Spatial Acoustic-Function Partitioning in Gradient Porous Materials for Broadband Sound Absorption

The growing demand for noise control in transportation, urban environments, and advanced equipment systems is driving sound-absorbing materials toward broader bandwidths, lower weight, and more compact configurations. Conventional porous materials dissipate acoustic energy mainly through thermoviscous friction near pore walls. Under low-frequency and thin-layer conditions, however, their performance is often constrained by the need to reduce incident reflection while maintaining sufficient energy loss within a limited thickness. Existing reviews on porous sound-absorbing materials have mostly been organized around material categories, structural parameters, theoretical models, or fabrication routes, while the common physical basis by which different gradient architectures achieve broadband absorption has rarely been discussed from a unified sound-energy management perspective. Here, we use spatial acoustic-function partitioning as an interpretive framework linking depth-dependent structural variation to the frequency-dependent input impedance, internal acoustic field, and irreversible energy loss. These acoustic functions are not tied to fixed geometric layers and may shift with frequency, boundary conditions, and the gradient profile. The discussion draws on established acoustic models to connect depth-dependent structural variation with impedance, field redistribution, and irreversible loss. Three representative routes are considered: parameter gradients in dual-gradient microperforated networks, multiscale interfacial gradients in gradient fiber/aerogel–porous scaffold composites, and continuous topological gradients in triply periodic minimal surface (TPMS) structures. The roles of additive manufacturing and intelligent design in realizing and optimizing these structures are also discussed. Representative quantitative studies show that no single-gradient profile is universally optimal: Monotonic, reversed, and non-monotonic profiles can each be favorable depending on the complete absorber configuration and target frequency range. Manufacturing fidelity and physics-constrained inverse design are discussed in terms of whether numerically optimized structures can be fabricated reliably and tested experimentally.