Layered acoustic structures made of plastic microfibre pulp waste with metamaterial for low- and middle-frequency sound absorption applications

This study investigates the development and acoustic performance of a layered composite structure composed of plastic microfibre pulp (PMFP) and plastic acoustic metamaterial (PAM), designed for low-frequency sound wave control. Addressing the dual challenge of reducing plastic waste and enhancing low-frequency sound absorption, the research explores the use of recycled plastic materials in innovative acoustic applications. PMFP was tested in various thicknesses, while PAM resonators were 3D-printed with adjustable neck lengths and diameters to achieve optimal sound absorption characteristics. The acoustic performance of the composite materials was evaluated in an impedance tube according to ISO 10534-2 for their sound-absorbing properties. Results showed that with increasing thickness of the PMFP sound, absorption peaks shifted to the lower frequencies, with 50 mm thick samples exhibiting a peak absorption coefficient of 0.74 at 500 Hz. When combined with PAM, the structure’s effectiveness increased significantly, with the best-performing configuration (PAM with a 4.5 mm neck diameter and 3.0 mm neck length) achieving a highest absorption coefficient of 0.93 at 1250 Hz. Moreover, introducing air gaps behind the composite material further enhanced low-frequency absorption, with a 50 mm air gap shifting the peak sound absorption to 315 Hz, reaching a coefficient of 0.87. The study concludes that the layered composite structure, with recycled plastic waste, provides an effective and environmentally sustainable solution for low-frequency sound absorption. This makes it suitable for use in various industrial and environmental noise control applications, offering an innovative approach to control noise in indoor areas while addressing plastic waste concerns.