Hybrid metamaterials based on labyrinthine and Helmholtz resonators for low-frequency noise attenuation

Low-frequency noise remains a major challenge in industries such as automotive, aerospace, and building acoustics. Conventional solutions often rely on bulky, heavy materials, which are impractical where strict weight and space limitations apply. In this study, thin and lightweight gradient and non-gradient (NG) acoustic metamaterials (AMs) are proposed as innovative alternatives for efficient noise attenuation in such constrained environments. Inspired by Helmholtz and labyrinthine resonators, both types of structures were designed and fabricated via additive manufacturing and experimentally characterized for sound absorption and sound transmission loss using an impedance tube. The proposed configurations achieved an average transmission loss improvement of 15 dB below 750 Hz and exhibited enhanced resonance characteristics, with the first resonance shifted to 1000 Hz, approximately 400 Hz higher than the simple reference model, leading to superior blocking performance. In addition, several configurations generated multiple resonances within the same frequency band, further broadening their attenuation capacity. Strong absorption performance was also observed, with coefficients in the range of 0.7–0.8 in the 200–250 Hz frequency range, significantly enhancing low-frequency noise control. Despite their high performance, the samples maintained a thickness of only 1 cm and a weight of about 0.95 kg/m2, showing their potential as lightweight, space-efficient solutions for low-frequency noise insulation in demanding industrial applications.