Design, fabrication, and acoustic characterization of novel 3D knitted pyramidal sandwich composites: Experimental, analytical, and numerical investigation

Developing lightweight sound absorbers with effective mid-frequency performance and improved response toward lower frequencies remains challenging because conventional porous materials are often limited at lower frequencies, whereas resonance-based absorbers usually operate over narrow bandwidths. In this study, rigid sandwich composites based on three-dimensional weft-knitted pyramidal preforms were developed as tunable acoustic structures combining porous dissipation, resonance-assisted absorption, and geometry-assisted impedance adaptation. The structures consisted of knitted cores with diamond, square, and rectangular pyramidal geometries combined with a knitted face sheet acting as an equivalent micro-perforated layer. Different yarn combinations of high-tenacity polyester, textured polyester, and acrylic were used to tailor the internal architecture and structural density. Normal-incidence sound absorption coefficients were measured using a two-microphone impedance tube, and the results were interpreted using an equivalent Maa-based analytical framework and finite-element simulations in COMSOL Multiphysics. The addition of the knitted face sheet shifted the dominant absorption peaks toward lower frequencies relative to the corresponding core-only structures and improved broadband acoustic performance. The main absorption peaks of the sandwich structures occurred primarily in the mid-frequency range, between approximately 1707 and 2774 Hz. The maximum absorption coefficient reached 0.96, while the normal-incidence sound absorption average ranged from 0.130 to 0.441. Rectangular structures generally exhibited the highest broadband performance, reflecting the combined influence of cavity shape, effective depth, internal volume, and structural thickness. Acrylic-containing yarn systems provided the highest acoustic efficiency within each geometric series due to increased pore complexity and enhanced viscous–thermal dissipation. The Maa-based model provided a calibrated equivalent interpretation of resonance trends, while finite-element simulations supported the observed pressure-field localization and resonance-assisted absorption mechanism. Overall, the proposed 3D knitted pyramidal sandwich composites demonstrate acoustic potential as lightweight textile-derived structures with tunable mid-frequency normal-incidence sound absorption.