i developed this code for my program, it may help you
a=wavread('1.wav');
ah=a.*hann(length(a));
aN=length(ah);
indexa=[0:1:aN-1];
fs=8000;
fa=[0:1:aN-1]*fs/aN;
aff=abs(fft(ah))/aN;
figure % for siren 1
plot (fa,aff);
xlabel(' FFT of Siren 1');
n = 1; % Filter order
lf = .5e3; % low frequency cutoff for bandpass
hf = 1.5e3; % high frequency cutoff for bandpass
[b,a] = butter(n, [lf hf]/(fs/2));
fout = filter(b,a,ah);
% Plot Spectrum figure
spec = fft(fout,nfft);
pfspec = spec(1:fix(nfft/2)); % Extract postive frequencies
plot(pfax/1e3,abs(pfspec))
xlabel('Hz')
ylabel('Magnitude')
title('Filter Signal Spectrum ')
