A noise barrier is an exterior structure, normally made of masonry or earth, designed to protect sensitive land uses from noise pollution. Other commonly used terms for noise barrier are: "soundwall", "sound berm", "sound barrier" and "acoustical barrier". Noise barriers are the most effective method of roadway noise mitigation or industrial noise sources (other than cessation of the source activity or use of source controls). In the case of surface transportation noise, very little can be done to further reduce the source noise intensity (other than increase the percentage of hybrid vehicles and electric vehicles, a strategy that works only at low to moderate traffic flow speeds). Extensive use of noise barriers began in the United States after noise regulations that were introduced in the early 1970s.

History

The best of these early computer models considered the effects of roadway geometry, topography, vehicle volumes, vehicle speeds, truck mix, roadway surface type and micrometeorology. Several research groups within the U.S. developed variations of the computer modelling techniques: Caltrans Headquarters in Sacramento, California; the ESL inc. group in Palo Alto, California; the Bolt, Beranek and NewmanJohn Shadely, ‘’Acoustical analysis of the New Jersey Turnpike widening project between Raritan and East Brunswick’‘, Bolt Beranek and Newman, 1973 group in Cambridge, Massachusetts and a research team at the University of Florida. Possibly the earliest published work that scientifically designed a specific noise barrier was the study for the Foothill Expressway in Los Altos, CaliforniaC.M. Hogan and Harry Seidman, Design of Noise Abatement Structures along Foothill Expressway, Los Altos, California, Santa Clara County Department of Public Works, ESL Inc., Sunnyvale, California, October, 1970.

Rapidly following, there ensued numerous case studies across the U.S. addressing dozens of different existing and planned highways. These studies were commonly commissioned by State Highway Departments and were mostly conducted by one of the four research groups mentioned above. The U.S. National Environmental Policy Act* U.S. National Environmental Policy Act, enacted January 1, 1970 had arrived and effectively mandated the quantitative analysis of noise pollution from every Federal-Aid Highway Act Project in the U.S., propelling noise barrier model development and application. With passage of the Noise Control Act of 1972Public Law No. 92-574, 86 Stat. 1234 (1972)Noise Pollution and Abatement Act of 1972, codification amended at 42 U.S.C. 4901-4918 (1988), demand for noise barrier design soared from a host of noise regulation spinoff.

By the late 1970s there were over a dozen research groups in the U.S. applying similar computer modelling technology and addressing at least 200 different locations a year for noise barriers. Today in 2006, this technology is considered a standard in the evaluation of noise pollution from highways, but, remarkably, the nature and accuracy of the computer models used is nearly identical to the original 1970 versions of the technology.

Theory of noise barrier design

The acoustical science of noise barrier design is more complex than imagined. Firstly, the theory is based upon blockage of sound ray travel toward a particular receptor; however, diffraction of sound must be addressed. That is to say, sound waves bend (downward) when they pass an edge, such as the apex of a noise barrier. Further complicating matters is the phenomenon of refraction, the bending of sound rays in the presence of an inhomogeneous atmosphere. Wind shear and thermocline produce such inhomogeneities.

The sound sources modelled must include engine noise, tire noise and aerodynamic noise, all of which factors vary by vehicle type and speed. One can begin to visualize the complexity of the resulting computer model, which is based upon dozens of physics equations translated into thousands of lines of computer code. The associated graphic to the right some original 1970 calculation results showing sound level contour lines as reductions in sound intensity realized by a specific soundwall.

Most commonly a noise barrier is constructed from masonry, earth, or a combination thereof, such as a wall atop an earth berm. The noise barrier must either be constructed on private land or on a public right-of-way. Since sound levels are measured using a logarithmic scale, a reduction of nine decibels is equivalent to elimination of about 90 percent of the unwanted sound. The bottom line is that noise barriers can be extremely effective tools for noise pollution abatement, although theory also calculates that certain locations and topographies are not suitable for use of any reasonable noise barrier. Clearly cost and aesthetics play a role in the final choice of any noise barrier.

Tradeoffs in noise barrier design

There are clear disadvantages of noise barriers, most prominently:

• Aesthetic impacts for motorists and neighbors, particularly if scenic vistas are blocked.
• Considerable costs of design, construction and maintenance.
• Necessity to design custom drainage that the barrier may interrupt.

Normally the benefits of noise reduction far outweigh aesthetic impacts for residents protected from unwanted sound. These benefits include lessened sleep disturbance, improved ability to enjoy outdoor life, reduced speech interference, stress reduction, reduced risk of hearing impairment and reduction in blood pressure (improved cardiovascular health).

With regard to construction costs, a major factor is the availability of excess soil in the immediate area which could be used for berm construction. If the soil is present, it is often cheaper to construct an earth berm noise barrier than to haul away the excess dirt, provided there is sufficient land area available for berm construction. Generally a two to one ratio of berm cross sectional width to height is required. Thus, for example, to build a six foot high berm, one needs an available width of 24 feet.

As a specific example, note the earth berm noise barrier at the top of this article. It was constructed solely of excess earth from grading pads for a residential development it will protect. Thus its entire construction cost is negligible; arguably, it may pay into the project, since offhaul of earth may have been needed. A further nuance of this particular project is that the residential side of the berm is overexcavated, which gives more privacy between highway and homes and also enhances noise benefit. Finally, note the aesthetics of the earth berm which blends with scenic elements of the natural hills of Annadel State Park in the background. It may be a surprise to find out this berm is over six feet in height, since the aesthetics of earth mounding reduce the visual impact of the structure, compared to a soundwall.

As a minor embellishment to noise barrier design, one may note the concept of constructing a louver or cap atop the wall that is directed back toward the noise source. This concept follows the theory that such a design should inhibit shadow zone diffraction filling in sound behind the noise barrier. In actual experience the benefits are slight compared to the benefits of a higher barrier and the costly construction tecniques necessary to create and maintain such a device.

References

See also

• Noise health effects
• Noise regulation
• Roadway noise

Environmental engineering | Noise pollution | Noise reduction | Road infrastructure | Acoustics | Sound

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