Scalable and eco-friendly flexible loudspeakers for distributed human-machine interactions

The increasing development of human-machine interaction has led to a growing demand for electronics to disseminate information. Of the five human senses (i.e., vision, audition, gustation, olfaction, and touch) machines are most adept at providing visual and auditory experiences. Loudspeakers, as vital hardware for transmitting acoustic information, are crucial to creating immersive experiences in applications such as augmented reality and virtual reality. The rise of the Internet of Things (IoT) and web 3.0 has rendered centralized and single-sourced audio devices insufficient for delivering complex and context-specific audio effects. To address this need, the industry is increasingly turning to distributed loudspeakers, which can be conveniently and labor-savingly applied in concerts. For large-scale IoT applications, it is essential to consider factors such as production and recovery costs, biodegradability, affordability, customizability, and scalability. In this regard, flexible and eco-friendly loudspeakers are more suitable for distributed applications on various object surfaces, as they are scalable and can create an immersive environment without being rigid or bulky. Flexible loudspeakers are typically powered by electrostatic, piezoelectric, or thermoacoustic transductions. Electrostatic flexible electret loudspeakers, in particular, offer structural simplicity, light weight, ease of production, and cost-effectiveness, making them ideal for human-machine interactive applications. However, many existing flexible electret loudspeakers use non-biodegradable fluorocarbon polymers (e.g., PTFE, FEP, PP), raising environmental and recovery-cost concerns for large-scale deployment. In this work, we present an eco-friendly flexible electret loudspeaker that utilizes polylactic acid (PLA) electret film, paper substrates, and carbon electrodes. PLA is a biodegradable, biocompatible thermoplastic biopolymer capable of storing significant electrostatic charge for extended periods. Paper and carbon are eco-friendly materials. A degradation test shows that the entire loudspeaker decomposes significantly after 227 days. Key innovations include: (1) material and structure optimization (validated by simulation and experiment) to improve SPL and frequency response; a 50 × 40 cm² loudspeaker generates ~60 dB SPL at 50 m and maintains relatively consistent SPL up to 15 kHz (<8 kHz covers human voice range); (2) high sensitivity of 0.444 mPa V⁻² cm⁻² at 70.7 Vrms; (3) easily customized shapes without performance loss; (4) flexibility demonstrated by uniform SPL directivity when rolled and durability by 11 h of continuous operation; (5) integration behind curtains or hung like posters for immersive audio; and (6) a portable self-made micro-voltage amplifier enabling operation with similarity to mobile-phone audio output.

Flexible loudspeakers generally rely on electrostatic, piezoelectric, or thermoacoustic mechanisms. Electrostatic flexible electret loudspeakers are attractive due to simplicity, low mass, and cost-effectiveness. Traditional electret loudspeakers and some flexible versions have used fluorocarbon polymers such as PTFE, FEP, and polypropylene (PP), which, while offering good performance, are not biodegradable and pose environmental and recovery-cost concerns when scaled. This motivates eco-friendly alternatives. Prior works on electrostatic and piezoelectric flexible loudspeakers often require relatively high driving voltages and use non-degradable materials, whereas thermoacoustic devices can be driven at low voltages but have compromised low-frequency performance.

