It is important to stress the logarithmic character of acoustics, since the sum in logarithms measns that the results are not always the expected ones. In a barrier where we have an isolation of 100 dB in 90 % of the surface and airborne of dB in 10 % of the surface, we will obtain airborne global reduction of 19,99 dB.
That is to say, a small defect in the isolation of a partition can cause a drastic fall in the isolation of that partition. That is the case where there are cable ducts, etc.