Lightweight sound-absorbing metastructures with perforated fish-belly panels

• The perforated fish-belly panel provides a new degree of freedom for sound absorption design. • Effective impedance theory is developed which can be extended to other sandwich structures. • Four-units coupling metastructure with reinforcement learning design is proposed for broadband absorption. • A figure of merit revealing a global multifunctional evaluation is discussed. We propose a perforated metastructure combining fish-belly panels and a corrugated sandwich structure for broadband sound absorption. The curved panels provide a new degree of freedom for sound absorption while the sandwich structure ensures a very good mechanical strength. We develop an effective impedance theory to study the relationship between sound absorption performance and geometric factors. To achieve a broadband absorption, we further propose a second original metastructure based on a 4-units coupling mechanism and the optimal structure is designed with the help of reinforcement learning algorithm. Finally, we define and discuss a figure of merit that reveals a global evaluation of both the acoustical and mechanical performances. Our work provides a promising platform, namely lightweight multifunctional structures for noise and load-bearing control in various industries with large-scale and inexpensive commercial manufacture technologies.