Acoustic Performance of Circular Silencers with Sequential Mixed-Perforation Liners: Experimental Insertion Loss and Maa Impedance Analysis

This study investigates the acoustic and aerodynamic performance of cylindrical silencers incorporating uniform and sequential mixed-perforation liners under grazing-flow conditions. Five uniform perforated panels with hole diameters ranging from 0.4 to 5 mm and four mixed configurations (0.4/1, 0.6/1, 0.8/1, and 1/5 mm) were analysed. The mixed silencers consisted of two consecutive 450 mm sections. Acoustic performance was experimentally evaluated in the 100–10,000 Hz frequency range at flow velocities of 4, 8, and 12 m/s, including insertion loss, flow-generated noise, and pressure loss. The experimental analysis was complemented by the Maa impedance model and the flow-dependent Rao–Munjal model. Impedance contrast ratios were additionally introduced to characterise differences between consecutive sections of the mixed liners. Maximum insertion-loss values of 35.5–41.0 dB were observed around 2000 Hz. Neither theoretical model exhibited a corresponding characteristic feature around 2000–2500 Hz, indicating that the attenuation maximum cannot be attributed to perforation impedance alone. The results demonstrate that perforation geometry primarily controls local resistive and reactive behaviour, whereas maximum attenuation results from the combined acoustic response of the perforated liner, porous backing, silencer geometry, and wave-propagation conditions. Sequential perforation therefore provides an additional design parameter for tailoring silencer performance without increasing its overall dimensions.