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Theme: acoustics

Acoustics is an area of design that is often neglected and equally often approached as some form of 'black art'. Here we explain how it should be incorporated into the design process to ensure conceptual aims are successfully achieved As far as acoustics are concerned, the conceptual aims of any development fall into two categories. The first is the perception of the acoustics of the completed development in the eyes (and ears) of its occupiers and visitors. Satisfying their needs can require actions from the adequate control of external noise intrusion from a nearby railway line or overflying aircraft, to the provision of suitable acoustic privacy between adjacent cellular offices. It can range from the provision of the desired internal acoustic, such as good speech intelligibility in a lecture theatre, to the creation of an impressive reverberation time in a grand entrance atrium designed to impress visiting clients. The second is the control of the acoustic impact on the surrounding environment, so that the development can operate without fear of a valid noise nuisance action being brought.

In delivering both, the designer must satisfy the legislative requirements as well as the client's or end user's 'wish list' of acoustic behaviour.

The word of the law In 1909, local authorities were given the statutory power to restrict developments that would have an unreasonable impact on the local environment. This came after years of gradually worsening environmental noise pollution brought about largely by the industrial revolution. Since then, the acoustics of developments have increasingly been subject to legislative requirements, together with the need to satisfy other guidelines of good practice. As recently as 2001, the government planning Green Paper, Planning: Delivering a Fundamental Change, proposed a wholesale review of the planning system in England. This growth in regulation is due mainly to increased awareness of the potential negative impact of noise on the existing environment and the amenity of local residents.

Perhaps the most familiar component of acoustic legislation is the planning condition.

In considering whether to give the go-ahead for any new development, the determining planning authority must decide whether the predicted noise impact of the proposed development is acceptable when compared with other benefits. Benefits may include an increased availability of local housing, an increase in local employment opportunities or a wider environmental benefit, such as that provided by a wind farm.

In these instances it is common for planning consent to be granted subject to conditions. These conditions may limit the permissible level of noise output from the development, along with the hours in which the noise may be produced. If the noise impact of a new development is not controlled adequately through the planning process, the same local authority responsible for consenting to the development may find its own environmental health department called upon to instigate action against the operator. The developer and planning authorities must, therefore, look very closely at the noise impact of a development when a planning application is made; failure to do so may result in the serving of a notice on the operator. The serving of a notice will, at the very least, require the operator to implement possible management measures, such as restricted hours of operation, or physical measures, such as the installation of noisecontrol equipment. Failure to comply with a notice could ultimately result in the enforced cessation of operation.

For a typical development, planning conditions refer only to the control of noise and vibration impact on the existing environment, and possibly the adequate control of noise intrusion from external noise sources, such as transport and industry, into internal living spaces. It is left to Building Control, via implementation of the Building Regulations, to ensure adequate provision for the acoustic performance of the internal building fabric, including the control of sound transmission between separate dwellings.

In some cases, particularly for larger developments, an Environmental Impact Assessment (EIA) is called for as part of the planning process. This study looks at all possible environmental impacts associated with the development. An EIA is a useful means of assessing the overall impact of the development rather than examining individual aspects in isolation.

The basis for assessment of noise and vibration impacts tends to be the guidance given in the relevant British Standards and other guidance documents. For instance, reference would be to BS4142 1to define maximum acceptable levels of noise emission from a commercial development, to BS8233 2to set the noise-intrusion criteria for different internal areas within a development, and to PPG24 3to evaluate the suitability of a site to accommodate noise-sensitive development. Reference should also be made to any development plans that relate to the area.

Additionally, it is generally good practice to refer to the guidance offered by the World Health Organisation concerning the potential health effects of noise, although noise limits contained in most existing UK and EU standards and policy documents are generally already based on the WHO guidelines.

4In this context, health effects need not necessarily relate to direct physical effects but may include indirect ones, such as increased stress through sleep disturbance, speech and other activity interference or annoyance.

Environmental noise BS41421, along with many local-authority standard planning conditions, looks at controlling environmental noise by limiting the noise output of a new development to levels that are not expected to create an adverse response from existing residents. It is generally acknowledged that an increase of around 3dB(A) is the smallest practically perceptible noise increase. However, should more than one development be allowed in any area, with each development having a noise allowance of +3dB(A), the cumulative impact of all the developments will increase the overall noise level by a subjectively significant margin.

There is a trend, therefore, particularly in central London boroughs, to limit the noise from all individual developments more strictly - to 1dB(A) or less. Although this undoubtedly offers a robust environmental policy, the approach can be extremely difficult to achieve. For example, air-conditioned buildings will need more costly and spacehungry noise control for mechanical and electrical plant. Similarly, the external building envelopes of noisy developments, such as music bars, will need to provide a higher acoustic performance to limit adequately any noise breakout to the local environment.

