Anoise barrier (also called asoundwall,noise wall,sound berm,sound barrier, oracoustical barrier) is an exterior structure designed to protect inhabitants of sensitiveland use areas fromnoise pollution. Noise barriers are the most effective method of mitigatingroadway, 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 ofhybrid andelectric vehicles, improvingautomobile aerodynamics andtire design, and choosing low-noisepaving material. Extensive use of noise barriers began in the United States afternoise regulations were introduced in the early 1970s.
Noise barriers have been built in the United States since the mid-twentieth century, when vehicular traffic burgeoned. I-680 inMilpitas, California was the first noise barrier.[1] In the late 1960s, analyticacoustical technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specificroadway. By the 1990s, noise barriers that included use of transparent materials were being designed in Denmark and other western European countries.[2]
The best of these early computer models considered the effects of roadwaygeometry,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 inSacramento, California; the ESL Inc. group inSunnyvale, California; theBolt, Beranek and Newman[3] group inCambridge, Massachusetts, and a research team at theUniversity of Florida. Possibly the earliest published work that scientifically designed a specific noise barrier was the study for theFoothill Expressway inLos Altos, California.[4]
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 ofnoise pollution from everyFederal-Aid Highway Act Project in the country, propelling noise barrier model development and application. With passage of theNoise Control Act of 1972,[5] demand for noise barrier design soared from a host ofnoise regulation spinoff.
By the late 1970s, more than a dozen research groups in the U.S. were applying similarcomputer modeling technology and addressing at least 200 different locations for noise barriers each year. As of 2006[update], this technology is considered a standard in the evaluation ofnoise pollution from highways. The nature and accuracy of thecomputer models used is nearly identical to the original 1970s versions of the technology.
Small and purposeful gaps exist in most noise barriers to allowfirefighters to access nearbyfire hydrants and pull throughfire hoses, which are usually denoted by a sign indicating the nearest cross street, and apictogram of a fire hydrant, though some hydrant gaps channel the hoses through smallculvert channels beneath the wall.
The acoustical science of noise barrier design is based upon treating an airway or railway as aline source.[dubious –discuss] The theory is based upon blockage of sound ray travel toward a particularreceptor; however,diffraction of sound must be addressed.Sound waves bend (downward) when they pass an edge, such as the apex of a noise barrier. Barriers that block line of sight of a highway or other source will therefore block more sound.[6] Further complicating matters is the phenomenon ofrefraction, the bending of sound rays in the presence of aninhomogeneousatmosphere.Wind shear andthermocline produce such inhomogeneities. The sound sources modeled must includeengine noise,tire noise, andaerodynamic noise, all of which vary by vehicle type and speed.
The noise barrier may be constructed on private land, on a publicright-of-way, or on other public land. Because sound levels are measured using alogarithmic scale, a reduction of ninedecibels is equivalent to elimination of approximately 86 percent of the unwanted sound power.
Several different materials may be used for sound barriers, including masonry, earthwork (such as earthberm), steel, concrete, wood, plastics, insulating wool, or composites.[7] Walls that are made of absorptive material mitigate sound differently than hard surfaces.[8] It is also possible to make noise barriers with active materials such as solarphotovoltaic panels to generate electricity while also reducing traffic noise.[9][10][11]
A wall with porous surface material and sound-dampening content material can be absorptive where little or no noise isreflected 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.[12]
Noise barriers can be effective tools fornoise pollution abatement, but certain locations and topographies are not suitable for use of noise barriers. Cost andaesthetics 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 thecut-and-cover method.
Potential disadvantages of noise barriers include:
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Roadside noise barriers have been shown to reduce the near-road air pollutionconcentration levels. Within 15–50 m from the roadside, air pollution concentration levels at thelee side of the noise barriers may be reduced by up to 50% compared to open road values.[13]
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 verticaldispersion of the plume. The deceleration and the deflection of the initial flow by the noise barrier force the plume to disperse horizontally. A highlyturbulentshear 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.[14]
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