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Graphene diaphragms could give humans 'bat-like' capabilities

12 July 2015

UC Berkeley physicists have used graphene to build lightweight ultrasonic loudspeakers and microphones with extremely wide frequency responses.

Image: Shutterstock

“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,” says UC Berkeley physicist Alex Zettl. “Until now, we have not had good wideband ultrasound transmitters or receivers. These new devices are a technology opportunity.”

Speakers and microphones both use diaphragms, typically made of paper or plastic, that vibrate to produce or detect sound, respectively. The diaphragms in the new devices are graphene sheets a mere one atom thick that have the right combination of stiffness, strength and light weight to respond to frequencies ranging from subsonic (below 20Hz) to ultrasonic (above 20kHz). Humans can hear from 20  to 20,000Hz, whereas bats hear only in the range 9 to 200kHz. The grapheme loudspeakers and microphones operate from well below 20Hz to over 500kHz.

Zettl, a senior scientist at Lawrence Berkeley National Laboratory, says the microphone and loudspeaker that his team has developed are among those graphene based devices that are closest to commercial viability.

Two years ago, Zettl's co-researcher and UC Berkeley post-doctoral fellow, Qin Zhou built loudspeakers using a sheet of graphene for the diaphragm, and since then has been developing the electronic circuitry to build a microphone with a similar graphene diaphragm.

An atom-thick layer of graphene (the black mesh in this diagram), provides the vibrating diaphragm for both an ultrasonic microphone and loudspeaker (image: UC Berkeley)

“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,” says Zhou.

Graphene membranes are also more efficient, converting over 99 percent of the energy driving the device into sound, whereas today’s conventional loudspeakers and headphones convert only 8 percent into sound. Zettl anticipates that in the future, communications devices like mobile phones will utilise not only electromagnetic waves, but also acoustic or ultrasonic sound, which can be highly directional and long-range.

Zhou was encouraged to use the ultrasound microphone and try to capture the sound of bats. Subsequent recordings were slowed to one-eighth normal speed, converting the high frequencies to an audio range suitable for humans. The team were amazed at the quality and fidelity of the bat vocalisations. “This is lightweight enough to mount on a bat and record what the bat can hear,” Zhou says.

Zettl believes that audiophiles would also appreciate the graphene loudspeakers and headphones, which have a flat response across the entire audible frequency range.

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