Noise barrier

Exterior structure on infrastructure used to prevent loud sounds from escaping

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, 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 and electric vehicles, improving automobile aerodynamics and tire design, and choosing low-noise paving material. Extensive use of noise barriers began in the United States after noise regulations were introduced in the early s.

History

[

edit

]

Noise barriers have been built in the United States since the mid-twentieth century, when vehicular traffic burgeoned. I-680 in Milpitas, California was the first noise barrier.[1] In the late s, analytic acoustical technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specific roadway. By the s, 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 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 Newman[3] 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.[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 , 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 ,[5] demand for noise barrier design soared from a host of noise regulation spinoff.

By the late s, more than a dozen research groups in the U.S. were applying similar computer modeling technology and addressing at least 200 different locations for noise barriers each year. As of , this technology is considered a standard in the evaluation of noise pollution from highways. The nature and accuracy of the computer models used is nearly identical to the original s versions of the technology.

Small and purposeful gaps exist in most noise barriers to allow firefighters to access nearby fire hydrants and pull through fire hoses, 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.

Design

[

edit

]

The acoustical science of noise barrier design is based upon treating an airway or railway as a line source.[dubious &#; discuss] The theory is based upon blockage of sound ray travel toward a particular receptor; 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 of refraction, the bending of sound rays in the presence of an inhomogeneous atmosphere. Wind shear and thermocline produce such inhomogeneities. The sound sources modeled must include engine noise, tire noise, and aerodynamic noise, all of which vary by vehicle type and speed.

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 decibels is equivalent to elimination of approximately 86 percent of the unwanted sound power.

Materials

[

edit

]

Several different materials may be used for sound barriers. These materials can include masonry, earthwork (such as earth berm), 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 solar photovoltaic 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 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.[12]

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.

Disadvantages

[

edit

]

Potential disadvantages of noise barriers include:

• Blocked vision for motorists and rail passengers. Glass elements in noise screens can reduce visual obstruction, but require regular cleaning
• Aesthetic impact on land- and townscape
• An expanded target for graffiti, unsanctioned guerilla advertising, and vandalism
• Creation of spaces hidden from view and social control (e.g. at railway stations)
• Possibility of bird&#;window collisions for large and clear barriers

• Noise abatement walls often block rail passengers' or road users' view and attract graffiti.

• 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.

• Low walls close to the track avoid optical impact.

Effects on air pollution

[

edit

]

Roadside noise barriers have been shown to reduce the near-road air pollution concentration levels. Within 15&#;50 m from the roadside, air pollution concentration levels at the lee 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 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 turbulent 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.[14]

See also

[

edit

]

References

[

edit

]

• Noise barriers at Wikimedia Commons

Noise Barriers: How do they work?

Outdoor noise barriers can effectively reduce the transmission of noise from source to receiver. When placed between source and receiver, the barrier diffracts the sound transmitted to the receiver. This reduction is frequency dependent: Noise barriers block high frequencies more effectively than low frequencies.

What determines the effectiveness of a noise barrier?

A noise barrier&#;s effectiveness is determined by the degree to which it forces sound to bend to reach the receiver. The following sketches show the general principles involved and how changing the barrier height and location effects this critical angle.

As long as the barrier interrupts the straight line path from source to receiver, noise will attenuate as it diffracts around the barrier (shown as dotted lines).

There are a number of ways to increase the effectiveness of a barrier. The first is to construct the barrier close to the source. This is one of the best methods as it benefits all locations past the barrier.

If the barrier can&#;t be located near the source, the next best location is near the receiver. This is equally effective for that receiver, but the benefit diminishes for receivers at a greater distance. When a noise barrier is close to the receiver, the critical angle also increases.

Generally, the least effective location for a barrier is midway between source and receiver. Yet regardless of location, a barrier&#;s acoustical benefit improves when the barrier height is increased.

The mass of the barrier is usually not a critical element. The barrier should be constructed so sound that penetrates through the barrier is sufficiently lower than the sound that diffracts over the top. For example, the sound transmission loss of the barrier should be at least 10 decibels lower than the attenuation planned for above the barrier. A solid barrier that supports itself and withstands wind loading will often provide more than adequate sound transmission loss.

Calculating noise barrier attenuation

The actual calculation of barrier attenuation is based on formulas developed by Fresnel, using the following geometrical format:

When the barrier blocks line-of-sight between the source and receiver, the Fresnel Number (N) is found using this formula:

Using the Fresnel Number and the following chart, you can determine the excess attenuation (Ae4 in the chart below) of the barrier.

Other factors

Remember that other factors can influence a barrier&#;s effectiveness. Varying air speeds (increasing with height above ground) caused by light winds travelling from source to receiver can refract the sound passing through the air and bend it downward. This action tends to reduce the effectiveness of a barrier. Trees with foliage above the top the barrier can have a similar detrimental effect.

Noise barriers can also cancel the beneficial attenuation that results from ground effect, a phase cancellation effect that can occur when both source and receiver are very near the ground. This loss of ground effect attenuation must be subtracted from the barrier attenuation to determine the net attenuation of the barrier.

The barrier should block all paths from source to receiver. This means that if a barrier is 3 metres high, it should extend 6 metres horizontally past the point where the barrier blocks line-of-sight from source to receiver. This prevents sound from flanking the barrier&#;s edge. In addition, there can be no gaps or low points in the barrier. An opening, for a roadway for example, would significantly diminish the acoustical performance of the barrier unless it was carefully designed.

Before installing a noise barrier, it&#;s important to consider the acoustical factors we describe in this article. Yet there are other factors that can&#;t be overlooked, factors that can affect how the barrier is perceived by residents. During the design stage, ask the following questions:

• Will the barrier create a safety issue by blocking the view of drivers to other cars or to pedestrians?
• Will the barrier block a view that is important to the residents?
• Will the barrier itself look unattractive?
• Will the barrier obstruct a breeze that helps cool residences&#; yards?
• Will the barrier adversely effect plants or gardens? Will it cause unacceptable shading?
• Will the barrier require passages for pedestrian and/or vehicle access?
• Will the barrier become a personal security hazard by creating areas where criminals can hide?
• Will the barrier demand ongoing maintenance costs for the municipality?
• Will there be drainage or snow removal issues?