The noise matrix: time to reload?

Practitioners from AMEC examine the strengths and weaknesses of using matrices to assess the significance of noise effects in developments

Environmental impact assessment (EIA) requires the identification of “likely significant” effects, but since most guidance documents used by acousticians do not directly define significance, it is for practitioners to develop methods to determine what is a significant noise effect.

To provide clarity and rationalise assessments, many EIA practitioners have developed assessment techniques based on matrices. This aids in the determination of significance of noise effects and provides a framework for both the assessments that are undertaken and the conclusions they present.

The evolution of the matrix

In general, matrices have evolved over time from simply indicating that an effect, such as noise, needs to be assessed, to more complex matrices which take into account features such as: the likelihood of an effect occurring, effect weightings, spatial dispersal of effects and sensitivity of receptors.

It’s important to remember that magnitude (the size or degree of change) does not always equate with significance (the importance for decision making). For example, a large increase in noise levels may be considered to have a high magnitude, but it may not be significant if the activity causing the increase noise is of very short duration.

The magnitude of effects is objective, whereas the determination of significance is more subjective as it involves value judgments.

The matrix revolution

Over time guidance documents have increasingly provided methodologies which align with the matrix approach. For example, early guidance presented example categorisations of significance based on noise-level change but did not introduce a matrix-based approach to assessment. More recent guidance, on the other hand, provides an example as to how the magnitude of noise effects can be combined with the sensitivity of receptors in a matrix to conclude the overall level of significance.

Further evidence that matrices are increasingly viewed as good standard practice is evident within the 2009 revision to BS 5228.

A review of environmental or acoustic consultancies, revealed that some have adopted matrix-based approaches which determine levels of significance on a sliding scale, whereas others simply use matrices to conclude effects as either significant or not. It also found marked differences in the structure of the matrices adopted by consultants and the terminology used.

Strengths and weaknesses

Like all tool, matrices have potential strengths and weaknesses. Benefits include:

  • They are clear and easy to understand – providing an assessment method which can be followed by those with limited expertise in assessing noise and allowing guideline noise values, benchmarks, noise level thresholds and noise level changes to be consolidated into levels of effect
  • They are easy to reproduce – different practitioners can undertake assessments of similar developments in a consistent manner.
  • They are particularly appropriate for noise assessment methodologies – such techniques lend themselves to matrix approaches and allow matrix-based assessments to be automated based upon the output of computational noise models.
  • They allows consideration of receptor sensitivity – assessment matrices aid this through a variety of approaches and guide assessments towards considering receptor groups allowing separate assessment criteria to be developed specifically for them.

However, weaknesses include:

  • Due to the objective and subjective nature of noise, it is not possible to develop a matrix which can take into account all of the factors that require consideration within EIA.
  • Consideration of multiple noise indicators is difficult – it is challenging to account for all factors, such as times of day or noise sources of differing character, within a single matrix.
  • Reliance on conclusions from assessment matrices can be dangerous – professional judgement must be made as to the overall significance of effects.
  • Site-specific factors can be overlooked – an assessment matrix developed for one planning application may not be as appropriate for another, regardless of the development similarities.

The matrix reloaded

Clearly matrix approaches are viewed as good practice providing clarity, traceability and reproducibility to EIA, but they have limitations and should be viewed as one of many assessment tools available to practitioners and not as a narrow assessment framework or the complete assessment methodology in itself.

It is critical that practitioners are allowed to set out an assessment matrix they feel is appropriate, without being constrained by it. It is acceptable to make conclusions based on subjective professional judgements which perhaps address factors not included in the matrix, however, any deviation away from the conclusions suggested by a matrix approach must be fully justified.

“Unfortunately, nobody can be told what the matrix is…” – the practitioner should develop an applicable matrix which reflects the nature of the proposed development, the locality of the site, and relevant guidance and policy.

Once developed the matrix should be used only as an aid to assessment, not the sole driver. “The answer is out there and it’s looking for you...”

This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.

Ian Hepplewhite, James Trow and Gail Hitchins work for AMEC E&I UK.

This article was adapted from a paper produced for Proceedings of the Institute of Acoustics Vol 33, part 4, 2011, pp.156-162

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