code: def _calc_sdof_tf(self, osc_freq, damping=0.05):
          
          return (-osc_freq ** 2. / (np.square(self.freqs) - np.square(osc_freq)
                                     - 2.j * damping * osc_freq * self.freqs))
docs: Compute the transfer function for a single-degree-of-freedom
          oscillator.
  
          The transfer function computes the pseudo-spectral acceleration.
  
          Parameters
          ----------
          osc_freq : float
              natural frequency of the oscillator [Hz]
          damping : float, optional
              damping ratio of the oscillator in decimal. Default value is
              0.05, or 5%.
  
          Returns
          -------
          tf : :class:`numpy.ndarray`
              Complex-valued transfer function with length equal to `self.freq`.