Alternative solutions, for example allowing more internal plant room areas or locating entrances to noisy areas away from noise-sensitive neighbours, are becoming increasingly popular. However, these solutions must be considered from the outset of the design when planning the basic building layout, thereby increasing the importance of considering noise issues at the earliest possible stage.

Manufacturers of noise-control equipment are responding to these changes by designing more slimline and lightweight solutions, such as IAC Slimshield acoustic louvres.

Keep quiet on site Large-scale construction, particularly in built-up urban areas, can create significant levels of disturbance not only to the local residents but also to users of neighbouring commercial buildings. Environmental health departments are assigning increasing importance to the control of construction noise on a site's neighbours. This can be seen by the increasing imposition of 'code of construction practice' documents, which impose strict noise and vibration requirements on contractors. The situation is also reflected by an increasing use of Section 61 agreements.

5A Section 61 agreement may be made between the main contractor and the environmental health department of the local authority in which the construction works are to take place. The agreement sets out allowed times of work, maximum noise levels and systems for reporting and dealing with any noise and vibration problems. Through such an agreement, the contractor is assured that construction will be allowed to continue, providing the noise and vibration criteria are not exceeded. The local authority is assured that the noise and vibration will be controlled, and is provided with relevant information regarding duration and types of operation in the event of complaints.

All change As legislative bodies become more aware of the need for acoustic design, new guidance documents are being produced and changes introduced to existing documents. We have seen a revision to Building Regulations Approved Document E (ADE) (AJ 6.02.03), which has defined new requirements for the acoustic performance of separating walls and floors, including an increased low-frequency bias. This updates the old document, which was issued before the majority of people had home-entertainment systems capable of producing high levels of low-frequency noise.

The revised ADE also applies to hotels, hostels and halls of residence, reflecting the idea that people are entitled to a reasonable level of acoustic privacy in any building that may be used for residential purposes, no matter how transient their stay. An entirely new departure is the inclusion of an acoustic rating for partitions within a dwelling, such as between adjacent bedrooms. Although this is not a particularly onerous criterion, it requires housebuilders to pay attention to the acoustics within a dwelling as well as between dwellings.

The revised ADE additionally makes reference to Building Bulletin 93, which defines the levels of noise intrusion and acoustic privacy required for school developments. In this era of increasing awareness of the potentially detrimental effect of noise on learning achievements, maximising the acoustics of learning environments can make a significant improvement to the concentration and discipline levels of a class. As guidelines increase the requirement for acoustic performance of building elements such as facades and partitions, greater pressure is placed on manufacturers of acoustic systems to achieve enhanced performances without reducing the aesthetic impact of the development. Such systems include Pilkington's Audioscreen glass, which provides enhanced sound reduction at a standard glazing thickness, and Lorient's acoustic door seals, which limit the reduction in partition performance that can result from installing a door.

Feeling comfortable In addition to the changes in legislation regarding noise and vibration, people are becoming more aware of the general acoustic environment around them and their right to expect a reasonable environment in which to live and work. This trend has not passed developers by. Consequently, creating the desired acoustic environment in a development is an increasingly important aspect of the design.

The first step to achieving an acceptable acoustic environment is to determine what 'feel' is required for each area of the building. The acoustics of areas with different uses will need to be tailored to fit. A developer wanting a bar with a vibrant, lively atmosphere will have failed if a customer feels they are the only person talking even when the bar is full. This could be the result of an excessively 'dry' or non-reverberant acoustic. In contrast, a restaurant needing a more intimate environment, where people want to be able to talk without having to raise their voices, would require a less reverberant acoustic, plus an appropriate level of continuous background noise.

It is worth remembering that two different developers may have completely different concepts of what they are trying to achieve from the same type of development.

Imagine the noise in a luxury executive saloon car; the low hum of the road that hardly interferes with the driver listening to music, or the soft clunk of doors closing to give an impression of smoothness and quality. This is in complete contrast to an equally expensive performance sports car, which is more likely to be designed to produce a huge engine roar, which enhances the feeling of speed and performance. Both are high-performance, high-cost cars, but they each have very different acoustic specifications.

It is essential that, to deliver a product that satisfies the end users' acoustic expectations, the designer understands their requirements from the outset. The main problem lies in converting subjective expectations into objective specifications. It is often very difficult to describe an acoustic environment in words, even if armed with the appropriate technical vocabulary.

