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A noise barrier (also called a soundwall, noise wall, sound berm, sound barrier, or acoustical barrier) is an exterior structure designed to protect inhabitants of sensitive land use areas from noise pollution. Noise barriers are the most effective method of mitigating roadway noise, railway, and industrial noise sources – other than cessation of the source activity or use of source controls.
In the case of surface transportation noise, other methods of reducing the source noise intensity include encouraging the use of Hybrid vehicles and , improving automobile aerodynamics and tire design, and choosing low-noise paving material. Extensive use of noise barriers began in the United States after were introduced in the early 1970s.
The best of these early computer models considered the effects of roadway geometry, topography, vehicle volumes, vehicle speeds, truck mix, road surface type, and micro-meteorology. Several U.S. research groups developed variations of the computer modeling techniques: Caltrans Headquarters in Sacramento, California; the ESL Inc. group in Sunnyvale, 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, California.C.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
Numerous case studies across the U.S. soon addressed dozens of different existing and planned highways. Most were commissioned by state highway departments and conducted by one of the four research groups mentioned above. The U.S. National Environmental Policy Act, enacted in 1970, effectively mandated the quantitative analysis of noise pollution from every Federal-Aid Highway Act Project in the country, propelling noise barrier model development and application. With passage of the Noise Control Act of 1972,Public 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, more than a dozen research groups in the U.S. were applying similar technology and addressing at least 200 different locations for noise barriers each year. , this technology is considered a standard in the evaluation of noise pollution from highways. The nature and accuracy of the used is nearly identical to the original 1970s versions of the technology.
Small and purposeful gaps exist in most noise barriers to allow to access nearby fire hydrants and pull through , which are usually denoted by a sign indicating the nearest cross street, and a pictogram of a fire hydrant, though some hydrant gaps channel the hoses through small culvert channels beneath the wall.
The noise barrier may be constructed on private land, on a public right-of-way, or on other public land. Because sound levels are measured using a logarithmic scale, a reduction of nine is equivalent to elimination of approximately 86 percent of the unwanted sound power.
A wall with porous surface material and sound-dampening content material can be absorptive where little or no noise is reflected back towards the source or elsewhere. Hard surfaces such as masonry or concrete are considered to be reflective where most of the noise is reflected back towards the noise source and beyond.Federal Highway Administration "Highway Traffic Noise" 6/05
Noise barriers can be effective tools for noise pollution abatement, but certain locations and topographies are not suitable for use of noise barriers. Cost and aesthetics also play a role in the choice of noise barriers. In some cases, a roadway is surrounded by a noise abatement structure or dug into a tunnel using the cut-and-cover method.
LSW Graffiti wm.jpg|Noise abatement walls often block rail passengers' or road users' view and attract graffiti.
Geluidscherm Overschie.jpg|This noise abatement wall in the Netherlands has a transparent section at the driver's eye-level to reduce the visual impact for road users.
Rieder 360° - die niedrige Lärmschutzwand (3).jpg|Low walls close to the track avoid optical impact.
Noise barriers force the pollution plumes coming from the road to move up and over the barrier creating the effect of an elevated source and enhancing vertical dispersion of the plume. The deceleration and the deflection of the initial flow by the noise barrier force the plume to disperse horizontally. A highly Turbulence Wind shear zone characterized by slow velocities and a re-circulation cavity is created in the lee of the barrier which further enhances the dispersion; this mixes ambient air with the pollutants downwind behind the barrier.Bowker, G.E., Baldauf, R., Isakov, V., Khlystov, A., and Petersen, W. (2007). The effects of roadside structures on the transport and dispersion of ultrafine particles from highways. Atmos. Environ. 41, 8128–8139
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