The increasing sophistication of human-machine interaction necessitates innovative methods for information dissemination. While sight and sound are the most effective senses for machine-human communication, current audio delivery systems often fall short, particularly in scenarios requiring immersive experiences or large-scale deployment (e.g., augmented or virtual reality, Internet of Things applications). Centralized audio solutions are insufficient for the complex and context-specific audio demands of the modern technological landscape. This has fueled a demand for distributed loudspeaker systems that can be easily and affordably integrated into various environments. Traditional rigid loudspeakers are unsuitable for distributed applications due to factors such as cost, bulkiness, and lack of adaptability. Flexible and eco-friendly loudspeakers offer a significant advantage, enabling scalable and sustainable solutions. These devices can be seamlessly incorporated into diverse surfaces, creating immersive audio environments without the limitations of rigid counterparts. Existing flexible loudspeaker technologies often rely on materials such as fluorocarbon polymers (PTFE, FEP, PP), which, while exhibiting promising performance, lack biodegradability and pose environmental concerns. The disposal and recovery costs associated with these materials hinder large-scale adoption and contribute to electronic waste problems. This research focuses on addressing these limitations by developing an eco-friendly flexible electret loudspeaker. This approach leverages the advantages of electrostatic transduction, offering structural simplicity, lightweight design, and cost-effectiveness. The core innovation lies in the utilization of polylactic acid (PLA), a biodegradable and biocompatible thermoplastic biopolymer derived from renewable resources. PLA's moldability and mechanical strength, combined with its proven electret capabilities, make it an ideal material for the loudspeaker diaphragm. The use of paper substrates and carbon electrodes further enhances the eco-friendly profile of the design. The integration of these sustainable materials addresses the environmental impact concerns associated with traditional loudspeaker technologies while maintaining or exceeding the performance of existing alternatives.

Existing flexible loudspeakers utilize various transduction methods, including electrostatic, piezoelectric, and thermoacoustic approaches. Electrostatic flexible electret loudspeakers are particularly attractive for their simplicity, lightweight nature, and ease of production. However, many utilize non-biodegradable fluorocarbon polymers, raising significant environmental concerns. While piezoelectric and thermoacoustic options exist, they often present challenges such as high driving voltages (electrostatic and piezoelectric) or compromised low-frequency response (thermoacoustic). The research reviewed highlights a gap in the market for a high-performance, eco-friendly, and easily scalable flexible loudspeaker technology. This paper aims to fill this gap by proposing a novel design based on biodegradable and sustainable materials.

The proposed eco-friendly flexible electret loudspeaker consists of a three-layer structure: perforated paper substrates coated with carbon electrodes, a PLA electret film as the diaphragm, and an air gap between the layers. The fabrication process is detailed as follows: 1. **Paper Substrate Preparation:** Perforated holes are created on the paper substrates using a laser module of a 3D printer to optimize sound pressure release. The carbon electrodes are applied evenly to one side of the substrates using a pencil. Conductive wires are then attached to the electrodes. 2. **PLA Electret Film Fabrication:** PLA films are fabricated through a hot-pressing and quenching process using a 3D printer. This method helps to increase the surface potential of the PLA film, a crucial factor in loudspeaker performance. The thickness of the PLA films are varied to study their influence on loudspeaker output. 3. **Corona Charging:** The PLA film is corona-charged to create a stable electric field essential for sound generation. Different charging voltages are applied to explore the effects on the surface potential. 4. **Assembly:** The PLA film is adhered to the electrode surface of one paper substrate, and the second paper substrate is then placed on top, forming the three-layer structure. Double-sided tape is used as a spacer to maintain the appropriate distance between the electrodes and the PLA film. The use of a fractal curve shaped double-sided tape spacer in larger speakers is also explored. 5. **Simulation and Experimental Characterization:** COMSOL Multiphysics simulation is utilized to model the loudspeaker's performance and optimize key parameters such as the thickness of paper substrates, surface potential of the PLA electret, and the perforation pattern on the paper substrates. A variety of experimental tests were conducted to characterize the performance, including sound pressure level (SPL) measurements using a multifunctional noise analyzer, vibrating displacement measurements using laser Doppler vibrometry, and a degradation test to assess the biodegradability of the loudspeaker. Various geometries and dimensions are also tested in experiments. The experimental setup involves a soundproof shed for SPL measurements, a function generator and power amplifier to drive the loudspeaker, and a laser Doppler vibrometer to measure the diaphragm's vibration. The frequency response and SPL are measured at various voltages and distances. A customized micro-voltage amplifier is also developed to demonstrate the loudspeaker's portability and performance in real world conditions.

