It is a self frequency double laser in some hosts.

选择合适的基质，可产生自倍频可见激光。

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The variable speed constant frequency double-fed wind power system is taken as the research object and the control problem of maximum wind energy tracing below the rated wind speed is studied.

以变速恒频双馈风力发电系统为研究对象，研究了额定风速以下风能的最大追踪控制问题。

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Another experiment demonstrated that physical exercise, which was sufficiently vigorous to double the rate of breathing, had no effect on the frequency of yawning.

另一项实验表明，体育运动——尤其是剧烈的运动——可以使呼吸频率增加一倍，但这对打哈欠的频率没有影响。

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The Swiss Federal Railways has improved efficiency in recent years too, by using double-decker trains and increasing its trip frequency.

BBC: Are the Swiss really that efficient?

The SS-AR2's incorporated drivers support a maximum input power of 100 watts through a 25 mm (one inch) soft dome, a 130 mm (five inch) sliced-paper cone and a 165 mm (six and a half inches) aluminum cone (double-drive system), producing a frequency response from 42 Hz to 60kHz.

ENGADGET: Sony reveals SS-AR2 loudspeaker, wants an invite to your next house party