Effects of Two DOF Lining Tolerances on Modeled Inlet Acoustic Attenuation

The effect of manufacturing tolerances on realized attenuation for two degree of freedom linings is investigated with the use of lining models and finite element duct propagation codes. Acoustic linings are created for a small turbofan engine that optimizes attenuation at takeoff/sideline and approach conditions. Lining physical and geometric parameters are set which best meet the optimum impedance requirements at two target frequencies. Variations of these parameters representing realistic manufacturing tolerances are used to systematically examine the effect on installed impedance and predicted attenuation. Attenuation at sideline and approach conditions is found to sensitive to manufacturing tolerances around optimum conditions. It is found that local lining impedance variation due to local Sound Pressure Level also has a significant effect on realized attenuation in the sideline case. The results of the study are case dependent; however the analysis scheme presented provides a method for cost-benefit analysis of maintaining tight tolerances on manufacturing processes.