Assessment of noise: technical advice note

Chapter 3: Appropriate NIA Methodology

3.1 The following sections illustrate the assessment methodology applied to various types of development. Appendix 3 provides a series of Excel Workbooks for use in carrying out assessments for different types of developments according to the methodology described in Chapter 2.

Example 1: New Residential Development Close to an Existing Transport Noise Source.

3.2 In this example, a new housing development is proposed in the vicinity of an existing railway line. The following procedure illustrates how the noise impact from the railway may be assessed

Stage 1: Initial Process:

3.3 The initial procedure is to identify all the noise-sensitive receptors ( NSR) where noise impacts are to be assessed. In this example, all residential properties within the housing development would be considered, although an initial assessment may only include those properties which are most exposed to the noise from the railway. The sensitivity to noise for residential properties would be rated as 'high'.

Stage 2: Quantitative Assessment:

3.4 For a noise-sensitive development, a quantitative assessment is based on comparing absolute noise levels with an appropriate noise target or criteria level.

3.5 In deriving appropriate criteria noise levels, consideration should be given to the avoidance of the adverse effects of noise including non-auditory effects such as annoyance, sleep disturbance and possibly health effects such as cardio-vascular disease. Guidance noise levels issued by authoritative organisations such as WHO and relevant planning and health guidance issued by Government should be sought in applying appropriate criteria levels, although context is important.

3.6 It should be noted that external noise level criteria should be expressed as free-field noise levels i.e. noise levels which are unaffected by reflecting surfaces other than the ground (at least 10m away from any hard reflecting surface other than the ground). Where internal predictions are made, these should be based on free-field noise levels.

3.7 For illustrative purposes only, the WHO precautionary guideline levels (assumed to be free field):

• 55 L _Aeq,16h for day and
• 45 L _Aeq,8h for night

are used as criteria noise levels for the relevant time periods defined below:

• Day - 07:00 to 23:00 hours;
• Night - 23:00 to 07:00 hours

3.8 The choice of appropriate criteria noise levels and relevant time periods are the responsibility of the local authority. Although this may lead to inconstancies between different Local Authorities and, indeed, across areas within a given Local Authority, it does provide flexibility, allowing particular circumstances to be taken into account and the use of the latest guideline values to be included where appropriate.

3.9 Table 3.1 illustrates the criteria selected for assessing railway noise showing the relationship between magnitude of impacts and exceedance levels for both day and night periods.

Table 3.1 Magnitude of Impacts Associated with Night and Day Exceedance Levels.

Night Noise Level 1, x = (Existing - 45) L Aeq,8h	Day Noise Level 1, x = (Existing - 55) L Aeq,16h	Magnitude of Impact
> 15	> 10	Major adverse
10 = x = 15	5 = x = 10	Moderate adverse
5 = x < 10	3 = x < 5	Minor adverse
0 = x < 5	0 = x < 3	Negligible
x < 0	x < 0	No adverse impact

1 Corresponding facade levels are 2.5 dB(A) higher

3.10 Estimates of the noise levels outside the selected properties are derived from predictions or baseline surveys and the appropriate magnitude of impacts determined from Table 3.1.

Stage 3: Qualitative Assessment:

3.11 For this example, the quantitative assessment is deemed to adequately address the noise impacts on the amenity associated with residential properties and no further adjustment to the magnitude of impacts is required. However, if for example, the development was near to a level crossing there may be concerns with the impact from horn noise. In such circumstances the magnitude of impacts based on L _Aeq,T noise levels alone may not be adequate and an assessment including L _Amax,F levels would need to be considered, particularly for assessing impacts at night. The standard, Calculation of Railway Noise ( CRN) prediction methodology does not included horn noise evaluation.

Stage 4: Level of Significance:

3.12 The level of significance of the noise impacts for each residential property is then determined from Table 3.2, which for this case, where all the sensitive receptors are residential properties, the 'High' category is relevant.

