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What’s the Deal with Electrostatic Headphones? Are they better than Planar Magentic?

Charge and Bias and Stators-- Oh My!

While dynamics and planars make up the majority of headphones used by audiophiles, the different flavor of resolution and airy soundstage offered by electrostatics sometimes makes them the headphone of choice. Since electrostatics have no magnets and are very light weight, the transducers seem to just melt away, providing an extremely comfortable and immersive auditory experience.

The principle behind the operation of an electrostatic headphone is quite simple. The electrostatic transducer consists of a diaphragm with a constant charge, sandwiched between two acoustically transparent stators separated by spacers. The music signal is applied to the stators in the form of equal but opposite high voltage pulses. In other words, when a positive voltage is applied to one stator, an equal but opposite negative voltage is applied to the other stator. As a result, the diaphragm is pulled towards the stator with negative voltage and pushed away from the stator with positive voltage. Thus the diaphragm moves with the music signal and this movement produces sound.

A transducer designed in this fashion is said to be in push-pull configuration. When properly implemented, this results in the same amount of force over the diaphragm, irrespective of where the diaphragm is between the stators, which produces a linear response that does not change with gain and has vanishingly low distortion.

In a conventional electrostatic transducer, the diaphragm consists of a thin film with resistive coating that holds a charge. Variations in the thickness of this resistive coating cause non-uniform driving force which results in distortion. Coating both sides of the diaphragm can make this worse because thickness variations between the two coatings will lead to even more distortion. The resistive coating material needs to be very stable and not deteriorate or lose adhesion over time, since this will result in degradation of performance.

Designed with Purpose

The original source of inspiration for our exploration into electrostatic technology was to improve the quality of care for MRI patients. We collaborated with researchers from the UCLA Semel Institute of Neuroscience and Human Behavior, along with the industrial designers at BoomBang, to create a patient-friendly headset for use in MRI machines. The goal was to make it easier for doctors to deliver instructions to their patients while improving the accuracy and comfort of the MRI scan process. [Read more about the challenges of making the medical headset here.] Based on the high performance of the medical headset, we decided to transpose this technology to a consumer audiophile headphone, which we call CRBN. 

There are a number of design considerations to get the best out of an electrostatic transducer. With CRBN, we set our goals very high. We wanted to design an electrostatic headphone that is efficient, with imaging, clarity, resolution and transparency that are best-in-class. We also wanted a frequency response that is neutral, with smooth treble and well extended bass (this last one is not easy!). We optimized every aspect of the electrostatic transducer design to meet these design goals.

Why Carbon Nanotubes?

CRBN is the first consumer electrostatic headphone in the world to use our innovative, Patent-pending carbon nanotube suspended diaphragm technology. This carbon nanotube technology solves many of the major challenges associated with electrostatic transducers.

Instead of using a resistive coating for the diaphragms, CRBN uses an ultra-thin diaphragm with carbon nanotubes directly suspended inside the material. By controlling the density of carbon nanotubes in the film, we are able to adjust the resistivity of the diaphragm material. Because the charge is uniformly distributed throughout the film, this results in a uniform driving force without the distortions associated with typical resistive coating. Unlike resistive coating, the carbon nanotube suspended diaphragm is immune to the elements and does not degrade over time.

OK Cool, but Why is CRBN Special?

Electrostatic headphones have a reputation for less than stellar bass response. The Audeze solution to this well-known issue involves several angles. Our carbon nanotube diaphragm allows us to achieve high efficiency by holding more charge and preventing charge migration. This allows us to reduce diaphragm tension and increase the space between the stator and diaphragm, which provides for higher excursion and lower resonant frequency without the risk of the diaphragm coming into contact with the stator. This low resonant frequency, combined with a large diaphragm surface area, acoustically transparent damping material, and custom designed earpads provide extremely well-extended bass response down to 20Hz.

In designing the CRBN driver, we chose an optimal diaphragm and stator size large enough to provide excellent bass and imaging without compromising on treble. Our stators are made using a proprietary PCB manufacturing technique that allows us to achieve high rigidity, uniform thickness and consistent flatness, which is important for equal force distribution and linear response.

The perforation of the stator is optimized for excellent high frequency extension and its conductive surface is specially coated to ensure high dielectric strength, allowing for an open airy sound while maintaining high efficiency and durability. Our super lightweight diaphragm has naturally low interia that allows for extremely fast acceleration. We employ a special edge-damping technique to further reduce distortion and smooth the frequency and phase response.

As a result, CRBN is transparent and highly resolving across the entire audible spectrum, with excellent transient response, sparkling clarity, and bass response that’s unmatched in the electrostatic arena. Another example of Audeze’s commitment to Uncompromised Audio.

UPDATE: Our lead engineer has chimed in with this excellent comparison of Electrostatic and Planar Magnetic as it applies to Audeze headphones:
Planar magnetic and electrostatic headphones are very similar by nature of how they produce sound: both use a flat, flexible diaphragm which is moved by electromagnetic or electrostatic force. All similarities end here, since the nature of these two forces is completely different.

Electrostatic force is limited by the physical dimensions of the design, reducing the distance between the diaphragm and stators increases the force but also increases the possibility of sticking the diaphragm to one of the stators. By increasing the tension on the diaphragm, you reduce the stickiness but you lose low frequencies. It's not easy to find the right balance between these two contradictory demands. Electromagnetic force is limited only by the strength of the magnets used and the configuration of the magnetic circuit. No stickiness or arcing and if your amplitude is within the mechanical limits you can achieve much higher sound levels than with electrostatics. With our more efficient headphones (LCD-MX4, LCD-X, LCD-4z) you can easily reach 130 dB SPL (not recommended!) and we did it unintentionally on some occasions. Basically, planar headphones can have much higher dynamic range than electrostatic headphones and that is their big advantage. This comes with a heavy price though - the added weight of the magnets. Here are some examples:

CRBN driver - 75 grams each
LCD-5 driver - 97 grams each
LCD-2 driver - 158 grams each
LCD-4 driver - 186 grams each

On the other hand, the laws of physics are on the side of electrostatic headphones. Namely, acceleration is directly proportional to the moving mass. Electrostatic headphones have much lighter diaphragms than planar headphones. How much lighter depends on the type of film which is used as a substrate and how much metal (usually aluminum) is attached to it. In our nano-scale diaphragms the metal (not always aluminum) is vacuum deposited directly on the film. In all other cases aluminum is laminated to the film, introducing a layer of adhesive in between. There is a difference between metal thicknesses as well, aluminum is very difficult to find thinner than 4.5 microns (at least in quantities needed for headphones production, if you are willing to order hundreds of tons it’s a different story) while a vacuum deposited layer of metal can be easily applied in sub-micron thicknesses. Generally speaking, electrostatic diaphragms are about half the weight of nano-scale diaphragms which in turn are approximately five times lighter than laminated (ultra-thin) diaphragms. A lighter diaphragm leads to faster, more accurate sound.

There is one more factor in favor of electrostatic headphones, driving force is uniform across the diaphragm while planar headphones have driving force only under the surface of the conductors, which never cover the whole diaphragm surface. There are always parts of diaphragm that are just passively following and sometimes even partially moving out of phase. Another disadvantage of planar drivers is that sound waves have to go around the magnets and when they combine with the sound coming from directly exposed portions of the diaphragm it can lead to some loss of details and transparency. We mostly alleviated that problem with Fazors, but it's not completely gone.

Advantages and disadvantages are on both sides, what is more important is up to everyone to decide for themselves.