Understanding Headphone Driver Units

Headphone driver units are the integral parts of every headphone advertisement, presentation or purchase. Drivers are mentioned every single time someone wants to point out the advantages of a certain headphone/earbud model. This leads us to the conclusion that driver units must be very important elements of every pair of headphones, regardless of their type or purpose. 

However, if you ask a random person in the street about driver units, they won’t be able to give you the exact explanation because most people don’t really understand how these devices work. This is why we want to ask you a simple question. Have you ever wondered how your headphones work and how they produce the sound you love so much? If you have and you are looking for the right answer, you’ve come to the right place.

In this article, we will try to introduce you to all the existing driver unit types and explain their working principles in layman’s terms. We are confident that this will be more than useful the next time you decide to buy a pair of headphones because you will be more aware of the type of device you need.

What are Drivers and What Kinds of Drivers Exist?

Drivers are small loudspeakers hidden inside headphone earpieces/housings, which convert the electrical signal into sound waves that our ears perceive as sound (music, voice, etc.).

Even though all driver types are different, most of them have a very similar structure consisting of a magnet, voice coil, and a diaphragm. When the electric current passes through the coil, it makes the diaphragm move/vibrate, which actually leads to sound production. Regardless of their structure and technology used, all drivers have one thing in common – they produce sound by transforming vibrations into sound waves.

When it comes to driver features, they are usually classified and judged by their diameters. This data is often inadequately used to estimate the headphone sound quality as well as its loudness. Although some numbers may imply that drivers with larger diameters help produce the sound of better quality (in comparison to smaller drivers), we have to reassure you because the process of assessing sound quality is more complex and it’s affected by more factors, not only by the driver diameter.

However, we shall leave the discussion about headphone sound quality for some other articles and focus our attention on our topic and help you understand headphone driver units.

A Short Guide to Understanding Headphone Driver Units

Dynamic Drivers

Dynamic drivers are probably the oldest and most common driver type on the global market. As you can assume, they are extremely popular and it’s for a reason. These drivers are very simple and cost-efficient. They also deliver a very good performance.

The work of these drivers is based on the rules of magnetism and electromagnetism. They consist of one neodymium magnet, a voice coil (also called wire coil) and a large diaphragm. When the electric current passes through the dynamic driver, it goes through the voice coil, which lays near the magnet. Together, they create a magnetic field that becomes electromagnetic once the device is connected to the source of power. The current makes the coil move and the movements are passed onto the diaphragm, which is attached to the coil. Subsequently, the diaphragm starts to vibrate displacing the air from headphones and producing sound waves.

Dynamic driver

Dynamic driver (source – Pioneer HDJ-X5-K)

Dynamic drivers are popular because of their simple structure, affordable materials used to build them, and because of their great performance. This leads to the mass production of inexpensive headphones and earbuds that produce the sound many customers like. As some of you may know, the amount of bass, as well as sound loudness, are proportional to the amount of the displaced air. Unlike some other drivers, dynamic drivers have vents used for air displacement. When the diaphragm moves, it displaces the air through the vent. The drivers capable of displacing more air usually deliver louder and more bass-heavy sound than others. Dynamic drivers also manage to cover all the frequency spectrum but this comes at the expense of major detail loss. They also have problems with distortion at high volumes as well as inconsistency and inaccuracy across different frequencies. 

In addition, there’s a negative side of dynamic driver affordability. Being cheaply glued together, dynamic driver parts fall apart due to the glue wear off, which can have a huge negative effect on the sound.

Balanced Armature Drivers

The greatest and most important advantage of BA drivers is their size. They are so small that multiple drivers can be installed in a single earpiece and this makes them adequate for smaller headphones (earbuds) and particularly for hearing aids and in-ear monitors.

Balanced armature drivers consist of a tiny arm (aka armature), a voice coil wrapped around the arm, two magnets on both of its sides as well a small diaphragm. When you connect a BA driver to the power source, the electric current passes through the driver (and through the coil), making the coil to move. Subsequently, the arm starts to move back and forth from one magnet to another, which changes the created magnetic field. As the moving arm is attached to the central part of the diaphragm, the vibrations are transferred to the diaphragm, and its movements create the sound.

