What Are Speaker Drivers? (How All Driver Types Work)

Speakers are these incredible devices that turn electrical audio signals into sound waves for our listening pleasure (or displeasure), and they wouldn’t be able to do so without drivers. So, it goes without saying that speaker drivers are essential to speaker design and that we should understand how they work.

What are speaker drivers, and how do they work? Speaker drivers are the transducer elements of speakers that are ultimately responsible for converting the audio signal (electrical energy) into sound (mechanical wave energy). Though there are several driver types, they practically all use a conductive element to move a diaphragm and produce sound.

In this lengthy article, we’ll go through each of the speaker driver types and describe how they act as transducers and produce sound waves.

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Table Of Contents

• Speaker Drivers Are Transducers
• The Alternate Definition Of Speaker Drivers
• The Dynamic Speaker Driver
• How Does The Dynamic Speaker Driver Work?
• Subwoofers, Woofers, Tweeters &More
• Component Vs. Coaxial-Speakers
• Amplifiers & Crossovers
• The Other Speaker Driver Types
  • Magnetostatic/planar magnetic
  • Ribbon
  • Electrostatic
  • Moving-iron
  • Piezoelectric
  • Magnetostrictive
  • Bending wave
  • Heil air motion
  • Transparent ionic conduction
  • Plasma arc
  • Thermoacoustic
  • Rotary woofer
• Related Questions

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Speaker Drivers Are Transducers

The most basic and important definition of a speaker driver is that it is a transducer.

Transducers are devices that transform one kind of energy into another kind of energy. In the case of speaker drivers, as we've alluded to, this transformation converts electrical energy into mechanical wave energy.

The electrical energy is in the form of audio signals. More specifically, it is in the form of analog audio.

To learn more about analog and digital audio and their relationship to speakers, check out my article Are Loudspeakers & Monitors Analog Or Digital Audio Devices?

The mechanical wave energy is best described as sound waves.

To learn more about the differences and similarities between sound and audio, check out my article What Is The Difference Between Sound And Audio?

These analog audio signals are complex alternating currents made up of frequencies typically in the audible range (20 Hz – 20,000 Hz). That being said, audio signals could have information out of this audible range.

These analog audio signals are generally amplified and passed through crossover networks before they reach their drivers.

The speaker driver is designed, in part, with a conductive element that is to be part of a circuit that passes the AC voltage of the audio signal.

In one way or another, this alternating current causes the driver to, in turn, produce sound waves. Quality driver designs will accurately reproduce the audio waveform as sound.

We'll discuss the various types of drivers and how they are designed to convert audio into sound. For now, it's key to know that speaker drivers are transducers.

For more information on speakers as transducers, check out my in-depth article titled How Do Speakers & Headphones Work As Transducers?

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The Alternate Definition Of Speaker Drivers

Note that the term driver may take on a different meaning when dealing with consumer-grade “digital” computer speakers or built-in computer speakers.

In this case, “driver” could refer to the hardware audio driver.

This driver is a set of files that allows the computer and the audio devices, including the speaker output device(s), to communicate with each other.

For more information on speakers and computers, check out the following My New Microphone articles:
• Are Speakers (& Studio Monitors) Input Or Output Devices?
• How To Connect Speakers To A Computer (All Speaker Types)

This is a short but notable aside. When we discuss speaker drivers, we rarely, if ever, are talking about computer hardware drivers. Nonetheless, this could be interpreted as the definition and is worth mentioning in this article.

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The Dynamic Speaker Driver

The vast majority of speakers are designed with dynamic driver elements.

If you've ever taken apart a computer speaker, peered beyond the grille of a PA speaker, checked out an automobile speaker, been in a studio with studio monitors, etc., then you've most definitely seen a dynamic driver.

Dynamic drivers are also incredibly popular in headphone and even microphone designs (though the transduction happens in reverse).

For more information on dynamic headphones and microphones, check out my articles Complete Illustrated Guide To Moving-Coil Dynamic Headphones and The Complete Guide To Moving-Coil Dynamic Microphones, respectively.

So what is a dynamic speaker driver?

Let's begin answering this question by having a look at the design of the dynamic speaker driver. Below is a simplified cross-sectional diagram of a typical dynamic speaker driver:

As we can see, the dynamic speaker driver is made of the following components (I'll add the names of the larger elements that is made of the above-labelled components):

• Diaphragm
  • Cone
  • Dust cap (dome)
• Suspension
  • Surround
  • Spider
• Voice Coil
• Magnetic structure
  • Magnet
  • Top pole plate
  • Yoke (bottom pole plate + pole piece)
• Basket

Let's define each of these components in more detail before getting into how the dynamic speaker driver works.

