The problem isn't the engine itself; that is actually relatively quiet. It's the rotors that make the classic "whup-whup" sound.

问题不在于相对来说更安静的发动机，而在于发出经典的“呼呼”声的旋翼。

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The same principles can be applied to quiet down a jet engine or silence the open bays of a military craft.

同样的原理可用来使喷气发动机变得安静或使军用飞机在打开舱门时变得安静。

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You don't get the usual rumbling of an idling engine, just the whisper-quiet whirl of the e-tron's motor (Audi is currently engineering an "e-tron sound" so that outsiders will be able to hear the car) and wind whizzing by.

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And with no engine noise, the Model S rolls with a quiet coolness.

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Two-and-half-hour stints are a breeze, thanks to the surprising suppleness of the suspension, potent engine, smooth-shifting seven-speed automatic transmission and quiet interior.

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