How effective are noise barriers?

Drive down the interstate highways bisecting many busy cities and suburbs, and you&#;ll likely no longer see the homes, buildings, or vistas that used to be a staple of roadway views. That&#;s because in most populated places, massive sound walls have been installed. These noise barriers, typically made of concrete and standing an average of 14 feet, turn the backs of neighborhoods into prison-like yards, and, on narrower stretches of road, encase drivers in roofless tunnels. Since the s, when the barriers first started sprouting, nearly three thousand linear miles have been erected. According to Department of Transportation officials, California alone has 760 miles of sound walls; Florida, 252 miles.

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By and large, residents say they want these walls. California has a waiting list for them. And at meeting last June with representatives of the Florida Department of Transportation (FDOT), which this journalist attended, many residents of Boca Raton were eager to know whether sound walls would be erected to buffer their homes from a planned turnpike expansion.

One man&#;s concern, however, stood out at that meeting. He talked about his prior house, which he claimed was quiet until a highway noise wall was installed a few blocks away &#; and it turns out that this isn&#;t so unusual. For homes several streets from the barriers, or for those uphill of sound walls &#; and for everyone in certain weather conditions &#; the walls don&#;t effectively block the sound, and may even help to amplify it. And what&#;s worse is that these aren&#;t new insights. Engineers and acousticians have known for years that the sound barriers bracketing America&#;s urban and suburban highways are only marginally useful, and that a variety of better technologies could be developed.

The problem: Nobody has an incentive to get them on the road.

&#;Walls are not a very effective solution,&#; said Robert Bernhard, vice president for research at the University of Notre Dame and an expert on noise control. Because the federal government pays for noise walls &#; and only noise walls &#; as part of highway expansion projects, he said, there is little incentive for researchers to keep testing and perfecting the alternatives.

Sound moves in not-so-mysterious ways, meaning that typical sound barriers have only limited effectiveness. Visual: Wisconsin DOT

Noise that bothers a community must be at least considered for mitigation thanks to the Noise Control Act of . It was passed as part of the federal government&#;s efforts to better protect the environment &#; noise being one of many pollutants coming under scrutiny. Typically, when an interstate is widened or newly built, and in a small number of cases, when no additional construction is done, the state highway agencies determine whether they should mitigate the ruckus to area neighborhoods.

That ruckus tends to come from three separate elements: the roar of the vehicles &#; primarily the exhaust and engine; the whooshing aerodynamics around the vehicles; and the slapping of the tires against the road. At highway speeds, the predominant sound for cars is that of tire-pavement; for trucks, engine and stack sounds are also a factor &#; at least for now.

States use a specific noise model to predict the sound once the road will be expanded, and for several decades after. The complex formula includes the mixture of cars and trucks expected on the road; the buildings and vegetation in the area that would block some sound; the configuration and ground quality of the land between the road and the homes; the ways the sound is expected to diffract around the wall; and other key factors.

Based on the formula, if the noise is projected to go over the government threshold of approximately 67 decibels (dB) during the noisiest hour of the day &#; and it is &#;reasonable and feasible&#; to reduce it at least 5 dB for some percentage of homes &#; the government requires that walls be included if the surrounding community wants them. Just what constitutes &#;reasonable,&#; of course, is interpreted in different ways by each state, which is why the use of sound walls varies greatly from one state to another.

Even with the sound reduction, however, roadside residents are unlikely to hear crickets chirping. A dishwasher running in the next room is 50 dB, as are the ambient sounds of a laid-back city. The noise criteria aim to allow people to talk over their backyard picnic table, or shout at someone several feet away. &#;It&#;s not a situation where meeting the standard makes for a great backyard environment,&#; Bernhard said.

Of course, some of our ability to process sound is psychological: If people can see the tops of trucks over the wall they say it&#;s noisier, something people in the field call &#;psycho-coustics,&#; explained Bruce Rymer, a senior engineer at the California Department of Transportation. Just by ensuring a wall breaks that line of sight, &#;we achieve a reduction of 5 decibels,&#; said Mariano Berrios, environmental programs coordinator at FDOT.

