Low-frequency sound transmission in a cross-laminated timber apartment building can result in annoyance, even if the acoustic requirements of the building code are fulfilled. Cross-laminated timber is an upcoming material in the building industry. But there are also problems arising when building with this relatively new material. CLT is a lightweight building material, which means that the mass itself is insufficient for meeting the acoustical requirements. The total sound transmission depends on the amount of direct sound and flanking sound transmission. The ISO 12354 standard used to determine the amount of sound transmission between rooms does not include frequencies below 100 Hz. Even though these low-frequency sounds are the main cause of annoyance in lightweight construction buildings. The low-frequency sound transmission is hard to measure because of the long wavelength. In this research, a numerical model was developed and used to determine the low-frequency sound transmission in CLT apartment buildings. The effect of the following sound-reducing measures are studied: the material properties of the material CLT, additional linings on the room separating elements, use of elastic interlayers between CLT panels and the type and number of connectors connecting the CLT panels. These are all common sound-reducing measures for lightweight constructions. The effect of the material properties of the CLT and the effect of additional linings are computed by a numerical direct sound transmission model. The effects of elastic interlayer and connectors between the CLT panels are computed with a numerical vibration reduction index model. The results of the numerical models are compared to measurements found in literature and the sound transmission according to the ISO standard. The results showed the importance of Young’s modulus in the y and z-direction, these influence the location of the resonance induced dips in the sound insulation. The internal loss factor of the CLT panels influenced the height of the dips. A loss factor of 20 % resulted in results most similar to the measurements. The direct sound transmission through a bare CLT panel can be predicted within a range of 3 dB difference with measurements. The prediction of the ISO standard is within a range of 5 dB with the measured values. The vibration reduction index between panels without interlayers is modelled with frictional contact regions between the panels. The numerical results showed similarities with the measurement results for the vibration reduction index of panels with screwed connectors. The ISO standard significantly overpredicts the vibration reduction index of CLT junctions. The effect of the elastic interlayer showed insignificant improvements in the frequency range 50-500 Hz, the additional reduction stays below 3 dB. The ISO standard does not include a method to determine the effect of elastic interlayers or connectors between CLT panels. The numerical models prove that is possible to predict the low-frequency sound transmission in CLT apartments. Important notes are that the CLT lamellas need to be modelled separately. Only in this way the model is able to capture the sound that goes through a structure within a range of 3 dB. In order to test the effect of additional lining, the material properties need to be known. The vibration transmission between panels in the junctions is more complex, as it depends on more design parameters. A frictionally bonded contact region between the panels results in vibration reduction indices that are in line with measurement results of panels with several connectors. The effect of the elastic interlayer is minimal in the low-frequency range, but the results are similar to measurement results. Both the direct sound transmission model and the vibration reduction model were influenced by the boundary conditions.