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Noise Barriers

Noise barriers can be much more effective than distance in reducing noise from road or rail traffic. In its simplest form a noise barrier can be a continuous close boarded wooden fence, with a mass of not less than 16 kg/m2. There is relatively little point in increasing the weight of the barrier beyond this because a significant proportion of the noise passes over the top, or round the ends, of the barrier. However, the requirements should be checked with an acoustics consultant.

The attenuation of a barrier is a function of the path difference, that is, the extra distance that the sound has to travel to pass over the top of the barrier, relative to the direct sound path from the source to the receiver, as shown in Figure. Barriers are less effective at reducing low frequency noise than mid and high frequency noise. Hence, to calculate the effectiveness of a noise barrier it is necessary to know the source noise levels in octave or one-third octave bands (see Figure).

Hedges or single trees (or rows of trees) do not, in themselves, make effective noise barriers, although a noise barrier can be located within a band of trees to create an acceptable visual effect. Barriers can also be formed by other buildings, or by landscaping using earth bunds. The path difference and, hence, the attenuation of a barrier will be affected by whether the road or railway is in a cutting or on an embankment.

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Figure: Attenuation by a noise barrier as a function of path difference

The required sound insulation category specified by BS EN 1793-2 shall be determined by the addition of 15 dB(A) to the maximum insertion loss specified for the barrier in the environmental statement or environmental assessment report.

For example, if the insertion loss for a barrier was determined as 3 dB(A), the required sound insulation is to be 18 dB(A), requiring a class B2 barrier.

Where the most environmental statement or environmental assessment report states that an absorptive barrier is required, the sound absorption shall be determined by BS EN 1793-1, and:

1)  where the barrier is determined as a single barrier, a minimum performance of class A1; or,

2)  where the barrier is determined as a parallel barrier, a minimum performance of class A3.

Discretionary or ancillary (site-specific) characteristics that can affect the design or choice of product may include:

1)   brush fire, shatter (wilful damage) properties, light reflectivity,

2)   climatic conditions in the proposed location, including wind, flooding and snow,

3)   aesthetics and sustainability with reference to local regulations.

A generic product shall be selected to be used as the basis for design assumptions. The generic product shall:

1)    have an acoustic durability of a maximum of 0.25 dB loss per year,

2)   have a non-acoustic durability of at least 20 years,

3)   meet the requirements for earth bunds used as noise barriers, specifying the sound insulation category, and sound absorption for required absorptive barriers.

The characteristics of the generic product shall be used to derive self-weight. The self-weight shall be used to calculate the following loading on the structural elements and foundations:

1)   vertical loading,

2)  normal (90o) wind, static and self-weight loading,

3)   where snow blowers are used in the barrier location, maximum bending from snow clearance.

References:

  1.  Acoustics of Schools: a design guide, published jointly by the Institute of Acoustics (IOA) and the Association of Noise Consultants (ANC), November, 2015.
  2.  Design Manual for Roads and Bridges, LD 119 Roadside environmental mitigation and enhancement, Rev.0, 2020

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