Noise Walls Frequently Asked Questions

General Noise Wall FAQs

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How is noise defined?
Noise is usually defined as unwanted or unacceptable sound. The intensity of sound, or sound level, is described using decibels. A decibel is a unit of measurement that quantifies the sound pressure differences in the air that we perceive as sound (or noise) on a scale ranging from zero decibels on up.
Zero decibels is the threshold of human hearing, 40 to 50 decibels is normal for a peaceful neighborhood, 70 to 80 decibels is the level adjacent to a busy urban street or 50 feet from a major freeway, and 120 to 140 decibels is a typical level at which sound is painful.
For highway traffic noise studies, noise levels are quantified in terms of the equivalent sound level, or Leq. The Leq is essentially the average noise level over a period of time, usually one hour.
When is a traffic noise analysis required?
A traffic noise analysis is required for proposed N.C. Department of Transportation projects of a certain scope, such as building a highway on new location, substantially changing the location of the road either in its vertical dimension (raising it or lowering it) or in its horizontal dimension (moving it over), or adding through traffic lanes or long auxiliary lanes.
Noise analysis is also required with certain kinds of interchange improvements or if there is an addition of a new, or a substantial change to an existing, weigh station, rest stop, ride-share lot or toll plaza.
Minor projects, such as normal roadway resurfacings (without adding new lanes), do not require a noise analysis. More details of which type of projects require noise analysis are provided on the Electronic Code of Federal Regulations’ website (see “Type I Project”).
How are traffic noise levels determined?
NCDOT uses the Federal Highway Administration’s Traffic Noise Model version 2.5 (TNMv2.5) to predict noise levels from a proposed highway project. TNM is the Federal Highway Administration’s computer program for highway traffic noise prediction and analysis. TNM computes highway traffic noise at nearby receptors (see next question for an explanation of what a receptor is) and aids in the design of highway noise barriers.
What are the noise level thresholds?
NCDOT considers noise walls where traffic noise “impacts” occur at a "noise sensitive receptor" (e.g., homes, parks, schools, places of worship). There are two criteria that are evaluated when considering whether an impact occurs: the absolute noise level and the change in noise level.
For an exterior residential land use, the absolute noise level threshold at which an impact occurs is 66 decibels or higher.
The change in noise level threshold compares noise levels today to noise levels predicted 20 years out with the project under traffic. This difference in noise level must be 10 decibels or greater in order to be considered an impact. Thus, a traffic noise impact occurs when the absolute future traffic noise level is 66 decibels or higher OR when the difference between existing noise levels and future noise levels is 10 decibels or greater. Noise walls are considered when either of these two impact criteria is met.
More information can be found at the following links:
- NCDOT: Reducing Noise Pollution
- Connect NCDOT: Traffic Noise Policy
How far out does a noise study go? How far away are noise walls effective?
Noise levels along a highway are a function of the distance from that highway. The closer the homes are to a highway, the louder the noise levels will be. As the distance from a highway increases, the noise levels drop off. The noise study extends far enough from the highway to ensure that all traffic noise impacts have been identified (see the above question). Beyond this distance, there is no value in continuing the noise study.
Noise walls are most effective for residences that are in close proximity to a highway. For the first one to two rows of residences, noise walls are generally very effective. As the distance from the highway increases, the more unlikely it is that a residence would notice substantial benefit from a noise wall. The degree to which a noise wall provides shielding from traffic noise is not only a function of distance but is also influenced by terrain, other buildings, dense vegetation, ground cover (pavement, soil, lawn, water, etc.), and whether the residences are higher or lower than the highway. For these reasons, the noise levels behind a noise wall can vary greatly from site to site.
What is noise abatement?
Noise barriers (in the form of walls or berms) are the most commonly used form of noise abatement and are the only form of noise abatement required for consideration on federal-aid highway projects. Noise walls or berms are solid barriers built between the road and the receptors along the road.
What does NCDOT consider "feasible and reasonable"?
A noise wall or berm must be both feasible and reasonable if it is to be constructed with a highway project. Feasibility and reasonableness are determined by criteria that are measurable.
As a result, noise abatement is not automatically provided where noise impacts have been identified. A wall or berm is feasible if it can provide a noise reduction of at least 5 decibels and if it can be constructed without major engineering or safety issues.