A report full of numbers defining the reverberation time, room constant, speech intelligibility and background noise levels will not be much help when trying to get a developer and/or end user to approve a particular design. It is certainly unlikely to persuade anyone to part with the extra money needed for the potential cost of additional acoustic treatments required to achieve numerical specifications. In this respect, nothing beats the identification by the end user of a similar development that either exhibits the desired acoustic or has some undesirable aspect of the acoustic that can be identified clearly and avoided. However, such opportunities do not always exist. This has led to an increased reliance on computer modelling capable of allowing interested parties to experience the subjective acoustics of a development before it is built. Using these modelling packages allows us to recognise the benefits of installing acoustic surface treatments such as Obersound acoustic wall panels and Abso acoustic blinds, that, in the past, may have been difficult to justify on grounds of cost.

Hearing is believing Acoustic computer-modelling techniques can demonstrate both visually and aurally the results of design proposals. There are two main approaches. Noise mapping provides a visual indication of relative noise levels in an external environment in the form of coloured contours. Ray tracing provides models of internal acoustics, and includes the potential to produce audio files that can be replayed on a standard PC. These enable the client or end user to hear what their completed development will actually sound like.

Noise mapping is very useful when investigating environmental noise levels radiating from a development with multiple or extensive noise sources, such as a large building with significant numbers of external plant items. Packages such as IMMI, Cadna-A and B&K Predictor are equally useful for assessing the impact of transport noise on the requirements for facade sound insulation. Different levels of noise are indicated with coloured noise-contour bands at various distances from the development, providing an indication of the levels at all locations of interest.

The model takes into account parameters such as distance, ground height and type, barriers and ground-reflection characteristics. It provides accurate data much faster than can be calculated by hand. This is presented in an easily interpreted visual format, which gives details of the whole area, rather than the more traditional tabulated list of results at a restricted number of locations.

Ray-tracing packages are indispensable when designing internal acoustics for large and complicated spaces. Standard calculation techniques that are straightforward to undertake by hand are not particularly accurate and can be very time-consuming for unusually shaped spaces, like an atrium with a suspended mezzanine or a shopping mall with a large open core area with several branches leading off it. Simplistically, in a package such as Odeon Room Acoustics Software, a wire-frame model of the room under investigation is created. Each surface is then assigned a finish with defined acoustic characteristics. Public-address systems, people or other noise sources can be introduced to the model. Acoustic characteristics of the room can then be calculated by throwing out individual 'rays' of noise in all directions from one or more sources and tracing their progress as they are reflected or absorbed by the room surfaces.

From this ray tracing you can immediately see which surfaces are likely sources of noticeable echoes. You can calculate the reverberation time of the room, and assess the distribution of noise throughout a room or the transfer of noise between interconnected rooms. Taking this calculated acoustic response, you can then produce an auralisation of the room. In other words, you can hear what the completed room will sound like.

With the appropriate computing power it is possible to create 'real time' auralisations of a room, allowing you to walk in a virtual sense around the space and listen to how the sound changes from location to location.

This technique of auralisation is extremely useful when presenting a proposed design solution to a developer or other members of the design team. Once the basic model is set up, it is a fairly quick process to alter the acoustic properties of the room's surfaces and then auralise the resulting difference. This means the developer can make an informed decision as to whether introducing a particular surface finish or acoustic panel is sufficiently important to warrant the financial impact of that change.

Don't do it by the book Recent years have seen an increasing awareness of acoustics as a design issue. This is reflected by many changes to existing legislation and guidance documents, along with the production of new requirements and associated documents.

Unfortunately, purely complying with legislation will not necessarily satisfy the increasingly high demands of the end user. It is only through the satisfaction of all legislative requirements and compliance with enduser expectations that a wholly successful end product will be delivered. Most importantly, time for consideration of acoustic design issues is being brought ever forward in the design process. To assist in the process and in achieving agreement on end-user acoustic requirements, new computer-based design techniques are becoming available that enable visual or aural output. Through the appropriate use of these tools, designers and end users alike can make more informed decisions regarding the cost benefits of their preferred design solutions.

Jo Edwards is executive acoustic engineer and Dr Andrew Bullmore is acoustics group principal with Hoare Lea Acoustics References 1BS4142: 1997, Method for Rating Industrial Noise Affecting Mixed Residential and Industrial Areas 2 BS8233: 1999, Sound Insulation and Noise Reduction in Buildings - Code of Practice 3 World Health Organisation Guidelines for Community Noise, 1999 4 PPG24 Planning Policy Guidance Note 24:

Planning and Noise, 1994 5Section 61, Control of Pollution Act, 1974 READER ENQUIRIES Abso 1500 Cadna-A 1503 Ecophon 1504 B&K Predictor 1501 British Gypsum 1502 IAC 1505 IMMI 1506 Lorient 1507 L S Longden 1508 Obersound 1509 Pilkington 1510 Polyflor 1511

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