Acoustics and noise transfer in buildings are becoming an important issue in the construction industry. There are several issues that must be understood to fully appreciate the transfer of noise. Noise is an airborne vibration, which has an effect on the eardrum. Noise has two characteristics, its level and its frequency.
There are two types of noise: airborne and impact. Airborne sounds are those that are transferred in the air. These noises include traffic, conversations, and music. Impact sounds are those that are propagated in the walls and floors of a building and include noise such as footsteps and drills. Most noises encountered in a building consist of both airborne and impact sounds.
The two major influences in the transfer of noise are absorption and reflection. There are three main areas in a room that influence the transfer of the noise: ceiling, walls and floor (for a suspended floor).
- The ceiling is the major sound surface in many rooms. As the room size increases so does the importance of the ceiling for sound transference. Ceilings in a commercial application are often constructed from, or covered by, some form of sound absorbing mineral tile. However, ceiling tiles do not provide a uniform surface due to joints between tiles and light fittings whether recessed or suspended. Flat lucite/perspex lenses over fluorescent tubes are the worst fittings for sound reflection. Parabolic, deep cell diffusers are the best for sound absorption.
- Walls – These are usually the next most influential surface. Their importance increases as room size decreases. Typically walls have very poor sound absorbing qualities and this is often made worse by putting sound reflectors against the walls such as filing cabinets.
- Floor – carpeting the floor will only slightly increase the NRC (Noise Reduction Coefficient). Moving to thicker carpeting is often not a cost-effective solution because much of the floor area is covered with furniture with a worse NRC. Carpeting will however reduce impact noise.
|Sound Pressure Level (dB)||Description of Activity||Typical Subjective Description|
|0||Threshold of hearing||Absolute silence|
|20||Whispering at 1 metre||Very quiet|
|40||Quiet residential neighbourhood||Quiet|
|60||Conversation at 1 metre||Moderately noisy|
|80||Food blender, garbage disposal||Noisy|
|100||Use of outboard motor, power lawn mower, motorcycle or farm tractor||Very Noisy|
|140||Aircraft Carrier Deck||Intolerable|
It must be understood that the decibel rating is a logarithmic scale. The mass law equation predicts that each time the frequency of measurement or the mass per unit area of a
single layer wall is doubled, the transmission loss increases
by about 6dB.
Effects of Materials
The acoustic behaviour of materials is influenced by several parameters: its mass, reactivity, and vibration absorption. The increased weight per unit area of panel decreases sound transmission. The increased frequency of incident sound decreases sound transmission.
Other factors that affect sound transmission are:
- Panel stiffness – at very low frequencies the stiffness (i.e. resistance to deformation) may have more effect than its weight. In this part of the frequency range insulation is termed stiffness controlled.
- Rigid panels – if a rigid panel is struck it will continue to vibrate at frequencies determined by its size, shape, and thickness – this is its “natural frequency” (natural mode of vibration).
A good rule of thumb is the stiffer the panel, the more sound it will transmit. A profiled steel sheet of a given thickness can produce the same structural strength as a flat panel of substantially greater weight and thickness. That is because a profiled panel will have a higher bending stiffness than a flat one.
Acoustic data is usually quoted in terms of either sound absorption or sound transmission loss. Sound transmission loss is the more relevant value and is expressed as a function of frequency. This is Rw (alternatively know as Sound Transmission Coefficient). It is a constant for any particular material and is measured in decibels.
The density and spacing of fixing elements and their separation from the interior support can also impact on the Rw values of a system. Insulation can play the biggest factor in increasing the Rw values of the system.
|Single skin construction using 0.35mm COLORBOND® steel *||17dB|
|Single skin construction using 0.42mm COLORBOND® steel *||19dB|
|Single skin construction using 0.48mm COLORBOND® steel *||21dB|
|Single skin construction using 0.80mm COLORBOND® steel *||26dB|
|Single skin construction using 1.25mm steel||27dB|
|A twin skinned system with 50mm of insulation||37dB|
* BlueScope Steel data is from testing of panels without sealing between sheets.
** Corus data is theoretical only.
A single layer of poured concrete 150mm thick gives an Rw of about 55.
Effects of Systems
If a higher Rw value is necessary, and it often is in high quality construction, it is not economical to continually double the wall or floor thickness to achieve it. Double layer assemblies are a more practical way of getting high Rw values without excessive weight.
A built-up steel cladding system comprises an external profiled sheet, a perforated or non-perforated internal liner sheet and an in-fill between the two sheets, such as mineral fibre.
The transmission loss of such a system can be modified in a number of ways:
- The total mass can be increased by using a denser or thicker layer of in-fill
- Better performance is possible through using dense, soft rolls of mineral fibre in-fills, rather than rigid ones
- Transmission loss is also improved via an air-gap inside the outer face of the cladding as it minimises transmission of reverberations from the external sheets to the insulation
If sound absorption in a room is important, then the internal sheet of the cladding system should be perforated to allow deadening by the insulation. However this can also have a reverse effect on the transmission loss characteristics of a structure.
The ideal double layer assembly has no rigid mechanical connection between its two surfaces. Rigid mechanical connections are the acoustic equivalent of an electrical short circuit or a thermal bridge in an insulated wall and should be avoided.
The mechanical connection between layers of wallboard can be reduced by the use of staggered wooden studs, separate rows of wooden studs, or a single row of wooden studs with resilient metal furring strips to support the wallboard layers independently of each other. Non-load-bearing steel studs are usually resilient enough to provide adequate mechanical de-coupling between the layers. Good results have also been obtained using 150mm load-bearing steel studs in conjunction with resilient channels.
Connection between a floor system and a ceiling should be via acoustic resilient mounts.
Small openings, such as fixing points and penetrations, allow airborne sound to pass through the element. Therefore to increase the insulation properties of the element it is important for that element to be airtight. To seal perimeters and penetrations for services, dense, flexible material such as a mastic should be used. For areas where large movement is expected, a polyurethane fire and acoustic rated sealant should be used.
The use of Permastop or Vapa-Chek under a metal roof provides acoustic benefits to a building in two different ways:
- Impact. Installing Permastop or Vapa-Chek against the underside of a metal deck roof greatly dampens the impact noise from rain and hail, providing superior sound absorption and sound transmission control at minimal cost
- Airborne Reverberation. Installing Permastop or Vapa-Chek under the roof and against the walls will also help to control the reverberation from noise created within the building. Table DD DCM NR 001 shows the noise reduction performance of standard 75mm Permastop or Vapa-Chek with light duty Sisalation foil. The use of a perforated Sisalation foil will further enhance the acoustic performance. This is not recommended where condensation may be an issue.