Balanced armature driver

Balanced armature driver (source – Knowles)

BA drivers deliver slightly different sound in comparison to the dynamic drivers. They are not really capable of covering the whole audible spectrum and can’t deliver great bass. Balanced armature drivers use crossovers to split the signal they receive in different frequency bands and send them to the matching drivers. They can’t produce powerful bass because they don’t have vents and their air displacement is not great.

All these facts imply that BA driver performance is limited but they do have many advantages that make them a great choice depending on the device’s purpose. For example, they are more adequate for smaller headphones. BA drivers are so small that multiple units can be installed in one earpiece, which increases sound clarity and preserves most of the details. There is even a solution for the lack of bass. As BA drivers are very small, they allow manufacturers to add one dynamic driver to the group of several BA drivers and enhance the bass performance. In addition, the fact that these separate BA drivers are responsible for specific frequency bands ensures better midrange and treble production. BA drivers also have low energy consumption, which makes them great for wireless devices because your batteries will last longer.

The most important disadvantage, along with poor bass reproduction, are high prices caused by their complex structure.

Planar Magnetic Drivers

Planar magnetic drivers got their name after planar magnetic headphones and their work is based on planar magnetic technology which was first developed for NASA purposes and, after that, it was used in the headphone industry. This is the advanced technology that hasn’t been perfected yet but it has already helped the manufacturers to overcome some limitations of dynamic drivers.

These drivers actually gather all the greatest qualities of electrostatic and dynamic drivers. They consist of two large and strong magnets and a thin diaphragm between them. The magnets create a magnetic field, which is surrounded by electric conductors (wires) and positioned parallelly to the diaphragm. The magnetic force is equally distributed all over the driver but the change happens when the electric current passes through the driver. Once this happens, every conductor creates its own magnetic field, which interacts with the isodynamic field. The interaction causes diaphragm vibrations. You already know the rest – diaphragm vibrations lead to the creation of sound waves. Thanks to the isodynamic field, these movements are constant, uninterrupted and equally distributed across the whole diaphragm, which ensures perfectly balanced sound reproduction.

Planar magnetic driver

Planar magnetic driver (source – Audeze)

PM drivers are popular due to their great sound quality and increased durability. First of all, they ensure less distortion as well as enhanced accuracy. Unlike dynamic drivers, planar magnetic drivers have a diaphragm that moves uniformly. Due to the wires running through the entire diaphragm surface, all of its parts move equally, which leads to improved dynamics, better frequency response, decreased distortion and increased accuracy. Thanks to its thinness, the diaphragm moves much faster which extends the treble response and enables more detailed reproduction.

Planar magnetic drivers also deliver dynamic and punchy bass, which comes as a consequence of a great amount of air going through the driver caused by the large driver surface and its strong magnetic force.

In addition, PM drivers have better responsiveness. This means they react faster to the input signal changes than the other types of drivers. This happens because the diaphragm is thinner and lighter than the diaphragm in dynamic drivers, which enables it to move faster and have a better response to signal variations.

We also have to mention that these drivers have fewer moving parts in comparison to dynamic drivers which improves their durability. The equal force and current distribution lead to equal heat distribution.

On the negative side, planar magnetic drivers are heavy and quite large, which means that planar magnetic headphones are bulky and heavy, too. This also means they are quite uncomfortable and unpleasant for long listening sessions. In addition, since their work is based on a relatively new and rare technology, planar magnetic headphones are quite expensive. They are usually hard to drive and require an external amplifier, which makes them quite useless when connected to a simple smartphone or tablet. If you want a pair of planar magnetic headphones to reach their full potential, you better prepare for some additional costs and get a proper amplifier.

Electrostatic Drivers

If you have thought that planar magnetic drivers have the most advanced working principle, you have been wrong. Electrostatic drivers use even more advanced technology, which is, at the same time, both simpler and more complex than planar magnetic technology. 

