2.4  What information is needed to construct a noise model?

Approaches to environmental noise modelling vary in terms of the complexity with which each element of the model is described and analysed. However, irrespective of the chosen approach, the key information to all predictive studies is the systematic representation of the noise sources to be investigated, and the physical environment through which noise will transmit to the receivers. Once these are defined, an estimate of the way in which noise will travel from the noise sources to the receivers is also required. Table 1 shows the requirements for specifying a noisy environment:

Table 1:  Requirements of the specification of a noisy environment

To estimate the way in which noise will travel from the noise sources to the receivers, a range of sound propagation methodologies may be employed. Methods vary widely in their complexity and the scope of applications for which they can offer meaningful predictions.

The most basic form of propagation methodology is described as 'spherical' or 'hemi-spherical' spreading. This method simply accounts for the reduction in sound intensity as a sound wave front spreads over a larger area.

For common types of noise sources in relatively simple environments, as may be the case where separating distances are relatively small and there are no intervening structures to impede noise propagation, this type of method is often sufficient for estimation purposes.

In instances where the noise sources are more complex and/or account must be made of the influence of significant features of the physical environment, more robust and detailed information is needed to describe the propagation of noise. In most types of practical applications, engineering methods will provide the most viable basis for predicting environmental noise levels. These methods rely on a combination of acoustic principles and empirical knowledge to provide a means of estimating the influence of a range of phenomena, including:

• The absorption associated with the passage of noise through the atmosphere
• The change in noise level that occurs as a result of interactions between the sound wave travelling directly to the receiver and those reflected from the ground, accounting for influence of the ground cover type
• The attenuation offered by obstacles that fully or partly obstruct line of sight between a source and a receiver location
• The influence of atmospheric conditions that can change the direction of an advancing sound wave front by refracting the wave at points where there are significant changes in wind speed and/or temperature
• The influence of reflecting surfaces that re-direct an advancing sound wave front

Engineering methods can therefore take account of a wide range of factors that influence noise propagation, and their use for multi-source industrial/commercial installations can become complex when all relevant paths of sound transmission are taken into account. Whilst these methods provide a robust way of describing sound propagation in many general applications, it must be recognised that there will be more complex situations where their description does not properly account for what will actually happen in practice. Example situations include assessments relating to noise sources with very distinct frequency characteristics and, in particular, situations where the influence of different aspects of the physical environment cannot be considered in isolation.

It is therefore important that they are used with proper regard to their limitations. This may mean that their descriptions are only relied upon for very broad value estimates of the noise where feasible to do so (e.g. in instances where there is a large margin between the predicted value and the decision threshold). Alternatively, it may be a case of correcting the predicted values where the limitations can be quantified. In some instances though, it may be necessary to discard the results provided by engineering methods, and either refer to more advanced analytical methods or seek an alternative to predictions as a basis for informing the assessments. The limitations of engineering methods, and possible alternative approaches, are discussed further in subsequent sections.