Of all the headphone types in the world, the electrostatic headphones are likely the least understood and the most exotic. These high-end headphones are enjoyed by many audiophiles who are willing to spend the money on a pair.
What are electrostatic headphones? Electrostatic headphones are transducers that convert audio signals (electrical energy) into sound waves (mechanical wave energy) and work on electrostatic principles. Audio signals cause varying electric fields in the driver that move the diaphragm and produce corresponding sound waves.
In this article, we’ll dive into how electrostatic headphones work in more detail and discover how these rare but incredible headphones function.
What Is A Headphone Driver?
Before we get into the finer details of how electrostatic headphones work, let’s go over the definition of a headphone driver.
Headphone drivers are the transducer elements that convert the audio signals (electrical energy) into sound (mechanical energy). The vast majority of headphones will have two drivers designed to accept stereo audio.
Headphone drivers require analog audio (rather than digital) to function properly. Analog audio signals are electrical signals with alternating current.
When the headphones are properly connected, each driver becomes part of a circuit that passes these AC signals from the connected audio source (headphone amp, smartphone, etc.) to the headphones.
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
Digital audio must be converted into analog audio to drive the headphones. DACs (digital-to-analog converters) are typically 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
The driver is designed, in one way or another, to accept the audio signal and use its energy to vibrate a diaphragm that produces sound. The diaphragm ought to create sound waves that accurately depict the information of the audio signal.
How does this happen? Well, there are multiple working principles for different headphone driver types.
To read about all the different headphone driver types, check out my article What Is A Headphone Driver? (How All 5 Driver Types Work).
The vast majority of headphones work on principles of electromagnetism. Moving-coil dynamic headphone drivers, which are the most common, use conductive coils while planar magnetic headphones use conductive wires embedded onto the diaphragm. Balanced armatures use a conductive coil wrapped around a moveable armature.
Bone conduction headphones utilize piezoelectricity and piezo crystals.
Electrostatic headphones, which we’ll get to now, work on electrostatic principles.
The Electrostatic Principles Governing Electrostatic Drivers
Let’s cover some theory before we get into the design details of electrostatic headphone drivers.
Let’s start with the basics.
Static electricity is the collection of electrically charged particles on the surface of a material.
Like electrical charges repel each other while opposite electrical charges attract each other.
This is similar to how magnets work. Remember that electomagnetism is the working principle of most headphone drivers. There are definitely parallel to be drawn between electrostatic and electromagnetic headphone driver principles.
If two positively charged objects are brought near to one another, they will experience forces that push them apart due to the electric fields that surround them. Similarly, if two negatively charged objects are brought near one another, they, too, will repel.
Conversely, an object with a positive charge and another with a negative charge will attract one another and they will experience forces that pull them together due to the electric fields that surround them.
Direct current is defined as the unidirectional flow of an electric charge. DC bias can be used to produce a fixed electric charge within a material. This is one of the ways in which the diaphragms of electrostatic drivers maintain a fixed electric charge.
Electret material is defined as a dielectric material that has a quasi-permanent electric charge and can also be used to hold a fixed charge on an electrostatic diaphragm.
The diaphragm of an electrostatic headphone driver is generally a thin polyester film with a conductive coating. As mentioned, it can be biased with an external high voltage supply or by an electret design.
Though not an electrostatic principle, it’s worth restating that analog audio signals, which drive headphone drivers, are electrical signals with alternating currents. Their signal strength is typically measured in volts (V or mV); decibels relative to 1 volt (dBV), or decibels relative to 0.775 volts (dBu).
This is important because capacitors block DC but allow AC to pass. The stator plates in the electrostatic driver design are essentially parallel plate capacitors.
Capacitors are passive two-terminal electronic components that store electrical energy in an electric field.
What an alternating current (like an audio signal) passes in a circuit with a capacitor (like the stator plates of an electrostatic headphone driver), the capacitor’s plates, at any given moment, have equal but opposite electrical charges.
So when the current flows in one direction, the plates are charged in a certain polarity. The amount of charge is relative to the strength (voltage) of the signal.
When the current flows in the opposite direction, the charges flip. Again, the amount of charge is equal but opposite and the strength of the charge is relative to the voltage of the AC audio signal.
