FFT变换结果为对称型，矩形窗是使信号突然截断，旁瓣会很大，且衰减较慢，旁瓣的第一个负 峰值为主瓣的21％，第一个正峰值为主瓣的12.6％，第二个负峰值为主瓣的9％，效果一般，泄漏较大。

-FFT, the result is symmetric, rectangular window is suddenly cut off the signal, the side lobe would be great, and the slow decay, the first negative sidelobe peak of the main valve　to 21 , the first positive peak of the main lobe of the 12.6　, the second negative peak of the main valve 9 , the effect is general, larger leaks.