a diaphragm with 2 layers of transparent conductive gel and a layer of transparent rubber.
- Plasma arc: Uses electrical plasma as a radiating element (rather than a diaphragm) in an electrostatic design. The audio signal alters the electric field which causes the plasma to move and produce sound waves.
- Thermoacoustic: Works on the thermoacoustic effect. Audio signals are used to periodically heat the carbon nanotube thin film which would then produce sound waves.
- Rotary woofer: A fan with blades that constantly change their pitch.
Let’s now get to the more practical loudspeaker transducer designs.
Magnetostatic/Planar Magnetic Loudspeaker Transducer
Magnetostatic/planar magnetic loudspeaker transducers work similarly to planar magnetic headphone transducers, only on a larger scale.
These transducers work on electromagnetic principles.
The thin planar (often rectangular) diaphragm of a planar magnetic loudspeaker driver has embedded conductive wire that effectively passes the alternating current of the amplified audio signals.
This causes a varying magnetic field in the diaphragm.
The diaphragm is positioned between two magnetic structures or, in some designs, in close proximity to a single magnetic structure. It will oscillate back and forth as the audio signal is passed through it.
The diaphragm moves in a planar bipolar manner without much bending or wrinkling and therefore, with low distortion.
Magnepan is the industry standard producer of planar magnetic loudspeakers. The Magnepan MG 1.7 is an excellent example of a planar magnetic loudspeaker.
Ribbon Loudspeaker Transducer
The ribbon loudspeaker is similar to the aforementioned planar magnetic speaker but is different in a few key ways.
The ribbon loudspeaker diaphragm is fully conductive (rather than having a conductive element embedded into it). It has a very low mass in order to move accurately.
The diaphragm is ideally corrugated to increase its transverse rigidity and reduce its resonant frequency.
To work will require an amplifier with a step-down transformer (or similar transformerless circuit) to drop the voltage of the audio signal and boost the current. The magnets of the ribbon driver are to the sides of the diaphragm rather than to the front and back and must be very powerful if the ribbon is to produce much sound.
Ribbon diaphragms/drivers are cherished for their accuracy but are notorious for their low sensitivity/efficiency ratings and their fragility.
They are typically used in conjunction with moving-coil drivers to produce the full range of audible frequencies.
The Atlas Sound EM806A-B (link to check the price at B&H Photo/Video) is a full range line-array speaker system with 8 x 6.5″ (16.51 cm) high-frequency ribbon drivers and 8 x 6.5″ (6.51 cm) moving-coil dynamic drivers.
There are microphones that utilize ribbon transducers (only in reverse). To learn more about ribbon microphones, read my article The Complete Guide To Ribbon Microphones (With Mic Examples).
Electrostatic Loudspeaker Transducer
Electrostatic loudspeaker drivers work very similarly to electrostatic headphone transducers, only on a larger scale with greater amplification requirements.
The electrostatic loudspeaker driver works on electrostatic principles.
It has a large, thin and often rectangular diaphragm that is conductive across its entire area. It must be positively charged via a high-level DC biasing voltage or a strong electret material.
This diaphragm is effectively sandwiched between two large perforated stator plates that act as a parallel-plate capacitor.
The driver has the diaphragm and plates insulated from each other via spars around the perimeter of the diaphragm and plates.
A specialized amplifier must crank up the voltage of the intended audio signal while knocking down the current. This is to properly charge the high-impedance “capacitor” that is the stator plates.
As the amplified audio signal is connected to the plates, the stators will posses equal but opposite electrical charges at any given point.
This means that, at any given point, the positively-charged diaphragm will be pulled toward one plate while being pushed by the other plate.
Therefore, as the audio signal passes through, the diaphragm will oscillate according to the audio waveform and the electrostatic loudspeaker transducer will produce sound!
The MartinLogan Motion ESL 9 (link to check it out at MartinLogan) is a great example of an electrostatic loudspeaker.
There are microphone equivalents to electrostatic headphone/loudspeaker drivers. They are known as condenser microphones (“condenser” used to be the term for “capacitor”). They come in externally-polarized “true” and pre-polarized/electret designs.
For more information on condenser microphones, check out the following My New Microphone articles:
• What Is A Condenser Microphone? (Detailed Answer + Examples)
• The Complete Guide To Electret Condenser Microphones
Moving-Iron Loudspeaker Transducer
Moving-iron loudspeaker transducers use conductive coils and electromagnetism. However, unlike the popular moving-coil dynamic drivers, the moving-iron coils are stationary.
As the audio passes through the coil, it vibrates a magnetized piece of metal called the iron.
This piece of metal is either coupled to a dedicated diaphragm or acts as the diaphragm itself.
The moving-iron loudspeaker transducer is a primitive design and was actually the first loudspeaker to ever be produced. It has limited bandwidth and accuracy.
The aforementioned balanced armature headphone drivers utilize the same working and design principles as the moving-iron loudspeaker transducers.
Piezoelectric Loudspeaker Transducer
Piezoelectric loudspeakers are the odd-balls of the bunch since they do not technically have “diaphragms”.
Rather, they convert electrical audio signals into coinciding mechanical vibrations through piezoelectric crystals.
These transducer are typically found in beepers and as the tweeters of cheap speakers. However, they’re worth mentioning here.
As the audio signal is applied to a piezoelectric crystal, a voltage is produced across it. This voltage causes the piezoelectric crystal to deform as its molecules attempt to find electrical equilibrium and a neutral electrical charge across the crystal.
As we can imaging, deforming the crystal at audio frequencies between 20 Hz – 20,000 Hz will cause it to vibrate and interact with its surrounding medium to produce sound waves.
Piezoelectric loudspeakers are relatively poor at producing sound but their low-cost and durability make them excellent choices for specific applications.
The aforementioned bone conduction headphone drivers work on the same principles as piezoelectric loudspeaker transducers.
Magnetostrictive Loudspeaker Transducer
Magnetostrictive loudspeaker transducers are similar to piezoelectric drivers in the fact that they are based on the deformation of material to produce sound.
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape during the process of magnetization.
The sound of magnetostriction can be heard as the dreaded 60-cycle hum (or 50-cycle hum depending on where you live) that you’ll hear from power mains transformers.
These durable transducers utilize many thin magnetic plates stacked together in a core. A conductive coil is then wrapped around them and the entire driver is housed within a canister.
As the audio signal is passed through the coil, a varying magnetic field is produces and transmitted to the core.
The thin plates of the core change shape ever-so-slightly and propagate sound waves that way. When the coil is not experiencing an audio signal, the core returns to its original shape.
These transducers are often best suited for ultrasound. However, with technological advances, they may become a viable and durable option as a normal audible audio transducer-type.
How is the human ear a transducer? The human ear, which is made of many components, acts as a transducer that converts mechanical wave energy (sound waves and the vibrations they in the ear) into electrical energy (the impulses that are sent from the inner ear to the brain for us to “hear” as sound.
How do I turn my speakers into a microphone? We can turn a loudspeaker into a microphone by simply reversing the signal flow. Speakers are often wired the same as an unbalanced microphone would be. Therefore, we can often simply plug the speaker into a mic input on a mixer and it will act as a mic. Note that we may need to replace the connector of the speaker with a suitable plug for the mic input.
I’ve written more on how to turn a speaker into a microphone in my article How To Turn A Loudspeaker Into A Microphone In 2 Easy Steps.