Imaginary Part of Desired Impulse Response

Apply window to ideal impulse response:

hw = wzp .* h; % Final FIR coefficients
Hw = fft(hw);  % Frequency response we'll see

Total stopband energy:

ierr = norm(Hw(N/2+2:N))/norm(Hw)
disp(sprintf(['Stop-band energy is %g percent of',...
             total spectral energy'], 100*ierr));