NMS Project 2.2
NMS Project 2.2
Project Team
Key Technical Issues
Documents
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NMS Project 2.2
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UK NMS Programme
NMS Project 2.2
Project Team
Key Technical Issues
Documents
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The measurement and assessment of environmental noise can give rise to many issue. These may range from the most fundamental considerations of what are the basic aims of the exercise, and whether or not the measurement and assessment standards being adopted are actually appropriate to the noise under consideration, through to the precise mechanics of how the noise measurements are undertaken (e.g. equipment, location, time, meteorological effects etc.). Often, the answers to many of the issues raised will be specific to the situation being considered.
This section of the website presents a general overview of some of the major issues that presently face those undertaking environmental noise assessments.
The most fundamental of issues is whether or not the assessment of environmental noise should be based on the relative level o a specific source of noise above the measured background?
This is the approach adopted in the UK through the application of BS4142. A recent European Environmental Agency report by one of the consortium members, Dr Ian Flindell, has concluded that the UK stands almost alone in the EU (with the exception of Ireland and Portugal) in adopting this approach. All other EU countries prefer to adopt a zoned land use approach, whereby acceptable noise levels are assigned to different land use areas. In its instruction, BS 4142 does note that the adoption of an absolute noise limit may be more appropriate in certain circumstances, but no guidance is given as to what these circumstances may be. Also, what should the practitioner do when the background noise level falls below 30 dB(A)?
Assuming that the relative assessment methodology is appropriate, and therefore the measurement of background noise is required, what considerations should be given to designing the noise measurement exercise? Considerations might include locations, times, averaging periods, noise indices, meteorological conditions, etc. These considerations apply equally to the measurement of specific noise as they do background noise.
Once the background and/or specific noise has been measured, how should the data be used to assess potential noise impact. For example, should the lowest background noise level be compared against the highest specific noise level, or would it be more appropriate to use the noise levels averaged in some way over a given time period (day, evening, night)?
Another issue concerning the design of any noise measurement exercise is the identification of all potential sources of uncertainty. In environmental noise measurement and assessment, there are numerous source of uncertainty but these can be divided into two groups:
While the policy recommended in the Salford guidelines should reduce the technical uncertainty associated with individual measurements in many situations, it will not necessarily have any effect at all on the generally much greater uncertainty that currently exists within the field of environmental noise measurement. This is because existing standards, regulations and guidelines in the field of environmental noise measurement assessment often allow considerable flexibility for practitioners to exercise individual discretion when interpreting the precise requirements of the standard as written down. Using equipment operating within accepted calibration standards, it is the common experience of all members in this consortium that practical uncertainties arising from different interpretations of existing standards and regulations are usually much greater than the uncertainties arising from the use of different measuring instruments or from repeated measurement using the same measurement conditions.
The key issue that falls out of consideration of the foregoing is the need for robust, well thought out design of any noise measurement exercise. It is essential that, before any measurements are actually undertaken, the objectives and desired outcome goals of the exercise are clearly identified. Only then can the measurement exercise be designed successfully. The fundamental issue, therefore, is the clear identification of the purpose of the exercise.
There is also the issue of balancing an idealised, academic approach to measuring noise against the practical reality of the actual situation facing the practitioner. Ordinarily a compromise must be reached between practical reality and academic ideals. The work specification refers to ‘metrology’ issues arising from ‘real-life’ measurements of environmental noise, and in particular a need to gain further understanding of uncertainties from various sources in practical application of standards such as BS 4142. Furthermore, the specification states that the NMS programme should ensure mesurment is ‘fit for purpose’. This highlights a very important issue – the difference between academic based methods and the real-life practical application of such methods.
Much work has been conducted on the development of objective methods for identifying the presence of tonal and impulsive acoustic features in noise. The academic research approaches are based on auditory theory and the concepts surround the processing capabilities of the ear. These in turn lead themselves to fairly sophisticated techniques, often requiring narrow band or FFT analysis, for confirming the presen of these features in a noise complex.
However, real life practical measurements require measurement techniques that are easy to implement, must suit the available instrumentation and should describe the noise features that are actually responsible for an adverse response. In some cases, subjective evaluation may suggest that the noise is clearly a problem, but on other occasions it may be more marginal. The latter cases may require the more sophisticated objective measurement techniques. This proposed work must therefore balance the needs of practicality with the more academic sophisticated measurement approaches i.e. the principle of ‘fit for purpose’ needs to be considered. Without such consideration, the face validity of environmental noise measurements cannot be improved.
BS 4142 proposes the imposition of a +5dB character correction when the noise under consideration is judged subjectively to have a particular character that draws attention to it. Likewise ISO 1996 proposes a variable correction factor of up to 6dB(A). However, neither is specific as to the appropriate methodology for determining whether or not a correction factor should be applied.
There exist a plethora of objective methods for detecting features in noise and/or their character correction, the latter being related to the supposed subjective annoyance likely to result from the feature. However, no guidance exists as to which method to use under which circumstances. Indeed, there has been no robust inter-comparison of the various objective methods, either in terms of their relative effectiveness or in terms of their ease of implementation.
A previous DoE project related to subjective listening tests on the judged annoyance of specific types of specific types of industrial noise to explore the effect of the acoustic features impulsivity and tonality on subjective annoyance and to assess the performance of objective rating and assessment methods. An outcome was the confirmation of the uncertainties relating to the assessment of acoustic features. The conclusions recommended that future work should develop better descriptors for the identification and the physical magnitudes of various features. This proposed project aims to establish objective methods for classifying acoustic features, which is in line with the work and recommendations of this previous project.
The ultimate aim of identifying features is to provide an accurate and comprehensive description of the noise that is meaningful in terms of the subjective characteristics it represents, yet the results of subjective experiments to validate the methods is not extensive. Perhaps a better approach would be to concentrate first on deriving objective methods to detect the presence of audible features and to impose a penalty based on this fact alone, rather than attempting to determine a formal relationship between feature level and increased subjective response.
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| Comments to: Justin Adcock | |