This guide has been produced to assist people in the selection of a masonry construction system to satisfy statutory requirements for acoustic performance, and to meet the specific objectives of individual projects.
Guidelines for maximising sound performance by attention to correct detailing and the application of a variety of surface finishes are also described.
What the Building Code of Australia Requires
The Building Code of Australia (BCA) requires a wall separating sole occupancy units, or between a sole occupancy unit and a public corridor, plant room, lift shaft, stairway, hallway or the like, to have a Weighted Sound Reduction Index (Rw) not less than 45.
Where a habitable room such as a living room, dining room, family room, bedroom, study and the like, but not including the kitchen, in one sole occupancy unit is situated next to a bathroom, sanitary compartment, kitchen or laundry in an adjoining unit, the wall separating the units must have an Rw not less than 50. In addition, the dividing wall construction must provide a “satisfactory” level of impact sound isolation.
Soil and waste pipes which serve or pass through more than one sole occupancy unit must be separated by a construction which provides a minimum of Rw 45 if adjacent to a habitable room (other than a kitchen), or Rw 30 if the adjacent room is a kitchen, bathroom, laundry or the like.
Note: Rating of sound insulation in buildings and of building elements now refer to a “Weighted Sound Reduction Index (Rw )” to replace “ Sound Transmission Class (STC)”. This was changed in amendment No. 6 to BCA 96.
Weighted Sound Reduction Index (Rw)
The weighted sound reduction index is a single number value which describes the overall acoustic performance of a construction system. It is a measure of the degree of acoustic separation or noise reduction that can be expected of a wall, floor or roof.
Rw ratings are determined by laboratory tests of a specimen of the construction system which is fixed within a frame to form the wall between two test chambers. A high noise level is generated in one room and the difference in sound level between the source room and the receiver room represents the transmission loss through the test specimen. Measurements are conducted in one-third octave bands over all frequencies between 100 Hz and 4000 Hz (inclusive). To determine the Rw rating of a system, the measurement results are compared with reference curves. The total of the deficiencies of the measured values below the reference curve cannot exceed 32db when they are added together.
Impact Sound Insulation
Impact sound insulation is also determined by laboratory testing. A standard ‘tapping machine’ is set to operate on a horizontal steel plate which rests against the test specimen. The sound transmission through the wall is then measured and the impact sound levels are converted to normalised levels using a reference equivalent absorption area of 1.0m2. Adequate impact sound insulation is achieved if the performance of the test specimen is equivalent to, or better than, the performance of a cavity brick construction of two leaves of 90mm bricks under the same test conditions.
Sound Transmission Class (STC)
Sound Transmission Class rating is a single number value which describes the overall acoustic performance of a construction system. It is a measure of the degree of acoustic separation or noise reduction that can be expected of a wall, floor or roof.
STC ratings are determined by laboratory tests of a specimen of the construction system which is fixed within a frame to form the wall between two test chambers. A high noise level is generated in one room and the difference in sound level between the source room and the receiver room represents the transmission loss through the test specimen. Measurements are conducted in one-third octave bands overall frequencies between 125 Hz and 4000 Hz (inclusive). To determine the STC rating of a system, the measurement results are compared with reference curves. The total of the deficiencies of the measured values below the reference curve cannot exceed 32 db when they are added together, and the maximum deficiency allowed for any single value below its corresponding reference value is 8 db.
Noise Reduction Between Spaces
The noise levels received from an adjoining premises are dependent upon the level of the noise generated and the STC rating of the construction between the two dwellings. The perceived loudness of the sound depends not only on the received sound level but also on the background noise level within the receiving room. With the typical background levels in most suburban areas, an STC 45 wall construction, would usually ensure television, telephone ringing and conversation will sound "muffled", but still audible. Unless the background sound level within the receiving room is very low, the transmitted sound should from these sources not be intrusive.
