The Complete Guide To Balanced Armature IEMs/Earphones


If you’ve ever seen a big-time professional musical performance live or on video, you may have recognized one or more balanced armature in-ear monitors in the ears of the performers. In-ear monitors often preferred over foldback monitors due to feedback considerations and a great number of professional in-ear monitors utilize balanced armature designs.

What are balanced armature IEMs/earphones? IEMs (in-ear monitors) and earphones are transducers that convert audio signals (electrical energy) into sound waves (mechanical wave energy). Balanced armature (BA) models work on electromagnetic principles and utilize BA drivers that work with a coil wound around a moveable armature coupled to a diaphragm.

I know I know, this definition may be a bit confusing without a diagram and further explanation. This article will provide just that and even more information to help us better understand balanced armature IEMs and earphones.


Primer On Headphone Drivers

Let’s begin with a general discussion on headphone drivers before we get into the specific details of balanced armature drivers.

The drivers are the most critical elements in and headphone design. They are the transducers that convert the audio signals (electrical energy) into sound waves (mechanical wave energy).

Analog audio signal, which drive the headphone drivers, are electrical signal with alternating current.

The audio source (headphone amp, smartphone, etc.) connects to the headphone’s drivers and passes these AC signals through the drivers. It is then the drivers’ job to reproduce these AC signals as sound waves.

Note that for digital audio devices, the audio signals must absolutely be converted from digital audio to analog in order to properly drive the drivers. This is achieved with DAC (digital-to-analog converters). DACs are found within headphone amplifiers and in close proximity to the headphone jacks of digital devices.

To learn more about headphone amplifiers and headphone jacks, check out the following My New Microphone articles:

• What Is A Headphone Amplifier & Are Headphone Amps Worth It?
• How Do Headphone Jacks And Plugs Work? (+ Wiring Diagrams)
• Differences Between 2.5mm, 3.5mm & 6.35mm Headphone Jacks

As you may have assumed, headphones always have two drivers (that’s why they’re name is pluralized). Each driver is tasked with reproducing its own signal when a stereo audio signal is applied. Alternatively, each driver may receive the same copy of a signal when a mono audio signal is applied.

For more information on how headphone drivers receive their intended audio signals, check out the following My New Microphone articles:

• An In-Depth Look Into How Headphone Cables Carry Audio
• How Do Wireless Headphones Work? + Bluetooth & True Wireless

As an audio signal flows through the driver, the driver causes a diaphragm to vibrate and produce sound waves that mimic the audio.

Each driver type has its own methods of transducing the audio signals.

To read about all the different headphone driver types, check out my article What Is A Headphone Driver? (How All 5 Driver Types Work).

Electrostatic headphones work on electrostatic principles while bone conduction headphones work with piezoelectricity.

The most common working principle of headphones is electromagnetism. Moving-coil and planar magnetic headphones work on this principle.

Balanced armature drivers, which we’ll be discussing in the article, also work with electromagnetism. This working principle is best described through electromagnetic induction.


Electromagnetic Induction

Electromagnetic induction states that an electrical current flowing through a conductor will cause a magnetic field to be produced in (and around) the conductor.

Similarly, a voltage will be produced across an electrical conductor if that conductor experiences a changing magnetic field.

A conductive coil has an alternating current audio signal pass through it. It extends this current and changing magnetic field to the movable armature it is wrapped around.

The armature is balanced between two magnets. As it experiences a varying magnetic field, it interacts with the magnets and vibrates between them.

The armature is mechanically coupled to a diaphragm that moves in accordance with the audio signal to produce sound based on the audio signal.


How Do Balanced Armature Earphone Drivers Work As Transducers?

With that primer, let’s get into the finer details of how balanced armature IEMs/earphones work.

As I’ve alluded to in the above sections, the defining component of a balanced armature earphone is its BA driver.

Actually, it’s the case that many BA IEMs or earphones have multiple drivers since each BA driver has a limited frequency response. We’ll discuss that in more detail in this section.

BA drivers are very small and have size limitations. Therefore, we’ll only ever find earphone-type balanced armature “headphones.”

Related article: What Are The Differences Between Headphones And Earphones?

The main point is that we cannot understand balanced armature IEMs and earphones without comprehending how their drivers function.

