Noise-canceling Electrostatic Headphones for fMRI

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Electrostatic Transducers

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Electrostatic Transducers

What are Electrostatic Transducers?

Electrostatic Transducers can easily be identified by their flat, thin (often tall) profile. They basically consist of a very light membrane (often Polyester film) suspended between two conductive plates (perforated metal sheeting/wire mesh). In operation, the membrane is charged to a high voltage, and a differential audio signal is sent to the two conductive plates, varying the charge on the plates and exerting an electrostatic force on the membrane. The membrane then moves, moving the air around it.

Why use electrostatics?

Aside from the limitations within the MRI chamber, conventional dynamic loudspeakers suffer from a number of inherent problems. A dynamic driver design is a tradeoff between inversely related driver mass and rigidity; higher mass equals higher driver inertia (limited frequency response, poor transient response), and lower rigidity equals more driver deformation (harmonic distortion). The necessity of having multiple drivers to reliably cover a wide frequency range is related to these properties of mass and rigidity, and comes with its own problems in the form of time/phase alignment issues, and dispersion.

Electrostatic transducers circumvent the issues with mass and rigidity. The driving diaphragm is very thin and light--with mass on the order of a few millimetres thickness of air. Because of this, diaphragm inertia is insignificant. Also, because driving force is uniform over its entire surface, diaphragm deformation is not an issue. The only design-related nonlinearity is at the edge of the diaphragm, where it is suspended. The flexibility of the diaphragm makes that insignificant as well.

Problems with electrostatics

First and foremost, the electrostatic drive principle requires very high voltages to operate. At the headphone level, bias and signal voltages range from 200 to 800 volts, while large electrostatic loudspeakers can use up to 8000 or 9000 volts. Conventional amplifiers must be coupled through a high-quality transformer to raise the output voltage to suitable levels, and even then, must be exceptionally stable to perform well because of the reactive load (electrostatic transducers act as large capacitors). Bias voltage must be supplied separately--in the case of loudspeakers, an additional wall plug is usually required for each speaker.

In loudspeakers, frequency response on the low end is limited by a very low diaphragm excursion--necessarily so because the relatively low-magnitude electrostatic force requires a short gap between diaphragm and stator (on the order of millimeters). High bias voltages also run the risk of arcing between diaphragm and stator, damaging both. The transducer's dipole nature also results in cancellation of longer wavelengths, as most loudspeakers are built without a baffle.

Electrostatic headphones

In a headphone, many electrostatic-related issues are irrelevant. At the headphone power level, designing a stable, suitable high voltage amplifier is easier (though not trivial by any means). The concern of having open (high) voltages around your head is unsettling, but not particularly serious.

Low frequency response problems are more or less irrelevant in a headphone design, as far less excursion is required of a headphone driver. With acoustic dampening, it isn't too difficult to get a headphone with superb frequency and transient response, and very low distortion. In effect, applying electrostatic principles to headphone design (with good workmanship) almost guarantee high performace.

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