Note that there are instants when there is no current when the flow switches directions. At this point, the charges on the plates will quickly dissipate to zero as well before switching to mimic the flow of the current.
No electrons travel between the plates. Therefore, there is no electrical current between the stator plates, even though they can effectively close a circuit.
With that primer, let’s get into the details of how electrostatic headphones work.
How Do Electrostatic Headphone Drivers Work As Transducers?
Let’s begin by stating that the defining factor of electrostatic headphones is the electrostatic driver.
Therefore, we’ll be focusing more so on the driver than the form factor (open-back vs. closed-back; supra-aural vs. circumaural, etc.). The being said, the majority of electrostatic headphones are indeed circumaural. Many of which have open-back designs.
Note that that overwhelming majority of electrostatic headphones have two independent drivers: one designed to transduce the left channel of a stereo audio signal and the other to transducer the right channel.
To learn about 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
Electrostatic drivers have incredibly high impedance. Therefore, electrostatic headphones require specialized amplifiers to boost the voltage of the audio signal way up while also dropping the current of the signal to safe levels. These amps also often play the role of biasing the diaphragm with a high DC voltage (unless the driver has an electret design).
Note that electret headphones are pretty much relegated to vintage Numark, STAX, AKG and Audio-Technica designs.
The Electrostatic Headphone Driver
Let’s begin by having a look at a simplified cross-sectional diagram of an electrostatic headphone driver:
As we can see, there are 3 key components of the electrostatic headphone driver:
- Stator plates (also known as grid plates)
- Insulators (sometimes called spars)
The diaphragm is a very thin movable membrane that is typically made of a polyester film. This diaphragm is coated in a conductive coating.
In the electrostatic driver design, the diaphragm must hold a constant electrical charge. This is essential if the diaphragm is to move relative to the audio signal applied at the stator plates.
DC biasing is one method of providing the fixed electrical charge on the diaphragm. The specialized amplifier will supply a high-level DC voltage to the diaphragm and the conductive coating will hold a fixed positive electric charge.
This diaphragm is effectively “sandwiched” between two stator plates. The diaphragm and plates are insulated from each other via spars at the outer edge of the diaphragm. There is just enough space between the diaphragm and the plates for the diaphragm to vibrate and produce sound without touching the plates.
The insulation is incredibly important to keep the DC biasing voltage from reaching the plates while also keeping the audio signal at the plates from reaching the diaphragm.
Speaking of the stator plates, these grids act as a sort of parallel capacitor.
This capacitor becomes part of the circuit that passes the amplified audio signal when the headphones are connected to their specialized amplifier. One electrical lead wire connects to one of the stator plates and another lead wire connects to the other plate.
These stators are made of conductive metal and are designed to hold electric charges. At any given point when an audio signal (alternating current) is flowing through the plates, one plate will have a positive electric charge while the other has an equal but negative electric charge.
So, the diaphragm hold a fixed positive electric charge and the audio signal alternates the charge of the plates between negative and positive.
At any given time, one stator plate will have a positive charge with an electric field strength relative to the voltage of the amplified audio signal. At the same time, the other stator plate will have an equal but opposite charge and electric field strength.
These electric fields interact with the positively charge diaphragm. The diaphragm will be attracted to the negatively charged stator while simultaneously being repelled by the positively charged stator.
Because the diaphragm is lighteight, thin and movable, the alternating electric fields of the stator plates causes the diaphragm to move back and forth between the plates.
This diaphragm movement can be summed up by the following illustration.
So as the audio signal alters the charges on the stator plates, the stator plates cause the diaphragm to move and produce sound.
This is how the electrostatic headphone driver transducer converters audio signals (electrical energy) into sound waves (mechanical wave energy)!
Because the diaphragms are completely coated with a conductive material, the entire diaphragm is subjected to the attraction and repulsion of the stator plates.
This seemingly small detail causes the diaphragms to produce planar sound wavefronts. This is in contrast to the moving-coil drivers, where the coil is focused toward the centre of the diaphragm and causes spherical wavefronts.