The Balanced Armature Earphone Driver

The design of the balanced armature earphone driver is pretty involved. Let’s start off with a simplified cross-sectional diagram to help explain how they are made to function:

Simplified Cross-Sectional Diagram Of A Balanced Armature Driver

In the above diagram we see 7 different noteworthy components of the balanced armature driver design:

  • Diaphragm
  • Drive pin
  • Armature
  • Conductive coil
  • Magnets
  • Case
  • Sound outlet

The balanced armature design may also resemble something similar to this illustration:

Simplified Cross-Sectional Diagram Of A Balanced Armature Driver

In this example, the diaphragm is a stiff solid material much like the armature. It is connected to the inner case via a thin flexible membrane that allows the diaphragm to move while keeping the two air volumes (on either side of the diaphragm) separated.

The working principle is the same in both general designs. So how does the balanced armature driver work?

It begins with connecting the headphones to the audio source. When properly connected, the audio signal is passed through a circuit that includes the conductive coil.

As we know, the audio signal is an AC electrical signal which causes a varying magnetic field in the coil.

The coil is wrapped around a conductive armature and the AC electrical signal and accompanying varying magnetic field is extended to the armature.

This armature is balanced between two magnets, hence the name “balanced armature.” The magnet to the “top” has one magnetic pole facing the armature and the “bottom” magnet has the opposite magnetic pole facing the armature.

The varying magnetic field within the armature interacts with these two permanent magnets. Therefore, as the current flow in one direction, the armature will be attracted to the top magnet and repelled by the bottom magnet and when the current switched direction, the armature will be attracted/repelled in the opposite direction.

For more information on headphones and magnets, check out my articles Why & How Do Headphones Use Magnets? and Are The Magnets In Headphones/Earbuds Bad For You?

This moving armature is mechanically coupled to a thin diaphragm via the drive pin. As the armature vibrates up and down, it causes the relatively large surface area of the diaphragm to push and pull air. This pushing and pulling of air produces sound waves.

The components within the balanced armature are rather fragile and a protective case is required. The sound waves are produced within this case and escape through a sound outlet.

That’s essentially how a balanced armature driver works!

The frequencies at which the armature and diaphragm vibrate should be consistent with the waveforms of the applied audio signals. Unfortunately, balanced armature drivers have notoriously poor frequency responses.

This means that, although the audible range of human hearing is between 20 Hz – 20,000 Hz and quality audio signals have the potential to cover this entire range, balanced armature drivers cannot reproduce all these frequencies.

This is partly due to the inherent resonance frequencies that come with a balanced armature. The armature will be more easily moved at some frequencies compares to others. On top of that, the volume of air inside the case has its own resonances due to the dimensions of the case.

Tuning The Balanced Armature Driver

Extensive damping is often required to level out the frequency response. Even still, many BA transducers require multiple drivers to effectively produce a wide frequency response.

There are many different factors that manufacturers tune (or are at least aware of) that affect the sound of the BA drivers, units, and ultimately the hearing aid, earphone or in-ear monitor.

These balanced armature driver tuning factors include:

  • Size and internal volume
  • Armature mass and stiffness
  • Membrane stiffness and diaphragm mass
  • Armature damping
  • Acoustic filters
  • Sound outlet
  • Tube length and diameter
  • Tube damping

Size & Internal Volume

The size and internal volume of a BA driver is a major factor in determining the maximum output level. Larger volumes allow larger diaphragms and greater diaphragm movement which results in higher sound pressure levels.

Some BA receivers are designed with small vents in the housing behind the diaphragm to increase the “internal volume” in order to produce higher SPLs.

Armature Mass & Stiffness

Increasing the mass of the armature permits greater magnetic flux and improves low-end frequencies.

Reducing the mass and increasing the armature stiffness increases the bandwidth of the system while worsening the low-end response.

Getting an extended frequency response with decent bass is difficult in a BA driver. For this reason, there are BA drivers dedicated to acting as “woofers” to produce the low-end. Some balanced armature earphones designed even include a moving-coil dynamic driver to produce the low-end rather than relying on a BA driver.

Membrane Stiffness & Diaphragm Mass

All else being equal, larger diaphragms push more air but their higher mass limits their ability to produce high-frequencies.

The mass and stiffness of the diaphragm are key factors in determining the position, relative amplitude, and Q of the BA driver’s resonant peaks. Adjusting the stiffness and mass is, therefore, a way to tune the BA driver.

The placement of the diaphragm also plays a role in determining the overall loudness of the driver. Designing the diaphragm higher in the housing increases the rear volume (internal volume) of the driver and yields higher SPLs.

