揭示出冷空气的向南传播主要是一种低频模态的10—20天周期的振荡。
It has been revealed that the propagation of cold air mainly shows a mode of 10—20 day period oscillation.
三波共振相互作用低频振荡周期与初始扰动振幅有很强的相关关系；
The low-frequency fluctuation periods and the initial amplitudes have close relationship in the resonance interaction.
反映滚动轴承故障的特征周期信号处于较低频带内，容易被噪声淹没，难以检测。
The fault characteristic periodic signals of the rolling bearing are in low frequency band and often buried in the noise.
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