We were called in recently by a construction company to see if we could help in solve some noise transmission problems they had on some new school buildings.
The problem had arisen with the first floor level, inter classroom walls where these walls had failed their Part ‘E’ building regulation target. We eventually traced the problem to the noise flanking due to it being able to pass through the perforated purlins into the roof void and then back into the next room via the perforated purlins in the other classroom. The internal walls were not closed up to the external roof and the fact that there was only mineral wool above the wall joint meant that there was insufficient mass to prevent the noise transmission.
The client was facing two choices. The first was to take the roof off and construct a mass barrier between the inter classroom walls and the roof sheet or try and fix the problem internally. We finally came up with a solution which, in theory, would provide the extra performance needed.
The internal roof of the top floor classrooms was made using a continuous folded corrugated purlin sheet arrangement. Above these perforated purlins was 300mm mineral wool with a glass tissue to stop the fibres entering the classroom. This arrangement was a cost effective way of reducing reverberation within the room. The purlins were spaced about 150mm apart and their depth was about 125mm. There was no way into what were effectively perforated tubes passing from one room to another so our solution was as follows.
We are all familiar with the normal splitter attenuator arrangement where mineral wool splitters are fixed into a case leaving air gaps along there length. We decided that if we boarded each side of the walls with 600mm wide laminate board, to act as the case, and then were to pack the gaps between the purlins between the boards and the roof, this would form a 600mm long attenuator. The result was a neat, compact looking job which actually worked.