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.
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.
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.