Diaphragm

The speaker diaphragm is the large movable membrane designed to oscillate back and forth according to the audio signal waveform. As the diaphragm moves forward and backward, it pushes and pulls air, respectively.

Pushing and pulling air causes compression and rarefaction in the air/medium and sends sound waves propagating through the air/medium.

The diaphragm is made up of the cone and the dust cap (dome).

Cone

The cone is the largest part of the diaphragm and is often even referred to as the diaphragm.

This relatively thin membrane is attached to the voice coil at the inner circumference and is designed to move inward and outward along with the movement of the voice coil.

At the outer circumference, the cone is attached to the surround. The surround effectively connects it to the basket (housing/chassis) while allowing the cone to oscillate back and forth.

Speaker cones can be made of a wide variety of materials and combinations of materials. They can be treated with many different types of resins and lacquers in different concentrations.

The cone's shape, thickness, stiffness, weight, damping, and resilience play major roles in determining the speaker driver's frequency response and sonic character. A lot of care is taken in high-quality speakers to get the cone design just right for optimal speaker performance.

Because the cone has the largest mechanical demands of all the speaker components, it also influences the speaker driver's power rating.

Dust Cap (Dome)

The speaker driver's dust cap (also known as the dome) keeps dust and other airborne debris out of the voice coil. To do so, it must attach to the cone and, in effect, become part of the speaker's diaphragm.

Therefore, the dome has a noteworthy effect on the speaker driver's frequency response and the diaphragm's overall behaviour. The dome will play a part in reducing some cone resonances while introducing others.

Suspension

The suspension connects the diaphragm to the housing/basket of the speaker. It allows for the proper amount of diaphragm excursion while keeping the diaphragm and voice coil in their appropriate range of motion. This means supporting movement along the Z-axis while restricting movement in the X and Y-axes.

The speaker suspension is made up of the surround and the spider.

As an aside, the suspension of a brand new speaker is relatively stiff and must be broken in before the speaker reaches its optimal performance. This is known as speaker burn-in.

For more information on speaker burn-in, check out my article Do Speakers Need To Be Broken-In/Burned-In? (Fact/Fiction?).

Surround

The surround (sometimes called the front suspension) is a ring-shaped component that connects the cone/diaphragm to the basket/chassis of the speaker driver.

As part of the suspension, the surround helps to control cone excursion (the outward and inward movement of the cone/diaphragm) and plays a major role in determining the limits of the diaphragm excursion.

The surround also absorbs energy from the cone before it would reach the basket, thereby reducing the overall resonance of the speaker.

The surround, unfortunately, is prone to mechanical failure due to the repetitive stress of being flexed inward and outward over and over again.

It is often the first component to deteriorate and cause speaker blow-out. This will lead to audible distortion and further mechanical failure in the speaker if not repaired.

For more information on speaker blow-out, check out my article Loudspeaker Blow-Out: Why It Happens & How To Avoid/Fix It.

Fortunately, surrounds are relatively easy to replace, and their specifications are typically more universal than the other speaker components, making them easier to find.

Spider

The spider makes up the interior portion of the speaker suspension. Its main purpose is to keep the voice coil where it should be, allowing movement along the Z-axis and restrictive movement in the X and Y axes.

Without a spider, there would be a serious risk of the voice coil hitting and/or sticking to the magnet. This would cause significant non-linearities in the speaker's movement, which would, in turn, produce distortion and potentially even damage to the speaker.

The spider is connected to the voice coil to its interior and the basket at its exterior.

The suspension also plays a role in determining the low-frequency response and power handling of the speaker.

Voice Coil

The voice coil is a tightly-wound (typically cylindrical-shaped) coil of conductive wire. It has a lead wire attached to either of its ends and becomes part of a circuit that passes the AC audio signal.

As an AC voltage is applied across the voice coil, a coinciding varying magnetic field is induced around it. This magnetic field interacts with the permanent magnet's field and causes the voice coil (and attached diaphragm) to move.

The consistency of winding tension, application of enamels and adhesives, and wire composition all influence coil performance and play a role in determining the efficiency and power rating of the speaker driver.

Magnetic Structure

The magnetic structure provides a concentrated permanent magnetic field in the driver and, in particular, the voice coil.

