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

Abstract

In this work, acoustic damping performances of double-layer in-duct perforated plates are studied at low Mach (Ma) and Helmholtz number (He) to evaluate the effects of (1) Ma, (2) the porosities (i.e., open-area ratio) σ1 and σ2 of the front and back plates, and (3) the axial distance Lc 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 Ma = 0, α is experimentally found to oscillate with He, whatever the porosities of σ1 and σ2 are set. However, when Ma is increased to and above 0.037, the power absorption troughs, i.e., local αmin of the double-layer plates with σ1,2 ≤ 9% are more separated and shallower. Furthermore, when σ1= 9% or σ2 = 9%, the damping performances are quite different in terms of the local αmax peaks and their number. In addition, increasing Lc with respect to the downstream pipe length Ld gives rise to an increase of αmin and αmax 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.