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1 4.0 NOISE ANALYSIS <br />4.1 Existing Acoustic Environment <br />Various noise fundamentals are introduced below followed by a <br />discussion of (1) the harmful effects of noise, (2) guidelines <br />for achieving land use compatibility with noise, and (3) the <br />current and future noise environment in the vicinity of the <br />proposed Bristol Street Widening Project. <br />Fundamentals of Noise <br />Noise levels are measured on a logarithmic scale in decibels <br />which are then weighted and added over a 24 -hour period to re- <br />flect not only the magnitude of the sound, but also its duration, <br />frequency, and time of occurrence. In this manner, various <br />acoustical scales and units of measurement have been developed. <br />A- weighted decibels (dBA) approximate the subjective response of <br />the human ear to a broad frequency noise source by discriminating <br />I_ against the very low and high frequencies of the audible spec- <br />trum.. They are adjusted to reflect only those frequencies audi- <br />ble to the human ear. <br />I} Examples of the decibel level of various noise sources are shown <br />in Figure 8. They include: the quiet rustle of leaves (10 dBA), <br />I a soft whisper (20 to 30 dBA), the hum of a small electric clock <br />j (40 dBA), ambient noise outdoors or a house kitchen (50 dBA), <br />normal conversation (50 dBA), or a busy street (70 to 80 dBA). <br />[ Equivalent sound levels are not measured directly but are calcu- <br />lated from sound pressure levels typically measured in A- weighted <br />decibels (dBA). The equivalent sound level. (Le q) is the constant <br />level that, over a given time period, transmits the same amount <br />of acoustic energy as the actual time- varying sound. Equivalent <br />sound levels are the basis for both the day -night average sound <br />I level [Ldn) and the Community Noise Equivalent Level (CNEL) <br />] scales. <br />Harmful Effects of Noise <br />Approximately 20 million people in the United States currently <br />have some degree of hearing loss. in many of these cases, expo- <br />sures to very loud, impulsive or sustained noises caused damage <br />to the inner ear which was substantial even before a hearing loss <br />was actually noticed. To prevent the spread of hearing loss, a <br />desirable goal would be to minimize the number of noise sources <br />which expose people to sound levels above 70 decibels. <br />I. - The Appendix provides additional information on the funda- <br />mentals of noise (refer to pages 11 and 12). <br />4 -1 <br />