Assessment of noise: technical advice note

Chapter 2: Noise Impact Assessment

2.1 The basic principle of any noise impact assessment is to assess the change in the acoustic environment that will be brought about by the proposed development. It is important to appreciate that the assessment of change can, and should be, both qualitative and quantitative. This Technical Advice Note aims to provide guidance on the assessment of significance of noise impacts for various common situations.

2.2 Where a possible quantitative change in noise level is to be assessed, it is essential to ensure that the most appropriate noise metrics, sampling periods and survey duration are used. For example, it would be inappropriate to assess the level of change in noise likely to occur following the introduction of a bus depot into a suburban area by comparing the predicted L _A10,18h with existing L _A10,18h noise levels; when the main noise level changes are likely to arise due to large numbers of buses leaving, or returning, to the depot over shorter periods of time outside the standard 0600 to 2400 time period.

2.3 A qualitative noise change may be described in various ways. Typically, a useful qualitative guide when assessing noise impacts is whether or not there are likely to be changes in behaviour as a consequence of the noise generated by, associated with, or potentially impacting upon the proposed development, for example, will changes in the noise climate be such that it causes people to change their behaviour by closing windows, raising their voice or not using their gardens as before. The impacts can also be positive.

2.4 Before undertaking a noise impact assessment, it is important that the person undertaking the assessment has a thorough understanding of the project and its context. This would involve:

• understanding the nature of the development;
• understanding the nature and character of the prevailing noise environment;
• identifying all the potential new noise sources that will arise from the proposal, during the construction, operation and, where relevant, decommissioning phases;
• understanding the nature of the new noise sources that will arise from the proposal, including such features as tonal characteristics, intermittency, duration and timing (diurnally and seasonally)
• Identifying potential noise sensitive receptors.

2.5 For a quantitative assessment of the noise impacts, the noise level change needs to be related to the sensitivity of the receptor so that the significance of the noise level change can be determined. Hence, the significance of the noise impact at a particular receptor can be determined from the magnitude of the noise change and the sensitivity of that receptor to the change in noise. The magnitude of the noise level change can be assessed relative to an absolute threshold level or relative to the pre-existing ambient noise level.

2.6 Issues which may be relevant when considering noise in relation to a development proposal include:

• Type of development and likelihood of significant noise impact;
• Sensitivity of location ( e.g. existing land uses, Noise Management Areas, Quiet Areas);
• Existing noise level and predicted change in noise level;
• Character (tonal, impulsivity etc), duration, the number of occurrences and time of day of noise that is likely to be generated; and
• Absolute level and possible dose-response relationships e.g. health effects, if robust data available.

2.7 When noise impact assessments are being prepared, the recommended approach is to consider both the likely level of noise exposure at the time of the application and any increase that may reasonably be expected in the foreseeable future using the most appropriate parameters. The extent to which it is possible to mitigate the adverse effects of noise should also be considered.

Assessment Methodology

2.8 The following sections set out a framework for assessing the noise impact(s) that could potentially arise when either:

• a noise source is planned to be developed or, an existing noise source is to be further developed - referred to as noise generating development ( NGD);

or

• a noise sensitive development is planned or, an existing noise sensitive development is to be further developed - referred to as noise sensitive development ( NSD).

Overview of Assessment Methodology

2.9 The assessment methodology consists of five stages which can be applied to either type of development described above (Paragraph 2.8). Although the processes within each stage may differ depending on the type of development, the final output from this process will be similar across all developments. Figure 2.1 shows a schematic diagram of the various stages in the assessment procedure.

2.10 Stage 1: Initial Process: The initial process requires the identification of all noise sensitive receptors ( NSR) that may potentially be affected by the development and to prioritise each NSR according to their level of sensitivity. The following steps are then carried out for each NSR identified.

2.11 Stage 2: Quantitative Assessment: The procedure within a quantitative assessment depends on the type of development i.e.NGD or NSD. The final procedure in this stage is to determine the magnitude of the impact.

2.12 Stage 3: Qualitative Assessment: A qualitative assessment allows additional factors to be included in the assessment procedure to augment the quantitative evaluation. The outcome from this process allows the magnitude of impacts determined from the quantitative assessment to be adjusted accordingly.

2.13 Stage 4: Level of Significance: The level of significance of the noise impact at the NSR is obtained through the relationship of the receptor's sensitivity to noise and the magnitude of the noise impact. The result of this process is entered into the Summary Table of Significance of Noise Impacts.

