7 NOISE FROM TWO-BLADE PROPELLERS other. The result is distortion and the introduction of noise to decrease 7 decibels. It is interesting to note new frequencies that will contribute to the loudness that the drop occurs mostly in front, where the high- level. Especially is this true if the… … Figure 8.—Loudness levels about propeller at five distances. instrument readings are slightly higher than the calcu- components are modulated at the frequency of rota- lated values at 120° where the noise consists almost tion. Such high intensity levels have been studied in the N. A. C. A. sound…
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