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Schematics of the graphene-based EDGS speaker. A graphene diaphragm, biased by a DC source, is suspended midway between two perforated electrodes driven at opposite polarity. The varying electrostatic force drives the graphene diaphragm which in turn disturbs air and emits sound through the electrodes. The light mass and low spring constant of thegraphene diaphragm, together with strong air damping, allow for high-fidelity broad-band frequency response. Such a speaker also has extremely high power efficiency.
Images of (a) 7 mm diameter graphene diaphragm suspended across annular support frame, (b) actuating electrodes, and (c) assembled speaker.
Frequency response of various miniature audio speakers. (a) Graphene diaphragm EDGS speaker (this work); (b) commercially available Sennheiser® MX-400 magnetic coil speaker; and (c) thermoacoustic speaker. The points in panel (c) are experimental data from Ref. 13 , while the solid red line is the theoretically predicted behavior for an ideal thermoacoustic speaker. The EDGS speaker performs noticeably better than the commercial voice-coil speaker at high frequencies, both in terms of maintaining high response and avoiding sharp resonances (the slow oscillations in the EDGES curve are due to sound wave interference in the space between the speaker and microphone and they depend on the relative position of the speaker and microphone, but the main trend is consistent). In the low frequency region, both EDGES and the MX-400 perform well, while the thermoacoustic response falls precipitously already below 15 kHz. The decrease in the response curves in (a) and (b) at very low frequency is largely due to limited capability of the microphone and the inefficient coupling between the speaker and microphone.
Vibration velocity of graphene diaphragm in EDGS v.s. frequency, measured by LDV. Such a measurment is useful because it eliminates extrinsic effects (e.g., acoustic structural design, sound card, microphone response), and represents the “pure” response of the graphene diaphragm itself. Within the error limit of the LDV setup, the response curve appears to be quite flat, indicating that graphene serves as an ideal key component for loudspeakers.
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