2.14 Stages 2, 3 and 4 are repeated for each NSR.

2.15 Stage 5: The Decision Process: The number of noise sensitive receptors within each level of significance is totalled to complete the Summary Table of Significance. The Summary Table will normally form only part of the information required to inform the decision process when applying for planning permission.

Figure 2.1: Flow Chart of Assessment Procedure

Details of Assessment Procedure

2.16 The following provides further details of the procedures for each of the stages comprising the assessment procedure.

Stage 1: Initial Process

2.17 The initial process is to identify all noise sensitive receptors which are likely to be adversely affected by the development.

2.18 For noise generating developments ( NGD), it may be sufficient to identify the nearest noise sensitive receptors. However, in general, the distance over which a noise source may have a significant impact on NSRs will depend on the magnitude of the noise source, the existing noise level and the influence of site features on sound propagation.

2.19 Generally, in the case of noise sensitive developments ( NSD), the noise sensitive receptors will be those associated with the development. Although other noise sensitive receptors may be identified in cases where a noise sensitive development adversely affects existing noise sensitive receptors due, for example, to an increase in traffic associated with a planned large housing development.

2.20 For each NSR, the level of sensitivity associated with the type of NSR needs to be assessed. Table 2.1 shows the level of significance associated with typical NSRs.

2.21 There are three levels of sensitivity 'High', 'Medium' and 'Low'. The ranking is primarily based on the relationship between the amenity associated with a NSR and its susceptibility to noise. NSR's which have amenities associated with low noise levels, such as residential properties, are allocated with a 'High' level of sensitivity, whereas nightclubs would be allocated with a 'Low' level of sensitivity.

2.22 This stage is completed when all NSRs have been allocated a level of sensitivity as illustrated in Table 2.1.

Table 2.1: Level of Sensitivity Associated with Various Examples of NSRs

Sensitivity	Description	Examples of NSR
High	Receptors where people or operations are particularly susceptible to noise	Residential, including private gardens where appropriate. Quiet outdoor areas used for recreation Conference facilities Theatres/Auditoria/Studios Schools during the daytime Hospitals/residential care homes Places of worship
Medium	Receptors moderately sensitive to noise, where it may cause some distraction or disturbance	Offices Bars/Cafes/Restaurants where external noise may be intrusive. Sports grounds when spectator noise is not a normal part of the event and where quiet conditions are necessary ( e.g. tennis, golf, bowls)
Low	Receptors where distraction or disturbance from noise is minimal	Buildings not occupied during working hours Factories and working environments with existing high noise levels Sports grounds when spectator noise is a normal part of the event Night Clubs

Stage 2: Quantitative Assessment

2.23 The procedure in carrying out a quantitative assessment depends on the type of development.

2.24 In the case of a noise generating development ( NGD), a quantitative assessment will be based on the change in noise climate before and after the new noise is introduced.

2.25 For a noise sensitive development ( NSD), a quantitative assessment will be based on comparing an absolute noise level with an appropriate noise target.

2.26 Irrespective of which type of development is under consideration, a common method for assessing the magnitude of noise impacts needs to be established.

2.27 To assist in this common approach the following descriptors and the corresponding generic criteria, as shown in Table 2.2, provides a classification of magnitude on noise impacts.

Table 2.2 Classification of Magnitude on Noise Impacts

Descriptors for Magnitude of Impact	Generic Criteria of Descriptor
Major	Loss of resource and/or quality and integrity of resource; severe damage to key characteristics, features or elements (Adverse).
	Large scale or major improvement of resource quality; extensive restoration or enhancement; major improvement of attribute quality (Beneficial).
Moderate	Loss of resource, but not adversely affecting the integrity; partial loss of/damage to key characteristics, features or elements (Adverse).
	Benefit to, or addition of, key characteristics, features or elements; improvement of attribute quality (Beneficial).
Minor	Some measurable change in attributes, quality or vulnerability; minor loss of, or alteration to, one (maybe more) key characteristics, features or elements (Adverse).
	Minor benefit to, or addition of, one (maybe more) key characteristics, features or elements; some beneficial impact on attribute or a reduced risk of negative impact occurring (Beneficial).
Negligible	Very minor loss or detrimental alteration to one or more characteristics, features or elements (Adverse).
	Very minor benefit to or positive addition of one or more characteristics, features or elements (Beneficial).
No change	No loss or alteration of characteristics, features or elements; no observable impact in either direction.

2.28 The following examples illustrate how this process may be applied to either a noise generating development ( NGD) or a noise sensitive development ( NSD).

