Noise Transmission

Noise transmission through wall and floor systems is important because the better the system is at reducing noise transmission the more comfortable the living environment.

The NZ Building Code (section G6) specifies minimum requirements for noise transmission reduction for walls and floors that are common between two occupancies (intertenancy). The code specifies a Sound Transmission Class (STC) of not less than 55 for walls & floors. It also specifies an Impact Insulation Class (IIC) for floors of not less than 55. STC rating relates to airborne noise transmission whereas IIC rating relates to impact noise transmission.

Although noise transmission requirements of the building code do not apply to stand alone homes, the living environment within these homes can be significantly improved by using concrete wall and floor systems which inherently have good noise transmission reduction properties.

Typical STC values for concrete walls

The following are typical STC values that have been obtained from testing conducted by the Prestressed Concrete Association in the USA for normal weight concrete.

Wall Systems STC Rating
100 mm concrete wall 44
150 mm concrete wall 55
200 mm concrete wall 58

Floor Systems

STC Rating

150 mm precast or insitu concrete with carpet on underlay 52
200 mm precast or insitu concrete with carpet on underlay 56

Openings

Even small openings can significantly reduce effectiveness. It is important to ensure there are no gaps that sound can pass through. Small gaps around services and unfilled joints must be adequately filled

Indirect noise transmission

Connection details and sound transmission properties of floors and ceilings are also important. The effectiveness of a wall in reducing sound transmission may be reduced if the sound can travel through the ceiling into the next room.

Concrete Masonry

The following figure from the New Zealand Concrete Masonry Manual illustrates the relationship between STC and the mass of the wall.

Sealing masonry walls

Because concrete masonry blocks are porous there will be some 'leaking' of sound. Sealing a block wall (usually with paint) has been shown to increase the STC rating by 3-5 STC points.

Kg/m² of wall area

Block Density 1750 kg/m³ All cores filled 2nd cores filled 3rd cores filled 4th cores filled No cells filled
100 mm 170 150 145 135 110
150 mm 290 210 185 170 130
200 mm 400 280 245 220 160
Block Density 2200 kg/m³  
100 mm 205 180 175 165 140
150 mm 335 285 260 250 225
200 mm 450 340 310 290 240