Effect of protective layer on the performance of absorptive ducts

Acoustically absorptive lining in ducts and parallel-baffle mufflers is invariably protected from the abrasive effect of grazing flow and dust by means of a protective layer in the form of highly porous thin metallic plate or cloth, or a very thin impervious layer of synthetic material like Mylar. An investigation of effects of protective layers on the acoustic performance of lined circular ducts and parallel-baffle mufflers (modeled as two-dimensional rectangular ducts) is the subject of this paper. Analytical models are presented for both cases with bulk reacting as well as locally reacting absorptive linings protected by a perforated or un-perforated layer, These simple models based upon a stationary inviscid medium are then used in a parametric study to provide noise control engineers with necessary design information, The effect of grazing flow on the impedance of a perforated protective plate is duly accounted for in the model.