This magnetic field is required for proper voice coil (and, therefore, diaphragm) movement in the dynamic speaker driver. As a varying magnetic field is induced across the voice via the AC audio signal, the coil interacts with the permanent magnetic field and oscillates relative to the audio signal waveform.

The magnetic structure is made of the main magnet and several pole pieces (top pole plate/ring, bottom pole plate and pole piece).

To learn more about speakers and magnetism, check out my article Why And How Do Speakers Use Magnets & Electromagnetism?

Magnet

The main magnet is the main source of the speaker driver's magnetic field (the pole piece simply extends the magnetic poles of the main magnet).

The magnet's shape, size, and strength should be designed to best suit the speaker driver and the enclosure in which the driver finds itself.

For more information on speaker enclosures, check out my article Why Do Loudspeakers Need Enclosures?

Top Pole Plate

The top pole plate extends one magnetic pole of the magnet to the exterior of the voice coil.

The shape of the top pole plate has major effects on the speaker's performance.

If the top pole plate is too thin, the driver will lose efficiency and begin saturating/distorting. If it's too thick, the transient response of the speaker will suffer.

The space between the exterior of the voice coil and the top pole plate is critical as well. Larger gaps are easier to produce but will underperform due to lower magnetic field strength and poorer heat dissipation. Smaller gaps run the risk of the magnet and voice coil touching, which will reduce the longevity of the speaker.

Yoke

The bottom pole plate and upward-reaching pole piece are often referred to together as the yoke.

This component is the back of the loudspeaker and extends the opposite magnetic pole to the interior of the voice coil.

So not only does the yoke after the stability of the magnetic structure, but it also affects the overall efficiency of the speaker driver.

Similar to the top pole plate, the shape and strength of the yoke, along with the gap between the yoke and the voice coil, affect the thermal dissipation and power handling of the driver.

Basket

The basket (also known as the chassis or housing) is the stationary physical housing that connects to the magnetic structure at the bottom, the surround at the top, and the spider somewhere in the middle.

Each basket has its own acoustic properties (absorption, resonance, etc.) and should be designed to benefit not only the driver itself but the loudspeaker performance at large.

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How Does The Dynamic Speaker Driver Work?

Now that we understand how the dynamic speaker driver is constructed, let's further our understanding of how the driver works.

I'll repost the cross-sectional diagram of the dynamic speaker driver again to help improve your experience:

Let's begin with the audio signal.

As previously mentioned in the section Speaker Drivers Are Transducers, the audio signal must be analog. This means the signal is a continuously variable AC voltage.

Audio signals from playback devices are generally at line level and must be amplified to speaker level before effectively driving a speaker driver.

Note that crossovers are also nearly always used with speaker drivers to separate a specific frequency band of the audio signal that the driver is designed to produce with greater clarity.

More on amplifiers and crossovers in the section titled Amplifiers & Crossovers.

Once properly processed, the audio signal is sent to the voice coil of the speaker driver via electrical lead wires. This produces an AC voltage across the coil.

Electromagnetic induction states that an electrical current through a conductor will produce a magnetic field. The AC though the voice coil, then causes an alternating magnetic field.

The varying magnetic field of the voice coil happens within the permanent magnetic field of the driver's magnets. This interaction causes the voice coil to oscillate in a way that mimics the audio signal waveform.

Since the voice coil is attached to the diaphragm, the diaphragm will also move with the voice coil.

Remember that the spider keeps the voice coil moving along the Z-axis only and, combined with the surround, limits the excursion of the diaphragm.

So the voice coil and diaphragm move according to the audio signal. The movement of the diaphragm pushes and pulls the air around it and produces increases and decreases in localized pressure.

These pressure oscillations are propagated outward from the driver and are known as sound waves.

And that's how a dynamic speaker driver works!

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Subwoofers, Woofers, Tweeters & More

As mentioned, the dynamic speaker driver is the most common speaker transducer type.

However, these drivers are not overly effective at reproducing the entire audible range of human hearing (20 Hz – 20,000 Hz). Therefore, speakers are often designed with various types of dynamic speaker drivers.

The most popular dynamic speaker driver types are as follows:

• Subwoofer: for very low frequencies
• Woofer: for low frequencies
• Mid-range: for mid-range frequencies
• Tweeter: for high frequencies
• Super-tweeter: for very high frequencies

These driver types all work on the same principle and general design philosophy as the aforementioned dynamic driver.

The differences in their build are based on the frequency range they are designed to reproduce. In general, deeper frequency reproduction requires a larger driver diameter and a more robust driver design.

