Discussion
Which of the following would not reduce jet noise?
Increasing the exhaust velocity
Raising the bypass ratio
Fitting chevrons to the nozzle
Adding acoustic lining to the nacelle
Answer: A
Correct answer: Increasing the exhaust velocity
Raising the exhaust velocity increases jet noise, and steeply, because acoustic power varies as the eighth power of that velocity. The other three all reduce noise: raising bypass ratio lowers the jet velocity directly, chevrons promote a gentler mixing between the streams, and acoustic lining absorbs fan and turbine noise propagating in the ducts - though lining cannot reach the jet noise generated outside the nacelle.
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