Noise Generating Development

2.29 This example deals with the situation where a new road is planned near to residential properties. To assist in developing an appropriate classification of the magnitude of noise impacts, advice from the Highway Agency Design Manual of Road and Bridges is sought.

2.30 The classification of the magnitude of noise impacts is shown in Table 2.3 and is based on the change in the noise index L _A10,18h dB.

Table 2.3: Example of Associating Changes in Noise Levels with Magnitudes of Impacts for a New Road in a Residential Area.

Change in Noise Level, x L A10,18h dB	Magnitude of Impact
x = 5	Major adverse
3 = x < 5	Moderate adverse
1 = x < 3	Minor adverse
0 < x < 1	Negligible adverse
x = 0	No change
-1 < x < 0	Negligible beneficial
-3 < x = -1	Minor beneficial
-5 < x = -3	Moderate beneficial
x = - 5	Major beneficial

Noise Sensitive Development

2.31 In this example, a housing development is planned where the impact from a nearby road is to be assessed. The impact of the noise from road traffic during the day is to be assessed. A target noise level of 55 L _{Aeq,16h(07:00 - 23:00)} dB (free-field facade level) based on WHO precautionary guideline value for serious noise annoyance has been selected as appropriate.

2.32 Table 2.4 shows the classification of the magnitude of noise impacts based on the difference in noise between the existing noise level and the target noise level.

Table 2.4: Example of Associating Exceedance Noise Levels with Magnitudes of Impacts for a New Residential Area.

(Existing - Target 1) Noise Level, x L Aeq,16h(07:00-23:00) dB	Magnitude of Impact
x = 10	Major adverse
5 = x < 10	Moderate adverse
3 = x < 5	Minor adverse
0= x < 3	Negligible adverse
x < 0	No change

1 In this example the target noise level is 55 L _{Aeq,16h(07:00-23:00)} dB

Stage 3: Qualitative Assessment

2.33 A qualitative assessment is based on perception and how noticeable the noise impact is in affecting the amenity value of the noise sensitive receptor.

2.34 Where a new noise source is planned, then, the assessment will be based on the effect the new noise climate may have on the amenity value of the existing noise sensitive receptors.

2.35 Where a new noise sensitive receptor is planned the assessment will be based on the effect the existing noise climate may have on the amenity value of the proposed development.

2.36 The aim of the qualitative assessment is to provide additional information which may support the outcome under the quantitative assessment or indicate that the classification of the magnitude of the noise impact needs to be modified.

2.37 To assist in this process it is important to understand the extent to which the noise impact affects the amenities associated with the noise sensitive receptor under consideration. For example, in the case of residential properties, the associated amenities would include qualities which are conducive to:

• undisturbed sleep;
• ability to relax;
• ability to concentrate i.e. reading-listening to radio/ TV;
• able to converse;
• use of outdoor facilities - garden etc

2.38 The initial step in carrying out a qualitative assessment is to understand what impact the noise will have on the amenities associated with the NSR in regard to the perception of noise.

2.39 Table 3.5 shows an example of the relationship between perception and the impact of noise on the amenities associated with residential properties.

2.40 As the noise becomes more noticeable, the level of disruption increases leading to significant changes in behaviour with a subsequent loss in the amenities associated with the NSR.

2.41 In order for a qualitative assessment to assist in supporting or modifying the outcome reached from the quantitative assessment, descriptors for the qualitative impacts that correspond with those used for assessing the magnitude of impacts need to be assigned.

2.42 An example of this process is illustrated in Table 2.5, for the purposes of noise impacts on residential properties.

2.43 A similar process would need to be derived for noise impacts associated with other types of NSRs which may have different types of amenities. For example, a designated area of Special Scientific Interest may have amenities associated with protecting bird populations. To construct a table similar to that shown in Table 2.5 would require advice from relevant specialists.

Table 2.5: Example of Assigning Descriptors for Qualitative Impacts from Noise on Residential Properties.

