image: Fig. 1. The physical mechanism of ultra-broadband MMAs based on MRM systems.
Credit: ©Science China Press
Controlling low-frequency broadband sound using compact structures remains a core challenge in acoustics. Achieving efficient, wideband sound absorption with lightweight, thin materials is vital for noise reduction in aerospace systems and for managing complex acoustic environments. Conventional absorbers—such as porous materials and resonant elements—are constrained by size, mass, and modal limitations, making it difficult to achieve both low-frequency performance and ultrabroadband response within limited space. These challenges have significantly restricted their application in advanced noise control technologies. Overcoming the physical limits of compact, broadband low-frequency absorption by engineering tailored modal distributions and energy dissipation mechanisms has thus become a central focus in acoustic research.
Recently, the research team led by Professor Yong Li from Tongji University, along with Professor Jie Zhu’s team from Tongji University and Professor Din Ping Tsai’s team from City University of Hong Kong, successfully developed a metamaterial absorber (MMA) spanning seven octaves (100 Hz–12,800 Hz). The study introduced a quality-factor-weighted (Q-weighted) mode density modulation mechanism, enabling precise control of mode density, resonance frequencies, and loss characteristics. This approach significantly enhanced bandwidth and absorption efficiency, resulting in an absorber whose operational range nearly covers the entire audible spectrum. The findings were published in National Science Review under the title "Seven-octave ultrabroadband metamaterial absorbers via Q-weighted mode density modulation", with Tongji University doctoral candidate Nengyin Wang and City University of Hong Kong postdoctoral researchers Sibo Huang and Zhiling Zhou as co-first authors.
By precisely tuning resonance characteristics using cascaded Helmholtz resonators and an ultrathin wire mesh, the absorber demonstrates an average absorption coefficient of 0.944 across the 100 Hz–12,800 Hz frequency range, with a thickness approaching the minimum required by causal constraints.
This study establishes a new paradigm for designing high-performance broadband absorbers. Its compact profile and ultrabroadband performance show promising applications in aerospace noise reduction and precision acoustic environment control. Furthermore, the proposed Q-weighted mode density modulation theory exhibits universal applicability, potentially extending to other wave systems such as electromagnetic and elastic waves, offering new insights into wave modulation in multiple fields.