CONTROLLING HIGHWAY NOISE
Highway noise is the cumulative sound levels produced by motor vehicles and their components and is affected by many variables such as vehicle condition, road surfaces, topography, and weather. Highway noise is the most prevalent source of environmental noise pollution. Numerous studies have indicated that the most common sources of noise in our environment today are those associated with transportation. Studies on highway traffic noise began in the 1960’s, and it has been of increasing concern for both the public and government municipalities over the years. The Federal Highway Administration (FHWA) has developed noise regulations as required by the Federal-Aid Highway Act of 1970. These regulations apply to highway construction projects where a State department of transportation has requested Federal funding for participation in the project.
Highway traffic noise is not constant, so noise level criteria is based on a time weighted average or “leg”. The FHWA uses a 1-hour leg in determining noise thresholds. Traffic noise is reported as dBA to better represent human hearing. Very high and very low-pitched sounds are adjusted or "A-weighted" (dBA). The perception of how loud a sound is varies from person to person, but a change in 3 dBA is considered to be the minimum audible difference between sound levels, and a change of 10 dBA will be perceived as half as loud or twice as loud.
Effective control of highway traffic noise requires a three part approach:
- Land Use and Planning - State and local governments have the authority to regulate land use planning and development in the vicinity of highways to avoid future noise impacts and the need to provide noise abatement for future highway projects. Local government authorities regulate land development in such a way that noise-sensitive land uses are either prohibited from being located adjacent to highways, or that the developments are planned, designed, and constructed in such a way that noise impacts are minimized.
- Source Control - The Noise Control Act of 1972 gives the Federal Environmental Protection Agency (EPA) the authority to establish noise regulations to control major sources of noise, including transportation vehicles and construction equipment. This legislation requires the EPA to issue noise emission standards for motor vehicles used in Interstate commerce and requires the Federal Motor Carrier Safety Administration (FMCSA) to enforce these noise emission standards.
- Noise Mitigation – The FHWA regulations contain noise abatement criteria, which represent the upper limit of acceptable highway traffic noise for different types of land uses and human activities. State and local municipalities may also have defined limitations of noise levels.
The most common outdoor noise reduction method is the construction of noise barriers. Noise barriers can be in the form of man-made walls or topographical changes in the form of earth berms or hills, or natural barriers such as dense tree lines between the roadway and surrounding communities. To be effective, a noise barrier must be solid and of sufficient mass, and be high enough and long enough to block the road from the receiver. Since outdoor noise barriers are open to the air above and around them, sound will diffract above and around them limiting noise reduction to an average of 5 to 10 dBA. A noise barrier can be reflective or absorptive depending on the material used. Reflective barriers can diminish noise reduction capabilities if they are on both sides of the highway and are closer than 100 feet apart. In this case, traffic noise can reflect off of the opposite sides decreasing their effectiveness by as much as 3dBA. In these cases an absorptive material can be added to the face of the barriers to reduce the unwanted reflected sound.
In general, a highway barrier should meet the following criteria:
- Reduces the loudness of traffic noise by as much as half.
- Must be tall and long with no openings.
- Must be designed to be visually appealing.
- Must be designed to preserve aesthetic values and scenic vistas.
- Does not increase noise levels perceptibly on the opposite side of a highway.
- Substantially reduces noise levels for people living next to highway.
Many times residential homeowners may find themselves in a situation where publicly funded highway barriers are either cost prohibitive or ineffective to their location. In this instance it may be possible to reduce the sound levels with the installation of exterior sound curtains. These are typically modular panels constructed of a one or two pound per square foot reinforced mass loaded vinyl barrier, and a fiberglass absorber stitched in an exterior grade vinyl facing. Exterior sound curtains can be attached to an existing structure such as a chain link fence or wall. Noise barriers are most effective when they are placed as close as possible to either the source or the receiver.
Additional steps can also be taken to insulate a home from intrusive highway noise, such as replacing doors and windows with acoustically rated models, and increasing the transmission loss (TL) of exterior walls with the installation of mass loaded vinyl barrier and resilient isolation clips. Portable sound masking units sometimes referred to as white noise generators, can also be used in quiet areas such as bedrooms. Sound masking provides a fixed and constant level of unobtrusive background sound which is set to cover as well as soften other noises so they appear as much smaller sound fluctuations. The result is that the unwanted noise does not register to the human ear as a distraction.
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