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1 4.3 Noise Mitigation Measures <br />Noise Attenuation With Distance <br />In an area which is relatively flat and free of barriers, the <br />noise level resulting from a single "point source" of noise drops <br />by 6 decibels for each doubling of distance or 20 decibels for <br />each factor of ten in distance. This applies to fixed noise <br />sources such as industries, refrigeration/air conditioning units, <br />and bells or buzzers at schools. It also applies to individual <br />mobile noise sources such as an airplane, train or idling motor <br />vehicle. <br />For. a "line source" of noise, such as a heavily travelled road- <br />way, the noise level drops off by a nominal value of 3.0 decibels <br />for each doubling of distance between the noise source and noise <br />�~ receiver. Environmental conditions such as the wind direction <br />and speed, temperature gradients, the characteristics of the <br />ground (hard or soft) and the air (relative humidity), the pres- <br />ence of grass, shrubbery and trees combine to increase the actual <br />attenuation achieved outside of laboratory conditions to 4.5 <br />decibels per doubling of distance. However, for a "worst- case" <br />analysis, a 3.0 decibel reductiori with doubling was assumed for <br />arterials throughout this report. <br />General Methods to Reduce Acoustic Impacts <br />There are several basic techniques available to minimize the <br />adverse effects of noise on sensitive noise receivers. Classical <br />engineering principles suggest controlling the noise source when- <br />ever feasible and protecting the noise receptors when noise <br />source control measures are inadequate. <br />Many of the noise source control mechanisms are being applied by <br />State and Federal governments. Acoustic site planning, archi- <br />tectural design, acoustic construction techniques and the erec- <br />tion of noise barriers are all effective methods for reducing <br />noise impacts when sourcl control mechanisms are insufficient to <br />achieve desired results. <br />--------------------- <br />1. In its "Noise Assessment Guidelines ", the U.S. Department of <br />Housing and Urban Development uses a 4.5 decibel drop for <br />each doubling of distance in assessing roadway noise. Thus, <br />a noise level of 74.5 decibels at 50 feet from the highway <br />centerline would be attenuated naturally to 70.0 decibels at <br />100 feet, 65.5 decibels at 200 feet, 61.0 decibels at 400 <br />feet and so forth. <br />2. A more detailed discussion of available methods to reduce <br />noise impacts is included in the Appendix (refer to page <br />16). <br />4 -25 <br />