Armature Damping

Putting damping material between the armature and the magnets helps to damp resonant peaks by limiting armature movement. This, unfortunately, decreases the output levels of the driver but is most often a good trade-off.

Armature damping also has the added benefit of improving the durability of the BA driver by greatly reducing the risk of the armature hitting against the magnets.

Acoustic Filters

An acoustic filter can be placed in the sound outlet to help reduce resonant frequencies and cut unwanted highs and lows from the BA driver output.

Sound Outlet

The shape and length of the sound outlet will cause its own resonances that will colour the sound of the driver.

Tube Length & Diameter

The tubes that extend from the drivers to the acoustic output of the earphone add further colouration to the sound via their own resonances.

A longer tube length introduces lower-frequency resonances that are still typically above the resonances of the driver itself.

A larger tube diameter narrows and increases the resonances of the tube.

Tube Damping

Tube damping filters restrict airflow within the tube and can help to flatten out the resonance peaks in the overall earphone design. This works similarly to decreasing the tube diameter.

With all that being said, even with damping and tuning, the resonant peaks inherent in BA drivers are often too great to yield accurate sound production.

In many cases, multiple BA drivers with appropriate crossover systems are required in a single earpiece to produce a flatly-tuned frequency response.

Using Multiple Drivers With CrossOvers

The limited frequency responses of balanced armatures make them sound great in specific frequency ranges but they often perform poorly when tasked with covering the entire audible frequency range.

Their tuned resonances may work to their advantage in hearing aids if a patient has a specific null point in their hearing sensitivity. However, with in-ear monitors, which are designed to reproduce a full audio mix for a performer, a single BA driver may underperform.

Therefore, many in-ear monitors utilize multiple balanced armature drivers to produce their wide frequency responses.

For example, the Audiosense T800 (link to check the price on Amazon) uses 8 Balanced Armature for each side.

Audiosense T800

Each Audiosense T800 earpiece is described as having the following BA drivers:

  • Knowles SWFK-31736-000 (dual-tweeter)
  • Knowles HODVTEC-31618-000 (dual-woofer)
  • 4 custom Knowles drivers
Audiosense T800 Knowles 8 Balanced Armature Drivers

As we can see, the 8 drivers are consolidated into 3 cases.

  • The SWFK-31736 has 2 drivers in 1 case with 1 sound outlet.
  • The HODVTEC-31618 also has 2 drivers in 1 case with 1 sound outlet.
  • The 4 custom Knowles drivers are designed into 1 case with 2 sound outlets.

Each of the drivers is responsible for its own smaller range within the T800’s 5 Hz – 22,000 Hz frequency response.

The drivers are consolidated inside the earpiece with the proper tubing that combines their sound outlets and leads the sound to the output of the earpiece.

Now we run into the situation of sending a singal audio signal to an earpiece with multiple different drivers. In order for the earpiece to function properly with all these different drivers, we must essentially split the audio signal into multiple frequency bands and send the appropriate “pieces” of the audio signal to the appropraite BA drivers.

This splitting is made possible by a crossover. A crossover effectively portions out different frequency bands from a signal audio signals and sends these smaller bands to the drivers that are best-suited to reproducing them.

Crossovers are commonly used in speaker systems, where they split a single input signal to create two or three output signals consisting of separated bands of high-, mid-, and low-range frequencies. The different bands are then used to drive different speakers in a sound system, namely the tweeters, woofers, and subwoofers, respectively.

Crossover systems optimize the sound of multi-driver transducers. For example, they don’t send high-frequency signals to a woofer that would not be able to recreate them properly. This added audio input would just waste energy in the woofer.

They also protect the smaller tweeter speakers from blow-out that would occur from high-energy low-frequency signals.

So how do these crossover systems/networks do what they do? The simple answer is: with filters.

A crossover network with include a high-pass filter and low-pass filter and, depending on the transducer it is feeding, a band-pass filter. These 3 different filter types perform the following:

  • High-pass filter: allow high-frequencies to “pass” by cutting out frequencies below a certain threshold. The outputs of these filters are typically sent to tweeters.
  • Band-pass filter(s): allow frequencies within a specified band to “pass” by cutting out frequencies below one threshold and frequencies above another threshold. The outputs of these filters are generally sent to mid-range speakers.
  • Low-pass filter: allow low-frequencies to “pass” by cutting out frequencies above a certain threshold. The outputs of these filters are typically sent to woofers.
Basic Audio Crossover Network Frequency Filtering

There is much more to know about filters than the basics listed above but that’s enough to know in order to understand balanced armature earphone designs.