Subwoofers will generally be built in their own separate enclosures and have their own amplifiers. They require lots of energy and space to produce the low end of the audible frequency spectrum.

2-way speakers (having 2 drivers) typically use a woofer to cover the low-end and a tweeter to cover the high-end.

3-way speakers (having 3 drivers) typically use a woofer for the low-end, a mid-range driver for the mid-range, and a tweeter for the high-end.

4-way speakers will generally add a super-tweeter to produce the very top-end of the audible frequency spectrum to help take some load off the regular tweeter.

For a detailed article on the roles of subwoofers, woofers, tweeters and the other speaker driver sizes, check out my article Differences Between Mid-Range Speakers, Tweeters & Woofers.

Note that there are no absolute standard crossover frequencies for the various dynamic driver types.

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Component Vs. Coaxial Speakers

The dynamic speaker driver types mentioned above are generally found in one of two speaker designs: component or coaxial.

Component speakers have their drivers separated inside a single speaker housing and/or enclosure.

The Sony SSCS5 is an example of a 3-way component speaker with a woofer, mid-range driver, and tweeter:

Coaxial speakers have their drivers stacked one-on-top-of-the-other.

The JBL GTO629 is an example of a 2-way coaxial speaker with a woofer and tweeter:

Coaxial speakers are generally cheaper, easier to install and produce the full range of frequencies in a more compact package. This makes them ideal for automobile audio.

Component speakers are typically more expensive but have better designs to produce high-quality sound.

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Amplifiers & Crossovers

As mentioned, the audio signals that ultimately drive speaker drivers must be processed. In particular, their voltage must be brought up via an amplifier, and their frequency bands must be limited via an audio crossover.

This article is not about amplifiers and crossover, but it's important to mention them, at least when discussing speaker drivers.

A subwoofer crossover will essentially act as a low-pass filter that sends only the low frequencies of an audio signal to the subwoofer driver.

A 2-way speaker will have a 2-way crossover that splits the audio signal into lows and highs.

A 3-way speaker will have a 3-way crossover that splits the audio signal into lows, mids and highs.

So on and so forth.

Passive speakers have crossovers but do not have built-in amplifier(s). Therefore, a power amplifier must be put inline between the audio source and the passive speaker(s) to boost the audio signal to speaker level.

Active and powered speakers have active crossovers that split line level signals and send the individual frequency bands to dedicated power amplifiers.

To learn more about active and passive speakers, check out My New Microphone's article titled What Are The Differences Between Passive & Active Speakers?

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The Other Speaker Driver Types

Although the dynamic speaker driver makes up the vast majority of driver designs, there are other speaker driver types worth mentioning.

The terminology here is arguable. When using the term driver, we generally are referring to dynamic speaker drivers. The other types may be called drivers though we may refrain from naming them that to avoid confusion.

An alternative, then, is to call these alternative driver types “transducers.” I'll use both terms interchangeably in this section.

These speaker drivers are built with different design philosophies and often have different audio and power requirements than the typical dynamic drivers.

The other speaker driver/transducer types are:

• Magnetostatic/planar magnetic
• Ribbon
• Electrostatic
• Moving-iron
• Piezoelectric
• Magnetostrictive
• Bending wave
• Heil air motion
• Transparent ionic conduction
• Plasma arc
• Thermoacoustic
• Rotary woofer

Of the speaker driver types listed above, the first 6 or so are practical designs that have seen varying amounts of success in commercial and/or niche applications. The last 7 are more experimental and theoretical but worth discussing nonetheless.

Let's deepen our understanding of these alternative speaker driver types, shall we?

Magnetostatic/Planar Magnetic Loudspeaker Transducer

Planar magnetic loudspeakers have been brought to the consumer market largely in part by the niche manufacturer Magnepan. Check out their official website by clicking here.

Magnetostatic/planar magnetic speaker drivers, like the dynamic drivers, mentioned previously, work on electromagnetic principles.

However, rather than having a voice coil attached to a cone-like diaphragm, the planar magnetic driver has a thin planar (often rectangular) diaphragm with an embedded conductive wire (also planar).

This wire is typically serpentine and passes through the majority of the diaphragm area. We can envision this in the following illustration:

As the AC audio signal passes through the conductive traces of the diaphragm, an alternating magnetic field is induced in and around the diaphragm.

The diaphragm is positioned between two magnetic arrays/structures or, in some designs, in close proximity to a single magnetic structure. This can be envisioned in the following illustrations:

So as the audio signal induces an alternating magnetic field in the diaphragm, it will be attracted/repulsed by the magnets around it, causing it to move.