Table 3.2: Significance of Effects

Magnitude of Impact	Sensitivity of Receptor
	Low	Medium	High
Major	Slight/Moderate	Moderate/Large	Large/Very Large
Moderate	Slight	Moderate	Moderate/Large
Minor	Neutral/Slight	Slight	Slight/Moderate
Negligible	Neutral/Slight	Neutral/Slight	Slight
No change	Neutral	Neutral	Neutral

3.13 The level of significance and their relevance to the decision making process are explained in Section 2.52 above.

Stage 5: The Decision Process:

3.14 When all the noise impacts for each residential property have been assessed following the above procedure, a Summary Table of Significance is presented, an example of such a table for this type of impact is shown in Table 3.3.

Table 3.3: Example Summary Table of Significance

Level of Significance	Number of Residential Properties
	Night	Day
Large/Very Large	0	0
Moderate/Large	0	1
Moderate	5	5
Slight/Moderate	0	0
Slight	10	9
Neutral/Slight	0	0
Neutral	0	0

3.15 The results from the assessment of the noise impact from the railway on the proposed development of residential properties, as shown in Table 3.3, indicates that for both night and day time periods the level of significance of the impact for the majority of properties is slight to moderate. However, there is one property, during the day period, where the significance of noise impact is moderate/large.

Example 2: New Noisy Development (incl. commercial and recreation) Affecting a Noise-Sensitive Building

3.16 This example illustrates the procedure for assessing the noise impact on a residential property where an industrial development is proposed. The assessment is based on the principles described in BS 4142:1997: Method of rating industrial noise affecting mixed industrial and residential areas, but does not adhere to the BS 4142: 1997 method of evaluation.

Stage 1: Initial Process:

3.17 BS 4142: 1997 can be described as a context, comparison based, noise assessment. The assessment is carried out by comparing specific noise levels from a source, against appropriate indicators for the situation without the specific noise source operating or influencing the ambient noise level. Examples of context comparisons might include comparing Rating Level (L _Ar,Tr) (derived from the Specific Noise Level (L _Aeq,T) with an appropriate character correction, as necessary) with the existing background noise level L _A90,T (The LAeq,T of the Residual Noise Level (L _Aeq,T)) Where the Rating Level, as defined in BS 4142: 1997, exceeds the pre-existing noise levels this is taken as indicative of the likelihood of adverse complaints arising.

3.18 With BS 4142:1997 the measured background noise level (L _A90,T) is subtracted from the Rating level (L _Ar,Tr) under investigation. Paragraph 9 of BS 4142:1997 advises that as the difference increases, the likelihood of complaints increases, and that:

• A difference of around 10 dB indicates that complaints are likely
• A difference of around 5 dB is of marginal significance

3.19 If the rating level is more than 10 dB below the measured background noise level this is a positive indication that complaints are unlikely.

3.20 In deciding if a significant impact occurs in regard to the assessment of industrial noise, or noise of an industrial nature, using the methodology of BS 4142 (where appropriate); the Scottish Government consider impacts are normally not significant (in a quantitative sense only) the difference between the Rating and background noise levels is less than 5 dB(A), and that usually the threshold of minor significant impacts is when the difference between the Rating and background noise levels is at least 5 dB(A); and commonly do not become sufficiently significant to warrant mitigation until the difference between the Rating and background noise levels is more than 10 dB(A).

3.21 In assessing the noise impact from an industrial development, the noise sensitivity of a receptor is described in terms of the level of exceedance of the rating level, L _Ar,Tr above the background noise level, L _A90,T, where the sensitivities are defined as follows:

• Rating Level (L _Ar,Tr) - Background (L _A90,T) <5 dB(A), the sensitivity is Low
• Rating Level (L _Ar,Tr) - Background (L _A90,T) = 5 dB(A), but less than 10 dB(A), the sensitivity is Medium
• Rating Level (L _Ar,Tr) - Background (L _A90,T) = 10 dB(A), the sensitivity is High

Stage 2: Quantitative Assessment:

3.22 In this example, a quantitative assessment is based on an estimate of the change in noise level, L _Aeq,T before and after the industrial development is operational. Table 3.4 shows the criteria used to define the magnitude of noise impacts for this example.

