Loudspeakers, as vital hardware for transmitting acoustic information, have become indispensable in daily life. Traditional rigid loudspeakers usually consist of at least one vertebral body, a voice coil attached to the apex of the vertebral body, a permanent magnet fixed to the frame, and a shell. Such centralized, single-source loudspeakers are not sufficient in the rise of the Internet of Things (IoT) and Web 3.0 to deliver complex and context-specific effects. Therefore, easily customized distributed flexible film loudspeakers have become the direction of industry development.
Flexible loudspeakers are typically powered by electrostatic1,2, piezoelectric3,4, or thermoacoustic5,6 transductions, of which electrostatic and piezoelectric flexible loudspeakers usually need a high driving voltage (tens or hundreds of volts) and have a good frequency response; Thermoacoustic flexible speakers require a low drive voltage (as low as a few volts), but the response is poor in low frequency audible bands. Specifically, the electret loudspeaker is a flexible electrostatic loudspeaker, which has the advantages of simple structure, light weight, easy production, and high-cost performance, and has great potential in human-machine interaction applications.
Although traditional rigid electret loudspeakers have been around for decades, the development of flexible electret loudspeakers has not been as rapid. A few flexible electret loudspeakers that currently exist use fluorocarbon polymers such as polytetrafluoroethylene (Teflon, PTFE)7, fluorinated ethylene propylene (FEP)8, and polypropylene (PP)9. While these polymers show promising performance, they are not biodegradable and not eco-friendly. Large-scale production and application of these loudspeakers could therefore pose a threat to the environment, leading to electronic waste problems or increased recovery costs. As such, developing an eco-friendly flexible electret loudspeaker holds significant commercial and environmental potential.
In our paper, "Scalable and Eco-Friendly Flexible Loudspeakers for Distributed Human-Machine Interactions", we present an eco-friendly flexible electret loudspeaker that utilizes polylactic acid (PLA) electret film, paper substrates, and carbon electrodes. The loudspeaker is a three-layered structure, with perforated paper substrates coated with carbon electrodes on one side and a polylactic acid (PLA) electret film as the main diaphragm in the middle layer. After the PLA film is polarized using the Corona charging method36, the negative and positive charges on the two surfaces generate a high surface potential as a built-in electrical field (Ein). When combined with the external electrical field (Eout) from the AC driving voltage, the loudspeaker vibrates to release sound, and the vibration is controlled by the AC driving voltage.
To achieve optimal performance, key design parameters are optimized through finite element modeling and experimental verification. We investigated the impact of design parameters on loudspeaker performance by using axisymmetric and three-dimensional Multiphysics finite element models. Simulations evaluated the electrostatic field, vibrating displacement, and sound pressure level (SPL) using a fully coupled method. The simulations were conducted in a spherical air domain with a perfect matching layer, enclosing the entire device. The loudspeaker configuration of the paper substrates, either with or without holes, is shown in Fig. 2a. Results in Fig. 2b revealed that while vibrating displacement was not uniform, the SPL response was relatively uniform in the loudspeaker's surrounding space. We further extended the simulations to explore the SPL response under driving voltage amplitudes up to 100 Vrms. We considered key design parameters such as the thickness of paper substrates (at a frequency of 6 kHz), the surface potential of the PLA electret (at a frequency of 6 kHz), and the presence of holes on the paper substrates (hole diameter of 1.2 mm, distance between hole centers of 4 mm, at a frequency of 10.42 kHz). For substrate thickness and electret surface potential simulations, simplified axisymmetric models without holes have been used. A general simulation of configurations with and without holes showed similar results, which would give useful clues as to design optimization. Conclusions drawn from the simulations (Fig. 2c-e) are as follows: 1) Thin top substrate and thick bottom substrate favor larger SPL output. 2) Higher electret surface potential enhances SPL response. 3) Holes on substrates aid in sound pressure release.
Simulation results indicate that a thin top substrate, thick bottom substrate, high surface potential of PLA electret, thin PLA film thickness, and appropriate hole density on paper substrates improve the SPL outputs, which has been proved in experiments. A rectangle-shaped loudspeaker with a size of 50×40 cm2 can produce an SPL of 60 dB even at a distance of 50 meters. The SPL response of our loudspeaker is stable with driving frequency of up to 15 kHz, covering the normal frequency range of human voices (<8 kHz). Our loudspeaker's shapes are easily customizable, and the paper substrate and PLA film can be easily scissored without affecting output performance. The flexibility is verified by the uniform SPL directivity of a rolled loudspeaker, and the durability is demonstrated with stable outputs during 11 continuous hours of operation and after 108 days storage in lab conditions. As demonstrated in the article, our loudspeakers can be integrated behind a curtain in the classroom or hung up like a poster in the corridor, providing good sound quality and volume while greatly improving space utilization compared to traditional loudspeakers. For portability, our loudspeakers can be driven by a micro voltage amplifier, producing audible audios with sound wave patterns and spectrograms similar to the original audio and ones played from a mobile phone. Future work to improve our eco-friendly flexible electret loudspeakers includes modifying PLA electret or developing new eco-friendly electret materials to improve surface potential and mechanical properties. Additionally, ensuring the durability of our loudspeakers in outdoor environments is another area for improvement.
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