The diaphragm is carefully connected to the housing around its perimeter and moves in a near-perfect planar bipolar manner. There is little to no banding or wrinkling of the diaphragm except at its perimeter. This yields a very accurate response and low distortion.

As suggested earlier, Magnepan is the industry standard producer of planar magnetic loudspeakers. The Magnepan MG 1.7 (pictured below) is an excellent example of a planar magnetic loudspeaker.

Magnetostatic/planar magnetic loudspeaker transducers work similarly to planar magnetic headphone transducers, only on a larger scale. For more information on planar magnetic headphones, check out my article The Complete Guide To Planar Magnetic Headphones (With Examples).

Ribbon Loudspeaker Transducer

The ribbon speaker driver also works via electromagnetic principles. However, it differs from the aforementioned dynamic and planar magnetic designs and is its own type of speaker driver.

Let's have a look at a simple illustration of the ribbon speaker transducer:

In ribbon designs, the diaphragm (known as the ribbon) is fully conductive. It is made of a conductive material rather than have an embedded conductive wire or an attached coil.

The ribbon diaphragm is ideally corrugated to increase its transverse rigidity and reduce its resonant frequency. It also has a relatively low mass, which improves the accuracy of its movement.

The audio signal is applied across the diaphragm itself, and an alternating magnetic field is induced.

In terms of amplification, the ribbon diaphragm generally requires less voltage and more current than traditional speaker drivers to keep the relatively fragile ribbon safe.

Many ribbon drivers, then, have a step-down transformer (or an equivalent transformerless circuit) to drop the voltage of the signal while boosting the current.

The magnets, which are to the side of the diaphragm rather than to the front and rear, must be extra powerful to make up for the lower voltage and less-than-ideal positioning. These magnets are generally designed extremely close to the diaphragm to improve magnetic flux and keep air from passing through the driver.

Ribbon diaphragms/drivers are cherished for their accuracy but are notorious for their low sensitivity/efficiency ratings and fragility.

These drivers are often used as tweeters and in conjunction with moving-coil drivers to produce the full range of audible frequencies in multi-way speaker design.

The Atlas Sound EM806A-B 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

The electrostatic loudspeaker transducer/driver is unlike the other types in that it works on electrostatic principles rather than the typical electromagnetic principles.

These speakers are generally capable of producing the entire range of audible frequencies though some are enhanced with a dynamic woofer/subwoofer to cover the sub-bass and bass frequencies.

The electrostatic speaker diaphragm is typically larger and thinner than other speaker diaphragms and is typically rectangular in shape. It is coated with a conductive material across its entire area.

The diaphragm must hold a positive electric charge for the speaker to work properly as a transducer. It is generally charged via a high-level DC biasing voltage or a strong electret material.

The all-important diaphragm is effectively sandwiched between two large perforated stator plates that act as a parallel-plate capacitor.

The stator plates and the diaphragm are electrically insulated from each other by the use of spars around the perimeter of the diaphragm and the plates.

The electrostatic speaker driver, then, could look something like the following illustration:

To better visualize the electrostatic speaker driver/diaphragm, check out this simplified cross-sectional diagram:

The audio signal is sent to the stator plates, which act as a sort of parallel-plate capacitor.

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.

Once connected to an audio source, the stators, at any given time, will be equally but oppositely charged.

The positively charged diaphragm will be pulled toward one plate while being pushed by the other plate at any given instant. This causes the diaphragm to move back and forth and produce sound waves as it does so.

These sound waves mimic the audio signal and escape the driver through the perforated stator plates.

The diaphragm of the electrostatic transducer, like that of the planar magnetic driver design, moves in a bipolar fashion and produces little to no distortion.

The MartinLogan Motion ESL 9 is a great example of an electrostatic loudspeaker.

There are headphone equivalents to electrostatic speakers. To learn more, check out my Complete Guide To Electrostatic Headphones (With Examples).

There are also microphones that utilize the same general principle. They are known as condenser microphones. 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 loudspeaker 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.

Although this design has lost favour in loudspeaker design, it is still used regularly in in-ear monitors under the design name “balanced armature.” To learn about balanced armature IEMs, check out my article The Complete Guide To Balanced Armature IEMs/Earphones.

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 where sound quality is not a major concern.

Piezoelectric drivers are also used in bone conduction headphones. For an in-depth article on bone conduction headphones, check out My New Microphone's Complete Guide To Bone Conduction Headphones (With Examples).

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.

Let's have a look at a simplified cross-sectional diagram of a magnetostrictive speaker driver:

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.

