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Graphene paves the way for electrostatic earphones

26 Feb 2014  | Ramkumar Ramaswamy

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What this tells you is that the SPL is constant with frequency if k and m are small enough so that ζ is the only significant parameter. Of course that is an idealisation but it gives you the sense. The thinner the diaphragm, the smaller become k as well as m.

Enter graphene
Graphene is a recently discovered material that can be described as a one-atom thick layer of graphite. This material has exceptional mechanical strength, with a breaking strength over a hundred times greater than a hypothetical steel film of the same thickness. It would take an elephant, balanced on a pencil, to break through a sheet of graphene the thickness of cling film! The Wikipedia entry on this miracle material lists a large number of uses for it. What is interesting is that a graphene diaphragm of the same size and thickness as a diaphragm made of Mylar has a very high spring constant – something that is not obvious from the paper. What graphene allows you to do is to build the diaphragm enormously thin and therefore reduce k as well as m, because k is proportional to thickness. This is what made Zhou and Zettl reason that it should work well as a small diaphragm for an electrostatic transducer. Let us see what they found.

The researchers fabricated a graphene diaphragm no larger than 7mm in diameter and a mere 30nm in thickness. The details of the fabrication process are described in their paper. Graphene is an excellent conductor of electricity, so we do not need a further impregnation or coating of conductive material on the diaphragm. The conducting graphene diaphragm was biased at a fixed DC voltage of 100V (which is substantially lower than the kilovolts voltages used with typical electrostatic transducers), and the diaphragm was positioned between two fixed silicon electrodes onto which the AC signal of about 10V was applied. This is illustrated in the diagram above taken from their paper.

They compared the performance of this earphone with a commercially available, non-electrostatic device. As the graph from their paper shows, the performance is rather good for what is essentially a crude laboratory prototype with no special acoustic design. What is also very interesting is that the efficiency of this transducer, operating at a few nanoamperes, was close to 100%, in stark contrast to conventional earphones that operate at less than 1% efficiency.

If you are not in the habit of reading research journals, don't be misled by the fact that this work was published in a journal that goes by the name Letters. This is cutting edge physics, immediately applicable to the real world. Even with no further progress on fidelity enhancement, one can immediately foresee the use of this technology in the design of a new generation of super-efficient hearing aids that are powered by, say, motion, or ambient light.

As I see it, one of the aspects that would make for some interesting future research is whether one could get better fidelity with an insulating film reinforced with graphene in such a manner as to make a diaphragm with high surface resistivity as with current electrostatic speakers, but much thinner and stronger (therefore making a smaller size possible) rather than a pure graphene diaphragm which is highly conductive.

About the author
Ramkumar Ramaswamy earned his M.Sc in Physics from Delhi University in 1990 and his PhD in Operations Research from the Indian Institute of Management Calcutta in 1994. He built his first audio amp at the age of 12 using the popular TBA810 which, much to his delight then, instantly became a local MW receiver too.

To download the PDF version of this article, click here.

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