A common way to construct Cross Laminated Timber (CLT) dwellings is to mount the floor slabs continuous over several apartments, thereby reducing the number of CLT elements and work time with the crane on site. There are some crucial acoustic factors to consider if you are thinking about this route. CLT is sensitive to flanking transmission in the middle frequency range from about 125-160 Hz and upwards. If CLT elements are mounted between apartments and are exposed within the dwelling, the resulting sound insulation will not fulfill Swedish requirements. The resulting sound insulation R´w will likely be in the 40-50 dB range and not 52 dB as it should be. A common way to solve this issue is to install a resilient ceiling, and the solution can be optimized in different ways depending on the rest of the structure.
If you are considering an installation floor, a wise approach is to add a suspended ceiling with 22 mm wooden lath, 25 mm acoustic steel profiles and one layer of gypsum. This solution will take care of the horizontal flanking transmission through the CLT ceiling. Another common solution is to use a CLT slab, impact sound insulation 30-40 mm and a heavy screed of 50-60 mm. With this solution, the suspended ceiling will also be necessary to deal with the vertical sound insulation. A typical solution is to use a larger suspension height, perhaps 150-200 mm and two layers of gypsum. This will also take care of the horizontal transmission path through the continuous CLT in the ceiling. A good rule of thumb when working with CLT dwellings is to verify that there are no continuous and exposed CLT elements between apartments! The CLT elements must always be hidden behind sound insulating layers or separated between each apartment (which requires more lifting operations by the crane).
When mounting resilient constructions in general, it is very important to allow for movement. A suspended resilient ceiling must be able to move up and down to properly absorb sound energy. If walls are connected towards the resilient ceiling, its movement will be prohibited, resulting in a large reduction in sound insulation. Although it is tempting from a production perspective to simply mount all of the ceiling first, and then erect the walls, you cannot do that. It must be mounted one room at a time, after the inner walls. The same principle goes for floating ceiling structures by the way, such as the above-mentioned installation floor system or floating heavy screed. It is just as important to not allow any contact whatsoever around the floor boundaries.
Vertical movement can also be prohibited by installations in the ceiling cavity. When mounting a cross pattern of wooden lath and steel acoustic profiles, the largest gaps are thus 22 mm. Any installation that is to be mounted in that cavity (such as electrical) must not exceed this measure and preferably also have some margin of error. If larger installations must pass, one could create a local gap in the wood/steel profile pattern to permit the installation to be mounted. But regardless of solution, the quality control of workmanship becomes a crucial factor to verify to end up with good sound insulation.
There are technical solutions of all acoustic problems in wooden buildings today, but it is still easy to step on acoustic “land mines” if you are not careful. Conclusion: Always seek good consultation in your project as early as possible. You will save a lot of money in the long run.