Abstract
Results of ground runup, flyover, and interior noise-level surveys of DC-8's equipped with suppressed JT3C-6 turbojet and unsuppressed JT3D-1 turbofan engines are shown. At takeoff power during ground runup, the turbofan engine, producing 3500 pounds more thrust than the turbojet engine, generated a total acoustic power level which was about 2 dB lower. Flyover noise measurements showed that the over-all sound-pressure levels were about 5 dB lower with the turbofan installation; the perceived noise levels were about 5 PNdB lower. These differences were due to the change in exhaust-nozzle configuration and jet-exhaust velocity. Cabin-noise levels at cruise conditions were about the same in both types of airplanes, due to the dominance of boundary-layer noise.
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