下面的代码中出了什么问题呀？滤波前和滤波后频谱没发生什么变化，为什么会滤不了波
%设计带阻滤波器
[x0,Fs]=audioread('luyin.wav');
N=length(x0);
Fs2=8000;
fp1=1000;
fp2=3500;
fs1=1500;
fs2=2800;
Wp1=fp1/Fs2*2*pi;
Wp2=fp2/Fs2*2*pi;
Ws1=fs1/Fs2*2*pi;
Ws2=fs2/Fs2*2*pi;
Wp=[Wp1,Wp2];
Ws=[Ws1,Ws2];
Rp=0.01;
Rs=80;
[n,Wn]=buttord(Wp/pi,Ws/pi,Rp,Rs,'s');
[b,a]=butter(n,Wn,'stop');
% [H,w]=freqz(b,a);
% magnitude=20*log10(abs(H));
% plot(w/(2*pi)*Fs2,magnitude)
y=filter(b,a,x0);
Y=fft(y,N);
n=(1:N);
f=n*Fs/N;
subplot(3,1,2)
plot(f,abs(Y))
xlabel('f/Hz');
ylabel('magnitude');
title('滤波后音频信号傅里叶变换后的幅度')