The sound, which is sent by the air, enters into the fibrous insulation material and stays in the pores of the material.
The sound also stays inside of the holes of the hollow cellulose fibers of the insulation. The cellulose wool is a strong sound insulation material.
Moreover, the places with bad insulation reduce the sound insulation of the dividing wall. The sound bridges appear in the places of the slab insulation connections.
It should be noted that the slabs shrink during the operation. If you use the cellulose wool, which is a joint-free, and shrink-resistant insulation material, the above way of sound penetration will be excluded.