Guide to Predictive Modelling for Environmental Noise Assessment
Stage 3: Detailed model design
Given the wide variety of uses of noise modelling, as well as the wide variety of factors that influence environmental noise levels, there is no procedure for defining a detailed model design that is suitable for every application. The process of developing a detailed model design will often consist in a gradual refinement of the predictions of the screening study. Since practical and technical constraints will often prevent the ideal modelling approach from being pursued, the detailed design must identify the most effective refinements that strike the best compromise between the resources required to construct the model and the reliability of the outcome for decision-making purposes. Reaching this compromise will need consideration of the criticality and financial impact of the decision that depends on the assessment outcome.
The detailed model design will define the features of the sources and environment whose description will have a significant effect on the calculated noise levels. These features will often determine the type of algorithm required.
The following sections discuss the types of technical factor that should be considered in developing the detailed model design.
The first aspect of the physical environment to be defined is the scale of the assessment area for which the detailed model needs to be developed. This will be based on the assessment locations identified in the screening study.
The positional accuracy required decreases with increasing separating distance: a 10% change in separating distance equates in general to less than a ±1 dB change in the calculated noise level.
The other aspects of the environment to be defined relate to the presence of screening and reflecting surfaces, the location and extent of any absorptive ground coverings, and the atmospheric conditions. The level of information required depends on the sophistication of the propagation algorithm to be used.
In some instances, a decision that the assessment should consider atmospheric conditions that are favourable to the propagation of sound will reduce the required precision of details of screening structures and ground cover as their influences are significantly reduced under such conditions.
The importance of precisely defining such attributes must be considered in the context of the importance of small changes in calculated noise level to the assessment outcome. However, when using engineering methods to calculate the influence of these features, it must also be recognised that the validity of the methods is often restricted to general characterisations, particularly when describing features such as acoustically soft ground covers. It therefore does not follow that continually increasing the precision with which the physical environment is described will necessarily translate to any increased precision of calculated noise levels.
Based on the findings of the preliminary screening study it should be possible to identify all sound sources that may together result in total noise levels similar to, or higher than, the prevailing decision threshold or limit at the assessment locations. The contribution of relatively low-power unscreened sources should not be neglected if the model includes screening effects, as these may become significant if higher power sources are screened.
A range of source attributes may need to be defined in more detail for the purposes of the detailed model design. Consideration of the extent and nature of the physical environment can simplify this definition. For example, in instances where there is no screening and the separating distances are relatively small such that ground and atmospheric effects are minimal, the calculation of total levels may not require any information about the frequency profiles of the emission sources.
Attention must also be given to the range of emission levels that a source may produce and the time span over which its emissions may vary. Correspondingly, the relevant assessment time period must be clearly defined and related to the operating patterns of the sources. For example, the standard most frequently used for rating industrial noise in the UK defines time periods of 1 hour and 5 minutes for assessments made during the day and night respectively. Therefore, sources with an ‘on-time’ or pattern of time variation significantly less than the assessment period will need to be directly factored into the emission rating. Conversely, sources which could display differing emission levels in different assessment time periods will need to be rationalised according to whether the assessment relates to average, typical upper, or worst case conditions, as well as considering how the pattern of variations may relate to that of other assessment sources or atmospheric conditions (e.g. is the highest noise level likely to occur when favourable propagation conditions occur?).
Other important source characteristics such as frequency and directivity may also need to be defined, depending on the likelihood of those characteristics being significant to the assessment at the receiver location. For example, frequency information may be important in terms of calculating the effect of ground coverings or screening, as well as being a material consideration to the impact the noise may have at the location (i.e. is the noise dominated by individual frequencies at the receiver location and therefore potentially more disturbing than a wide-frequency source?).
In situations where barrier effects are to be factored into the calculation, careful consideration must be given to the assignment of representative source heights for large pieces of machinery. Conservative decisions may need to be made in order that unrealistic screening benefits are not factored into the calculation. However this will need to be balanced against the need for refinement of the calculation and may warrant closer inspection of the sound radiation properties of the source.
In most practical assessments, environmental noise propagation calculations will be performed using standard engineering methods such as ISO 9613 or CONCAWE. The CONCAWE method was originally developed for noise impact from large industrial (petrochemical) sites, but is now widely used in a range of environmental scenarios. For an example of its use, click here. In the future, the newly developed procedures formulated under the EU HARMONOISE and IMAGINE projects may be used. The use of these engineering methods provides a relatively efficient means of producing estimated noise levels that account for a significant level of detail. It must however be recognised that these methods’ validated application is for the calculation of overall total averaged noise levels under specified meteorological conditions. Not all of the engineering methods are able to directly estimate noise levels that may occur in differing meteorological conditions (e.g. ISO 9613 does not provide a method of calculating noise levels that occur upwind of a source). In situations where the noise model is to account for sources with very prominent and narrow frequency components, or where the variations which occur for different meteorological conditions are of interest, engineering methods should be used with a high level of caution. In some cases, the complexity of the situation may warrant the use of more intensive scientific methods or, ultimately, abandonment of predictions. To make informed decisions about the appropriate algorithm to adopt requires background knowledge in the principles of sound propagation and an overview of the relative merits and limitations of the various procedures.