Prior to the undertaking of a noise survey the acoustic consultant will need to know the noise parameters set by the local environmental protection team for the required noise impact assessment.
Step 1 – Baseline Situation
It will usually be necessary to have a full understanding of the existing noise levels in the vicinity of the receptor. This can be achieved by carrying out a planning noise survey of background or ambient noise levels over the period when the noise source will be operational. The baseline noise levels will usually be determined by measuring the LA90 or the LAeq, however in practice both will usually be measured simultaneously. The term ‘LA90’ is the ‘A’ weighted noise level exceeded for 90% of the measurement period. Typically this is called the background noise level if it relates to a period when the noise source is not operational.
The ‘A’ weighting is a correction applied within a sound level meter to adjust the response of the Class A noise meter to match the response of human hearing at different frequencies. This is used to exclude short-term noises, such as a vehicle passing from the measurement value, leaving only the underlying or background noise. The terms ‘LAeq’ is as an average noise level over the measurement period, although it relates to the average noise energy. It is a popular and universally used measure which correlates well with human annoyance.
Step 2 – Noise sources and its times of operation
A detailed knowledge of the noise source (or in the case of a proposed development, the likely noise source) is also essential. This is because noise levels are of less importance than the amount by which they exceed the baseline noise and the times or days of operation.
Step 3 – The Manufacturer Data for the Proposed Equipment
If the proposed development will create a source of noise, the noise consultant may need the manufacturer’s data for proposed equipment, times of operation and working practices.
Step 4 – Determining the impact
Noise impact is determined using a variety of methods, all of which will rely on comparing noise levels at a receptor against absolute noise level criteria or against existing baseline noise levels. Generally one of three approaches is used:
a. Where the proposal is for a development which will introduce an industrial type noise source, or where a receptor in a new development may be affected by this type of source, it is usual to assess impact by comparing the noise level, after making corrections for certain attributes of the noise, against background noise levels at the receptor (existing or proposed). The council expects that, at the receptor, noise from the source is a certain amount below existing background levels. Typical cases include new equipment in a business, an air conditioning unit or an extract and fan serving a restaurant. This is known as the BS4142 methodology.
b. In some cases the impact does not depend on a comparison between source levels and baseline levels. For example, a proposed housing development is close to an existing road, where generally the impact is determined by establishing whether the absolute levels due to the source are acceptable.
c. Applications for certain developments may require a more specialist approach. For example, the method for assessing a new nightclub combines both elements. Existing levels of noise in low frequency bands are measured and compared against levels in those same frequencies with music playing. The council may then require that the music does not cause any increase above existing levels.
Step 5 – Noise mitigation measures
Development proposals which are inherently noisy may include mitigation measures in the original scheme. However, the need for further mitigation may be necessary when the impact assessment indicates that the acceptability criteria are exceeded. Either way noise mitigation, or reduction measures, should be considered in the assessment in demonstrating how the acceptability criteria will be achieved.
The most effective measures will be those which reduce levels at source, rather than in transmission or at the receptor. However, in situations such as where the proposal is for new housing near existing sources of noise, it will not be possible to reduce source noise levels. Where the proposal will introduce a new noise source it is good practice to reduce levels at source as far as possible, before considering other mitigation measures, some of these may be:
a. Reduction of noise at source – Using equipment or systems with lower sound power levels is highly effective and can avoid the need for other more costly and intrusive mitigation options. Noise impact can be lessened by reducing total running times or by shifting operations to less sensitive times of the day. The use of acoustic silencers and enclosures around the source may also be effective at reducing the need for other mitigation methods.
b. Reduction of noise in transmission – The simplest way to reduce noise once emitted is by increasing the distance to receptors. For example, siting of plant and equipment within an industrial site as far away from sensitive receptors as possible and/or the new housing development may be designed so that properties are set back from a noise source. In many cases, a properly calculated buffer zone between source and receptor will represent the most cost effective solution.
c. Reduction of noise at a receptor – if the development is located in a busy urban area, this may be the only option to reduce the noise impact, also where the applicant has no access to land for the construction of a barrier. The way a development is designed can be an effective mitigation tool if the building faces away from the main noise source. Also, if noise sensitive rooms are located on the sheltered side of the building, the impact will be reduced at the most sensitive areas.
d. Acoustic Improvements to the building façade – Where no other options are available, improving the sound insulation of a building facade can be effective in reducing internal noise levels. However, often it will be necessary to provide acoustic treated mechanical ventilation to avoid the need to open windows in warm or humid weather, this is often the solution next to busy elevated railway lines.
If you have a new development and the local authority have requested you undertake a BS8233 Noise Assessment or a BS4142 Noise Assessment then please speak with our noise consultants today. The noise assessment process isn’t intrusive, lengthy or costly. Normally we would leave a meter over a 24 hour period which would gather the necessary data automatically, taking into account other factors such as construction, setting and external variables. We could then provide a detailed report and offer valuable advice how best to proceed.
If you would like to know more about or noise impact assessment service, please contact us at info@aptsoundtesting.co.uk or contact Darren directly on 07775 623464. Alternately if you would like more information on noise assessments then please visit our checklist page which has lots of useful information.