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Appendix H <br /> Anomalous Excess Attenuation <br /> Large-scale effects of wind speed,wind direction, and thermal gradients in the air <br /> can cause large differences in sound transmission over large distances. These <br /> effects when combined result in anomalous excess attenuation,which can be <br /> applied to long-term sound-level estimates. Additional sound attenuation on the <br /> order of about 1 dB per 1,000 feet can occur. <br /> Other Atmospheric Effects - <br /> Short-term atmospheric effects relating to wind and temperature gradients can <br /> cause bending of sound waves and can influence changes in sound levels at large <br /> distances. These effects can either increase or decrease sound levels depending <br /> on the orientation of the source and receptor and the nature of the wind and <br /> temperature gradient. Because these effects are normally short-term, it is — <br /> generally not practical to include them in sound propagation calculations. _ <br /> Understanding these effects,however, can help explain variations that occur <br /> between calculated and measured sound levels. <br /> Guidelines for Interpreting Sound Levels <br /> Various federal, state,and local agencies have developed guidelines for <br /> evaluating land use compatibility under different sound-level ranges. The <br /> following is a summary of federal and state guidelines. <br /> Federal Agency Guidelines <br /> The federal Noise Control Act of 1972 (Public Law 92-574)established a <br /> requirement that all federal agencies administer their programs to promote an <br /> environment free of noise that jeopardizes public health or welfare. The U.S. <br /> Environmental Protection Agency (EPA)was given the responsibility for: <br /> ■ providing information to the public regarding identifiable effects of noise on <br /> public health or welfare, <br /> ■ publishing information on the levels of environmental noise that will protect <br /> the public health and welfare with an adequate margin of safety, _ <br /> ■ coordinating federal research and activities related to noise control, and <br /> ■ establishing federal noise emission standards for selected products <br /> distributed in interstate commerce. <br /> The federal Noise Control Act also directed that all federal agencies comply with <br /> applicable federal, state,interstate, and local noise control regulations. <br /> H-6 <br />