Abstract
The aft interior of the DC-9 aircraft is generally considered to be uncomfortably loud (100–110 dBC). This noise is primarily periodic and is caused by imbalances in the fan and turbine of the Pratt Whitney JT8D engine. Lord Corporation has recently developed active and adaptive passive systems to combat this problem. The active system uses four force-generating actuators located on the left and right engine pylons to minimize the response at eight control microphones which are located in the rear of the cabin. The adaptive passive system consists of eight tuned vibration absorbers whose natural frequency is adapted to the engine speed. An overview of each system with theory will be presented as well as experimental results where the tonal noise reduction and engine speed tracking capabilities of each system were evaluated using an identical engine vibration input on a DC-9 fuselage. These experimental results showed that the active system achieved larger tonal noise reductions and had faster tracking ability than the adaptive passive system.
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