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Graphene paves the way for electrostatic earphones

26 Feb 2014  | Ramkumar Ramaswamy

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Electrostatic speakers have been around for some time already—in fact, the earliest ones apparently date back to the 1920s and were built using pig intestine covered with gold leaf. The image they conjure up is one of a large thin sheet which looks esoteric and sounds great, but is delicate and cumbersome to carry around. What if you had an electrostatic speaker that you could not just carry around with one hand but actually insert into your ear? That is what researchers Qin Zhou and A. Zettl at the University of California at Berkeley and the Lawrence Berkeley National Laboratory tried to prove could be possible. Their work was published in Applied Physics Letters, Vol. 102, Issue 22, June 2013.



Electrostatic speakers 101
The principle of operation is quite simple and is based on the fact that unlike charges attract and like ones repel. An electrostatic transducer has for its diaphragm a plastic film, coated or impregnated with an electrically conductive material such as graphite. The impregnation is done in a way that results in a very high surface resistivity though, because the intent is to distribute a static charge on this diaphragm that can stay evenly distributed as the diaphragm vibrates in the electric field. This diaphragm stands between two stators, which are perforated steel sheets coated with an insulator (to make sure the diaphragm does not discharge by touching it). The diaphragm is charged by connecting it to a fixed positive voltage (typically a few kilovolts) and the AC audio signal (again amplified to a few kilovolts) is applied to the stators. Because like charges repel and opposite charges attract, the diaphragm's positive charge will force it to move forward or backward depending on the polarity of the high voltage AC on the stator. One advantage of electrostatic speakers is the fact that since the damping force is almost entirely from the very air that is being driven, the efficiency is much higher than conventional drivers.

One key reason that electrostatic speakers are large is that they operate on the principle that air should provide the primary damping force for the diaphragm. The per-area air damping coefficient significantly decreases when the size of the diaphragm falls below the sound wavelength, and that is why, for sufficient low frequency response you need a large diaphragm with traditional materials such as metalized Mylar. The only way to make them smaller is to make the diaphragm thinner and lighter, and you cannot do that beyond a point because the material will simply break apart. A bit of math helps understand exactly what forces are responsible for SPL behaviour. In the equation on the left below, the left side represents the velocity of the diaphragm. On the right side, F is the driving force, ζ is the damping force, k the spring (restoring) force of the diaphragm, and m the mass of the diaphragm. In the equation on the right, c is the sound velocity and ρ the air density.



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