Perception	Criteria of Descriptor for residential dwellings	Descriptor for qualitative impact
Noticeable (Very disruptive)	Significant changes in behaviour and/or an inability to mitigate effect of noise leading to psychological stress or physiological effects, e.g. regular sleep deprivation/awakening; loss of appetite, significant, medically definable harm.	Major
Noticeable (Disruptive)	Causes an important change in behaviour and/or attitude, e.g. avoiding certain activities during periods of intrusion. Potential for sleep disturbance resulting in difficulty in getting to sleep, premature awakening and difficulty in getting back to sleep. Quality of life diminished due to change in character of the area.	Moderate
Noticeable (Mildly intrusive)	Noise can be heard and may cause small changes in behaviour and/or attitude, e.g. turning up volume of television; speaking more loudly; closing windows more often. Potential for non-awakening sleep disturbance. Can slightly affect the character of the area but not such that there is a perceived change in the quality of life.	Minor
Just Noticeable (Non intrusive)	Noise can be heard, but does not cause any change in behaviour or attitude, e.g. increasing volume of television; speaking more loudly; closing windows. Can slightly affect the character of the area but not such that there is a perceived change in the quality of life.	Negligible
Not noticeable	None	No Impact

2.44 Part of the process in carrying out a qualitative assessment is to ensure that the quantitative assessment has been comprehensively carried out when assessing noise impacts on all the amenities associated with the NSR under consideration.

2.45 Generally, a quantitative assessment, based on a simple change in noise level, in the case of a NGD or comparison with a target noise level for NSD, is not adequate in addressing the noise impact on all the amenities associated with a particular NSR.

2.46 For example, in the case of a NGD such as a new road where an assessment of the noise impacts on residential properties is to be carried out, the magnitude of impact may have been determined as 'minor adverse'. If this assessment is based on the change in the noise index, L _A10,18h dB, noise impacts during the night time period would not have been fully addressed.

2.47 If there is insufficient data to carry out a quantitative assessment of night time levels, then a qualitative assessment would need to address this issue. If, for example, it was anticipated that the road would carry a high number of heavy goods vehicles during the night, then there would be concerns that the subsequent noise impact may cause sleep disturbance. A qualitative assessment would then consider whether the classification of the magnitude of impact derived from the quantitative assessment was in agreement with the corresponding descriptor associated with the quality impact shown in Table 2.5. If there was sufficient evidence to indicate that the noise impact at night was likely to cause sleep disturbance, then the magnitude of impact derived from the quantitative assessment indicating only a minor adverse impact may need to be changed to 'moderate' or 'major' depending on the judgement of the assessor.

2.48 A qualitative assessment will need to consider several factors that influence the impact of noise on the NSR to supplement the quantitative assessment. These factors, which are both qualitative and quantitative, are listed as follows:

• Averaging time period
• Time of day
• Nature of sound source (intermittency)
• Frequency of occurrence
• Spectral characteristics
• Absolute level
• Influence of noise indicator

2.49 Appendix 2 provides some information on how these factors may influence the modification of the magnitude of impacts derived from the quantitative assessment.

2.50 The outcome from Stage 3 is to establish the magnitude of impact from noise on the NSR under consideration by carrying out a more comprehensive assessment than that based on a simple quantitative assessment.

Stage 4: Level of Significance

2.51 This next stage considers the level of significance the noise impact has on the decision process. Table 2.6 provides a framework in determining the level of significance relating the magnitude of impact with the sensitivity of the receptor.

Table 2.6: Significance of Effects

Magnitude of Impact	Level of Significance Relative to 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

2.52 The level of significance and its relevance to the decision making process is explained as follows:

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.

2.53 At the end of this stage the noise impact on the NSR will be allocated a level of significance which is entered into a Summary Table of Significance as shown Table 2.7.

Table 2.7: Summary Table of Significance

Level of Significance	Number of NSRs
	Low	Medium	High
Large/Very Large
Moderate/Large
Moderate
Slight/Moderate
Slight
Neutral/Slight
Neutral

2.54 The procedures described under Stages 2, 3 and 4 are then repeated for all the NSRs under consideration.

Stage 5: The Decision Process

2.55 Part of the decision process will include a completed Summary Table of Significance which provides an overview of the level of significance of the noise impact on all NSRs.

Noise Measurements

2.56 Generally noise measurements will be made in order to characterise the existing noise environment or to determine specific noise levels at one location so that predictions of noise levels likely to be generated at proposed developments can be made. For the former it is necessary to have a clear understanding of the existing environment. This will usually require the measurement of baseline noise levels at times of the day, night, week, season or year when the proposed project is likely to have an impact. Reasonable worst case impacts are normally considered. When undertaking environmental measurements, reference should normally be made to BS 7445-1:2003 Description and measurement of environmental noise. Guide to quantities and procedures for information in relation to the description and measurement of environmental noise; or the specific requirements of other guidance where appropriate e.g.BS 4142. For the latter case the level of noise expected to be generated by the different activities associated with the proposed project must be predicted. Where relevant, empirical noise prediction methodologies, such as CRTN¹ and CRN², are often employed to predict noise. When using these or other standard empirical prediction methods the guidance within these documents should be followed or, where deviations from the standard have occurred, reasons for, and consequences of, deviating from the standard should be explained, and any subsequent uncertainties in the predictions of noise levels quantified.

