Assessment of noise: technical advice note

Appendix 2: Factors to consider in assessing noise impacts

This Appendix provides additional guidance which needs to be considered in the assessment of noise impacts. Although the purpose of this advice is to provide guidance during the qualitative assessment stage of the assessment procedure, it does include useful information to assist in formulating appropriate criteria during the quantitative stage.

For most situations, a judgement on the magnitude of the impact of a noise source based solely on the absolute difference or relative difference between two noise levels may not be adequate. The impact of noise on the amenity value of any noise sensitive receptor is complex and a range of factors, in addition to noise level alone, need to be considered. To assist this process the following factors have been identified.

Averaging time period

Typically, a quantitative assessment may involve estimating noise indices which are averaged over a given time period. For example, when assessing noise annoyance from road traffic the noise index, L _A10,18h dB is commonly used. This index, by definition, is the arithmetic average of the 18-one hourly, L _A10,1h dB values between 06:00 to midnight. Where a quantitative assessment has been based on the change in this, the averaging process may not highlight a significant change in noise impact when the change in noise level occurs over a limited portion of the 18 hour period. To illustrate this problem, a quantitative assessment based on the change in the noise index, L _A10,18h dB shows an increase of 1 dB(A) and the magnitude of impact may be classified as 'minor adverse'. However, this increase may have arisen where noise over 15 hours of the 18-hour period had not altered but there is an increase of 6 dB in each of the remaining hours. In such circumstances, the impact may be re-assessed to 'major adverse', particularly if the increase occurred during the evening period when people are relaxing at home.

Time of day

In determining the magnitude of a noise impact, the effect on the amenity value of the noise sensitive receptor may depend on the time of day. For residential properties, the evening and night periods may be regarded as more important, when most people are at home, whereas, the day period for schools is more relevant in assessing noise impacts.

In addition to diurnal considerations there may be alternative periods which are relevant:

• weekdays/weekends
• Saturday/Sunday
• weekdays/public holidays
• seasonal effects

Nature of sound source (intermittency)

Generally, a noise which is more variable or intermittent is regarded as more annoying or disturbing than a noise which is continuous over the same time period despite equivalent noise exposure levels, L _Aeq,T.

For example, the noise from freely flowing traffic and noise from road traffic at a road junction may give equivalent L _A10,18h noise levels and assessed as equivalent in terms of noise impacts. However, it would be generally agreed that the noise from road traffic at a junction is more annoying because of the variability in noise levels and this should be taken into account when assessing impacts.

Similarly, where the noise source consists of a number of individual events, the impact on disturbance may be more complex than simply based on the overall noise exposure level, L _Aeq. For example, high noise level events which occur infrequently may be regarded as more annoying than more frequent noise events consisting of corresponding lower noise levels to give equivalent noise exposure levels.

Frequency of occurrence

The noise impact from a development which may include noise sources which do not occur every day may be dependent on the frequency and pattern of occurrences. For example a one-day music outdoor festival held every 6 weeks over a 6 month spring/summer period is likely to cause less disturbance than a 4-day concert held during the same period.

Spectral characteristics

If a new noise source is expected to exhibit a very different frequency spectrum from the existing noise climate, the impact on disturbance may not be fully assessed from a simple difference in dB(A) noise levels.

In BS 4142, when assessing industrial noise, distinct tonal content and other readily distinguishable acoustic characteristics are penalised by the addition of 5 dB(A) to the rated noise level.

Absolute level

Assessing the noise impact solely on the change in noise levels may give rise to gradual increases in noise as a result of a succession of small incremental increases in noise which individually may be regarded as insignificant, but cumulatively increase in significance. To address this problem the assessment should consider comparing absolute noise levels with recognised guideline target levels.

A proposal which would cause an increase on an existing level which is already above an existing guideline should probably be regarded as worse than if the existing level were below the guideline. For an area recognised for its tranquillity, even a small increase is an impact which should be considered as a potentially significant because the specific amenity value of the location will be eroded.

Influence of noise indicator

Generally, indicators used for assessing noise impacts may not be fully adequate, giving only a partial indication of the full impact of the noise on sensitive receptors. This is particularly apparent when comparing the change in noise levels before and after a noise generating development becomes operational, even when the before and after noise impacts are from similar source types. For example, a new motorway is planned some distance from a sensitive receptor and the existing ambient noise is from a minor road. An assessment of the noise impact based on the noise index L _A10,18h, may indicate only a small increase, indicating a negligible adverse impact. However, the actual change in the noise characteristics perceived at the sensitive receptor is quite different from that described simply in terms of the noise index, L _A10,18h. Prior to the motorway development, the noise from traffic on the minor road is intermittent and would be replaced by a more continuous noise caused by traffic on the motorway. The change in the noise characteristics from an intermittent to a more continuous noise may be perceived as less annoying than that based on an assessment of the L _A10,18h index, alone.

It is therefore important that the indicators used in describing the noise impact are appropriate and comprehensive; were relevant other indicators should be considered to refine the assessment, these include:

• L _Amax - An indication of the maximum noise level
• L _A,90 - An indication of the minimum or background noise level
• N - The number of distinct noise events in a certain time period

as well as the traditional indices based on L _Aeq and L _A10.