Complex composite structures manufactured using a low-pressure vacuum bag-only (VBO) method are more susceptible to defects than flat laminates because of the presence of complex compaction conditions at corners. This study investigates the contribution of multivariate processing parameters such as bagging techniques, curing profiles, and laminate structures on laminates’ shape conformation. Nine sets of laminates were produced with a concave corner and another nine sets with a convex corner, both with a 45° inclined structure. Three-way analysis of variance (ANOVA) was performed to quantify thickness variation and spring effect of laminated composites. The analysis for concave and convex corners showed that the bagging techniques is the main factor in controlling the laminate thickness for complex shape applications. The modified (single) vacuum-bag-only (MSVB) technique appeared to be superior when compared to other bagging techniques, exhibiting the least coefficients of variation of 0.015 and 0.016 in composites with concave and convex corners, respectively. Curing profiles and their interaction with bagging techniques showed no statistical significance in the contribution toward laminate thickness variation. The spring effect of laminated composites was investigated by calculating the coefficient of determination (R²) relative to that of the mold. The specimens exhibited a good agreement with R² values ranging from 0.9824 to 0.9946, with no major data offset. This study provides guidelines to reduce thickness variations and spring effect in laminated composites with complex shapes by the optimum selection of processing parameters for prepreg processing.

Selective frequency band filtration enables targeted acoustic wave attenuation, with applications in noise control, telecommunications, and vibro-acoustic systems. In passive noise cancellation earmuffs, selective frequency filtration can be used to address the challenge of workers needing to remove earmuffs for communication exposing them to harmful noise levels. By integrating periodic lattice structures, earmuffs can selectively allow safe low- to mid-frequency sounds to pass through while blocking harmful high-frequency noise. This study proposes a novel maltese-cross-honeycomb unit cell design, integrating Bragg scattering, and local resonance mechanisms through the Helmholtz resonator concept into a lattice structure to achieve broadband and selective sound filtration. Sound absorption and transmission loss were experimentally characterized using an impedance tube, analysing the effects of thickness, cell arrangement, and cell opening orientation. A validated finite element modelling model was used to study structural displacements and sound pressure levels (SPLs) within the impedance tube, correlating SPL values with absorption coefficients. The proposed metamaterial design exhibits a peak sound absorption coefficient of approximately 0.99 with high sound absorption retention across the frequency band of 3000–6400 Hz, with a transmission loss of 20–25 dB. On contrary, the published triangular reentrant honeycomb design could not exhibit retention of sound absorption across the frequency range, and the transmission loss value of only 10–12 dB was achieved. The optimized metamaterial design was then adopted as an insert for earmuff application demonstrating required hearing protection and speech intelligibility (up to 95 dB pink noise). Based on these findings, the maltese-cross-honeycomb unit cell metamaterial design is proposed to be adopted for earmuff applications to show selective frequency filtration by allowing low to mid-frequency waves (speech up to 3000 Hz) to pass through while blocking off the unwanted high-frequency sound waves.