A wall or berm is reasonable if it can be designed to achieve a noise reduction of at least 7 decibels, constructed in a cost-efficient manner, and if the community wants the wall or berm. All three of these criteria must be met for a wall or berm to be considered reasonable to build.
For more information, read NCDOT's Traffic Noise Policy.
What amount of noise reduction should be achieved with a noise wall or berm?
A noise wall or berm must provide a readily perceptible decrease in noise levels to adjacent receptors to be feasible. This is defined as a decrease of at least 5 decibels in traffic noise levels. As noise level changes of 3 decibels or less are not noticeable, it is not feasible to build a noise wall or berm that provides a noise level reduction that is not detectable or just barely detectable to adjacent receptors.
What are the most common types of noise abatement constructed?
Noise barriers are commonly constructed as walls, earthen berms, or a combination of the two. Walls are the most common noise barrier used and are normally constructed out of dense materials such as concrete or masonry block. Earth berms are a natural alternative to walls but require significantly more land in order to construct them. Walls can be constructed on top of berms in order to raise the overall height of the barrier.
How does noise abatement work?
Noise walls or berms reduce noise by blocking the direct travel of sound waves from a source (such as a highway) to adjacent homes or businesses. Some of the noise is reflected, some is absorbed, and much is deflected along a longer path as the waves are forced over the top or around the wall or berm. Sound that is forced to take a longer path dissipates with distance and is not as loud when it reaches the receiver.
The wall or berm must be high enough and long enough to block the view (line of sight) of the highway. This allows a noise barrier to provide a perceivable noise reduction. Noise walls or berms are not effective for homes on a hillside overlooking a road or for buildings which rise above the barrier. Openings or gaps in walls for driveway connections or street intersections reduce the effectiveness of the potential abatement.
Noise walls or berms are generally most effective for the first one or two rows of homes. As noise levels decrease with distance, there is a point away from the highway at which noise abatement is no longer effective. It is important to note that walls or berms are not designed to eliminate or block all noise.
What is the process once a noise study is complete?
Once the noise study is completed, the results indicating the locations where noise walls are recommended will be posted on the project website. Soon after the Design Noise Report is complete, the balloting process will begin where the property owners and tenants benefited by a recommended noise wall will vote on whether they would like a noise wall or not.
How does the balloting process work?
Once recommended noise wall locations are determined during the final noise analysis, all property owners and tenants who are benefited by a recommended noise wall (a benefitted receptor would have at least a 5 decibel decrease in traffic noise with the noise wall in place) will be asked to vote for or against construction of the wall. This is called balloting, and it is the last step in the noise wall approval process.
During this time, NCDOT will contact property owners and tenants who are eligible to vote and explain the balloting process and what they are being asked to vote on. Recommended noise walls that pass the balloting process will be built. Once balloting is complete for a noise wall, NCDOT will send a postcard to those who were asked to vote on the noise wall to let them know if it passed or not.
The ballots typically come in a large yellow envelope with red text on the envelope stating, ‘Noise Wall Ballot Enclosed.’ Inside the packet will be a letter that explains the voting process, a map that shows the general location, height, and length of the wall, and a postcard. The postcard will ask you to simply check ‘yes’ or ‘no’ to whether you would like a noise wall or not like a noise wall. There will be a date in red text and that is the date NCDOT asks that you return your ballot by. Generally, NCDOT allots roughly two weeks for ballots to be returned.
Voting by email is an option; if you’d like to receive your balloting information via email and/or submit your vote by email, contact Missy Pair at mpair@ncdot.gov and let her know you are interested in voting by email.
Who gets to vote during the balloting process?
Property owners and tenants “benefited” by a recommended noise wall will vote on whether they would like a noise wall or not. A benefit is defined as a receptor that receives a noise level reduction of 5 decibels or more through placement of a noise abatement measure (e.g. noise wall or earthen berm).
Will planting vegetation help reduce noise levels?
Vegetation is not effective for reducing noise levels. It is not feasible to plant enough vegetation along a highway to achieve sufficient noise reduction, although planting trees or shrubs can provide aesthetic benefit and visual screening. Studies have shown that vegetation must be at least 100 feet thick, at least 20 feet high, and dense enough (100 percent opacity year-round) to provide a 5 decibel noise reduction. For these reasons, the Federal Highway Administration does not allow the use of vegetation for the purposes of noise abatement.

