Ultrasonic, Highly Sensitive Graphene Microphones Developed By Researchers

You're on. suradach/Shutterstock
Robin Andrews 28 Nov 2015, 20:31

Graphene, a technological innovation from the University of Manchester that won the Nobel Prize in Physics in 2010, has been used to develop tough and flexible digital touchscreens, water filtration devices, drug delivery systems, and advanced night vision contact lenses. It’s a veritable wonder material, and just this year it’s formed the basis of another technological innovation – a microphone nearly 32 times more sensitive than regular ones. The new invention is described in the journal 2D Materials.

Most microphones have the same componentry as a loudspeaker – in fact, they’re loudspeakers working in reverse, turning sound into electrical currents. When you speak, the sound waves travel towards the microphone, which impact a membrane that then vibrates. These vibrations are transferred to a metallic coil that then moves back and forth across a permanent magnet. A temporary electromagnet is created by the interaction of the magnetic field with the coil, and an electrical current is generated, which travels to an amplifier or a sound recording device.

Normally, nickel is used in the construction of the membrane, but for this study, graphene was used. “We wanted to show that graphene, although a relatively new material, has potential for real world applications,” explains Marko Spasenovic, an author of the paper, in a statement. “Given its light weight, high mechanical strength and flexibility, graphene just begs to be used as an acoustic membrane material.”

The graphene membrane, just 30 carbon atoms thick, was grown on a nickel-based foil using a process known as chemical vapor deposition (CVD). During CVD, gaseous, reactive substances (such as methane, a carbon-containing compound) interact with a substrate, in this case the foil, in order to produce graphene. After the graphene sheets began to crystallize out on the foil, the nickel was carefully removed.

Microphone performance is normally tested and measured by recording a series of sound waves over a range of frequencies, from the very low 10 hertz to the far higher 24 kilohertz – roughly the entire human hearing range. Frequency is related to the “pitch” of the sound, whereas the amplitude is related to the “loudness.” Sound waves increasing in frequency but remaining at constant amplitudes (from the very low/quiet to the very high/loud) were blasted at the graphene foil.

The more the vibration of the membrane matches up with the wave pattern of the sound waves, the more sensitive it is deemed to be. The results, compared to conventional nickel-based membranes, were remarkable. It showed a 32-fold increase in sensitivity across a significant part of the audio spectrum: up to 11 kilohertz, across a dizzying array of amplitudes.

The graphene membrane had an extra sensitivity of 10-15 decibels up to 11 kilohertz. Luis Carlos Torres/Shutterstock

The researchers also simulated a 300-layer thick graphene membrane, which has the potential to be even more sensitive; it could hypothetically detect frequencies of up to one megahertz, which is in the ultrasonic part of the spectrum. This has yet to be tested experimentally, though.

“At this stage there are several obstacles to making cheap graphene, so our microphone should be considered more a proof of concept,” concludes Spasenovic. “The industry is working hard to improve graphene production – eventually this should mean we have better microphones at lower cost.”

Either way, this research shows that it is demonstrably possible for graphene to be used in a new generation of highly sensitive microphones, which will pick up far more sound detail than regular microphones do at present. Excitingly, highly sensitive ultrasonic microphones may also be on the cards.

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