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Self-powered Flexible Mic/Speaker is 0.1mm Thin

May 18th 2017

Man cupping his ear

Led by Nelson Sepúlveda, Associate Professor at the Michigan State University, a team of researchers who recently reported a breakthrough in ferroelectret nanogenerators (FENGs) was able to further characterize the behaviour of their thin-film FENG as a flexible acoustic transducer capable of operating both as a loudspeaker and a microphone.

Their paper "Nanogenerator-based dual-functional and self-powered thin patch loudspeaker or microphone for flexible electronics" published in Nature Communications examines the FENG's energy conversion mechanism under various sound pressure levels (SPL). Because the 0.1mm thin polypropylene ferroelectret (PPFE) includes compressible charged voids behaving as deformable dipoles in a metal-insulator-metal structure (with conductive silver layers for electrodes on the faces of the film), the alternating waves of sound pressure are faithfully transduced into alternating electrical signals. Here, rather than focus on energy generation, the study focused on reading out the analogue signal produced by the polymer-based FENG and the results are compelling.

Energy conversion mechanisms of a FENG. (a) The charge distribution and giant dipoles with upper and lower surfaces oppositely charged. (b) Electromechanical effect under direct pressure. (c) When the pressure is released, the giant dipoles restore their original sizes and a reverse current is generated. (d) Reverse electromechanical interaction: applying a positive potential further expands the giant dipoles. (e) The giant dipoles shrink as a negative potential is applied.

Performing measurements using three different configurations (the FENG as a freestanding film, affixed to a soft substrate, and attached to a soft substrate and rolled up into a cylinder), the researchers were able not only to accurately record sound in all three configurations, but also reversibly to play sounds by applying electric signals to the transducer.

The study shows the FENG-based microphone to be highly sensitive to a broad range of frequency including the full 20Hz to 20kHz human hearing range and extrapolates that such polypropylene ferroelectrets could be used as a thin, wearable and self-powered loudspeaker or microphone patch.

In one demonstration, the researchers recorded with high fidelity Verdi's symphony "La Traviata, Brindisi" with a piece of paper-like FENG-based microphone. In another demonstration, the thin flexible microphone was used to accurately perform voiceprint identity recognition (to secure access to a computer). Such films could easily be integrated secretly within a computer or personal device for added subtlety and perform either as a speaker or as a microphone. To prove the speaker functionality, the researchers embedded and connected together a matrix of square-shaped sheets of FENGs into a freestanding flag which they could use to play music.

Demonstrating the acoustic wave recording performances of a FENG-based microphone. (a) The transformation from acoustic energy to electric energy. (b) Recording " La Traviata, Brindisi". (c) Sound wave and acoustic spectrogram of the original music and (d) of the recorded music. (e and f) Expanded and detailed views of the original (e) and recorded (f) music.

Another benefit of such FENG-based loudspeakers is their symmetric output, producing identical sound pressure from both surfaces (hence radiating sound not only from the front side but from the backside at same level). In effect, this means more sound can be directed to more listeners, the paper notes. In the case of a cylindrical implementation, the paper reports that the rolled-up FENG forms an omnidirectional loudspeaker with the same directivity factor at every listener angle.

As for the energy-harvesting application initially reported, a single FENG-based device can be folded and stacked to form multilayer structures, further increasing its sensitivity and performance.

Because they are so thin and flexible, such FENG-based dual-functional acoustic devices could integrate many consumer applications with different form factors. Cited in the paper are FENG-based microphones integrated onto flexible electronic devices including smart cards (ID or banking) for authentication through voice recognition but also self-read e-newspapers (a flexible e-paper integrating the speaker into its laminated construction) or projection screens with integrated speakers. Pervasive active noise cancellation is another use case put forward, whereby such films could cover the cabin walls of cars, aircrafts and submarines.

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