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No more noise – CRB Noise barriers

Karin Johns, Freelance Journalist, investigates the effectiveness of pre-cast interlocking concrete block retaining walls in reducing highway traffic noise, as well as the urgency of reducing noise pollution – unwanted human-created sound that disrupts the environment – in South Africa.

Terraforce noise barrier of concrete retaining blocks in Century City

Terraforce noise barrier extension for a Rabie Property, Palm Royale Century City

How concrete acoustic barriers work

According to British Precast, trade body for the United Kingdom precast concrete industry, one very effective way to reduce levels of noise alongside a busy section of highway is by placing a barrier – a solid obstruction – in the direct path between the source and receiver that significantly reflects (transmitting it back across the highway) the sound energy reaching the receiver. Highway noise barriers can effectively reduce noise levels by five to ten dBA, cutting the loudness of traffic noise by as much as 50 percent.

Plants soften the noise barriers

The block’s open bottom allows root growth which further stabilises the berm, and also allows the plants to grow to their full height

However, the pressure waves hitting the top of the barrier will be diffracted downwards and some of this sound will still be received. A small amount of the incident sound may also pass through the barrier which needs to be sufficiently dense (at least 20kg per m²) and continuous to reduce this directly transmitted sound to an insignificant level compared with the sound diffracted at the top of the barrier. Concrete and masonry are ideal materials for this, as their inherent mass will always meet this requirement.

The key acoustic considerations are the height and length of the required barrier. A noise barrier can achieve a 5 dBA noise level reduction, when it is tall enough to break the line-of-sight from the highway to the home or receiver. After it breaks the line-of-sight, it can achieve approximately 1.5 dBA of additional noise level reduction for each meter of barrier height. In addition, its length must extend far beyond the area to be protected to avoid its performance being compromised by noise diffracted around the ends of the barrier.

Traffic noise is reflected back using Terraforce L11 blocks as a noise barrier

This noise barrier, constructed using Terraforce L11 blocks, works by reflecting traffic noise back across Stanhope Road, Cape Town

Barrier location is also a vital decision. Optimum performance can be achieved by maximising the difference between the direct line between the source and the receiver and the diffracted path over the top of the barrier. On level ground it is normally desirable to place barriers as close as possible to the noise source, but where roads and railways are in cuttings or an embankment, placing the barrier further away at the top of the slope is usually the best solution.

Interlocking concrete block retaining walls

Pre-cast interlocking concrete blocks, such as those supplied by Terraforce, well-known local concrete block manufacturer, lend themselves ideally to the construction of highway noise barriers. Terraforce blocks, having undergone rigorous testing in Canada and the United States, comply with I.C.B.O. (International Conference of Building Officials) standards (2000), and provide inherent properties of strength, durability and versatility that allow for aesthetically pleasing and cost effective noise barrier designs. Not only are they easy to maintain and resistant to rot, vermin, vandalism and fire, but they are also plant friendly, hollow concrete blocks that can be filled with a planting medium to accommodate suitable plants.

Read the full article here for more information on reducing highway sounds in residential areas, as well as a case study on a concrete noise barrier erected in Cape Town.

Reversable hollow-core retaining blocks

4 Huguenot Avenue Oranjezicht
Cape Town
Western Cape
South Africa

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