Table 3.4: Assigning Magnitudes of Noise Impact

Magnitude	Change in noise level, L Aeq,T dB (After - Before) 1
Major	= 5
Moderate	3 to 4.9
Minor	1 to 2.9
Negligible	0.1 to 0.9
No change	0

1 when evaluating noise levels for magnitude of impact purposes, all levels should be rounded to 1 decimal point i.e. when the 2nd decimal is 5 or more, round up.

3.23 If the local authority or representative carrying out the assessment considers that the above assignment of magnitudes of noise impacts is not appropriate for a particular situation an alternative assessment approach needs to be justified. In addition, should it be decided that the assessment would be more appropriately undertaken on the basis of internal levels it must be clearly stated whether or not the assessment is to be made with the windows open or closed and reference should be made to the use of internal design standards, corrected for the character of the noise source being assessed.

3.24 For example, the above magnitude of noise impacts could be derived on the basis of the ambient noise level, when the development is operational, exceeding a target noise level that is based on WHO precautionary guideline levels.

3.25 For a continuous noise source, the recommended WHO precautionary guideline internal noise levels within dwellings is 35 dB L _Aeq,16h for day and evening periods, this is to avoid critical effects of speech intelligibility and moderate annoyance, and to avoid sleep disturbance for the night time period 30 dB L _Aeq,8h is reported as being representative of the noise level at which the onset of effects occur, with no, single sound events exceeding 45 dB L _{Amax, FAST}. The effects on sleep become increasingly more marked at levels of 35 dB L _Aeq,8h and greater. However, the WHO levels are in respect of general environmental noise and not industrial/commercial in isolation. Therefore, the nature of the noise requires to be taken into account.

3.26 The World Health Organisation publication entitled Night Noise Guidelines advises that: considering the scientific evidence on the thresholds of night noise exposure, as indicated by L _night, external noise level, as defined in the Environmental Noise Directive (2002/49/EC), an external L _night of 40 dB should be the target of the night noise guideline ( NNG) noise level to protect the public, including the most vulnerable groups, such as children, the chronically ill and the elderly. However, the World Health Organisation also recognises that this is a very onerous and precautionary noise level, and further advises that a target L _night, external noise level of 55 dB is recommended as an interim target for the countries where the NNG cannot be achieved in the short term for various reasons, and where policy-makers choose to adopt a stepwise approach.

Stage 3: Qualitative Assessment:

3.27 The derivation of the magnitude of impact as described above may not be adequate. This is because the magnitude of impacts based on changes in noise level alone will be dependent on a number of factors including the nature of the noise source, in particular, the spectral content of the noise source and its absolute level. In addition, the period of the day will be a further factor that needs careful consideration in assigning magnitudes of noise impacts.

Stage 4: Level of Significance:

3.28 The level of significance of the noise impacts from the industrial development on the residential property is then determined from Table 3.5.

Table 3.5: Significance of Effects

Magnitude of Impact (After - Before) L Aeq,T dB	Sensitivity of Receptor based on likelihood of complaint x = (Rating (L Ar,Tr) - Background (L A90,T)) dB
	Low (x < 5)	Medium (5 = x < 10)	High (x =10)
Major (=5)	Slight/Moderate	Moderate/Large	Large/Very Large
Moderate (3 to 4.9)	Slight	Moderate	Moderate/Large
Minor (1 to 2.9)	Neutral/Slight	Slight	Slight/Moderate
Negligible (0.1 to 0.9)	Neutral/Slight	Neutral/Slight	Slight
No change (0)	Neutral	Neutral	Neutral

The sensitivity of the receptor is based on the likelihood of complaint as determined by the difference between the Rating level and the background noise level. The sensitivity of the receptor to noise increases from 'Low' to 'High' as the probability of complaint increases. The level of significance depends on the difference in noise levels as determined by the magnitude of the impact and the sensitivity of the receptor as determined by the probability of complaint.

Stage 5: The Decision Process

3.29 Depending on the outcome of the assessment, the decision making process would take into account the level of significance of the noise impact on the property according to:

Very Large: These effects represent key factors in the decision-making process. They are generally, but not exclusively associated with impacts where mitigation is not practical or would be ineffective.

Large: These effects are likely to be important considerations but where mitigation may be effectively employed such that resultant adverse effects are likely to have a Moderate or Slight significance.

