code: def many_psds(k=2,fs=1.0, b0=1.0, N=1024):
      
      psd=[]
      for j in range(k):
          print j
          x = noise.white(N=2*4096,b0=b0,fs=fs)
          f, tmp = noise.numpy_psd(x,fs)
          if j==0:
              psd = tmp
          else:
              psd = psd + tmp
      return f, psd/k
docs: compute average of many PSDs