Viscoelastic structural damping enables broadband low-frequency sound absorption

Low-frequency sound absorption has traditionally relied on air-resonant structures, such as Helmholtz resonators, which are made of stiff materials that undergo negligible deformation. In these systems, energy dissipation arises primarily from air motion and thermal-viscous effects, resulting in inherently narrowband performance and bulky, complex designs for broadband absorption. Here, we presented a composite acoustic metamaterial that replaces the high-stiffness neck of a Helmholtz resonator with a soft, viscoelastic cylindrical shell. This structural modification enables material deformation and shifts the dominant energy dissipation mechanism from air resonance to intrinsic viscoelastic damping. A single unit achieves over 97% absorption across a broad low-frequency range (227 to 329 Hz) with deep-subwavelength thickness (λ/15 at 227 Hz). We developed a discretized impedance model that quantitatively links material properties and geometry to absorption behavior. Our results established a materials-centered design paradigm in which both material selection and geometry serve as coequal, tunable parameters for compact, broadband low-frequency sound control.