Essentially, planar wavefronts sound more natural to our ears. This slightly more natural sound actually causes noticeably effects when the sound source (driver) is in close proximity to our ears (as is the case with headphones).
The AC audio signal is representative of sound and humans hear sound in the audible range of 20 Hz – 20,000 Hz (cycles per second). Sound waves, and the analog audio signals that represent them electrically, are made of many waves with frequencies within this range.
This means that the audio signal will complex diaphragm vibrations ideally in the range of 20 Hz – 20,000 Hz. That being said, electrostatic drivers are typically capable of producing frequencies that extend far beyond the human range of hearing.
To learn more about headphone frequency response, check out my article What Is Headphone Frequency Response & What Is A Good Range?
It’s critical for these plates/grids to be perforated so that the sound waves produced by the driver can actually escape the driver and reach the ears of the listener.
Before we get started on our discussion of the amplifiers, I’d like to mention that electrostatic headphones do not use the same phone connectors that are typically used by other headphones.
To learn more about the phone connectors of headphones, check out my articles How Do Headphone Jacks And Plugs Work? (+ Wiring Diagrams) and Differences Between 2.5mm, 3.5mm & 6.35mm Headphone Jacks.
Rather, electrostatic headphones generally use 4-pin or 5-pin connectors to connect to balanced outputs and/or to receive the DC biasing voltage for their diaphragms.
The Electrostatic Headphone Amplifier
Electrostatic headphone drivers require very high voltage signals. These signal levels are unnattainable with typical headphone jacks and even with professional headphone amplifiers.
Moving-coil dynamic headphones, which are by far the most common, have typical nominal impedance ratings between 8 Ω – 600 Ω and sensitivity ratings between 90 dB SPL/mW and 105 dB SPL/mW.
Converting these sensitivity rating to dB SPL/V using the 8 Ω – 600 Ω impedance range gives us an alternate sensitivity range of 92.2 dB SPL/V to 126.0 dB SPL/V. These calculations are based on hypothetical headphones with the the specs of 90 dB SPL/mW + 600 Ω and 105 dB SPL/mW + 8 Ω, respectively.
Equation used: SV = SP + 20•Log(sqrt(1000/Z)).
Electrostatic drivers, on the other hand, have extremely high impedance ratings typically above 100 kΩ (100,000 Ω) and sensitivity ratings around 100 dB SPL/100V.
Coverting 100 dB SPL/100V yields about 60 dB SPL/V.
This seemingly extreme impedance is required to keep the electric charge from escaping the driver.
The incredibly low-sensitivity tells us that the electrostatic driver needs audio signals with very his voltages. Amplification is the only way to boost the signals to the voltage levels required.
For much more detailed information on headphone sensitivity and impedance, check out my articles The Complete Guide To Headphones Sensitivity Ratings and The Complete Guide To Understanding Headphone Impedance, respectively.
The bottom line is that electrostatic headphones need specialized amplifiers to drive their drivers properly.
So how are electrostatic headphone amplifiers designed and how do they work?
Well, there are plenty of electrostatic headphone designs including solid-state and tube electronics.
Note that different electrostatic headphones amplifiers will sound different from one another.
The HIFIMAN Shangri-La SR electrostatic headphone amplifier (link to check the price at B&H Photo/Video) costs about $32,000 USD.
The variety of exact electrostatic headphone designs is beyond the scope of this article. Rather, we will discuss the 3 key functions of electrostatic headphone amplifiers.
The 3 key functions of electrostatic headphone amplifiers are:
- Amplify the audio signal
- Boost the voltage of the signal
- Suppy the diaphragm with bias voltage
The bulk of an electrostatic headphone amplifier works similarly to regular headphone amplifiers.
Op-amps; vacuum tubes; solid-state (transistor-based) electronics, and other common amp circuit components are used in electrostatic headphone amps as well.
This main amplifier stage brings up the signal to a healthy level with low impedance. Normally these amp stages are used to properly match the output to the impedance of the connected headphones.
In the case of electrostatic headphone amps, the main amp circuit readies the signal to be sent to the step-up stage before being sent to the electrostatic headphones.
To learn more about headphone amplifiers, check out my article What Is A Headphone Amplifier & Are Headphone Amps Worth It?