Device design and materials: The loudspeaker is a three-layer structure comprising perforated paper substrates coated on one side with carbon electrodes and a PLA electret film as the diaphragm between them. Typical layer thicknesses are: top paper substrate 38 μm, bottom paper substrate 437 μm, PLA film 80 μm, with an overall device thickness of ~700 μm including the air gap. Double-sided tape (70 μm per layer) is applied at the edges and center as spacers to set an electrode–PLA gap of 70–140 μm and prevent contact. For large devices, a fractal-curve-shaped spacer is used to improve frequency response. Paper substrates are perforated with 1.2 mm holes on a grid; center-to-center spacing a is commonly 4 mm. Carbon electrodes are formed by hand-drawing with a 6B pencil, yielding ~400 Ω resistance between center and boundary over a 16 cm² circular area. PLA electret preparation: PLA discs (20 mm diameter, 0.8 mm thick) are 3D printed and hot-pressed at 220 °C under 14 kg for 5, 8, or 10 min to obtain films 80, 100, or 120 μm thick. Films are quenched in water immediately or after cooling to ~100 °C, which increases electret surface potential. PLA film properties measured include Young’s modulus (~1194 MPa) and relative dielectric constant (~2.2). Films are corona charged for 5 min at up to −16 kV (film-to-needle distance 5 cm) to inject charges; stable surface potentials around −300 V (or −200 V/−100 V using −12 kV/−8 kV) are obtained. Large films are partitioned and sequentially polarized. Assembly: The PLA film is adhered to the electrode-coated face of the bottom paper substrate using double-sided tape at the boundaries. The top substrate’s electrode-coated face is placed facing the PLA film; perimeters are sealed with tape spacers. Device shapes and sizes are easily customized by cutting paper and PLA. Simulation: Axisymmetric and 3D multiphysics finite element models (COMSOL Multiphysics) simulate electrostatic field, diaphragm vibration, and SPL using a fully coupled approach (solid mechanics, electrostatic force, acoustics). A spherical air domain with a perfectly matched layer (diameter 1 m) surrounds the device; narrow region acoustics is used for air gaps. SPL is evaluated 5 cm from the loudspeaker center. Parametric sweeps study effects of top/bottom paper thickness, PLA electret surface potential, and presence/geometry of holes. Characterization: SPL is measured in a small soundproof setup with a multifunctional noise analyzer positioned 5 cm from the loudspeaker center; background noise ~35 dB. Unless specified, circular devices of 16 cm² with perforated substrates are driven at 70.7 Vrms over 20 Hz–20 kHz to obtain SPL–frequency curves, or at 6 kHz while sweeping 17.7–100 Vrms to obtain SPL–voltage curves. LDV measurements quantify diaphragm vibration under sinusoidal drive (e.g., 70.7 Vrms at 100 Hz), showing peak-to-peak displacement ~5 μm at maximum displacement location and decreasing displacement with increasing frequency, stabilizing above ~5 kHz. Directivity is measured for flat (9 cm²) and rolled configurations (same projected area) at multiple drive voltages. Distance attenuation is characterized up to 30 cm. Long-term stability is evaluated over continuous 11 h operation and after 108 days storage (~25 °C, ~60% RH). Large-format devices (up to 50 × 80 cm² and 50 × 40 cm²; 2000 cm² tested at 1 m) are demonstrated behind curtains and in corridors. A compact self-made micro-voltage amplifier (3 × 4 cm², single 9 V battery) is designed to drive the loudspeaker from audio sources; recordings are compared to original and mobile phone playback using waveforms and spectrograms.

- Optimized eco-friendly flexible electret loudspeaker using PLA electret film, perforated paper substrates, and carbon electrodes achieved high SPL and broad frequency response with simple, low-cost fabrication. - Large-size device performance: A rectangular loudspeaker (50 × 40 cm²) produced an average ~60 dB SPL at a distance of 50 meters during corridor demonstrations; frequency response remained high and relatively consistent up to 15 kHz (covering normal human voice range <8 kHz). - Sensitivity: 0.444 mPa V⁻² cm⁻² at 70.7 Vrms, superior to many reported electrostatic and piezoelectric flexible loudspeakers (per Supplementary Table 1). - SPL–voltage at 6 kHz (5 cm measurement): With perforated paper substrates, devices achieved ~80–85 dB at 70.7 Vrms; SPL increased with higher PLA surface potential (~100/200/300 V). Even at 17.7 Vrms, SPL reached ~65–70 dB (human voice range 40–70 dB). - Structural optimization insights (simulation and experiment agreement): • Thin top paper substrate (38 μm) and thick bottom substrate (437 μm) maximize SPL. • Higher PLA electret surface potential enhances SPL outputs. • Perforation holes (1.2 mm diameter, 4 mm spacing) facilitate sound release and increase SPL compared to non-perforated designs. • Thinner PLA diaphragm increases SPL across the band due to larger vibration amplitude under the same electrostatic force. - Frequency response characteristics: • Higher hole density raises SPL below ~15 kHz but can degrade performance in the 15–20 kHz range; a=4 mm chosen as a trade-off between SPL and mechanical strength. • Larger area devices produce higher SPL up to ~12 kHz (e.g., 2000 cm² vs 16 cm²) but can show lower SPL at 12–20 kHz. • Device shape (triangle, square, pentagon, hexagon, circle at 16 cm²) had negligible impact on SPL–frequency response. - Vibration: LDV measured peak-to-peak displacement ~5 μm at 100 Hz (70.7 Vrms); displacement decreases with frequency and stabilizes above ~5 kHz. Maximum displacement near diaphragm center matches simulations. - Directivity and flexibility: Rolled and flat configurations (same projected area) showed uniform, comparable SPL directivity patterns over drive voltages (17.7, 53.1, 88.4 Vrms). - Distance attenuation (up to 30 cm) followed expected decrease in SPL with distance for both flat and rolled devices. - Stability and durability: Stable output over continuous 11 h operation for both flat and rolled configurations; comparable outputs after 108 days storage under lab conditions (~25 °C, ~60% RH). - Eco-friendliness and biodegradation: Device showed clear degradation after 227 days in incubator at 60 °C and 100% RH; PLA film degradation dominated; measured biodegradation ~63.8%. - Power density: For a 50 × 80 cm² device, power density <0.03 mW cm⁻² across the audible range at 70.7 Vrms. - Portability: A compact, single-9V, self-made micro-voltage amplifier (3 × 4 cm²) successfully drove the loudspeakers to produce audio with waveforms and spectrograms closely matching original audio and mobile phone playback.