Complete 540 Noise Wall FAQs

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When will the noise walls on the Complete 540 project be built?
The construction of the seven noise walls for R-2828 (East of U.S. 401 to I-40) will be staggered. See the estimated construction timeline for each wall.
What will the noise walls on the Complete 540 project look like?
The noise walls along Complete 540 will be brick, on both sides, with concrete piles and concrete coping across the top. They will be similar in appearance to the noise walls along the existing Triangle Expressway (N.C. 540).
Will I have to pay for the noise wall that is recommended near my home? Will my taxes increase?
No. The cost of the wall will be paid for by the same funds being used to construct the highway project. Your taxes will not increase.
If there an appeal process if a noise wall is not recommended near my home?
No. NCDOT can only build noise walls that fall within the feasibility and reasonableness constraints of the Traffic Noise Policy. If you have a question about the noise studies done near your property, please send an email to complete540@ncdot.gov or call the Toll-Free Project Hotline at (800) 554-7849.
Are noise studies reevaluated once a highway project is completed to verify that the noise level predictions were correct?
NCDOT does not conduct additional noise modeling or field measurements once a final design noise study (documented in a Design Noise Report) is complete, for several reasons. They are:
1. Noise impacts, abatement decisions and noise wall design are based on future (2040 in the case of the Complete 540 project), loudest condition traffic noise levels. Therefore, taking additional field measurements would not be useful, since they would be current noise levels, and could not be conducted in such a way as to guarantee we are capturing loudest traffic conditions.
2. Sound level meters capture all noise in an area, including barking dogs, children playing, sirens, lawn mowers and are influenced by meteorological factors like wind, temperature and humidity. None of these are accounted for in our noise models (discussed below) since NCDOT is only responsible for providing abatement for highway traffic noise.
3. Federal Highway Administration regulations and NCDOT’s noise policy require the use of Traffic Noise Model (Version 2.5) to predict worst hour traffic noise levels, assess impacts, and design acoustically effective noise abatement. We do not use field measurements for these purposes.
4. The Traffic Noise Model (Version 2.5) has been validated by Federal Highway Administration as an accurate tool for predicting worst hour traffic noise levels.
5. Predicting design year worst hour noise levels can only be done with modeling. We cannot replicate these conditions with field measurements.
NCDOT does take field measurements at the start of a noise study. These are usually a series of short-term measurements that are used to validate our model. In other words, the measurements taken in the field should closely agree with model predictions at those same locations. If they do, then the model has been validated to local conditions and is suitable for identifying noise impacts and assessing abatement. In the case of highways built on new location, long-term field measurements are taken to determine the existing, or ambient, noise levels.
Who can I contact if I have questions about the noise study that was done for the Complete 540 project?
If you have a question about the noise studies done near your property, please send an email to Complete540@ncdot.gov or call the Toll-Free Project Hotline at (800) 554-7849.

Last updated Feb. 11, 2021

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Noise Walls Frequently Asked Questions

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General Noise Wall FAQs

Complete 540 Noise Wall FAQs