The step that is unique to the electrostatic headphone amplifier is the step-up stage that cranks up the voltage of the audio signal to properly charge the stator plates. As mentioned previously, the voltage of the signal that is sent to the electrostatic driver is magnitudes greater than the voltage sent to “regular” headphone drivers.
This step-up stage has historically been done with a step-up transformer. Some designs can do away with the transformer and replace it with a transformerless transistor-based active circuit.
We’ll use the step-up transformer to explain the step-up stage. A simplified diagram of a step-up transformer is presented below:
- MC: magnetic core
- P: primary winding
- S: secondary winding
The transformer is a passive component that has the same working principle that governs the drivers of “normal” moving-coil dynamic headphones. That working principle is electromagnetism.
We’ll go through transformers briefly here.
The audio signal (AC voltage) passes through the primary coil and causes a coinciding magnetic field.
This magnetic field affects the magnetic core and is essentially transferred to the secondary coil.
The varying magnetic field around the coil produces a voltage within the coil via electromagnetic induction.
The secondary coil is designed to have many more turns than the primary coil. More turns in a coil mean that a greater voltage can be induced across it given the same varying magnetic field.
In fact, neglecting any losses due to inefficiency, the turns ratio is linearly linked to the voltage ratio. The equation is as follows:
Vs/Vp = Ns/Np
Vs = voltage across the secondary coil
Vp = voltage across the primary coil
Ns = number of turns in the secondary coil
Np = number of turns in the primary coil
As the step-up transformer boosts the voltage, it simultaneoulsy drops the current. The current difference is also linearly linked to the turns ratio and is defined by the following equation:
Is/Ip = Np/Ns
Is = current in the secondary coil
Ip = current in the primary coil
Ns = number of turns in the secondary coil
Np = number of turns in the primary coil
A step-up transformer also increases the impedance of the output (secondary coil). The impedance is boosted much more than the voltage:
Zs/Zp = (Ns/Np)2
Zs = impedance of the secondary winding
Zp = impedance the primary winding
Ns = number of turns in the secondary winding
Np = number of turns in the primary winding
So even though the electrostatic headphone amp amplifies the audio signal to a healthy level, a step-up stage is required to crank up the voltage to the rather extreme levels required of the electrostatic drivers.
Therefore, if a transformer is being used, the turns ratio should be extremely high. Quality transformers with turns ratios of 25:1 or more are likely used in these amplifiers.
The iFi Pro iESL (link to. check the price on Amazon) is another example of an electrostatic headphone amplifier. This one is much more “affordable” than the aforementioned HIFIMAN model:
In addition to these two steps, the electrostatic headphone amplifier is commonly tasked with supplying the diaphragm with its required DC biasing voltage so that it holds a fixed charge.
These DC bias voltages are generally in the range of 200 VDC to 700 VDC (Volts Direct Current).
A decent amount of power is needed to run these amplifiers and so the amps are typically plugged into the wall.
The bare-bones setup of an electrostatic headphone driver is as follows:
In the above diagram, we see the step-up stage and the audio signal lead wires connecting to the stator plates. We also see the EHT (extra high tension) or bias voltage is applied directly to the diaphragm.
For more information on the power needs of headphones, check out my article How Do Headphones Get Power & Why Do They Need Power?
The electrostatic transducer design extends to electrostatic loudspeakers.
Additionally, the principles that govern the electrostatic transducers in headphones and loudspeakers also apply to microphones. These “electrostatic microphones” are known as condenser microphones and are designed to convert sound into audio rather than audio into sound.
Note that condenser microphones come in externally biased “true” designs as well as 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
Pros & Cons Of Electrostatic Headphones
As with everything in life, there are pros and cons to electrostatic headphones. The pros and cons are summed up in the following table:
|Low distortion||Require specialized headphone amplifiers|
|Excellent transient response||Poor portability|
|Wide frequency response||Expensive|
|Large diaphragm with planar sound wavefront||Larger and heavier|
|Adjustable sound via different amplifiers|
Pros Of Electrostatic Headphones
Electrostatic headphones are cherished for their low distortion and incredible accuracy in sound reproduction. The ultra-thin diaphragm is completely reactive across its entire area and the stator plates are directly connected to the audio signal.
