EFFECT OF PROTECTIVE LAYER ON THE PERFORMANCE OF ABSORPTIVE DUCTS

Acoustically absorptive lining in ducts and parallel-baffle mufflers i s invariably protected from the abrasive effect of grazing flow and du st by means of a protective layer in the form of highly porous thin me tallic plate or cloth, or a very thin impervious layer of synthetic ma terial 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 bu lk reacting as well as locally reacting absorptive linings protected b y a perforated or un-perforated layer, These simple models based upon a stationary inviscid medium are then used in a parametric study to pr ovide noise control engineers with necessary design information, The e ffect of grazing flow on the impedance of a perforated protective plat e is duly accounted for in the model. (C) 1997 Institute of Noise Cont rol Engineering.