code: def _calc_fourier_spectrum(self, fa_length=None):
          
  
          if fa_length is None:
              # Use the next power of 2 for the length
              n = 1
              while n < self.accels.size:
                  n <<= 1
          else:
              n = fa_length
  
          self._fourier_amps = np.fft.rfft(self._accels, n)
  
          freq_step = 1. / (2 * self._time_step * (n / 2))
          self._freqs = freq_step * np.arange(1 + n / 2)
docs: Compute the Fourier Amplitude Spectrum of the time series.