Excellent Transient Response
The larger diaphragm is very responsive which yield incredible transient response and a tight bass response that really allows music to shine.
Wide Frequency Response
Along with an incredibly accurate transient response, the typical electrostatic driver has an extended frequency response. Oftentimes, this frequency response spans multiple octave blow and above the range of human hearing (20 Hz – 20,000 Hz).
Large Diaphragm With Planar Sound Wavefront
The large, flat and completely conductive diaphragm moves very planarly and produces nearly perfect planer wavefronts. These sound waves react with our ears very naturally. This helps to explain the aforementioned-mentioned pros on this list.
Adjustable Sound Via Different Amplifiers
Electrostatic headphones are largely dependent on their amplifiers. Therefore, one pair of electrostatic headphones may yield different charactersitcs when paired to different amplifiers.
In a way, this can be considered both a pro and a con, since it opens the door even more for subjectivity. Additionally, electrostatic headphone amps are very expensive and owning several to experience the different flavours of the headphones may be way out of our budgets.
Cons Of Electrostatic Headphones
Require specialized headphone amplifiers
As we’ve discussed in this article, electrostatic headphones absolutely require their specialized amplifiers. Without them, the headphones are lacklustre at best and abosulately attrocious at worst.
Once again, the amplifiers are very expensive, which increases the cost of already costly headphones.
The power hungry amplifier and the rather large headphone design make electrostatic headphones quite stationary.
We’ll get into this more in the next section, but electrostatic headphones are very expensive and out of the question for many people.
Larger and heavier
The large diaphragms required large housings. Some people do not like the larger and sometimes heavier deisgns.
Electrostatic Headphone Examples
Now that we understand how electrostatic headphones work, let’s have a look at a few examples.
STAX SR-007A MK2
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.
These powerful and accurate headphones bring out the tiniest details in a recording when connected to the right amplifier(s).
- Frequency response: 6 Hz – 41,000 Hz
- Sensitivity: 580 VDC
- Impedance: 170 kΩ (at 10kHz)
- Bias voltage: 100dB / 100V r.m.s. 1 kHz
HIFIMAN Shangri-La Sr
The HIFIMAN Shangri-La Sr (link to check the price at B&H Photo/Video) is an electrostatic headphone with a circumaural closed-back design,
These rediculously expensive headphones are designed to reproduce audio with impeccable clarity and highly detailed dynamics and frequencies.
- Frequency response: 7 Hz to 120 kHz
- Sensitivity: No info
- Impedance: No info
- Bias voltage: 550-650 VDC
The Shure KSE150 (link to compare prices on Amazon and B&H Photo/Video) is a pair of electrostatic earphones that come with their own portable amplifier/DAC capable of converting resolutions up to 24-bit / 96 kHz.
The KSE1500 is a premium electrostatic sound-isolating earphone and amplifier system designed to deliver the high-resolution audio to “on-the-go” audiophiles and audio enthusiasts.
- Frequency response: 10 Hz – 50,000 Hz
- Sensitivity: No info
- Impedance: No info
- Bias voltage: 200 VDC
The STAX SR-009 (link to check the price on Amazon) is an electrostatic headphone with a circumaural closed-back design. It is considered by the few who have listened through it to be one of the greater pairs of headphones of all time.
- Frequency response: 5 Hz – 42,000 Hz
- Sensitivity: 145 kΩ (including cable, at 10kHz)
- Impedance: 101dB / 100V r.m.s. 1 kHz
- Bias voltage: 580 VDC
These brands are featured in My New Microphone’s list of the Top Best Headphone Brands In The World.
How does an electrostatic speaker work? Electrostatic loudspeakers convert audio signals into sound waves via electrostatic principles. They work very similarly to electrostatic headphones, though their drivers are much larger and require much higher signal voltage to charge their stator plates and cause their diaphragm to move and produce sound.
How do you stop headphone static/hiss? Static noise and hiss in good quality headphones are typically a result of a poor digital-to-analog converter in a digital audio device or a dirty headphone jack. Try upgrading the sound card or using a different audio device or cleaning the headphone jack with a cotton swab with isopropyl alcohol.