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`.

def _calc_sdof_tf(self, osc_freq, damping=0.05):
        """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`.
        """
        return (-osc_freq ** 2. / (np.square(self.freqs) - np.square(osc_freq)
                                   - 2.j * damping * osc_freq * self.freqs))