The study's key findings demonstrate that the designed eco-friendly flexible electret loudspeaker achieves remarkable performance and sustainability: 1. **High Sound Pressure Level (SPL):** A rectangular loudspeaker measuring 50 × 40 cm² produced an average SPL of 60 dB at a distance of 50 meters, easily audible to humans. This performance compares favorably to existing technologies. The SPL response is consistent across the human hearing range, extending to 15 kHz and well within the typical range of human voices (<8 kHz). 2. **Optimized Design Parameters:** Simulation and experimental results both indicate that thin top substrates, thick bottom substrates, high surface potential of PLA electret, thin PLA film, and optimal hole density on the substrates maximize SPL output. This optimization was achieved by systematically varying design parameters and measuring the resultant SPL response. 3. **Customizable Shape and Size:** The loudspeaker's design allows for easy customization of shape and size without compromising performance. Various shapes, including circular, rectangular, and more complex designs, were tested with comparable results. 4. **Flexibility and Durability:** The loudspeaker demonstrated remarkable flexibility and durability; maintaining performance when rolled or flat. Stable output was observed during 11 hours of continuous operation, and performance was only marginally affected after 108 days of storage in laboratory conditions. 5. **Scalability and Application Versatility:** The eco-friendly design was shown to be highly scalable, with larger loudspeakers (up to 50 × 80 cm²) producing even higher SPL. Demonstrative applications showed the integration of these loudspeakers behind curtains and as wall-mounted posters with good sound quality and volume. 6. **Portability and Low-Power Operation:** A custom-designed micro-voltage amplifier (3 x 4 cm²) successfully powered the loudspeaker, demonstrating the feasibility of portable applications. The audio quality from the miniature amplifier system was comparable to that of a mobile phone.

This research successfully demonstrates a highly effective and sustainable approach to flexible loudspeaker design. The use of biodegradable materials, such as PLA, paper, and carbon electrodes, significantly reduces the environmental impact compared to traditional loudspeakers using non-biodegradable polymers. The optimized design, validated through both simulation and experimentation, resulted in a loudspeaker with a high SPL output, consistent frequency response, and impressive durability. The flexibility and customizable nature of the design allows for integration into a wide range of applications, thereby addressing a significant challenge in human-machine interaction. The findings contribute to a growing body of work on sustainable electronics, highlighting the potential of bio-based materials for developing high-performance devices. The achieved sound pressure level at a distance of 50 meters showcases the practicality of this technology for applications requiring distributed audio sources, such as large-scale immersive installations, public announcements, and smart environments.

This study presents a novel eco-friendly flexible electret loudspeaker with significant improvements in performance and sustainability. The use of PLA, paper, and carbon electrodes offers a compelling alternative to existing technologies. Optimization of key design parameters resulted in a loudspeaker exceeding performance benchmarks in SPL output, frequency response, and durability. Future work could focus on enhancing the PLA electret's surface potential and mechanical properties, as well as investigating the loudspeaker's long-term performance in outdoor environments.

While the study demonstrates excellent performance in laboratory conditions, further research is needed to assess the long-term durability and performance of the loudspeakers in varied environmental conditions, such as high humidity or temperature extremes. The biodegradation rate was assessed under accelerated conditions (60°C, 100%RH), and further studies are needed to determine the biodegradation rate under typical ambient conditions. The current design's mechanical robustness could also benefit from further investigation to ensure sustained reliability and long-term performance in real-world applications.