Ultra-thin integrated structure for low-frequency sound absorption and muffling

Low-frequency resonant acoustic structures possess similar underlying principles in sound absorption and muffling, but absorption and muffling are applied in different contexts. In this study, we propose an innovative ultra-thin noise reduction structure that integrates sound absorption and muffling functionalities. By modifying the cavity depth distribution and utilizing Helmholtz resonance with an extended inner neck, the design significantly reduces structural thickness while maintaining high noise reduction performance. The sound absorption coefficient and sound transmission loss are calculated using the transfer matrix method. The structure, with a thickness of only 1/73 of the target wavelength, achieves a near-unity sound absorption coefficient under normal incidence. Furthermore, the results demonstrate that when implemented in a ventilation duct, the structure enhances sound transmission loss by 17 dB, leveraging cavity resonance. Under ventilated conditions, low-frequency sound insulation is further improved through Fano resonance, introducing additional insulation peaks. By fine-tuning the parameters of localized units, broadband resonance is achieved, effectively expanding the noise reduction bandwidth. These findings offer a novel approach for compact, high-performance low-frequency noise control, with potential applications in various engineering and industrial environments.