The crossover designed into the BA earphones sends the appropriate bands of the audio signal to the appropriate BA drivers.

Filters are used all the time in audio applications. From headphone, loudspeaker and microphone designs to mixing and mastering and musical modulation.

To learn more about the use of filters in microphones, check out my article What Is A Microphone High-Pass Filter And Why Use One?


Examples Of Balanced Armature Drivers

Let’s have a look at a few examples of individual balanced armature drivers/receivers from the two most established brands: Knowles and Sonion.

I’ll add frequency response and impedance graphs to show how different BA drivers exhibit different resonances.

We’ll also discuss the intended uses of the driver units and the number of drivers within the the BA units.

To learn more about headphone frequency response, impedance and sensitivity, check out the following in-depth My New Microphone articles:

What Is Headphone Frequency Response & What Is A Good Range?
The Complete Guide To Understanding Headphone Impedance
The Complete Guide To Headphones Sensitivity Ratings

Knowles HODVTEC-31618-000

  • Driver units: 2
  • Intended use: woofer
  • Impedance: 48 Ω
  • Sensitivity: 130 dB SPL/mW
Knowles HODVTEC-31618-000 Frequency Response
Knowles HODVTEC-31618-000 Impedance

Knowles SWFK-31736-000

  • Driver units: 2
  • Intended use: tweeter
  • Impedance: 6.9 Ω
  • Sensitivity: 100 dB SPL/mW
Knowles SWFK-31736-000 Frequency Response
Knowles SWFK-31736-000 Impedance

Sonion 26A005

  • Driver units: 1
  • Intended use: full range
  • Impedance: 50 Ω
  • Sensitivity: 104 dB SPL/mW
Sonion 26A005 Frequency Response

No Impedance Graph


Pros & Cons Of Balanced Armature Earphones

As is the case with any technology, there are pros and cons to balanced armature headphones. The pros and cons are summed up in the following table:

ProsCons
High efficiency/sensitivityStrong resonance peaks and poor frequency response
Miniature sizeBA earphones typically require multiple BA drivers
Wide variety of BA driver designsOnly available in earphones and in-ear monitors
Passive working principleFragility

Pros Of Balanced Armature Earphones

High Efficiency/Sensitivity

The balanced armature is rather unstably balanced between the magnets of the driver. It does not take much energy to make the armature to move which means it doesn’t take much energy to make the mechanically coupled diaphragm move and produce sound.

Therefore, low-level audio signals may driver rather high sound pressure levels out of the balanced armature driver.

This translates to the BA drivers not requiring amplification. However, their relatively low electrical impedances make it so that they are best driven by ultra-low impedance output sources.

Miniature Size

The small sizes of BA drivers make them excellent candidates for earphones and in-ear monitors. Their miniature designs allow for multiple BA drivers to be utilized in a single earpiece.

Balanced armature drivers can occupy less than half the total volume of the smallest dynamic headphone drivers.

Note that there are actual limitations to the sizes of BA drivers, which I would consider to be a con. We’ll define this shortly.

Wide Variety Of BA Driver Designs

As we learned in the section Tuning The Balanced Armature Driver, BA drivers can be tuned in a variety of different ways.

There are wide-range BA drivers; low-end woofer-style BA drivers; mid-range BA drivers, and high-end tweeter-style BA drivers. The combination of differently tuned drivers allows manufactueres to sculpt the frequency response of their earphones/IEMs.

Passive Working Principle

Balanced armature drivers work on principles of electromagnetism and require no power to function properly. The crossover systems used to delegate different bands of the audio signal to different drivers can also have a passive design.

Cons Of Balanced Armature Earphones

Strong Resonance Peaks And Poor Frequency Response

Balanced armature drivers tend to have multiple resonant peaks in their frequency responses that alter the accuracy of the sound the reproduce from the applied audio signal.

These resonant frequencies cause highly coloured and rather unnatural sounds. Again, these resonances can be caused by any of the following:

  • Size and internal volume
  • Armature mass and stiffness
  • Membrane stiffness and diaphragm mass
  • Armature damping
  • Acoustic filters
  • Sound outlet
  • Tube length and diameter
  • Tube damping

BA Earphones Typically Require Multiple BA Drivers

Due to the rather poor frequency responses of BA drivers, BA earphones/IEMs will often require multiple drivers to achieve a flat and extended frequency response that allows for an accurate reproduction of sound.

