说明：  IIR平滑器的作用和特点演示，包括b取何值时，信号部分通过系统后不变？噪声部分是否放大了？计算NRR和系统频响
(IIR smoother presentation of the role and characteristics, including the b take what value, the signal in part through the system after the change? Whether the part of amplified noise? NRR and system frequency response calculation)