Physicists at University of California, Berkeley have used graphene to develop innovative ultrasonic microphones and loudspeakers that are extremely lightweight. These devices allow humans to imitate the ability of dolphins and bats which use sound to communicate and measure the speed and distance of objects around them.
Traditional radio transmission using electromagnetic waves in places where radio cannot be employed, underwater for example, can be complemented by these wireless ultrasound devices which offer greater reliability when compared to present-day sonar or ultrasound devices. The wireless devices can even be used to communicate through objects, such as steel, which cannot be penetrated by electromagnetic waves.
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.
Until now, we have not had good wideband ultrasound transmitters or receivers. These new devices are a technology opportunity.
Alex Zettl
Physicist at UC Berkeley
Diaphragms, used by microphones and speakers, are often made of plastic and paper that vibrate to detect or emit sound. The diaphragms integrated into the novel instruments are graphene sheets which are one atomic layer thick and exhibit the right combination of strength, stiffness, and light weight to react to frequencies spanning from subsonic to ultrasonic.
Bats can hear in the kilohertz range between 9 and 200kHz, while humans can hear from 20 to 20,000Hz. The graphene-based microphones and loudspeakers work from under 20kHz to more than 500kHz.
An atom-thick layer of carbon atoms, called graphene (black mesh), provides the vibrating diaphragm for both an ultrasonic microphone and loudspeaker. Image credit: UC Berkeley.
Graphene contains carbon atoms that are arranged in a hexagonal structure, producing a strong yet lightweight sheet with remarkable electronic properties.
These properties have attracted considerable interest in the physics realm over the past two decades or so.
The graphene-based ultrasonic radio and microphone have been described in a paper published online in the Proceedings of the National Academy of Sciences.
A few years earlier, UC Berkeley postdoctoral fellow, Qin Zhou, used a graphene sheet to build loudspeakers for the diaphragm, and since that time has been working on the electronic circuitry to develop a microphone that has a similar graphene diaphragm.
One major benefit of graphene is that since the atom-thick sheet is light in weight, it can easily respond to a range of frequencies of an electronic pulse, in contrast to present-day piezoelectric speakers and microphones.
There’s a lot of talk about using graphene in electronics and small nanoscale devices, but they’re all a ways away.
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.
Alex Zettl
This proves useful when ultrasonic receivers and transmitters are used to transmit huge amounts of data via diverse frequency channels at the same time, or to determine distance, like in sonar applications.
“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,” Zhou said.
Over 99% of the energy driving the device is converted into sound in the case of these high-efficiency graphene membranes, while current generation of headphones and loudspeakers transform just 8% into sound.
Zettl expects that in the coming months, cellphones and other communications devices will use both electromagnetic waves and ultrasonic/acoustic sound that would be long-range and highly directional. “Graphene is a magical material; it hits all the sweet spots for a communications device,” he said.
When Zhou’s wife, Jinglin Zheng, heard about the ultrasound microphone, she asked Zhou to record the sounds of bats twittering at high frequencies – frequencies which cannot be heard by humans.
Hence, they brought the microphone to a Livermore park and switched it on. However, both were stunned at the quality and reliability of the bat vocalizations, when the recording was slowed down to one-eighth the standard speed and the high frequencies were changed to an audio range which can be heard by humans. “This is lightweight enough to mount on a bat and record what the bat can hear,” Zhou said.
Bat expert Michael Yartsev, a newly hired UC Berkeley assistant professor of bioengineering and member of the Helen Wills Neuroscience Institute, said, “These new microphones will be incredibly valuable for studying auditory signals at high frequencies, such as the ones used by bats. 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.”
Zettl observed that graphene-based headphones and loudspeakers would be appreciated by audiophiles as they have a flat response across the whole audible frequency range.
“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,” Zettl said. “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.”
The Office of Naval Research, the U.S. Department of Energy, and the National Science Foundation supported the study. Other co-authors involved in the research were Seita Onishi, Zheng, and Michael Crommie, a UC Berkeley professor of physics.