Noise and Planning: Mitigation
In the second of our three-part series on noise and planning written by LABC Acoustics, we’re taking a look at noise mitigation and what you can do to achieve internal ambient noise levels.
If you missed our Noise and Planning Introduction, click here.
We assume here that a planning requirement for a noise impact assessment has led to an acoustic survey of your site and because the analysis of the noise data obtained shows the road, railway or other commercial or environmental noise source levels are high, noise mitigation is required.
The mitigation is usually designed to achieve internal ambient noise levels in bedrooms and living rooms and reasonable levels in gardens as detailed in BS8233: 2014.
The way noise mitigation works is relatively straight forward. You can either;
- Screen the noise at source
- Protect the receiver
Starting with the source mitigation first, this usually involves a physical treatment in the form of a noise barrier at the site boundary.
In some cases, the noise barrier is going to be featured in the scheme design for practical reasons, for example for safety; to stop children venturing onto an adjacent rail line, or for aesthetics; to improve the visual aspect over a garden where a fence can be used to define the property line and screen the view to the adjacent road.
Barrier performance is dependent on interrupting the sound propagation path from source to receiver. For this to be effective the barrier must be:
- Continuous with no gaps between the panels and the posts, or between the panels and the ground (gravel boards).
- Of sufficient mass per unit area to ensure that the performance is due to the sound having to travel over and/or around it and not through it. A fence made of newspaper would not work no matter how high or wide it was!
LABC Acoustics can model and design a barrier solution that works for your site.
Figure 1: SoundPLAN contour plot of noise contours over an acoustic bund/fence.
Façade noise mitigation treatment is always considered and can benefit from the screening offered by barriers. It usually comprises the acoustic design of the traditionally weakest elements of the residential envelope:
- Ventilation openings
Glazing: most glazing used today is double glazing. Glazing performance is mainly dependent on the thickness of the glass, the airgap between panes and in the case of acoustic glass the laminated treatment used.
Figure 2: Typical double glazing primary glazing.
At LABC Acoustics we design the glazing based on its laboratory test certificate performance and there can be confusion attached to this specific number because glazing suppliers think all single figure glazing ratings are the same because they are reported in ‘dB’ (decibels).
Nine out of 10 times when we specify glazing it will be using the single figure weighted sound reduction index ‘Rw’ together with a correction called the spectrum adaptation term for road traffic ‘Ctr’. The term is useful because it allows glazing to be selected based on the usual frequency content of the dominant source, in this case, road traffic noise. The glazing rating is therefore specified as a Rw + Ctr dB rating e.g. sound insulation provided by glazing should perform to a minimum 32dB Rw + Ctr.
Ventilation: the building must be ventilated appropriately and in compliance with the associated Approved Document F requirements. Ventilation requires a penetration to be made in the building envelope. This hole in the façade will let in noise and needs to be acoustically treated in order to avoid it compromising the façade sound insulation performance.
The most common ventilation method is to use ‘passive’ trickle vents in the window frames or through the external wall. Where there is a specified acoustic performance requirement for the glazing the vents must be selected to complement that performance (otherwise you will have wasted your money on upgrading the window!) Like glazing acoustic vents are selected on their laboratory tested sound insulation performance and they are also measure this in ‘dB’ using the single figure weighted element-normalized level difference Dn,e,w . Although it is measured in decibels (dB) this is not a like for like measure with Rw + Ctr. Careful selection is required and in some cases a detailed composite sound calculation required over the normal frequency measurement range 100Hz – 3150Hz.
Typically, a rule of thumb would be that an acoustic vent should be chosen with a Dn,e,w rating 6dB higher than the glazing (Rw + Ctr). The performance of the vent should ALWAYS be taken when it is OPEN and doing its job. Some manufacturers only quote vent sound insulation performance when they are closed!
Our final part of our Noise and Planning articles looks at noise in relation to The National Planning Policy Framework.
This is a guest blog by LABC Acoustics who hold a comprehensive database of acoustic planning requirements from around the UK and have significant experience in dealing with both planning officers and the people that advise them from Local Authority Environmental Health or Pollution Control departments.
We are here to help you speed up the route to compliance and minimise the cost of development. Call us on 02476 545397 to discuss further.