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Part I Noise Fundamentals

Part II Noise Analysis

Part III Noise Abatement

Frequently Asked Questions

Glossary and Acronyms

Frequently Asked Questions - Noise Abatement

How much will the noise levels change after a noise wall has been built and will the change be noticeable?
The reduction in noise levels depends on the separation distance and elevation difference between the receptor and the noise wall. IDOT’s noise reduction design goal is to achieve at least an 8-dBA traffic noise reduction, which is considered a substantial noise reduction. For a receptor to be considered “benefited,” at least a 5-dBA reduction will occur. Changes in noise levels greater than 5 dBA are readily perceived by humans.

Why isn’t noise abatement designed to reduce noise levels below the NAC?
The noise abatement criteria (NAC) identifies the noise level at which noise abatement should be evaluated. It is not a noise abatement goal. The objective of noise abatement is to achieve a noise reduction that will result in a noticeable difference from the unabated traffic noise levels and can be implemented in a cost effective way. In typical noise abatement evaluations, a substantial noise reduction is considered to be an 8-dBA traffic noise reduction. The following table demonstrates this noise reduction goal for two sites.

Location Future Noise Level NAC Noise Reduction Goal Target Noise Level
Site 1 69dBA 67dBA 8dBA 61dBA
Site 2 78dBA 67dBA 8dBA 70dBA

Why aren’t noise barriers proposed in some cases?
A noise barrier may be proposed when a noise impact occurs and the noise barrier will be feasible and economically reasonable. A noise barrier is determined to be feasible if it achieves at least a 5-dBA traffic noise reduction for one impacted receptor. Issues, such as driveway access and elevation of the receptor, may prevent achievement of a 5-dBA reduction.

A noise barrier must also be economically reasonable, which includes three criteria:

If a noise barrier is not considered feasible and economically reasonable for an area, the noise barrier abatement measure may not be implemented as part of the project.

Other feasibility factors that influence if a noise barrier will be proposed include whether or not sufficient right-of-way is available for the safe placement of the barrier, impacts to the line-of-sight of approaching vehicles in the vicinity of on-ramps, off-ramps, and intersecting streets and/or interference with utilities and/or drainage design elements.

How long does the noise wall need to be?
Generally to be effective, the noise wall should extend 4 times the distance between the receptor and the noise wall. In other words, if the distance between the house and the noise wall were 50 feet, the noise wall would need to extend 200 feet beyond the receptor in each direction.


Would a berm be as effective as a noise wall in reducing noise levels?
Earth berms are just as effective as noise walls. Studies have shown that earth berms actually reduce noise levels to a greater extent than noise walls due to absorption and edge effects. However, the use of berms depends on available space. For instance, since IDOT requires at least a 3:1 slope for maintenance purposes, a 12-foot berm with a 3:1 slope would be approximately 72 feet wide at the base. The available area for abatement would need to accommodate this base width.

Why can’t we build a taller wall to get greater noise reduction?
The barrier height is just one element that affects the traffic noise reduction achieved. A noise wall that breaks the line of sight between the traffic noise source and noise receiver reduces traffic noise up to 5 dBA. Each additional two feet of noise wall improves the traffic noise reduction by approximately 1 dBA; however, there are diminishing returns. Beyond a certain height, incremental changes in height do not provide additional reduction in noise level. This occurs because the wall has already intercepted a high percentage of noise energy.

Reduction in Sound Level

Degree of Attainability

5 dBA Easily Attained
10 dBA Attainable*
15 dBA Very Difficult
20dBA Nearly Impossible

Does a noise wall absorb noise or does noise bounce off the wall? 
This depends on the type of noise wall constructed. An absorptive noise wall is designed to absorb sound and keep it from reflecting off the noise wall. A reflective wall is a wall not designed with an absorptive material and consequently, sound reflects off the wall back toward the source. The reflected sound level is less than the sound level coming directly from the noise source. This is due to the additional distance the reflected sound travels, thereby dissipating the sound (reducing noise energy). Generally, the increase in sound levels due to reflections is not perceivable and therefore negligible.  

If I live one block from the highway, will a noise wall reduce the noise in my yard?
Noise walls are generally most effective within 200 feet of the noise wall, and one block is generally a distance of 700 feet. This means there may only be minimal reduction but the magnitude will depend upon several factors, such as the distance from the roadway, the elevation of your home relative to the roadway, and the presence of buildings between the roadway and your home.

What about reflective noise?
Reflective noise is generated when sound waves reflect off a hard flat surface back toward the source and the opposite side. This situation may occur when a noise wall is constructed on one side of the highway, potentially reflecting noise to the opposite side. However, the increase in noise level on this opposite side of the highway is typically less than 3 dBA, which is not a perceivable change.

What is the cost of a noise wall?
The average unit cost of noise wall construction used for the barrier evaluation is $25 per square foot (including materials and installation). This cost is based on Illinois construction costs and walls built. In areas where there are utilities or drainage issues that may need to be addressed, additional costs may be incurred. Typical noise walls cost $1,500,000 per mile. The unit cost is re-evaluated by IDOT at least every five years and is based on actual construction costs.

Can alternatives to IDOT standard noise wall materials be used?
Based on material testing, IDOT has currently approved three types of materials for noise barriers:

            • Construction using concrete;

            • Construction using composite materials; and

            • Earth Berms.

Other materials may be considered if they meet IDOT’s criteria for noise abatement wall materials and are approved by the Illinois Highway Development Council.

“Non-standard” noise wall designs, such as alternative patterns for a concrete wall may be considered, but any costs exceeding that of a “standard” noise wall must be funded by the sponsor.

What are some of the positive and negative impacts of noise wall construction?
• Positive Impacts

–Easier conversation

–Better sleeping conditions

–Windows open more often

–Outside more in summer

–More privacy

–Increased safety

–Decreased stranded motorist instances


Negative Impacts

–Restricted view

–Feeling of confinement

–Loss of air circulation

–Loss of sunlight and lighting

–Eyesore if barrier not maintained


Can’t we just plant trees/vegetation to help reduce noise levels? 

  • Vegetation would need to be at least 200 feet wide and 18 feet high to reduce noise levels by 10 dBA.
  • In most cases, 200 feet of space between the roadway and receptors is not available.
  • Vegetation/trees can potentially help screen the highway traffic from view.

Can I get sound insulation for my house?
FHWA only participates in sound insulation of Activity Category D land use facilities. IDOT’s policy is established to address exterior areas where frequent human activity occurs and an impact has been determined based on the interior noise impact evaluation. Noise insulation will only be considered for Activity Category D if noise barriers are determined to be not feasible or reasonable. In addition, IDOT is prohibited from spending state money outside of state right-of-way.

Why can’t IDOT prohibit trucks along roads or reduce speed limits?  Won’t that reduce noise levels?
Both of these options may reduce noise levels; however, several factors must be considered.  If the road is a main route into and out of a city, or if there are commercial and industrial businesses along the route, a prohibition of trucks would have adverse economic impacts.  Also, truck traffic cannot be prohibited on state marked routes.  Lowering the speed limits may slightly reduce traffic noise levels, but the speed reduction would lower the capacity of the roadway, thereby increasing delays, air pollutant emissions, and the overall cost of transporting goods and services.  Speed limits are determined by the roadway design and speed studies.

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