The topic of sound insulation within acoustics can be a complicated subject for a variety of reasons including terminology, the sector it is being applied to, standards and rating values.
At AEI Acoustics we want to help you understand the subject easier, for the specific sector you are concerned about and without the confusion of complicated mathematical equations and science that is associated with the field of acoustics.
In this article we are going to concentrate on sound insulation within a building and how this is achieved with internal partitions or as they are better known walls, floors and ceilings. External facades and walls are also expected to meet certain criteria but we will look at those in a later article to try and keep things simple.
So what is “sound insulation?” It is simply how well a wall, floor or ceiling prevents noise from being transmitted from one room to another. You may also see it referred to as sound reduction, sound transmission loss or sound proofing. Sound insulation is determined by measuring and building the partition to control both airborne and impact noise.
Airborne noise is unwanted sound that is generated, transmitted through the air from a source such as a television, speech and music whereas impact noise is the sound generated through a floor structure. Walls do not need to meet any impact noise levels.
Throughout the various products, standards and literature the commonly seen units associated with sound insulation for partitions are: Rw (Measurement for a material based on lab testing)
L’nTw (On site measurement for impact sound performance)
DnTw (On site measurement for airborne sound performance)
+Ctr (adjustment factor for low frequency sound)
The next question asked is normally “What do we need to do?” The answer to this question may change depending on the purpose of the building you are constructing. For example in Building Bulletin 93 (BB93) different requirements are needed for both airborne and impact noise as opposed to those required under Approved Document E if you are developing a residential property. To further complicate matters there may be differences between new build, conversions and refurbishments. We would advise you to contact us if you need any assistance in identifying what is required (to avoid turning what is supposed to be a simple guide into a complicated one).
The third, and probably most important question, that needs to be answered is “How do we achieve the performance levels required?” Once you understand a couple of simple rules the solutions will be far easier to understand. The first rule is:
Add Mass – Simply the more mass to a partition the better it will perform. Care needs to be taken as the laws of diminishing returns do apply and after a certain level the increase in mass (and usually cost) does not provide an increase in performance that warrants the added costs or design requirements.
Add Mechanical Separation – Decoupling one or more layers of materials adds resilience in to the structure and prevents all the materials moving in sympathy together as the sound (vibrations) no longer have a direct path through the partition. When materials move in sympathy sound is conducted through the system far easier.
Do both – By adding mass and mechanically separating the layers at the same time you are ensuring that there is sufficient mass to provide the sound reduction whilst the mechanical separation ensures that there is no direct path through the partition.
Walls and floors require a slightly different approach. Firstly lets look at walls and how we can achieve the required performance levels.
A typical wall construction (as taken from the British Gypsum White Book) for a partition thickness of 97mm using a single layer of Gyproc Soundbloc to both sides achieves a sound insulation performance of Rw 45dB.
A staggered partition (again from British Gypsum’s White Book) requires 2 individual frames to be constructed that are independent of each other (hence staggered). Because the two sides are now mechanically separated the performance levels increase whilst the overall partition thickness does not. For instance a partition with an overall thickness of 102mm and treated with a single layer of Gyproc Soundbloc to both sides achieves a performance rating of Rw 49dB.
The increase in performance for a staggered partition is generally accepted especially given that the cost of achieving this is minimal in the terms of actual space lost (5mm difference). It is worth noting that the figures used are based on British Gypsum’s own tested products and requires the partition to be installed to their guidelines and standards. One point to remember is that we are only going to test for airborne noise between wall structures and not impact sound.
Where walls are being built on to an existing wall then an acoustic bracket should be used to decouple the boards (and metal frame if used) from the existing brick or blockwork.
Floors are slightly different as they need to designed to perform with regards to both impact and airborne noise both of which require a different approach, although the concepts of mass, separation or both still apply.
When designing a floor it is recommended that the ceiling is included in within the parameters, unless factors mitigate this possibility (such as no access to the property below). Adding a ceiling will aid with both airborne and impact noise and should be decoupled wherever possible to ensure that the maximum performance levels are achieved.
Floors may be either timber or concrete construction depending on the type of building being erected so we will look at both types individually.
For timber floors we need to ensure that sufficient mass is included to help with airborne related noise and this is provided by the floor board laid on top of the joist. This may be sufficient to achieve the desired performance targets however in some instances the need to improve on these is requested and to achieve this a specialist acoustic floor may be required. There are numerous different types available and a simple search will identify the manufacturers and the products available. Acoustic flooring generally falls in to 2 distinct categories that are overlay and structural. Overlay boards require a level base on which they are laid and structural boards can be fitted direct to joist (care needs to be taken when using engineered joists when using a system).
When using a timber floor the void between the joists should be filled with a mineral fibre (around 45kg/m³ is usually sufficient) and at least 100mm thick. The insulation should be at least 100mm thick and not be in contact with the floor or ceiling. The ceiling should be mounted on resilient bars (to decouple the plasterboard from the joists) and two layers of plasterboard used to form the ceiling (ensuring joints overlap).
For both the floor and the ceiling it is important that neither the floor boards or the plasterboard come in to contact with the walls and a flexible seal or other specialist product should be used.
Concrete floors require a different approach given the mass of the floor is already provided through the physical construction and therefore airborne noise should not be an issue, if laid correctly.
The main issue with concrete is it conducts impact noise extremely well and if not treated at the design and build stages becomes an expensive and time consuming process to undertake as remedial work.
A ceiling such as that described for timber floors above works very well (plasterboard ceiling on resilient bars) or there are a number of alternatives that can be incorporated at floor level:
Underscreed – This is flexible material with a high compression value that is laid on top of the concrete base prior to the screed level being applied. The principle acts like the resilient bar and physically decouples the concrete base from the screed and floor covering.
Underlay – Again this is a flexible material with good compression that is laid on top of the finished floor construction and underneath the actual floor covering to be laid. The same principles apply to this product as to underscreed.
In the majority of cases a rubber type product around 4.5mm thick would provide a performance of ΔLw 19dB (a difference of) that is generally implied to be sufficient to achieve a pass for impact noise. We recommend that this is not taken as granted given that additional performance requirements may be required as part of the planning process.
For impact noise the best advice anyone can give is get the design right and ensure the installation is correct as trying to resolve the issue once the building is complete will be a huge drain on time, resources and most importantly budget.
Finally we need to look at flanking transmission with respect to partitions as this is probably the hardest issue to resolve once a build is complete. Flanking transmission is the acoustic issue of noise bridging around a partition usually as a result of poor design or installation.
If an issue is identified then the first step is to try and identify where the problem originates which means if sound is travelling through a void within a wall or floor construction then the issue could be on a different floor or a number of properties away, it does not have to be at the adjoining property or room.
Once the origin has been identified then remedial work needs to be carried out to rectify the issue and having to lift floors, remove walls or ceilings becomes an expensive task especially when the dwellings are now occupied.
Manufacturers of acoustic systems should be able to advise the best methods for installing their own systems but if you need further advice or assistance with specifying, designing or installing partitions please contact us today at email@example.com or call us on 01777 717281