Researchers in the US have made a graphene loudspeaker that has an excellent frequency response across the entire audio frequency range (20 Hz–20 kHz). While the speaker has no specific design, it is already as good as, or even better than, certain commercial speakers and earphones in terms of both frequency response and power consumption.

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, the consistency of the sound it produces over the entire 20 Hz–20 KHz range.

"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," says team leader Alex Zettl of the University of California, Berkeley. He told physicsworld.com that 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 substantial 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, making them excellent for listening to music, for example," says 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," adds Zettl.

A preprint of the research is available on the arXiv server.