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Appendix H <br /> A doubling or halving of acoustic energy will change the resulting sound level by <br /> 3 dB,which corresponds to a change that is just perceptible. In practice,this <br /> means that a doubling of traffic volume on a roadway, doubling the number of <br /> people in a stadium,or doubling the number of wind turbines in a wind farm will, <br /> as a general rule,only result in a 3-dB,or just perceptible, increase in noise. <br /> Outdoor Sound Propagation <br /> There are a number of factors that affect how sound propagates outdoors. These <br /> factors, described by Hoover and Keith(1996),are summarized below. <br /> Distance Attenuation <br /> As a general rule, sound from localized or point sound sources spreads out as it <br /> travels away from the source and the sound level drops at a rate of 6 dB per <br /> doubling of distance. If the sound source is long in one dimension, such as <br /> - traffic on a highway or a long train,the sound source is considered to be a line <br /> source. As a general rule,the sound level from a line source will drop off at a <br /> rate of 3 dB per doubling of distance. If the intervening ground between the line <br /> source and the receptor is acoustically"soft"(e.g.,ground vegetation,scattered <br /> trees,clumps of bushes),an attenuation rate of 4.5 dB per doubling of distance is <br /> generally used. <br /> Attenuation from Barriers <br /> Any solid structure such as a berm,wall, or building that blocks the line of sight <br /> between a source and receiver serves as a sound barrier and will result in <br /> additional sound attenuation. The amount of additional attenuation is a function <br /> of the difference between the length of the sound path over the barrier and the <br /> length of the direct line of sight path. Thus,the sound attenuation of a barrier <br /> between a source and a receiver that are very far apart will be much less than the <br /> attenuation that would result if either the source or the receiver is very close to <br /> the barrier. <br /> Molecular Absorption <br /> Air absorbs sound energy as a function of the temperature,humidity of the air, <br /> and frequency of the sound. Additional sound attenuation on the order of 1 to 2 <br /> dB per 1,000 feet can occur. <br /> H-5 <br />