Moderate: These effects, if adverse, while important, are not likely to be key decision making issues.

Slight: These effects may be raised but are unlikely to be of importance in the decision making process.

Neutral: No effect, not significant, noise need not be considered as a determining factor in the decision making process.

Example 3: New Noise Sensitive Development (other than residential) Close to an Existing Noise Source

3.30 In this example a new school is planned to be developed near to an existing busy road.

Stage 1: Initial Process:

3.31 The sensitivity to noise for a school would be rated as 'high'.

Stage 2: Quantitative Assessment:

3.32 For a noise sensitive development such as a school, a quantitative assessment would be based on comparing absolute noise levels with an appropriate noise target or criteria level for the time period that the school is in use.

3.33 In deriving appropriate noise level criteria, consideration should be given to the avoidance of the adverse effects of noise including speech intelligibility and the ability to concentrate. Guidance noise levels issued by authoritative organisations such as WHO and relevant planning and health guidance issued by Government should be sought in applying appropriate criteria levels, see Appendix 1.

3.34 For illustrative purposes only, the WHO precautionary guideline levels for schools (assumed to be free field) of 55 L _Aeq,T where the time period, T, refers to the time period when the school is in use, is used as an appropriate criteria level.

3.35 The choice of appropriate criteria noise levels and relevant time periods are the responsibility of the local authority. Although this may lead to inconstancies between different Local Authorities and indeed across areas within a given Local Authority, it does provide flexibility, allowing particular circumstances to be taken into account and the use of the latest guideline values to be included where appropriate.

3.36 Table 3.6 illustrates the criteria selected for assessing road traffic noise showing the relationship between magnitude of impacts and noise exceedance levels for the relevant time period T.

Table 3.6: Magnitude of Impacts Associated with Relevant Levels

Day Noise Level 1, x = (Existing - 55) L Aeq,T	Magnitude of Impact
> 10	Major adverse
5 = x = 10	Moderate adverse
3 = x < 5	Minor adverse
0 = x < 3	Negligible
x < 0	No adverse impact

1 Corresponding facade levels are 2.5 dB(A) higher

3.37 Estimates of the noise levels outside the school are derived from predictions or baseline surveys and the appropriate magnitude of impacts determined from Table 3.6.

3.38 Stage 3: Qualitative Assessment:

3.39 The quantitative assessment is based on the average continuous noise level, L _Aeq,T dB. Although the assessment would take into account the likelihood of a speed restriction imposed on the road outside the school, the influence on vehicle noise levels caused by the installation of a traffic light controlled pedestrian crossing would need to be taken into account. Typically, the noise characteristics of vehicles would be different compared with freely flowing traffic and an assessment of the noise impact based on the variability of the noise taking into account vehicle maximum pass-by noise levels, L _Amax would be required to augment the quantitative assessment based on L _Aeq,T as indicated above.

Stage 4: Level of Significance:

3.40 The level of significance of the noise impacts for the school is then determined from Table 3.7, which for this case, the 'High' category is relevant.

Table 3.7: Significance of Effects

Magnitude of Impact	Sensitivity of Receptor
	Low	Medium	High
Major	Slight/Moderate	Moderate/Large	Large/Very Large
Moderate	Slight	Moderate	Moderate/Large
Minor	Neutral/Slight	Slight	Slight/Moderate
Negligible	Neutral/Slight	Neutral/Slight	Slight
No change	Neutral	Neutral	Neutral

Stage 5: The Decision Process:

3.41 Depending on the outcome of the assessment, the decision making process would take into account the level of significance of the noise impact on the school accordingly:

Very Large: These effects represent key factors in the decision-making process. They are generally, but not exclusively associated with impacts where mitigation is not practical or would be ineffective.

Large: These effects are likely to be important considerations but where mitigation may be effectively employed such that resultant adverse effects are likely to have a Moderate or Slight significance.

Moderate: These effects, if adverse, while important, are not likely to be key decision making issues.

Slight: These effects may be raised but are unlikely to be of importance in the decision making process.

Neutral: No effect, not significant, noise need not be considered as a determining factor in the decision making process.