Bending Wave

Bending wave speaker drivers utilize electromagnetism to cause dilatational waveforms on their diaphragms and produce full-range sound.

Unlike the dynamic driver motor, which pushes and pulls a cone with a constant shape, the bending wave driver's motor causes dilatational mechanical waves in the diaphragm itself.

The easiest way to envision this wave movement is by dropping a pebble in water. The resulting waves propagate outward in a circular fashion along the water's surface.

In a bending wave speaker driver, an electrodynamic motor (voice coil and magnetic structure) pushes and pulls the a smaller concentric part of the diaphragm. This movement causes outward waves to be formed on the diaphragm itself.

This can be visualized in the following cross-sectional illustration:

These waves cause changes to the sound pressure levels in the air and are heard as sound waves.

The rigidity of the diaphragm increases from the centre toward the outside. Shorter wavelengths (higher frequencies) radiate mostly from the centre of the diaphragm, while longer wavelengths (lower frequencies) are propagated across the entire diaphragm.

The German manufacturer Göbel is a leader in the space of bending wave speaker design. Check out their website here.

Heil Air Motion

The Heil air motion transformer (or simply “air motion transformer” or AMT) is a special type of speaker transducer.

It works similarly to a ribbon speaker but its pleated diaphragm resembles a bellows rather than a corrugated ribbon. The diaphragm is generally made of PET film and has its conductor etched into it or coated onto it (rather than being constructed of a conductive material like a ribbon).

This diaphragm is held within a dipole housing with 4 walls and no top or bottom. This allows for bidirectional sound propagation.

The AMT diaphragm and housing look something along the lines of the following illustration:

A permanent magnet is placed behind the bellows-like diaphragm.

As the audio signal passes through the diaphragm, the diaphragm moves as follows:

• When the audio signal has positive current, the front-facing pleats expand and the rear-facing pleats contract.
• When the audio signal has negative current, the rear-facing pleats expand and the front-facing pleats contract.

This diaphragm movement produces variations in the sound pressure of the medium, and sound waves are propagated from it.

Transparent Ionic Conduction

The transparent ionic conduction speaker transducer is effectively a see-through artificial muscle that can reproduce audio signals as sound waves, often across the entire audible range of frequencies.

A TIC driver is constructed with a thin sheet of rubber sandwiched between two layers of a saltwater gel. The electrolytes in the gel act as the conductor.

Note that the electrical charges in TIC speakers are carried by ions rather than electrons.

The audio signal is amplified to a high voltage level and sent to the surfaces of the gel layers. This voltage causes the rubber sheet to rapidly contract and vibrate according to the audio signal's waveform.

Plasma Arc

Plasma arc speakers do not have solid diaphragms. Rather, they oscillate highly-ionized plasma gas from an electric arc.

These gaseous diaphragms are virtually massless and are capable of producing high fidelity sound without distortion or resonances.

The high-voltage audio signal alters the electric field of the arc, which causes the highly ionized plasma to move and produce sound waves.

Thermoacoustic

Thermoacoustic speaker transducers work on the thermoacoustic effect which states that a temperature gradient in a tube can produce sound.

Audio signals are used to periodically heat a carbon nanotube thin film which then produces sound waves accordingly.

Rotary Woofer

A rotary woofer is a specialty type of loudspeaker that uses the motion of its voice coil to change the pitch of a spinning fan blade mechanism rather than to move a cone/diaphragm.

These woofers are rare but are capable of producing sound frequencies much lower than even the deepest subwoofers.

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Related Questions

What are the different types of speakers? There are plenty of different types of speakers on the market. They can be distinguished by their size and role in audio production, their transducer design, whether they are passive or active, and other factors.

Speaker types include:

• Subwoofer
• Woofer
• Mid-range
• Tweeter
• Super-tweet
• Component
• Coaxial
• Bluetooth
• Wireless
• Wired
• Active
• Passive
• Powered
• Dynamic
• Magnetostatic/planar magnetic
• Ribbon
• Electrostatic
• Moving-iron
• Piezoelectric
• Magnetostrictive
• Bending wave
• Heil air motion
• Transparent ionic conduction
• Plasma arc
• Thermoacoustic
• Rotary woofer

How many watts is a good speaker? The best wattage (power handling rating) of a speaker depends on the power output of the amplifier that is driver the speaker. It's best to match “big speakers” with “big amps” and “small speakers” with “small amps”. Mismatching speakers and amps can lead to poor signal output, distortion, and even blow-out.