Noise and vibration monitoring is often under the radar as a profession across the UK, but its implementation can play a major role in compliance – and act as a helpful early warning across various industries.
In this article, Dave Clarke of SRL Technical Services and Paul Shields of AECOM focus on its use in the construction industry, explaining how a carefully coordinated approach can mitigate complaints, avoid disturbance and even protect against potential damage to structures.
Many developments take place in close proximity to existing buildings and so either demolishing an existing building or structure or creating a new one can have significant implications for neighbours.
Contractors may adopt a monitoring strategy for various reasons – from being obliged to install monitors, or because they have received complaints, or it might just be that they are considerate constructors and want to be good neighbours.
Section 61 Agreements (Control of Pollution Act 1974) are a means commonly used in London to protect surrounding premises and their occupants from the impacts of noise and vibration – and more and more other major cities around the UK are adopting this methodology.
In simple terms, a Section 61 (often shortened to S61) Agreement is a ‘contractual’ agreement between the contractor and the local authority which allows the construction to take place in a certain predefined way without the contractor living under the constant threat of having a notice served on them (under Section 60 of the Control of Pollution Act 1974) to stop work or have restrictions applied.
The contractor specifies their construction methods along with any mitigation measures, and the noise and vibration impact of the works are then calculated at nearby sensitive properties.
“Contractors may adopt a monitoring strategy for various reasons – from being obliged to install monitors, having received complaints, or that they are considerate”
If the local authority is satisfied that ‘best practicable means’ is being implemented and the levels are acceptable, it will issue the contractor with consent, although these are often accompanied by various conditions.
Monitoring methods
A common condition is a requirement to monitor noise and, if considered necessary, vibration.
This can be required during the whole construction period, or just while the noisiest activities are taking place.
Monitoring, as the name suggests, means that the noise / vibration level is being checked, usually in real time, to identify potential issues and demonstrate that levels remain below agreed limits.
This should not be confused with an instrument which is set up to gather long-term data to be used for design work to supplement short term survey measurements.
The challenges
Monitoring noise and vibration pose various particular challenges. For example, just finding a suitable location for your monitor can be fraught with difficulties:
• Where is the best place to capture meaningful results?
• Is the location secure (and if not, are you insured if the equipment is damaged or stolen)?
• Can it be accessed safely for the duration of the project?
• Is there a power supply?
• Have you obtained (or can you get) the necessary permissions to leave it there?
Interpreting the data can also be problematic. As monitors are usually remote, the obvious issue is knowing what caused a threshold to be exceeded, so you need to find ways to identify what is going on. The two most common approaches are to use pre-set trigger levels which either:
• Start an audio recording so you can play it back and hear what caused the noise (all too often you find it is yet another police siren)
• Issue notifications (via texts or emails) to site personnel so they can go out and see whether anything particularly noisy is happening on site (perhaps it is just a road cleaner going past the monitor)
Accuracy on the radar
As technology advances, more and more sophisticated equipment is becoming available. For example, directional microphone arrays can (allegedly) create 360o noise maps to help you pinpoint the direction of the offending source.
The other main challenge is how to assess any exceedances in real time, as the limit is usually specified as a daily exposure level.
For noise, this is normally defined as LAeq,10h which is effectively the average noise level over a 10-hour day (8am to 6pm). Vibration limits can be specified in two very different ways:
• Peak Particle Velocity (PPV) is a maximum level so it is easy to assess
• Vibration Dose Value (VDV) changes with length of time and level so it is more challenging
VDV is used to assess human annoyance whereas PPV is primarily intended for assessing potential damage to structures, though more recently attempts have been made to use PPV as a way of assessing likely disturbance to people as well.
Although LAeq and VDV can both change over time, they work in very different ways.
LAeq is the average noise level and so can go up and down during the measurement period. VDV is a cumulative level so it can only increase.
VDV remains at the same value during periods when there is no (significant) vibration and then increases as further vibration occurs.
In this respect, VDV is similar to how rainfall is recorded. Rain is collected in a receptacle and every time it rains the water level rises a bit more.
On the weather forecast, they will tell you that “25mm of rain fell yesterday”. They never say that “rain fell at a rate of 1mm / hour over the last 24 hours”. Similarly, VDV can be considered to be the total amount of vibration someone is exposed to over the day (or night).
Sticking with the water analogy, imagine your daily noise allowance is a bucket and the noise you generate are the taps.
The more you turn the taps on, the quicker the bucket fills.
In the same way, the louder the noise, the quicker you use up your daily allowance – and once the bucket is full, you have to turn your taps off for the rest of the day.
Monitors can predict whether or not the limit will be exceeded and how quickly.
Such instruments process the measured data continuously and can be programmed to issue warning messages to tell the site manager that the daily limit will be exceeded, if work continues in this way.
