4: Practical Noise Criteria
4.1: Balancing the Desirable and Affordable
In general, practical noise targets or criteria are a compromise between the desirable and affordable. It is desirable for the noise environment to be as quiet as possible, but human activity itself generates sound. In setting targets we need to consider how much noise should be regarded as an acceptable by product of normal living.
The most recent draft revision of ISO 1996 part 0 recognises three and only three possible outcomes of any noise assessment ranging from acceptable to unacceptable;
Outcome class A: zero or negative noise impact where no action is required.
Outcome class B: intermediate noise impact in the range between zero and unacceptable outcome.
Outcome class C: completely unacceptable noise impact.
The division between these outcome classes can be illustrated by considering the effect consequences of increasing noise level. Let us return to our generic S curve that represents increasing noise effects against increasing noise levels (figure 5). We will consider a single effect for now so that our explanation is not confused with trying to 'total' all the effects.
b> Figure 5: Generic curve for noise effects versus noise exposure showing three assessment outcomes
The generic curve follows the same general shape irrespective of the particular effect being considered. At low noise levels (outcome class A) there are generally low or negligible noise effects, either in terms of the strength of the effect or in terms of the percentage of the population affected. At increasing noise levels the strength of the effect or the percentage of the population affected also increases (outcome class B). However, these effects are not such as to be over-riding in any assessment. At some increasing noise level, the magnitude of effects may become unacceptable (outcome class C) in which case noise control action becomes imperative regardless of any costs or inconvenience incurred.
Where existing noise limits are based on scientific evidence, they are necessarily indicative of a threshold X dividing outcome classes A and B, since the definition of a threshold of unacceptability (threshold Y, dividing outcome classes B and C) is essentially a matter of social, economic and political interpretation. Of course, the avoidance of all noise impact (noise levels below threshold X) would be a commendable aim, but this is rarely achievable without unacceptable costs or inconvenience in other areas. There is a tendency within parts of the scientific community at the present time to mis-represent scientific data which actually indicates the position of threshold X on the noise level scale as indicating the position of threshold Y, and this is a mistake.
4.2: Existing Methods Used to Assess Environmental Noise
As part of the review work the noise criteria adopted in the UK and some of other EU countries have been examined. Full details are reported in Appendix II by country and noise source.
The short timescale allowed for this project imposed some limitations on the amount of information which it was possible to collect and on the level of comparative analysis which it was possible to undertake.. It is recommended that additional resources should be directed towards the full analysis and completion of this review, to be carried out as a separate study. A comprehensive programme of verification and wider dissemination of the information collected could be of considerable value. It should include updated tables as given in the Porter and Gottlob papers (PORTER 95, GOTTLOB 95). For this work, a few brief conclusions are made below in the context of this project.
- Exposure limits vary for different receptor locations and types of source./li>
- Most adopted limits appear to refer to annoyance. The threshold values given are in general larger than those based on the scientific evidence alone./li>
- The limits are generally based on measurements, although prediction is used mainly in addition to measurements.
- The basis of the limits and procedures can be broken down into several groups:
1. Those based on the results of primary research (e.g. The Swiss limits for general abatement of noise from road, rail and industrial noise).
2. Those directed by social, economic, and political considerations (e.g. the general noise limits in Italy).
3. Those that use as a base the limits set in other countries (e.g. airport noise in Italy).
4. Those that are perhaps almost arbitrary in terms of their derivation or depend on historical origins (e.g. qualification for noise insulation from railway noise in the UK).- ome research is planned but there is an indication that member states are awaiting the outcome of EU activities on formulating an "EU Noise Policy" in planning future research.
These findings confirm that noise targets, criteria or limits are not usually based on assumed health effects (other than annoyance or sleep disturbance). Existing standards and regulations usually take the results of primary research into account to some extent, but social, economic, and political considerations are often at least as important as suggested in the last section. Furthermore, this review has revealed that the aetiology of many existing noise limits seems to be somewhat obscure, and might be considered somewhat arbitrary in the light of present knowledge.
Schultz in 1972 also noticed some of these distinctions in reviewing the literature on various noise rating methods (SCHULTZ 72). He noted two approaches; one largely based on what he defined as hindsight and the practical experience of the acoustical consultant, and the other mainly based on laboratory or field studies undertaken with the express purpose of discovering some relation between subjective response and objective physical measures. He suggested that the experience of the consultant lent itself to the development of 'state of the art' solutions consistent with time-scales and budgets. In turn these led to rules of thumb and consistent schemes for assessing a particular problem. Interestingly he concludes that the difference between the two approaches lies with the difficulty of tracing the lineage (or heredity) of a method or limit and hence establishing its pedigree.
The historical development of some standards has encouraged the development of entrenched positions by different institutions. People tend to argue about precise numeric values of noise exposure but this often misses the point of the whole exercise. Environmental noise control decisions are invariably about compromise where the most important issues are general concepts and broad objectives. It is very important to be clear about the role played by the social, political and historic factors in the aetiology of current standards and regulations in the future development of new standards and regulations. Social and political interpretations can change, even if the underlying noise exposure response relationships remain constant. In addition, the underlying noise exposure response relationships might also change as public expectations develop.
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Published 12 September 2000
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