Masonry Magazine January 1973 Page.19
which all traffic is assumed to be located. The distance from this line to the observer, denoted as DE, approximates the mean of the distances from the observation point to the lane center lines which are nearest (DN) and Farthest (DF). Hence DE = 1/DN x DF. As indicated on the graph, the maximum attenuation achievable by use of a barrier is 15 dB.
Results from actual barrier construction by the Ministry of Transportation and Communications, Ontario, Canada, were evaluated in Report RR 180, "Noise Barrier Evaluation and Alternatives for Highway Noise Control." Noise barriers constructed in Metropolitan Toronto, 10 to 12 feet high, were located either midway between houses and highway pavement, or at the highway shoulder 60 to 140 feet from the nearest houses. These barriers resulted in noise reductions on the order of 2 to 6 dBA measured 4 feet above the ground at the first row of houses. The report concludes that barriers of 20 to 25 feet in height would be required for significant sound protection of single story houses near highways. The report also concludes that because of refraction,
barriers are not effective in abating noise above the first story level.
Since barriers of 20 to 25 feet in height may not always be practical, some adjustment may be necessary in regard to noise reduction standards. Possibilities for achieving desired noise levels include building barriers at lesser heights at the crest of depressed highway cuts to increase the overall barrier height (Figure 3). Whatever method is employed, it is apparent that barrier wall construction alone or combined with highway cuts or berms will be more economical than using earthwork as the sole noise abatement measure. It is also clear that acoustical engineering studies should be made in conjunction with structural design of highway noise barriers.
Concrete Masonry in Noise Abatement
For a sound barrier to be effective, it must have a sound transmission loss less than the required noise reduction. Simply stated, the barrier must prevent noise from passing directly through itself. Concrete masonry is particularly well suited for this purpose because it resists the passage of sound over a wide range of frequencies.
In addition to blocking the passage of sound, concrete masonry is highly effective in acting as a sound absorbtion material. Sound absorbtion is a significant factor in noise abatement projects where sound barriers are built only on one side of a highway. A dense surface on a sound barrier would reflect sound back toward the source and increase the noise level on the unprotected side of the highway. Sound absorbtion is also a factor in the surface of tunnel linings. Reflected sound traveling along the length of the tunnel may give drivers the illusion that following cars are directly behind them.
The superior acoustical properties of concrete masonry, together with its inherent durability and economy, make it the ideal material for construction of noise barriers. Structural design of free-standing concrete masonry barriers will be influenced primarily by wind loading. For reinforced concrete masonry the design is done in accordance with the same techniques used for reinforced concrete. In the case of non-reinforced concrete masonry, building code requirements or arbitrary rules governing wall thickness and distance between lateral supports may produce satisfactory results. For a more thorough structural analysis, the lateral wind resistance of concrete masonry walls, or wall-pilaster combinations should be evaluated on the basis of the flexural and shear strength of the elements involved.
When concrete masonry is used in constructing depressed roadways for noise abatement, the design may require the use of cantilever retaining walls. In this case, the earth pressures acting on the wall will be the primary forces to be considered. Design and construction techniques for reinforced concrete masonry retaining walls up to 10' in height are available from NCMA. If the retaining walls used in depressed roadways are to be built up beyond grade level to increase the noise barrier effect, the design must be based on both the earth pressure and the wind load.