Acoustics In Buildings
A GUIDE TO THE SPECIFICATION OF MOVABLE WALLS
When considering what type of movable partition to use within a building it is important to bear in mind all aspects of the specification, particularly in relation to:
- The expected level of on-site performance regardless of how individual rooms/areas are to be used.
- The ease of operation of the movable wall.
- The ability to store the panels when the rooms are opened up.
Although the above are important in relation to general use, many other factors can have a significant impact on the overall performance of a product. It is of critical therefore that these factors are given full consideration before a specification is issued so that the installation provides the levels of acoustic performance that is acceptable in operation.
Before considering the complexities of controlling sound in buildings it is important to have an understanding of the basic principles and terminology of acoustics and how performance values are arrived at.
What is Sound?
Sound is a form of energy created when air is disturbed in some way causing changes in air pressure that radiate from the source of the sound in waves. Sound can be airborne; typically that of a human voice.
Sound waves vibrate at different rates or frequencies as they move through the air and are measured in cycles per second or Hertz; the faster a sound wave passes a given point, the shorter the wavelength and in turn the higher the frequency. Vibrations in the air caused by the sound determine how loud it is; the stronger the vibrations the greater the ‘amplitude’. The length or duration of a sound i.e. it’s reverberation time or echo, is determined by the extent to which the energy is expended on contact with surfaces within a room.
How is Sound Measured?
Sound is measured in decibels using logarithmic scales; the human ear is incredibly sensitive and the scale needs to reflect this. The table below shows the increase in sound intensity in relation to decibel levels.
Decibels ratings are used to measure many classifications of sound and are expressed in different ways to represent the different calculations used and measurements taken. When dB ratings represent sound reduction the dB figure used is the difference between the original sound on one side of a structure and the resulting sound once it has passed through the dividing element (after various adjustments).
Loudness is the human impression of the strength of a sound. The loudness of a noise does not necessarily correlate with its sound level and can be affected by the presence of another noise at a similar frequency.
How are we affected by Sound and Levels of Noise?
The human ear can hear sounds across the frequency range 20 to 20,000Hz; however it is most sensitive in the range 100 to 5000 Hz. There is a great deal of variation in the hearing abilities of individuals and the awareness of sounds can depend not only on physical but also psychological factors. Noise can be described as unwanted sound; however the level at which noise is tolerable is subjective – a specific noise that one person finds upsetting may go unnoticed by another.
The table to the right gives dB levels for a range of recognisable sounds.
What is Sound Absorption?
Sound absorption describes the ability of materials to restrict the reflection of sound, doing this by converting the energy of sound into heat. The greater the sound absorption the shorter the reverberation time or echo produced. Sound absorption is measured as a co-efficient from 0 to 1.0; the best rating being 1.0.
What is the Difference between Sound Insulation and Absorption?
Put simply sound insulation affects people in the room adjoining a sound source while absorption affects those within it.
Sound Insulation in Buildings
The movement of sound around a building is a complex process that can be affected by a whole host of factors both within and around a site. There are also various ways of measuring and expressing levels and performance.
Problems can occur with the level of acoustic control in a building and this may be because:
- The building design does not lend itself to acoustic control.
- The specification of individual components is incorrect.
- The interaction between individual elements is not given full consideration.
- One of the parties or suppliers involved has used inappropriate information to gain a commercial advantage.
The Role of the Partition Wall
In commercial buildings the layout of an office is required to create areas that provide varying degrees of function and privacy. In relation to sound insulation, a partition is expected to deal with airborne sound and prevent or reduce noise travelling from one room to another. The type of activity to be carried out and the level of mutual noise disturbance that can be tolerated determines the appropriate standard of privacy between rooms.
Where a partition is made up of different elements i.e. including doors and glazing, with different sound reduction characteristics the overall level of reduction will be lower than that of the highest rated part regardless of the relative size of lower rated product.
When sound travels through a partition between rooms this is known as Direct Transmission, however, the overall level of sound transmission is not governed simply by the insulation of the intervening partition. It is critical to consider the surrounding structure through which sound energy may travel – the leakage of sound through this path is known as Flanking Transmission. Areas typically prone to sound leakage in commercial buildings are: –
- Abutting Walls, window mullions and columns.
- Continuous suspended ceilings, access panels and common voids.
- Continuous raised access floors and common voids.
- Continuous perimeter piping or conduit systems.
- Ventilation grills and connected ductwork and air diffusers
- Doors common to corridors
The importance of restricting flanking transmission cannot be over stated. .Areas of weakness can have a disproportionately negative effect on overall performance in walls where a high dB rating is specified.
Sound Reduction Index & Standardised Level Difference
The sound reduction index (R) is essentially a simple one that expresses the difference between the sound intensity hitting one side of a structure and the resulting sound measured on the other side. The two quantities most commonly used to identify the performance of building products and in particular movable walls are:
- The weighted sound reduction index: dB (Rw) – generally used in laboratory tests.
- The weighted standardised level difference: dB (DnTw) – generally used in on–site tests.
The (Rw) rating is used to specify the performance of a particular structure, material or product and is useful when choosing products. The (DnTw) rating describes the acoustic performance of a completed part of a building. The method for calculating weighted values is defined in BS EN ISO 717-1:1997.
Laboratory tests only measure the performance of the material or component in question. Purpose designed isolated chambers are used to ensure that all the sound energy measured has passed through the test sample and not through the flanking structure with results expressed as dB (RW) The method for carrying out lab tests is described in BS EN ISO 140-3: 1995. When studying test reports in relation to movable walls it is important to consider the following:
- That the materials used in the fabrication of the test specimen are still current.
- That the construction and installation of the specimen are consistent with that to be supplied.
- That the test report is from an accredited laboratory.