Electrostatic drivers don’t have cones, coils or magnets. They are small and have extremely thin diaphragms (thinner than the one installed in planar magnetic drivers) suspended between two metal stators (metal stator plates shaped like a mesh). These drivers’ work is based on the principles of electrostatic force, which is quite different than magnetic force. 

If you aren’t sure what the electrostatic force is, just remember those school experiments that include combing human hair with a comb that was previously rubbed against a piece of cloth. This rubbing creates static electricity that enables the comb to attract different objects.

The diaphragm of an electrostatic driver is made from mylar and plastic. Just like in planar magnetic drivers, this diaphragm contains many thin wires covering the entire diaphragm surface. When we expose the stators to high voltages (over 500 V), they are filled with static electricity and they make the diaphragm float between them, moving back and forth, pushing the air and producing sound.

STAX SR-009 electrostatic headphones

STAX SR-009 electrostatic headphones

The interesting part is that the diaphragm manages to keep the constant charge and that the diaphragm moves without making physical contact with any of the driver parts, which eliminates harmonic distortion almost completely as well as the sound coloration.

Electrostatic headphones have become popular thanks to the fact that they produce extremely natural, detailed, and transparent sound with great deep bass and minimum distortion. The durability of all the driver parts is amazing. Since the parts aren’t glued together, they can’t wear off as fast as dynamic drivers. They also lack magnets, which makes them much lighter and more comfortable.

On the negative side, electrostatic drivers are quite expensive, which raises the price of electrostatic headphones over $5000. Apart from this, you will have to add some money and equip them with a matching amp, which also costs a few thousand dollars. You should also be aware that these headphones are made exclusively for indoor use and that they are not portable because of their open-back design but also because of the amp. 

Bone Conduction (Magnetostriction) Drivers

Bone conduction drivers represent a relatively new driver type but their working principle is based on a 2-century old technology. This statement is based on a story about Beethoven holding a stick in his teeth while playing the piano. The other end of the stick was attached to the piano and the vibrations traveled from it, through the stick, to his teeth and jawbones, making him able to hear the sound.

This is actually the working principle of bone conduction drivers and headphones. Their earpieces lay on your temporal bones (in front of your ears) and basically play the role of human eardrums. When the electric current runs through the headphones, it creates sound waves, which are then converted into vibrations and conducted through bones directly into your inner ear and brain. This way, the outer ear and middle ear are bypassed, which is amazing for all the people with hearing problems. However, the use of bone conduction headphones isn’t limited only to this group. Anyone who wants to listen to music while remaining aware of their surroundings can use these headphones. This is particularly great when it comes to outdoor workouts, jogging, etc.

AfterShokz TrekzAir bone conduction headphones

AfterShokz TrekzAir bone conduction headphones

150 years passed before this technology was used again and it was done for military purposes (developing communication devices). The soldiers had to be able to communicate while remaining aware of all the danger around them, so the headsets based on bone conduction technology came in handy. After this, it was used in medicine for hearing device development. As a result, the BAHA device was created and implanted into a human head enabling the people with certain types of hearing issues to hear again.

Even today, we can’t say that the technology is fully developed, especially when it comes to headphones. Bone conduction headphones were introduced in 2012 and there is still a lot to be developed and perfected. Despite the fact that they are medically very useful, we can’t avoid mentioning their low sound quality, bad bass response, very high prices, and significant sound leakage. Once the manufacturers find a way to solve these issues, bone conduction headphones will become even more popular. 

Hybrid Drivers

These drivers are relatively new and they are usually made to compensate certain shortcomings that all driver types have and deliver the best possible sound. The most common driver combination is the combination of multiple balanced armature and a single dynamic driver. In this hybrid, the dynamic driver is responsible for the bass reproduction while the BA driver (or drivers) is responsible for the mids and highs. 

The most common kind of hybrid drivers

The most common kind of hybrid drivers (Dynamic + BA driver)

Other kinds of hybrid drivers are rare but they still exist. For example, there are hybrid headphones with dynamic and electrostatic drivers (Mitchell and Johnson MJ2 or Cyberdrive HP112A) and there are also IEMs with planar magnetic and dynamic drivers (like oBravo Cupid). 

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