But because noise travels in waves, not straight lines, sounds can and do go over the walls. This is why even with barriers standing 16 feet, homes several blocks away can hear the highway. Part of the sound wave is absorbed, part is reflected away from the wall, and part is transmitted through, Berrios explained. &#;Most of it goes above the barrier and gets diffracted, and gets to the receiver,&#; &#; that is, to a resident&#;s ears &#; he said.

Because noise travels in waves, not straight lines, sounds can and do go over the walls. This is why even with barriers standing 16 feet, homes several blocks away can hear the highway. Visual: Virginia DOT

This is especially problematic during certain weather conditions. When the consulting firm Bowlby & Associates, in Franklin, Tennessee, measured sounds around a highway in a yet-to-be-published study, they found that residents hundreds of feet from the highway could hear sounds some 5 decibels louder if the wind was blowing towards them, said Darlene D. Reiter, the firm&#;s president.

Weather, however, isn&#;t taken into account by the regulations. The noise model &#;assumes neutral conditions &#; no wind and no temperature effects &#; when in reality that happens only occasionally,&#; Reiter said. In the early morning, if the ground is cool but the air warms up, for instance, sound that would normally be pushed up is refracted downward, causing homes some 500 or 1,000 feet from the road to hear it loudly.

Those living up on hills or near freeway openings sometimes find the noise actually worsens once walls are built nearby. It was a gap in the barrier near his suburban New Orleans home &#; partially to accommodate a highway exit &#; that substantially increased noise in the backyard of attorney Harry Molaison. Although his house is roughly 500 feet from the service road leading to the interstate, &#;you have all this rebounding sound from one parallel wall to another,&#; he said.

&#;We don&#;t have the same peacefulness we had before,&#; he added.

It&#;s with these problems in mind that the University of Pittsburgh recently received a grant to study whether walls could be made of materials that absorb, rather than reflect, more of the noise. But even if new materials were developed &#; in addition to the popular concrete, sound walls are currently made of everything from masonry and steel to wood and plastic &#; the question would remain: Is this the best use of taxpayer money?

Highway walls are expensive, running more than $2 million per linear mile &#; for one side of the highway, Rymer said. The total spent on sound walls through , the most recent government figures, tops $6 billion. Each state has a different threshold for what triggers the need for a &#;reasonable&#; intervention. According to Rymer, in California, which has one of the lowest thresholds, walls are justified when they cost federal taxpayers as much as $92,000 per impacted home. This is money that isn&#;t spent on mass transit, or fixing ailing tunnels or bridges, or other transportation needs.

&#;Three miles of sound barriers on both sides of an interstate would buy another M8 railcar for Metro-North [train service], and take 100 passengers off the state&#;s highways&#; wrote Jim Cameron, the founder of a Connecticut-based commuter advocacy group, in a newspaper editorial earlier this year.

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Mammoth barriers also block small animals &#; frogs, turtles, snakes &#; from getting from one habitat to another, said Elizabeth Deakin, professor emerita of city and regional planning at the University of California, Berkeley. This may affect wildlife communication, migration, and even reproduction.

Of course, it&#;s understandable why neighbors whose homes border a highway want something that mitigates the noise. Loud traffic interferes with the enjoyment of your yard. Having cars so close to a home can even cause health issues. According to a World Health Organization report, environmental noise leads to heart disease, hypertension, and cognitive impairment in kids. But if the bulk of the noise is caused by the tires and the roadway, some experts suggest that attacking the commotion at the source &#; or testing other methods that might absorb it &#; could be a more effective and less costly approach.

Some tire companies have done research on making tires quieter, but the bulk of their efforts are in keeping the noise from penetrating the inside of the car, not in silencing them outside, Bernhard said. And while electric cars are far quieter than cars with internal combustion engines, at highway speeds car engines aren&#;t much of a factor &#; though trucks could be a different story. Tesla&#;s recent introduction of its electric semi-truck will undoubtedly alter highway sounds going forward, since the engine and stack noises will be eliminated.

Companies in some European countries are experimenting with unconventional methods that could ultimately block highway sound. One, a luminescent solar concentrator (LSC), features colorful translucent sheets that not only don&#;t obstruct views and sunlight, they generate electricity to nearby homes. Another is researching whether dense bamboo or other plant species can be coaxed to form an effective vegetation wall.

