The inlet and exhaust noise control of industrial applications like gas turbines, locomotives and diesel generators calls for mufflers with large cross sectional area. These mufflers have overriding design constraints like back pressure, geometry envelope and weight. In this paper, a rectangular duct with lining similar to an absorptive muffler with airgap at the back has been proposed in order to improve the acoustic performance of dissipative muffler at low frequencies. A three dimensional model is presented for evaluating acoustic performance of this new configuration by modeling it as a symmetrical rectangular duct with bulk reacting absorptive lining, protected from hot exhaust gases by a perforated sheet. This computational model has been validated for self-consistency and then used in a parametric study to understand effect of the airgap on acoustic performance and also to provide noise control engineers with necessary design information.