Adding multiple drivers adds to the complexity, fragility and cost of the headphones.

Note that, sometimes, a moving-coil dynamic driver is even needed to cover the bass range of the frequency response.

Only Available In Earphones And In-Ear Monitors

This is not to say that BA drivers cannot be used in larger headphones. However, their fragility and overly coloured frequency responses are overly disadvantageous compared to moving-coil, planar magnetic and electrostatic drivers when it comes to use in larger headphones.

Fragility

Rememer that the armature a loosely balanced between two magnets. dropping the BA earphone will likely cause the armature to hit one of the magnets with a strong enough mechanical force to damage the armature or diaphragm. This will cause distortion in the signal at best and render the earphones unusable at worst.


Balanced Armature Earphone Examples

To better understand the practical uses of balanced armature driver in IEMs and earphones, let’s have a look at a few examples.

1MORE Quad Driver IEM

The 1MORE Quad Driver IEM (link to compare prices on Amazon and B&H Photo/Video) BA earphones feature a diamond-like carbon dynamic driver and three balanced armature drivers in each earpiece to produce immaculate transparency, spaciousness, and realism.

There is alos a built-in microphone for easy calls and voice recordings when plugged into smartphones, laptops, etc.

1MORE Quad Driver IEM
  • Frequency response: 20 Hz – 40,000 Hz
  • Sensitivity: 99 dB SPL/mW
  • Impedance: 32 Ω

FiiO FA7

The FiiO FA7 (link to compare prices on Amazon and B&H Photo/Video) BA IEMs has 4 drives in a 4-way crossover. This pair is well-suited for musicians, engineers, and audiophiles.

The design features 4 Knowles balanced armature drivers:

  • CI-22955 for bass (large diaphragm): quick response with low distortion.
  • ED-29689 for mids: full, sweet, and unfatiguing sound.
  • Dual-BA SWFK-31736 for treble: extended and detailed highs for a natural and clear sound.
FiiO FA7
  • Frequency response: 20 Hz – 40,000 Hz
  • Sensitivity: 110 dB SPL/mW at 1 kHz
  • Impedance: 23 Ω (Passive)

FiiO is featured in My New Microphone’s Top 14 Best Earphone/Earbud Brands In The World.

Westone UM Pro 30

The Westone UM Pro 30 (link to compare prices on Amazon and B&H Photo/Video) BA IEMs are designed for personal listening and professional stage monitoring applications. The monitors utilize triple balanced armature drivers in a passive crossover network.

Westone UM Pro 30
  • Frequency response: 20 Hz – 18,000 Hz
  • Sensitivity: 124 dB SPL/mW
  • Impedance: 56 Ω (at 1 kHz)

Shure SE535

The Shure SE535 (link to compare prices on Amazon and B&H Photo/Video) is a pair of high-quality earphones with 3 drivers per earpiece.

These sleek, compact, noise-isolating earphones are ideal for use at home or on-the-road. Each earpiece has three of what Shure calls MicroDrivers (balanced armature drivers). These MicroDrivers include a single “tweeter” and dual “woofers” for a rich and detailed response across the audible range.

Related article: Do Headphones Have Subwoofers & How Do HPs Produce Bass?

Shure SE535
  • Frequency response: 18 Hz – 19,000 Hz
  • Sensitivity: 119 dB SPL/mW
  • Impedance: 36 Ω

Shure is featured in the following My New Microphone articles:
Top Best Headphone Brands In The World
 Top 14 Best Earphone/Earbud Brands In The World
Top 11 Best Microphone Brands You Should Know And Use


Related Questions

What are Neodymium drivers? Neodymium drivers, whether in headphones or loudspeakers, are dynamic drivers that use high-strength rare-earth Neodymium magnets in their design. Typically the title refers to moving-coil dynamic drivers that have Neodymium magnets and work to turn audio into sound via electromagnetic principles.

For more information on headphone magnets, check out my article Why & How Do Headphones Use Magnets?

Are 40mm drivers better than 50mm? In theory, drivers with 50mm-wide diaphragms will produce greater sound levels and more bass than drivers with 40mm-wide diaphragms (assuming the proper amount of power is driving them). However, there are many factors that go into headphone design and the best driver size is typically the size chosen for the particular headphone.

To learn more about headphone driver sizes and the role they play in headphone quality, check ut my article What Is A Good Driver Size For Headphones?

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