But altering the pavement is where most of the potential seems to lie. Several states &#; Arizona, California, and Florida in particular &#; have experimented with such changes. These &#;quieter pavements&#; involve adding more porous surfaces to asphalt or altering the configuration of the tiny grooves in concrete. &#;When there is texture on the surface of the pavement, the trapped air inside the tire&#;s tread pattern doesn&#;t make the same clapping noise,&#; Bernhard explained.

Some states have laid thousands of miles of these road surfaces, and have seen results of up to a 9dB reduction in noise. Dana M. Lodico, a senior consultant with Illingworth and Rodkin, said engineers have been studying its effects since the s. Her firm alone conducted four major decade-long studies and many shorter ones. &#;There&#;s tons of research&#; showing its effectiveness, she said, especially in states with warmer climates. (The studded tires some drivers use in snowy states can break down the road surface more quickly.) One major report that her firm worked on examined the cost-benefit of sound walls versus pavement changes, and found many scenarios where a combination of lower walls &#; or no walls &#; were more effective and less expensive than a barrier by itself, she said.

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Despite all of these potential innovations, however, the current structure of federal highway subsidies is likely to keep them from widespread use anytime soon. As it stands, the Federal Highway Administration has not approved pavement as an accepted form of noise abatement. &#;We have uncertainty about how long the reduced noise level from the pavement will last, and there is no guarantee that the reduction can be achieved on a consistent basis nationwide,&#; said agency spokesperson Doug Hecox.

That means states that currently change their pavement still have to put up walls as part of their highway projects. And because maintenance of the pavement to keep it quiet &#; resurfacing perhaps every 15 years, Lodico said &#; would fall to the states, many state officials undoubtedly prefer the more-permanent walls, which are built almost exclusively with federal funds.

When it comes to mitigating highway noise, Bernhard noted, &#;The predominant culture is cost avoidance.&#;

Meryl Davids Landau is a Florida-based journalist whose work has appeared in a variety of publications, including U.S. News & World Report, Glamour, Vice Media, Parents, Reader&#;s Digest, Good Housekeeping, and Prevention, among others.

Find information about noise barriers, our process for determining where to build them and other options to reduce noise at your home.

If you live near a roadway, you may be concerned about traffic noise. We recognize this impact from our highways and work to reduce it where we can.

Noise barriers

We provide noise barriers to reduce traffic noise through two types of projects.

Type one noise barrier projects

When we build new improvement projects, we include noise barriers if noise standards are exceeded and other criteria are met. This type is triggered when we do at least one of the following:

• Construct a highway at a new location.
• Significantly change either the horizontal or the vertical alignment of a highway.
• Increase the number of traffic lanes.
• Alter roadside topography.

Type two noise barrier projects

We build &#;retrofit&#; barriers along highways in neighborhoods that existed before noise abatement regulations were established.

The effectiveness of a noise barrier depends on the distance between the listener and the barrier. For residences located directly behind a barrier, the noise level will often be cut in half. This benefit decreases as a listener moves farther away and is negligible at distances greater than 500 feet.

WSDOT uses earth, concrete, wood, and masonry block to build noise barriers. Earthen berms work the best and are the least expensive, but a lack of available right-of-way usually makes concrete walls the most practical solution. Most often we build noise walls &#; free-standing walls usually made of concrete. The walls range in height from 6 to 20 feet, but normally are 12 to 15 feet tall.

Trees and shrubs can decrease highway-traffic noise levels if high enough, wide enough, and dense enough (cannot be seen through), but are often impractical. It would take at least 100 feet of dense vegetation to provide the same benefit as our smallest feasible noise wall. Trees do provide a visual shield and some psychological benefit. The Federal Highway Administration (FHWA) has not approved using vegetation for noise abatement.

Our noise barrier process

We follow the following standard practice to determine where to build noise barriers and decide how they are configured.

Traffic noise analysis

Long before construction begins, acoustical specialists evaluate sources and patterns of noise in neighborhoods near the project. Noise evaluations take into account many factors, including:

• Highway noise.
• Area topography.
• Population density.
• Cost.
• Expected levels of noise reduction a wall would provide.

Computerized noise models are developed to predict future traffic-noise levels. To be conservative in our estimates, WSDOT typically models with rush hour traffic volumes, traveling at the posted speed limit.

