Discussion
Consider the following pairs of measure and its effect on noise:
Raising bypass ratio - Lowers jet velocity and greatly reduces noise
Chevrons - Promote gentler mixing at a small thrust cost
Acoustic lining - Absorbs fan and turbine noise in the duct
Raising jet velocity - Reduces noise power
How many of the pairs given above are correctly matched?
Answer: D
Correct answer: Only three
Raising bypass ratio, chevrons and acoustic lining are all correctly described. The fourth reverses the physics: raising the jet velocity increases the noise power, and steeply, since it goes as the eighth power. Every effective measure against jet noise works by reducing that velocity or by making the mixing with the surrounding air more gradual.
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