The study addresses the need for scalable, distributed, and eco-friendly audio transducers for human–machine interaction by demonstrating a biodegradable, customizable, and high-performance flexible electret loudspeaker platform. Through combined multiphysics simulation and experiments, the authors identify and validate key design parameters—top/bottom substrate thickness, PLA electret surface potential and thickness, and hole density—that govern SPL and frequency response. The optimized devices achieve strong SPL at modest driving voltages (audible at 17.7 Vrms; 80–85 dB at 70.7 Vrms, 5 cm), broad and relatively flat responses up to 15 kHz (covering voice frequencies), and maintain performance when rolled, supporting conformal deployment on diverse surfaces. Large-area implementations deliver intelligible audio at long range (60 dB at 50 m) and integrate seamlessly into environments (e.g., behind curtains, wall-mounted like posters), underscoring benefits for immersive and distributed audio in IoT settings. Compared with prior flexible electrostatic and piezoelectric devices, this system combines eco-friendly materials with high sensitivity and competitive SPL, while maintaining simple fabrication and low cost. The close match between simulation and measurement further provides a robust framework for device scaling and application-specific optimization.

This work introduces a low-cost, eco-friendly, and scalable flexible electret loudspeaker composed of PLA electret film, paper substrates, and carbon electrodes. Finite element modeling and experimental validation guided design optimization, revealing that a thin top paper substrate, a thick bottom substrate, higher PLA surface potential, thinner PLA film, and appropriate hole density enhance SPL and frequency response. The devices are shape-customizable without performance loss, flexible (maintaining directivity when rolled), durable over continuous 11 h operation, and capable of long-range audio transmission (∼60 dB at 50 m for a 50 × 40 cm² device). Frequency response remains high and relatively consistent up to 15 kHz, covering the normal human-voice band. The loudspeakers can be seamlessly integrated into everyday environments (e.g., behind curtains or on walls) and can be driven by a compact battery-powered micro-voltage amplifier, producing audio comparable to commercial devices. Future work includes modifying PLA electret or developing new eco-friendly electret materials to further increase surface potential and improve mechanical properties, and enhancing durability for outdoor environments.

- High hole densities, while improving SPL below ~15 kHz, can reduce performance in the 15–20 kHz band and may compromise mechanical strength (e.g., a=2 mm spacing can lead to structural failure). - Very large-area devices may exhibit reduced SPL at higher frequencies (above ~12 kHz) compared to smaller devices. - Spacer thickness must be carefully controlled; excessive electrode–PLA spacing degrades performance. - Although the devices achieve audible SPLs at relatively modest drive levels for electrostatic systems, the required AC drive (tens of Vrms) remains higher than some thermoacoustic approaches. - Long-term stability in outdoor or harsh environments requires further improvement, as noted by the authors.