Evaluations of acoustic damping performances of double-layer in-duct perforated plates at low Mach and Helmholtz number

In this work, acoustic damping performances of double-layer in-duct perforated plates are studied at low Mach (M_a) and Helmholtz number (He) to evaluate the effects of (1) M a, (2) the porosities (i.e., open-area ratio) σ₁ and σ₂ of the front and back plates, and (3) the axial distance L_c between these two plates. The orifices' damping is characterized by sound absorption coefficient α denoting the fraction of incident sound energy being absorbed. For this, a quasi-steady acoustic model is developed first and experiments are then conducted. When M a = 0, α is experimentally found to oscillate with He, whatever the porosities of σ₁ and σ₂ are set. However, when M a is increased to and above 0.037, the power absorption troughs, i.e., local α_min of the double-layer plates with σ_1,2 ≤ 9% are more separated and shallower. Furthermore, when σ₁ = 9% or σ₂ = 9%, the damping performances are quite different in terms of the local α_max peaks and their number. In addition, increasing L c with respect to the downstream pipe length L_d gives rise to an increase of α_min and α_max by 10%. Finally, the double-layer plates are shown to involve a larger α than that of single-layer one over a broader He range.