Any applicable area predicted to have a future traffic-noise level of 66 decibels (dBA) or greater, qualifies as an impacted area. Research shows that above 66 dBA, a conversation between two people standing three feet apart and speaking in a normal voice is impaired.

All impacted areas are considered for noise abatement. Analysts make every attempt to qualify these impacted locations for noise barriers based on the reasonable and feasible criteria. A noise wall must provide at least a seven-dBA noise reduction, but we try to design walls that provide a ten-decibel reduction.

Reasonable and feasible criteria

A barrier must meet both reasonable and feasible criteria to be constructed:

• Feasible refers to whether the barrier can provide at least a five dBA reduction at three first row receivers and meets other constructability requirements.
• Reasonable refers to whether the barrier can provide minimum seven-dBA noise reduction requirements while meeting cost effectiveness requirements.

Barriers do have limitations. For a noise barrier to work, it must be high enough and long enough to block the view of the road. Noise barriers do very little good for homes on a hillside or for buildings that rise above the barrier. Openings in noise walls for driveway connections or intersecting streets destroy their effectiveness. In some areas, homes are scattered too far apart for noise barriers to be built at a reasonable cost.

Noise wall costs

Current construction costs average $51.61 per square foot. This translates into a fourteen-foot high wall (typical) costing about 3.9 million dollars per mile. Construction costs for rural barriers may be lower and urban barriers may be much higher. The higher urban costs are associated with other infrastructure (like retaining walls, water pipes, etc.) that may need to be changed or moved to allow the placement of the barrier.

Many noise walls built alongside interstate highways receive partial federal government funds, and WSDOT pays the remainder. On other state routes, WSDOT or local jurisdictions, depending on who is sponsoring the project, pays for an entire wall. In special cases, if a local community would like to enhance the barrier with aesthetic treatments or to make the barrier longer or taller than recommended, the community may provide additional funding. The proposed improvements must meet WSDOT safety, maintenance, and right-of-way needs.

Public input on noise walls

WSDOT project design offices works closely with the impacted communities for a proposed noise wall to make sure all reasonable design requests get included in the project plans.

Quieter pavement

WSDOT has some of the most comprehensive data in the world on quieter pavement performance.

Because the noise reductions from quieter pavements decline long before the pavement typically needs to be replaced for other reasons, FHWA does not allow quieter pavement for noise abatement.

Options to reduce noise at your home

WSDOT does not have funds available to make modifications to residences to reduce traffic noise in the home. We only consider interior noise reductions for certain types of public structures defined as Category D in the WSDOT noise policy.

Residents can make changes at their home to reduce traffic noise impacts.

Reduce noise levels inside the home

Replace single pane windows with insulated double pane windows.

Reinsulate walls and ceilings.

Seal door, window and other cracks.

Use indoor fans or install air conditioning instead of opening windows.

Use noise-absorbing material in the walls of new buildings during construction, although this acoustic insulation is very expensive.

Noise masking &#; Many people have reported some success with white noise, such as a fan or waterfall, to drown out the traffic noise and make it less noticeable.

Reduce noise levels outside

Use visually interesting landscaping to obscure the roadway. Some plants help mask traffic noise by rustling in the wind. Even though plants do not effectively reduce noise levels, they give a sense of privacy and serenity. Talk to a landscape architect for ideas.

Enclose a favorite garden spot, deck, or patio with transparent plastic or other barrier.

Build a "do-it-yourself" noise barrier. If properly built with appropriate materials, you can get significant noise reductions around your home. Talk to an acoustical consultant to make sure the barrier you plan to build will provide the noise reduction you expect. To prevent a substantial amount of noise, consider the following:

For more Noise Barrier Panelinformation, please contact us. We will provide professional answers.

• The material used must weigh at least 4 pounds per square foot.
• The wall or fence can't have gaps.
• Blocking the line of sight to the noise source will usually result in a 5-decibel reduction.
• Increasing the height of the barrier will provide additional noise reduction until the fence fully blocks the line of sight to the roadway. Beyond that, the noise reductions from additional height are minimal.
• Either the fence must be long enough to prevent noise from coming around the ends, or it must continue around the property line to enclose the target area.