Technical Booklet G Resistance to the Passage of Sound·Page 53·2.106
Minimum mass requirement for masonry inner leaf with separating floor
Where there is a separating floor, the masonry inner leaf of an external cavity wall must have a minimum mass per unit area of at least 120 kg/m² excluding finish.
Where there is a separating floor the masonry inner leaf of the external wall should have a minimum mass per unit area of at least 120 kg/m2 excluding finish.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet G - Resistance to the Passage of Sound.pdf