Example 4: New General Development affecting a Quiet Area within an Agglomeration

3.42 This example assesses the impact of a new development such as a new road on the amenity value associated with a designated Quiet Area ( QA) within an agglomeration.

Stage 1: Initial Process:

3.43 The amenity value associated with Quiet Areas is generally associated during the day time period 07:00 to 19:00 hours. The designation of Quiet Areas within an agglomeration will depend on a number of factors including the existing ambient noise levels and the area within the designated QA area where target noise conditions are to be met. Generally, few people are annoyed by noise levels below 55 L _Aeq,12h.

3.44 In assessing the noise impact from a new development, the sensitivity of the receptor within a QA could be described in terms of the existing ambient noise over a given area within the designated QA. Table 3.8 describes such a scheme which could be applied to QAs. The philosophy of approach adopted here is to give higher priority to those areas with the lowest ambient noise levels as they are likely to have been selected as QAs based on noise levels rather than other qualities.

Table 3.8: Assigning Sensitivity of Receptors within QA for Different Existing Noise Levels

Existing Noise Condition across > 50% of QA	Existing Noise Level, x L Aeq,16h dB where
	X > 55	50 = x = 55	x < 50
Sensitivity of receptor	Low	Medium	High

Stage 2: Quantitative Assessment:

3.45 In this example, a quantitative assessment is based on an estimate of the change in noise level, L _Aeq,12h after the new road is operational relative to the existing noise level over the same area within the designated QA. In assigning magnitudes of noise impacts, the recently revised DMRB classification of noise impacts ³ has been followed to illustrate the method. Table 3.9 shows the criteria used to define magnitude of noise impacts for road traffic noise:

Table 3.9: Assigning Magnitudes of Noise Impacts

Magnitude	Change in Ambient noise level, L Aeq,16h dB (After - Existing) 1
Major	= 5
Moderate	3 to 4.9
Minor	1 to 2.9
Negligible	0.1 to 0.9
No change	0

1 when evaluating noise levels for magnitude of impact purposes, all levels should be rounded to 1 decimal point i.e. when the 2nd decimal is 5 or more, round up.

3.46 If the local authority or representative carrying out the assessment considers that the above assignment of magnitudes of noise impact is not appropriate for a particular situation an alternative assessment approach needs to be justified.

Stage 3: Qualitative Assessment:

3.47 When assessing the impacts for a noise generating development on existing noise sensitive receptors may not be significant when viewed on their own. However, the cumulative effect of incremental increases from several successive noise generating developments, or the intensification of existing sources in a locality may be to increase the overall background and ambient noise to unacceptable levels, and significant negative impacts might arise.

3.48 Areas vary in character and levels of noise that are acceptable in one location may not be acceptable in another.

Stage 4: Level of Significance:

3.49 The level of significance of the noise impacts from the noise generating development on existing noise levels within QAs is then determined from Table 3.10.

Table 3.10: Significance of effects

Magnitude of Impact (After - Existing)dB	Sensitivity of Receptor based on existing noise level within area > 50% of QA, x = L Aeq,12h dB
	Low (x > 55)	Medium (50 = x = 55)	High (x < 50)
Major (=5)	Slight/Moderate	Moderate/Large	Large/Very Large
Moderate (3 to 4.9)	Slight	Moderate	Moderate/Large
Minor (1 to 2.9)	Neutral/Slight	Slight	Slight/Moderate
Negligible (0.1 to 0.9)	Neutral/Slight	Neutral/Slight	Slight
No change (0)	Neutral	Neutral	Neutral

Stage 5: The Decision Process:

3.50 Depending on the outcome of the assessment, the decision making process would take into account the level of significance of the noise impact on the QA accordingly:

Very Large: These effects represent key factors in the decision-making process. They are generally, but not exclusively associated with impacts where mitigation is not practical or would be ineffective.

Large: These effects are likely to be important considerations but where mitigation may be effectively employed such that resultant adverse effects are likely to have a Moderate or Slight significance.

Moderate: These effects, if adverse, while important, are not likely to be key decision making issues.

Slight: These effects may be raised but are unlikely to be of importance in the decision making process.

Neutral: No effect, not significant, noise need not be considered as a determining factor in the decision making process.