Microphone Location and Orientation

2.57 There are two conventions in the presentation of environmental data, one which takes account of the effect of the presence of building facades (known as facade levels), the other does not (the results being known as free-field). Free-field noise levels are equivalent to the levels that would be measured on a flat open site at the position of a proposed dwelling, well away from any existing buildings (in practice at least 10m away from any reflecting surface other than the ground). Unless otherwise required by a specific prediction methodology free-field measurements and predictions are preferred.

2.58 Generally, for the determination of ambient noise levels and also for the purposes of prediction, measuring locations should be between 1.2 and 1.5 metres above the ground for a single storey development and between 1.2 to 1.5 metres above the proposed internal floor level for each additional storey. Levels of noise from road and rail traffic are often specified as one metre from a facade, and these facade levels should be assumed to be 2.5 dB(A) higher than levels measured away from the influence of acoustics reflections from buildings etc ( i.e., free-field), unless more accurate noise levels are available. For aircraft, the noise levels refer to aircraft noise exposure contour values (summer average L _Aeq,16h) that are specified at 1.2 metres above the ground and published at 3dB intervals. Because most aircraft noise originates from above, contours include the effects of ground reflection. For aircraft noise the effect of the height of the proposed development is not normally relevant.

Noise Monitoring Locations

2.59 Normally any noise limits associated with a proposed development are chosen to protect the nearest noise sensitive premises exposed to the specific noise source. Therefore, in general, the appropriate noise monitoring location(s) will be outside the sensitive premises. However, this does not mean that the monitoring point should always be adjacent to the most exposed sensitive premises. This is because, usually, any noise limits refer to noise from specific noise sources and not to the total measurable noise level at a particular location that may, for example, be exposed to extraneous transportation noise. In situations where extraneous noise makes monitoring difficult, it may be prudent to select a monitoring location near to the boundary of the site/property, rather than adjacent to the premises most likely to be affected by noise. This approach requires that the proxy monitoring location provides a reliable and representative noise level from the specific noise source which can then be used to calculate the specific noise level at the relevant noise sensitive premises using the appropriate prediction method for the specific noise source. Ideally, noise monitoring locations should be selected such that they are accessible to all parties concerned.

Day and Night-time Periods

2.60 The recommended time periods are 07.00-23.00 for daytime and 23.00-07.00 for night-time.

Instrumentation

2.61 All instrumentation should comply with the current versions:

IEC 61672 - Electroacoustics - Sound level meters Parts 1, 2 and 3;

IEC 61260 - Electroacoustics - Octave-Band and Fractional-Octave-Band Filters;

IEC 60942 - Electroacoustics - Sound calibrators.

Weather Effects

2.62 The noise level measured at a monitoring point will be affected by wind speed and direction, and temperature gradients, particularly when the monitoring point is remote from the source (for example, greater than about 50 metres). The size of these effects usually increases with distance from a source, but are hard to predict, and so measurements should be made under reasonably stable adverse conditions. A suitable condition is a light wind with a vector component up to 2 m/s from source to receiver; this will increase the noise level by about 2 dB(A) compared with the still conditions. Measurements should also be carried out under dry conditions and, in the case of road traffic, when the road surface is dry.

Noise Indices

2.63 Because noise levels and frequency content may vary over time, many indices have been developed to describe noise levels. The equivalent continuous noise level over a time period T (L _Aeq,T) has emerged as a good general purpose index for environmental noise. For road traffic noise L _A10,18h is still widely used; and to describe background noise the L _A90,T is appropriate noise metric. For those noises characterised by definite tonal characteristics the use of Noise Rating ( NR) may be applicable. These should not, however, be used to measure noise that is irregular or impulsive in character. To describe the sound insulation of a component of a building envelope ( e.g. a window) BSENISO 717-1 Acoustics Rating the sound insulation in buildings and of building elements. Airborne sound insulation is appropriate. It is more difficult to specify the insulation of the whole building envelope because the value depends on different insulation values for the various building elements such as windows, walls and roof structure, as well as the type of noise source and its location. All noise metrics are explained in the Glossary to this document. Additional information in relation to units used in the measurement of environmental noise may be found in BS 7445-1:2003: Description and measurement of environmental noise. Guide to quantities and procedures.