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.
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.