This article was updated on the 3rd October 2019.
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This guide has been produced to assist people in the selection of a masonry construction system, which will both satisfy statutory requirements for acoustic performance and meet the specific objectives of individual projects.
Guidelines for maximizing sound performance by attention to correct detailing and the application of a variety of surface finishes are also described.
Weighted Sound Reduction Index
The weighted sound reduction index (Rw) is a single number value that 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 from 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 across all frequencies between 100Hz and 4000Hz. 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 that rests against the test specimen. The standard tapping machine consists of five hammers in a line that are lifted and dropped in turn to create 20 impacts per second. The sound transmission through the wall is then measured, and the impact sound levels are converted to normalized 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
Sound transmission class (STC) 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 125Hz and 4000Hz. 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 32dB when they are added together, and the maximum deficiency allowed for any single value below its corresponding reference value is 8dB.
Noise Reduction Between Spaces
The noise levels received from 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 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 from these kinds of sources should not be intrusive.
The STC rating of construction is usually determined under strictly monitored test conditions within a laboratory, where great care is taken during the installation of the test material into the test frame. However, in practice, 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 a system to perform much worse in the field. This degradation should be recognized, and an appropriate allowance made when selecting a tested system to achieve a particular STC 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 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 13mm render increases the mass of the wall, which will cause an increase in acoustic performance. As the acoustic performance of a material or construction improves, the STC value will be higher.
When the predicted performance relating to the mass of the structure is used, the application of a layer of 13mm render to one side of the wall should give an increase of 1 in the overall STC rating of the construction. In a technical study, the tested performances were much better than this, indicating 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, the 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 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 a 13mm 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 same technical study, it appears that 215kg/m2 is the minimum mass per unit area required by an unrendered wall to ensure that an STC rating of at least 45 will be achieved when a layer of 13mm 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 that many wall constructions fail to achieve their tested performance 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 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 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 this out successfully, an expanding foam sealant that is non-shrinking and durable with a long lifespan can be injected into the gap.
Where pipes or ductwork penetrate a wall, the clearance between the pipe or duct wall and the structure should be equal to approximately 1% of the pipe diameter or smallest duct cross-sectional diameter. In the case of ductwork, this clearance should not exceed 25mm. Insert into this cavity one of the following materials where appropriate and subject to fire rating requirements:
- Rockwool pipe insulation 13mm minimum wall thickness
- Bitumen impregnated polyurethane foam sealant compressed at least 50%
- Sponge rubber
- Fiberglass or Rockwool
- Finish off with a non-setting mastic or synthetic rubber sealant
- Gaps between ductwork fire dampers and wall penetrations (25mm 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 1mm galvanized steel or copper sleeve is to be grouted or cast into the penetration
To maximize the acoustic performance of a masonry wall, it is important to avoid:
- Gaps and cracks through which sound can pass
- Gaps around service penetrations
- Poorly sealed doorways
- Back-to-back power outlets and cupboards