code: def find_xdb_bin(wnd, power=.5, res=1500):
    
    spectrum = dB20(rfft(wnd, res * len(wnd)))
    root_at_xdb = spectrum - spectrum[0] - dB10(power)
    return next(i for i, el in enumerate(zcross(root_at_xdb)) if el) / res
docs: A not so fast way to find the x-dB cutoff frequency "bin" index.
  
    Parameters
    ----------
    wnd:
      The window itself as an iterable.
    power:
      The power value (squared amplitude) where the x-dB value should lie,
      using ``x = dB10(power)``.
    res :
      Zero-padding factor. 1 for no zero-padding, 2 for twice the length, etc..