Noise Levels of Turbojet‐ and Turbofan‐Powered Aircraft

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.