Figure 9.1 illustrates the fundamental function of a noise barrier. The noise source is traffic, particularly large truck traffic, which generates noise by the action of tires on pavement, the drive train, the engine, and the exhaust. The receiver or receptor can be defined as the location where land use results in exposure to highway traffic noise for an hour or more per day. It may typically be set at 5 ft (1.5 m) above ground or at window level. Acoustical design includes controlling noise that passes over the wall and is diffracted to the receiver, noise that is transmitted through the wall, and noise that is reflected from the wall.
Noise levels are expressed in dBA, decibels measured with a frequency weighting network corresponding to the A scale on a standard sound-level meter. The ease of attaining increasing levels of attenuation has been estimated as follows:
5 dBA: simple 10 dBA: attainable 15 dBA: very difficult 20 dBA: nearly impossible
Designs for reductions greater than 15 dBA are usually not considered feasible because of unpredictable and uncontrollable atmospheric and terrain surface effects, scattering from trees and buildings, and other unknowns.
Diffracted Noise. The noise that passes over the barrier, which is the most important of the three types of noise, depends on the location and height of the barrier. Attenuation is directly related to the difference between the length of the path from the source to the receiver in the absence of a noise barrier, and the length of the path from the source over the top of the wall to the receiver by diffraction. At a given distance from the roadway, increasing the barrier height increases the attenuation achieved. However, this relationship is obviously nonlinear, and as the height of the barrier increases above some reasonable value, the attenuation that can be achieved decreases rapidly. Assuming a barrier height that just breaks the line of sight from the source to the receiver, and assuming that such a barrier provides a 5-dBA attenuation, a rule of thumb is to assume that an attenuation of dBA can be achieved with each additional foot of barrier height. But because the relationship is actually nonlinear, this approximation holds for only a limited range. Sometimes it is possible to take advantage of local terrain and locate a noise barrier on a
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FIGURE 9.1 Acoustical concept of noise wall. (From Handbook of Steel Drainage and Highway Construction Products, American Iron and Steel Institute, Washington, D. C., 1994, with permission)
stretch of land at a higher elevation. This reduces the required height and cost. Barrier heights are generally in the range of 6 to 25 ft (2 to 7.5 m). They are generally effective in reducing noise for receptors within approximately 200 ft (60 m) of a highway.
Traffic generates sound waves longitudinally as well as laterally. Thus, care must be taken to extend the length of the barrier sufficiently to achieve the desired end result. A rule of thumb states that the noise barrier should extend, in each direction from the boundaries of the receiver, 4 times the distance from the receiver to the noise wall. This length can be reduced by combining the ends of the barrier with other features, such as natural knolls, or by flaring the wall toward the land use area to form a barrier to the longitudinal sound waves.
Transmitted Noise. The noise that passes through the barrier depends on its surface characteristics and composition (density). Acoustical performance can be determined by testing in accordance with standards of the American Society for Testing and Materials (Test Designation E90). It is important that the wall not contain gaps or holes. Overlapping sections can be used to accommodate access through the wall for maintenance or other personnel when applicable. In such cases, the overlap should be at least 2.5 to 3 times the width of the opening.
Reflected Noise. There is a possibility that noise barrier effectiveness can be reduced by reflected noise, such as where longitudinal walls are located on either side of the roadway. To avoid this situation, it has been recommended that the width between two parallel barriers be at least 10 times the average height of the barrier above the roadway.
