Graphene-based audio devices allow communication at bat frequencies

In yet another first for graphene, physicists from the University of California, Berkeley, have employed this versatile material to create ultra-thin, lightweight ultrasonic microphones and speakers that enable high-quality, two-way communication in the audio range normally used by the likes of bats and dolphins.

The graphene diaphragms created especially for these new devices are just one atom thick, and combine the stiffness, suppleness, and low inertia that give them the capability to respond at an enormous range of frequencies and, according to the researchers, provide a flat frequency response across the entire audible spectrum.

Generally, speakers and microphones use either paper or plastic diaphragms that vibrate to either emit or detect sound. However paper, plastic, and even some of the more esoteric materials used in these devices do not have a large frequency range, and certainly not down to the the subsonic (below 20 hertz) and up to the ultrasonic (above 20 kilohertz)

Graphene is different. Being so incredibly thin, a taut sheet of this material is easily deflected by incoming noises or vibrated with little effort by electromagnetic energy to produce sound and, with so little inertia, is able to rapidly respond to changes in frequency. As a result, graphene microphones and speakers are capable of capturing or emitting exceptionally wide ranges of sound frequencies and at a claimed 99 percent efficiency. Standard speakers can barely achieve more than 8 percent.

"There’s a lot of talk about using graphene in electronics and small nanoscale devices, but they’re all a ways away," said UC Berkeley physicist Alex Zettl. "The microphone and loudspeaker are some of the closest devices to commercial viability, because we’ve worked out how to make the graphene and mount it, and it’s easy to scale up."

The UC Berkeley researchers believe that practical applications for graphene speakers and microphones may be in the realm of underwater communications where standard radio transmitters are impractical. They also believe that the new devices would offer far greater fidelity than current ultrasound systems on the market today, as well as be able to communicate through steel objects; something that radio waves can’t do.

"Sea mammals and bats use high-frequency sound for echolocation and communication, but humans just haven’t fully exploited that before, in my opinion, because the technology has not been there," said Zettl. "Until now, we have not had good wideband ultrasound transmitters or receivers. These new devices are a technology opportunity."

Current ultrasonic emitters use piezoelectric devices generally made from a thin wafer of ceramic to produce ultrasonic audio. Unfortunately, being made of such stiff material, these devices have relatively poor frequency responses, so audio communication is generally of low quality, whilst the rapidly alternating signals of some high-speed data transfer using this medium may overdrive the transmitters and risk clipping the data-transfer quality.

Using graphene in this situation in future, according to the researchers, should mean that not only is sound quality improved through the exceptional frequency response of such a lightweight material, but ultrasonic data transfer bandwidth may increase too.

"Because our membrane is so light, it has an extremely wide frequency response and is able to generate sharp pulses and measure distance much more accurately than traditional methods," said UC Berkely postdoctoral fellow Qin Zhou.

Since building graphene speakers around two years ago, Qin Zhou has been working the electronic circuitry to build a microphone with a similar graphene diaphragm. When Zhou explained this work to his wife, she suggested that trying to record the sound of bats would prove its worth. They took Zhou's creation to a park in Livermore where they recorded some of the local Western Pipistrelle bats and then replayed the sounds they captured at one-eighth normal speed. What they heard was a surprisingly clear and high-fidelity recording of natural bat vocalizations.

"These new microphones will be incredibly valuable for studying auditory signals at high frequencies, such as the ones used by bats," said bat expert Michael Yartsev, a UC Berkeley assistant professor of bioengineering. "The use of graphene allows the authors to obtain very flat frequency responses in a wide range of frequencies, including ultrasound, and will permit a detailed study of the auditory pulses that are used by bats."

Apart from bats, the researchers also believe that their research may result in a new range of ultra-high fidelity speakers that possess a completely flat frequency response over the entire audible spectrum. Converting almost 100 percent of the input to sound, graphene speakers would not only sound incredible, but also produce exceptional outputs from relatively small levels of amplification.

“A number of years ago, this device would have been darn near impossible to build because of the difficulty of making free-standing graphene sheets,” said Zettl. “But over the past decade the graphene community has come together to develop techniques to grow, transport and mount graphene, so building a device like this is now very straightforward; the design is simple.”