| Noise Barrier |
Article Index for Noise |
Website Links For Noise |
Information AboutNoise Barrier |
| CATEGORIES ABOUT NOISE BARRIER | |
| environmental engineering | |
| noise pollution | |
| noise reduction | |
| road infrastructure | |
| acoustics | |
| sound | |
|
A noise barrier is an exterior structure, normally made of Noise Mitigation or industrial noise sources (other than cessation of the source activity or use of source controls). In the case of surface transportation noise, very little can be done to further reduce the source noise intensity (other than increase the percentage of Hybrid Vehicle s and Electric Vehicle s, a strategy that works only at low to moderate traffic flow speeds). Extensive use of noise barriers began in the United States after Noise Regulation s that were introduced in the early 1970s. HISTORY Noise barriers have been built intuitively, but seldomly, in the United States since the mid twentieth century, when vehicular traffic burgeoned; however, in the late 1960s acoustical science technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specific roadway. Below, a researcher collects data to calibrate a Headquarters in Sacramento, California ; the ESL inc. group in Palo Alto, California ; the Bolt, Beranek And Newman John Shadely, ‘’Acoustical analysis of the New Jersey Turnpike widening project between Raritan and East Brunswick’‘, Bolt Beranek and Newman, 1973 group in Cambridge, Massachusetts and a research team at the University Of Florida . Possibly the earliest published work that scientifically designed a specific noise barrier was the study for the Foothill Expressway in Los Altos, California C.M. Hogan and Harry Seidman, ''Design of Noise Abatement Structures along Foothill Expressway, Los Altos, California '', Santa Clara County Department of Public Works, ESL Inc., Sunnyvale, California , October, 1970.
By the late 1970s there were over a dozen research groups in the U.S. applying similar Computer Model ling technology and addressing at least 200 different locations a year for noise barriers. Today in 2006, this technology is considered a standard in the evaluation of Noise Pollution from highways, but, remarkably, the nature and accuracy of the computer models used is nearly identical to the original 1970 versions of the technology. THEORY OF NOISE BARRIER DESIGN The Acoustic al science of noise barrier design is more complex than imagined. Firstly, the theory is based upon blockage of sound ray travel toward a particular Receptor ; however, Diffraction of sound must be addressed. That is to say, Sound Waves bend (downward) when they pass an edge, such as the apex of a noise barrier. Further complicating matters is the phenomenon of Refraction , the bending of sound rays in the presence of an Inhomogeneous Atmosphere . Wind Shear and Thermocline produce such inhomogeneities. ]] The sound sources modelled must include Engine noise, Tire noise and Aerodynamic noise, all of which factors vary by vehicle type and speed. One can begin to visualize the complexity of the resulting Computer Model , which is based upon dozens of physics Equations translated into thousands of lines of computer code. The associated graphic to the right some original 1970 calculation results showing sound level Contour Line s as reductions in sound intensity realized by a specific soundwall. Most commonly a noise barrier is constructed from Masonry , earth, or a combination thereof, such as a wall atop an earth Berm . The noise barrier must either be constructed on private land or on a public Right-of-way . Since sound levels are measured using a Logarithmic Scale , a reduction of nine decibels is equivalent to elimination of about 90 percent of the unwanted sound. The bottom line is that noise barriers can be extremely effective tools for Noise Pollution abatement, although theory also calculates that certain locations and topographies are not suitable for use of any reasonable noise barrier. Clearly cost and Aesthetics play a role in the final choice of any noise barrier. TRADEOFFS IN NOISE BARRIER DESIGN There are clear disadvantages of noise barriers, most prominently:
Normally the benefits of noise reduction far outweigh aesthetic impacts for residents protected from unwanted sound. These benefits include lessened Sleep Disturbance , improved ability to enjoy outdoor life, reduced Speech Interference , Stress reduction, reduced risk of Hearing Impairment and reduction in Blood Pressure (improved Cardiovascular health). With regard to construction costs, a major factor is the availability of excess soil in the immediate area which could be used for Berm construction. If the soil is present, it is often cheaper to construct an earth berm noise barrier than to offhaul the excess dirt, provided there is sufficient land area available for berm construction. Generally a two to one ratio of berm cross sectional width to height is required. Thus, for example, to build a six foot high berm, one needs an available width of 24 feet. As a specific example, note the earth berm noise barrier at the top of this article. It was constructed solely of excess earth from Grading Pad s for a residential development it will protect. Thus its entire construction cost is negligible; arguably, it may pay into the project, since offhaul of earth may have been needed. A further nuance of this particular project is that the residential side of the berm is overexcavated, which gives more privacy between highway and homes and also enhances noise benefit. Finally, note the Aesthetic s of the earth berm which blends with scenic elements of the natural hills of Annadel State Park in the background. It may be a surprise to find out this berm is over six feet in height, since the aesthetics of earth mounding reduce the visual impact of the structure, compared to a soundwall. REFERENCES SEE ALSO |
|
|