The Sound Transmission Class rating of a construction is usually determined under strictly monitored test conditions within a laboratory, where great care is taken during installation of the test material into the test frame. In practice however, any small gaps and cracks which permit even minor air leakage will also provide a means for sound transmission. This lack of ideal conditions frequently causes the much lower field performance of system. This degradation should be recognised and an appropriate allowance made when selecting a tested system to achieve a particular Sound Transmission Class rating when installed.
Acoustic Performance of Masonry Render and Other Finishes
For a wall to reach its optimum acoustic performance, the construction must be solid without gaps through which air, and therefore sound, can pass.
Rendering one side, or both sides, of a wall increases the STC rating primarily because the render seals the fine pores in the brickwork and also eliminates partially filled and unfilled mortar joints.
In addition, a layer of 13 mm render increases the mass of the wall and there will also be an increase in the acoustic performance. "As the acoustic performance of a material or construction improves, the higher will be the STC value."
When the predicted performance relating to the mass of the structure is used, the application of a layer of 13 mm render to one side of the wall should give an increase of 1 in the overall STC rating of the construction. In a CSIRO technical study, the tested performances were much better than this which indicates that factors other than just the mass of the structure play a part in the effect of applying a surface finish to a wall.
Once one side of a wall has been rendered, little acoustic benefit will be gained by rendering the other side. This is largely due to the fact that the relative increase in mass is small compared to the overall mass of the structure and also because the initial benefit of sealing the pores of the brickwork has already been largely achieved by the first layer of render. For highly porous masonry wall constructions, sealing one side with an application of cement-based paint will result in a noticeable increase in sound transmission loss. The application of 13 mm render to one side of a masonry wall will result in an increase in STC rating of between 2 and 5.
Semi-dry pressed bricks need a higher mass per unit area to achieve the same STC ratings as extruded bricks. From the CSIRO technical study, it appears that 215 kg/m2 is the minimum mass per unit area required by an unrendered wall to ensure that an STC rating of not less than 45 will be achieved when a layer of 13 mm render is applied to one side.
Attention to Detailing for Maximum Performance
To ensure a wall construction provides its best insulation, it is essential that it is of a high standard, free of gaps and cracks. Flanking transmission (through unfilled joints and gaps) is the major reason so many wall constructions fail to achieve their tested performance when in the field. Where air can pass, so too can sound. Even a very small penetration will dramatically reduce the insulation performance of a wall.
This is one reason why masonry wall constructions should be finished with render, or a cement-based paint. The render or paint acts as a sealant for the porous masonry and also fills any weaknesses in the mortar joints.
Another common sound path occurs at wall junctions such as at floor or ceiling level and also at the intersection with another wall. Larger voids should be solidly backfilled with mortar. However, where this is difficult to carry out successfully, an expanding foam sealant which is non-shrinking, durable and has a long life span, can be injected into the gap.
Where pipes or ductwork penetrates a wall, the clearance between the pipe or duct wall and the structure should be equal to approximately 1.0% of the pipe diameter or smallest duct cross-sectional diameter. In the case of ductwork, this clearance should not exceed 25 mm. Insert into this cavity one of the following materials where appropriate, and subject to fire rating requirements:
• Rockwool pipe insulation 13 mm minimum wall thickness.
• Bitumen impregnated polyurethane foam sealant compressed at least 50%.
• Sponge rubber.
• Fibreglass or Rockwool.
• Finish off with a non-setting mastic or synthetic rubber sealant.
• Gaps between ductwork fire dampers and wall penetrations (25 mm maximum) should be packed with mineral wool of appropriate service temperature and sealed off with vermiculite plaster, or similar sealant.
• Where fire-rated masonry wall penetrations occur, a 1 mm galvanised steel or copper sleeve is to be grouted or cast into the penetration.
To maximise the acoustic performance of a masonry wall, it is important to avoid the following:
• Gaps and cracks through which sound can pass.
• Gaps around service penetrations.
• Poorly sealed doorways.
• Back-to-back power outlets and cupboards.