增加旋翼桨叶数量也是一种方法，这使得每片桨叶负载的直升机重量降低了，因此产生的噪音更小。

Then there's increasing the number of rotor blades. This helps because each additional blade carries less of the weight of the helicopter, and thus produces less noise.

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与在高空飞行的飞机不同，直升机不容易被雷达发现，因为它的飞行高度较低，且常常被地形所隐藏，因此最大的问题是噪音。

Unlike high altitude aircraft, helicopters aren't so vulnerable to being seen by radar, as they fly low and are often hidden by the terrain. The big issue is the sound.

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最后一天的早上，学校举办了一场特殊的聚会，但盘旋在头顶的直升机让我们难以听到噪音以外的其他声音。

We had a special assembly that final morning but it was hard to hear with the noise of helicopters overhead.

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