We have performed measurements of the low-frequency spectra of the refractive index and the extinction coefficient of solutions of acetone in cyclohexane at concentrations ranging between 27 and 135 mM by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. We have verified Beer's law for the concentration range of our experiment, which establishes the fact that the solute-solvent interaction has a significant contribution to the observed spectra, whereas the interaction between the solute molecules is negligible in the present experimental conditions. Combining the data reported by Vij and Hufnagel (J. Phys. Chem., 1991, 95, 6142) with the results of the present experiments, an analytical model for the time correlation function for the total dipole moment of the system was examined in order to reproduce the low-frequency spectra. The molecular origins of the components in the TCF are discussed in light of the results of our simulation.