Graphene Earphone
December 16, 2014, IEDM, San Francisco—Wentian Mi from Tsinghua University described earphones that don’t use magnets for the conversion of electrical energy to sound. This change in the underlying conversion mechanism from electro-mechanical to thermal will be an influence in the $8 B earphone market that is still growing at about 5 percent a year.
The first thermo-acoustic earphones used carbon nano-tubes on silicon, but the technology lacked drive capabilities and fidelity. The second generation used a graphene film on silicon and were clear, but also had fairly poor performance. This latest version uses multiple layers of grown graphene on a PET substrate to overcome the limitations and performance issues of the previous versions.
The characterization of the new materials in the earphone shows interesting features. The membrane material is now clear, but transparency is a function of the number of graphene layers. The sheet resistance is a nonlinear function of the number of layers, starting at 90 ohms per square for a single layer of graphene dropping to about 30 ohms per square with 6 layers. The non-linear change in sheet resistance may be a function of inter-layer cross-linking.
Acoustic performance is very good. The earphone membrane is fairly flat from 20 to 50 kHz with some drop at the higher frequencies. One issue is some type of thermal leakage the exhibits in devices with 6 or more layers of graphene. This leakage may also be a result of inter-layer interactions.
The frequency response is broad and flat from20 to 20kHz but there are some frequency doubling artifacts in the response. The artifacts are minimized by applying a DC bias to the earphone, and other work will consider different shaped membranes to further reduce the artifacts. Open air tests indicate some directionality as a function of increasing frequency, but this should not be a problem for earphones that are driving a verily small volume of air. The current version can deliver a SPL of 70 dB with a 1 W drive signal.
The thermo-electric principle and relative efficiency make these earphones similar in performance to the state of the art earbuds. Future work will consider higher frequency response for ultrasound applications, pulse density modulation, and nano-scale sound generators.
