Noise assessment aids environmental planning

As environmental issues move up the political agenda, decision makers are becoming more sensitive to the implications of an increasingly noisy society. The possible effects of noise on people: increased stress, reduced ability to sleep and, in extreme cases, cardiovascular problems, have placed a greater emphasis on reducing exposure levels. Consequently, noise assessments are becoming a requirement for a growing number of all types of developments. Mike Brownstone, Principal and Head of Acoustics at SLR Consulting, gives an overview of the noise assessment process; how they are carried out and why they are important.

The principal reason for carrying out a noise assessment is to provide information to a decision maker in a format that is understandable and clear.

In most cases, the decision maker has a vested interest in the findings of the assessment, perhaps as the owner or operator of a site with noise-generating plant who is looking to reduce noise emissions, or as a representative of a local planning authority needing to make a decision about the future of a particular development.

All decision makers need a balanced, objective, overview of the likely consequences of their subsequent actions so it is important to decide the aim of the assessment at the outset.

The two most common reasons for carrying one out are to consider either the effect that a noisy development might have on the surrounding community, or how the existing noise climate might affect a noise-sensitive development, such as housing. In both instances, the aim of the assessment will dictate how it is approached.

Noise sources

The acceptability, or otherwise, of the likely noise emission levels from a development, will be a material factor in how a development is received by the wider community. An important stage in carrying out a noise assessment is to determine which aspects of the proposed development are likely to generate noise.

The noise sources might be static, on-site plant or processes, such as waste processing or air conditioning plant, on-site mobile plant as might be used to move objects around a site, or associated off-site sources such as delivery or staff vehicles.

To quantify overall noise emission levels, information is normally required for

all noise-emitting plant. Levels may be determined mathematically, in the form of noise predictions, or by on-site measurement with a noise meter, or a combination of the two.

There are standard prediction algorithms for general sources like road or rail traffic. These use quantifiable parameters, such as volume and vehicle type, to predict a noise level that will be generated at a specific location. Historic noise emission information exists for individual noise sources, such as construction equipment, and is presented in British Standard publications.

Manufacturers often publish the noise emission levels for proprietary plant, although this information should be used cautiously, as it is normally obtained under specific operating conditions and may not be representative of the proposed methods of use. In most instances, direct measurement of specific plant or operations is an option.

For an assessment to be balanced, it is essential that the source data is representative of the proposed operations. Failure to use representative data will result in incorrect conclusions, which leads to poor decisions and possibly unnecessary cost.

Assessment criteria

Assessment criteria assist in the judgement of whether noise is acceptable. Dest documents include legislation, such as the Noise Insulation Regulations 1975 (as amended 1988), governmental guidance in the form of planning policy guidance notes and British Standards or guidance produced by international or professional bodies such as the World Health Organisation or the Association of Noise Consultants. The selection of assessment criteria can be based on the scope of each of the source documents and a great deal of reliance is placed upon the judgement of the assessor.

Three main types of assessment criteria are used when considering noise-generating developments, these are:

  • Absolute or benchmark criteria
  • The change in noise levels
  • The context of the noise

    Absolute criteria are those where noise below a stated value is unlikely to cause problems and, conversely, where noise above a stated value is likely to cause problems. Absolute criteria tend to be based on social surveys that gauge the acceptability of certain types and levels of noise. These criteria can be constrained by the limits of the research upon which they are based.

    A change in noise levels can be directly related to average human responses, as defined by social surveys. For example, there is a commonly held judgement that a change in noise level of less than 3dB is imperceptible in most circumstances. As a rule, the greater the change in noise level, the more extreme the human reaction. However, individuals may not always conform to the expected norms of human behaviour. Changes in noise level that often result in a negative response from the community as a whole, may be acceptable to some individuals. The opposite is also true, with individuals exhibiting exaggerated responses to small changes in noise level, perhaps due to a higher than normal sensitivity to noise.

    Context assessments are those where the acceptability of a noise source is determined by comparing it to the existing noise level within the community. The increase in noise determines how annoying the community might find it. Context assessments are also based on social surveys of general opinions on noise and are therefore susceptible to the same statistical anomalies as assessments of the change in noise level.

    Noise surveys

    These are the mechanism through which “real” noise levels are obtained. Assessments of noisy sites normally require a survey of noise levels within the community around the proposed development site.

    It is important that the noise survey covers locations that are likely to be affected during periods representative of the proposed operating times. For example, if a development will operate at all hours of every day, the noise survey should cover, as a minimum, the quietest period at the closest sensitive property – typically a Sunday night at the nearest house or flat. The assessment will then be based on the period and location where the potential for adverse impacts is at its greatest.

    The type and period of use should be considered when selecting the receptors for the noise survey. For example, since schools are rarely used at night, only daytime measurements should be necessary. However, residential properties can be occupied at any time day or night and the most sensitive period should be considered.

    Weather plays an important role in noise surveys, as extremes of weather will lead to a wide variation in the measured noise levels, which may bias the assessment. For noise surveys to be considered valid, the weather should be dry with wind speeds of less than 2 metres per second. However, there are exceptions to this rule; the assessment method for wind farms requires background noise measurements over a range of wind speeds to derive a location-specific relationship between wind speed and background noise levels.

    The assessment

    The crux of the assessment is the comparison of the predicted noise emission levels with the assessment criteria. This process decides whether the noise is acceptable.

    A noise assessment is rarely the only factor in the decision making process. Alternative factors, such as environmental, economic or planning, must be balanced in order to arrive at the correct decision. Unless the findings of the noise assessment indicate that a development is so noisy it is totally unacceptable, it is rare for it to be the sole basis for a decision.

    If the assessment determines that the noise is likely to lead to adverse impacts, measures to mitigate or remove the impacts are necessary. In general terms, mitigation can either be physical measures that reduce noise or those that relate to the management of the operations.

    Physical mitigation measures can be applied at source, between source and receptor, or at the receptor. Applying the mitigation at source is normally the most effective method of reducing noise since a similar noise reduction will result at all affected receptors. Examples include the use of quieter plant and noise enclosures.

    Mitigation measures between the source and the receptor usually take the form of barriers or bunds, which are often effective as they screen multiple sources. However, they can be rendered ineffective by tall noise sources or receptors.

    Mitigation measures applied at the receptor are rarely an option for most developments as generally only large transport infrastructure projects have the legislative power to implement them. They can be expensive as the number of receptors to be considered is normally large compared to the number of noise sources.

    Management schemes are often a good way of controlling noise. However, these require the support and commitment of the owner or operator to implement them effectively, for example, designing a site so that vehicles do not need to reverse thus removing potentially annoying reversing alarms.

    Future trends

    As research into the effects of noise continues, assessment guidance is tending to become more stringent with the aim of reducing general levels of noise exposure. A consultation draft of new governmental advice in the form of a planning policy statement is due to be released in October 2005 and there are European Union moves to harmonise the methods of noise assessment across Europe. The EU is also pushing national governments to use map-based noise predictions for making strategic decisions.

    It is likely that the use of noise assessments will become more important and they will form an essential tool in the context of planning and environmental policy.

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