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*:/ :11-3 It WA <br />FUNDAMENTALS OF NOISE <br />Sound Propagation <br />Sound dissipates exponentially with distance from the noise source. This phenomenon is known as <br />"spreading loss." For a single point source, sound levels decrease by approximately 6 dB for each doubling of <br />distance from the source (conservatively neglecting ground attenuation effects, air absorption factors, and <br />barrier shielding). For example, if a backhoe at SO feet generates 84 dBA, at 100 feet the noise level would be <br />79 dBA, and at 200 feet it would be 73 dBA. This drop-off rate is appropriate for noise generated by on -site <br />operations from stationary equipment or activity at a project site. If noise is produced by a line source, such <br />as highway traffic, the sound decreases by 3 dB for each doubling of distance over a reflective ("hard site") <br />surface such as concrete or asphalt. Line source noise in a relatively flat environment with ground -level <br />absorptive vegetation decreases by an additional 1.S dB for each doubling of distance. <br />Psychological and Physiological Effects of Noise <br />Physical damage to human hearing begins at prolonged exposure to noise levels higher than 8S dBA. <br />Exposure to high noise levels affects the entire system, with prolonged noise exposure in excess of 7S dBA <br />increasing body tensions, thereby affecting blood pressure and functions of the heart and the nervous system. <br />Extended periods of noise exposure above 90 dBA results in permanent cell damage, which is the main driver <br />for employee hearing protection regulations in the workplace. For community environments, the ambient or <br />background noise problem is widespread, through generally worse in urban areas than in outlying, less - <br />developed areas. Elevated ambient noise levels can result in noise interference (e.g., speech <br />interruption/masking, sleep disturbance, disturbance of concentration) and cause annoyance. Since most <br />people do not routinely work with decibels or A -weighted sound levels, it is often difficult to appreciate what <br />a given sound pressure level number means. To help relate noise level values to common experience, Table 2 <br />shows typical noise levels from familiar sources. <br />Page 4 <br />PIa Workr <br />75C-223 <br />