信封的摆动频率是描述的灰色地带。
The envelope of the beat frequency is depicted as the grey areas.
信封的摆动频率将永远固定在空间；没有角速度。
The envelope of the beat frequency will always be stationary in space; no angular velocity.
通常当闸门的水平摆动频率接近索的固有频率时，将产生横向共振。
In general, the flexural resonance occurs when the frequency of horizontal motion of the gate approaches the natural frequency of the cable.
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