Loudspeakers work by vibrating a thin diaphragm. These vibrations then create pressure waves in surrounding air that produce different sounds depending on their frequency. The human ear can detect frequencies of between 20 Hz (very low pitch) and 20 KHz (very high pitch), and the quality of a loudspeaker depends on how flat its frequency response is – that is, on the ability of the design to deliver a constant sound pressure level from 20 Hz to 20 kHz.

“Thanks to its ultralow mass, our new graphene loudspeaker fulfils this important requirement because it has a fairly flat frequency response in the human audible region,” team leader Alex Zettl told nanotechweb.org. “The fact that graphene is also an exceptionally strong material means that it can be used to make very large, extremely thin film membranes that efficiently generate sound.”

Because the graphene diaphragm is so thin, the speaker does not need to be artificially damped (unlike commercial devices) to prevent unwanted frequency responses, but is simply damped by surrounding air. This means that the device can operate at just a few nano-amps and so uses much less power than conventional speakers – a non-negligible advantage if it were to be employed in portable devices, such as smartphones, notebooks and tablets.

High-fidelity sound

The Berkeley researchers made their loudspeaker from a 30 nm thick, 7 mm wide sheet of graphene that they had grown by chemical vapour deposition (CVD). They then sandwiched this diaphragm between two actuating perforated silicon electrodes coated with silicon dioxide (to prevent the graphene from accidentally shorting to the electrodes at very large drive amplitudes). When power is applied to the electrodes, an electrostatic force is created that makes the graphene sheet vibrate, so creating sound. By changing the level of power applied, different sounds can be produced. “These sounds can easily be heard by the human ear and also have high fidelity (excellent for listening to music, for example),” said Zettl.

The researchers have already tested their device against high-quality commercial earphones of a similar size (Sennheiser® MX-400) and found that its frequency response over the 20 Hz to 20 kHz range is comparable, if not better.

The Berkeley team says that its CVD technique for fabricating the speaker is very straightforward and could easily be scaled up to produce even larger area diaphragms and thus bigger speakers. “The configuration we describe could also serve as a microphone,” added Zettl.

Details of the new device have been published on the arXiv server.