- That the test conducted was for a fully operational movable wall and not an individual panel without head track.
- That the test conducted did not include acoustic sealants between individual panel elements.
- That the sound reduction value is expressed as dB(RW).
- Tests carried out at UKAS accredited laboratories for UK manufactures.
Laboratory test figures published by building product manufacturers are useful in helping specifiers compare one product or range with another. However this does not provide a true indication of the potential site performance as many other factors will affect it. On site testing includes the impact of both direct and indirect flanking transmission with test values expressed as dB (DnT,w,). The method for carrying out on site tests is described in BS EN ISO 140-4: 1995.
Acoustics & Building Regulations
Part E of the Building Regulations was updated in 2003 to include details of minimum levels of acceptable sound performance in certain types of buildings, particularly schools (Building Bulletin 93 [BB93] provides more detailed guidance for acoustic planning and design in schools). However there is little mention of In commercial buildings legislation covers maximum noise levels from a health and
safety perspective but there is no regulation for minimum levels of sound reduction; however the use of specific areas such as conference/meeting rooms etc. makes the need for good levels of sound insulation in practice no less important. Below is a list of the recommended maximum noise levels acceptable in various environments:
Sound Reduction in Practice
Before deciding on what type and standard of product to use, it is important to understand what sort of difference mean sound reductions will make. Once rooms have been allocated for specific purposes approximations can be made on the level of sound that is likely to be present. Privacy levels for individual rooms also need to be considered in order to arrive at the level of sound reduction required.
Typical dB (A) levels for human speech
The table below shows the type of effect various levels of sound reduction will have.
Background noise in buildings normally arises from sources like external traffic or equipment within the building. Machines like air conditioning units or fans can operate at moderate levels that make noise that is unobtrusive and is only noticed when turned off. This sort of noise is often unnoticed as it is steady and conveys little information about events around it. As an unintended consequence of normal activities, background noise can be beneficial in masking more sounds from an adjacent room without being loud enough to be noticed in their own right.
Specifications in Practice
In most buildings there will always be a degree of sound leakage through the structure (flanking transmission) and this is why there are separate sound reduction indices for laboratory and on-site measurements It is generally accepted that even with ideal site conditions the minimum loss of performance between dB(Rw) and dB(DnTw) will be between 4 & 6 dB(Rw).
When deciding on an right product to be used, the preferred site performance figure dB(DnTw) should be added to the minimum expected loss of 4 to 6 dB(Rw), to arrive at the lab test dB(Rw) figure required. Where high ratingsare specified, the loss of rating between lab and on site results may be greater than above; in this case and where products needs to be tested once installed, an acoustic consultant should be employed to provide a more accurate prediction of site performance .
Specifying Movable Walls
Movable walls are an increasingly important part of building design, allowing architects and designers the freedom to fully utilise space within buildings while providing end users with flexibility of usage. Focus on acoustic control and product performance is essential in the development of specifications that deliver buildings that are fit for purpose. The desired flexibility can only be achieved in practice if the Movable Wall product chosen reconciles the expectations users have with the space concerned.
End User Expectations
Building users expect movable walls to be able to:
- Prevent the transfer of sound from one room to another regardless of what is happening in different areas.
- Be quick and easy to operate.
- Be easily and unobtrusively stored away when larger areas are required.
- Look aesthetically pleasing or match the surrounding colour scheme.
- Be durable with minimal maintenance.
A good understanding of a product range will provide solutions to the appearance and operation of a movable wall;
however the specification/ and standard of the building or area where the product will be used will determine the levels of sound insulation. The mass/weight of a wall is also important in achieving acoustic performance, but, the elimination of gaps together with a reduction in flaking transmission is equally if not more important.
High Performance Movable Walls
In buildings where a very high degree of sound insulation is required, there may be a need for ratings in excess of 50dB(DnTw). Due to the influence of site conditions and flanking structures, this is unlikely to be provided by a single movable wall. In such cases two movable walls would need to be installed independently with the tracks separated by about 300mm. The existence of gaps, even a few millimetres, has an increasingly dramatic effect on overall sound insulation as the dB(Rw) rating of the product rises.
Where high performance walls are installed regular maintenance is essential to ensure that all seals continue operating to the highest standards.
In order for a movable wall to provide sound reduction at optimum levels it needs to be installed into openings that are tailor made for it. Identifying potential areas of weakness should take place prior to installation as once in place it can be extremely difficult and costly to rectify problems. In an ideal situation the head track of the chosen system will fit directly to the structural soffit, however in practice most products are fitted beneath suspended ceilings.
Sub Frame – As the highest sound rated products tend to be top hung (see page?), a suitable sub frame for the head track to be fixed to, is installed.
Suspended Ceilings Voids – As a continuous ceiling void is a potential route for sound energy to cross from one room to another, an acoustic barrier constructed to provide at least the same dB(Rw) rating as the movable wall should be installed directly above the ceiling track and sealed with a suitable sealant.
Floors – In order for the top and bottom panel seals to operate efficiently, the floor along the line of the
movable wall should be level. Barriers similar in performance to those fitted in the suspended ceiling void should be installed beneath flooring panels.
Abutments – Columns and external walls should be plumb, to allow ease of installation and operation.
Flanking Structure – The nature and design of the fl anking structure is key to the successful installation of a movable wall. Wherever possible and ideally at design stage flanking structures should be broken to minimise transfer of sound energy.
Trunking & Conduits – Where the inclusion of cable trunking, pipe work and conduits within the flanking structure is unavoidable, there will be significant differences between dB(Rw) ratings and achievable dB(DnTw) values, unless these pathways are insulated, and attenuators are included with any ducting.
Installation – Movable walls are a specialist product and should only be fitted by trained installers.