Influence of Bimodal Pore Architecture on Broadband Acoustic Absorption and Mass Reduction in AlSi Open-Cell Structures

Lightweight materials capable of simultaneously providing noise and mass reduction are increasingly required in aerospace applications. This paper investigates the acoustic performance of a novel hierarchical bimodal architecture in open-cell AlSi porous structures, integrating μm-sized macro-pores (200–400 μm) and mm-sized macro-pores (2–2.5 mm) at 60% porosity. The bimodal specimen was produced through replication casting, and its porosity was evaluated using gravimetric measurements and image-based surface analysis, while acoustic performance was experimentally characterized using the two-microphone impedance tube method in accordance with ISO 10534-2:2023, covering a frequency range from 500 to 6500 Hz. The resulting frequency-averaged sound absorption coefficients (αFA) were 0.518, 0.72 and 0.844 for the μm-sized macro-porous, mm-sized macro-porous and bimodal structures, respectively. Furthermore, the proposed architecture improved the mass-to-acoustic efficiency ratio η by 125% compared to the μm-sized macro-pores structure and approximately 28.5% relative to the mm-sized macro-pore sample. Based on these values, bimodal architecture is considered to be the most acoustic efficient lightweight solution, among all specimens studied, demonstrating that hierarchical pore architectures can effectively enhance acoustic absorption, while maintaining low structural weight, highlighting the potential of bimodal open-cell AlSi structures for lightweight aerospace noise-control applications.