More sophisticated monitors can even tell you that you will have to stop work in, say, an hour’s time. This might not be a problem if you are using the noisiest equipment for a short period at the start of the day, but would be if you are hoping to use it all day long.
Further considerations
It is also important to appreciate that the impact on people is not solely about how much noise or vibration they are exposed to.
Other factors come in to play, such as how long the works are going to last, what times of day or night will be noisy, how well the site keeps them informed and engaged, what other impacts they are exposed to (for example, road diversions, dust and light pollution from works) and whether they see the development as beneficial or not.
To demonstrate the benefits and issues associated with monitoring, some examples listed below provide insight across a range of environments.
Keeping developments on track
The vast majority of the developments in London are in densely populated areas and in close proximity to residential properties which are sensitive to noise and vibration.
These urban locations present significant challenges during construction.
Some of the construction works happen during night-time for various reasons such as engineering, health and safety.
Such night-time works can potentially cause sleep disturbance.
One example includes a project carried out by AECOM where the construction site was right across the road from large blocks of flats.
At one site, construction noise and vibration assessments and predictions of noise and vibration levels were undertaken 12 months prior to the start of the construction works to identify potential risks.
Noise insulation and temporary rehousing trigger levels have been established as part of the mitigation measures.
Continuously monitoring noise inside and outside of the residential properties throughout the construction period served several purposes
• To validate and update noise predictions
• To demonstrate compliance with a Section 61 consent condition
• Proactive management of noise in conjunction with site visual inspections / walkovers to determine whether Best Practicable Means (BPM) to minimise noise is being employed
• Complaint investigation
On a learning curve
New schools are often built in the grounds of existing ones and the staff and pupils move in once it is complete, then the old one is demolished.
It is perhaps surprising, therefore, that there are not more complaints of disturbance.
On one project SRL worked on, the new building was only a few metres from the existing one and staff were concerned that noise and vibration from construction works could potentially be detrimental to their health and adversely impact their teaching.
Construction noise and vibration was assessed using British Standards and relevant regulations to determine the likely impact on the teaching staff.
The results were used to help reassure them that levels would not harm their health – for example, by damaging their hearing – nor would they interfere with teaching.
The assessment involved measuring the sound insulation performance of the facade of the closest classroom in the old school as well as determining how much the vibration transferred from outside to inside (the vibration transfer function).
This information was used to set appropriate limits for the monitors located outside the classroom, which would identify if the levels inside were acceptable or not.
Noise and vibration were to be monitored throughout the construction period to check that the actual levels were in line with predictions and also alert the contractors with early warnings of any potential issues via email or SMS notifications.
The alerts were set at multiple tiers, with amber alert issued to allow pre-emptive action to reduce noise or vibration levels, and red alert to inform construction staff that the threshold has been exceeded and the works needed to be stopped.
Getting the levels right
In another example, a building hosting the headquarters of a blue chip manufacturing company was undergoing a main refurbishment, including the complete replacement of elevator shafts.
The client was concerned about the impact of the noise and vibration from the building works on their business activities and whether they
could continue to operate and avoid the cost and disruption of having to relocate to another site.
It was recognised that some activities would be noisier than others and quiet times were agreed with the contractor with much lower limits than at other times.
Through a review of relevant standards and guidance, limits were set for feelable vibration and noise from the works that would interfere with speech and concentration.
From these limits, a trigger action plan was developed with alerts being sent by SMS to both the client and the contractor when the limits were close to being breached.
Noise and vibration were monitored continuously at key sensitive locations. Using this approach, and the alerts, it was possible for the client to continue with their business while the works were undertaken.
Vibration impact on structures
It is worth noting that building damage normally only occurs at levels of vibration which are considerably higher than those which people can tolerate.
This means that the limits in occupied buildings are always determined by a desire to minimise disturbance to the occupants rather than concern over any damage to the building.
However, many people still fear that their building is about to collapse when the levels they experience become unpleasant.
On one project in London, a community centre was being developed directly next to a residential property and the main concern was damage to their house as it was literally a few metres away from the main works.
A vibration monitor was fixed to the wall of the adjacent property during the groundworks and frame construction stages, which were identified via assessment to be of greatest concern.
The thresholds were low in this instance and vibration levels were carefully reviewed on a reactive basis, followed by weekly reports summarising the results which could be shared with the neighbour to give them further reassurance.
“Noise and vibration were monitored continuously at key sensitive locations. Using this approach, and the alerts, it was possible for the client to continue with their business while the works were undertaken”
Making the right choice
Many acoustics consultants, such as members of the Association of Noise Consultants (ANC), have considerable experience in developing and implementing appropriate schemes of noise monitoring schemes.
They will work with clients to ensure appropriate monitoring equipment is used in representative locations.
Expert analysis and interpretation of data, which can otherwise become overwhelming with a large number of different parameters measured throughout every day at numerous locations, can also be provided.
Find out more at www.association-of-noise-consultants.co.uk