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amount of displacement of a particle from its rest position. The energy transported by a wave is directly <br /> proportional to the square of the amplitude of the wave. This means that a doubling of the amplitude of a wave is <br /> indicative of a quadrupling of the energy transported by the wave. <br /> Sound and the Human Ear <br /> Because of the ability of the human ear to detect a wide range of sound-pressure fluctuations, sound-pressure <br /> levels are expressed in logarithmic units called decibels(dB)to avoid a very large and awkward range in <br /> numbers. The sound-pressure level in decibels is calculated by taking the log of the ratio between the actual sound <br /> pressure and the reference sound pressure squared. The reference sound pressure is considered the absolute <br /> hearing threshold(Caltrans 1998).Use of this logarithmic scale reveals that the total sound from two individual <br /> 65 A-weighted decibel(dBA) sources is 68 dBA,not 130 dBA(i.e., doubling the source strength increases the <br /> sound pressure by 3 dBA). <br /> Because the human ear is not equally sensitive to all sound frequencies, a specific frequency-dependent rating <br /> scale was devised to relate noise to human sensitivity. This A-weighted scale performs this compensation by <br /> discriminating against frequencies in a manner approximating the sensitivity of the human ear. The basis for <br /> compensation is the faintest sound audible to the average ear at the frequency of maximum sensitivity. This dBA <br /> scale has been chosen by most authorities for the purpose of regulating environmental noise. Typical indoor and <br /> outdoor noise levels are presented in Exhibit 4.4-1. <br /> With respect to how humans perceive and react to changes in noise levels,a 1-dBA increase is imperceptible,a <br /> 3-dBA increase is barely perceptible, a 6-dBA increase is clearly noticeable, and a 10-dBA increase is <br /> subjectively perceived as approximately twice as loud(Egan 1988), as presented in Table 4.4-1. Table 4.4-1 was <br /> developed on the basis of test subjects' reactions to changes in the levels of steady-state pure tones or broadband <br /> noise and to changes in levels of a given noise source. It is probably most applicable to noise levels in the range of <br /> 50-70 dBA, as this is the usual range of voice and interior noise levels. <br /> Table 4.4-1 <br /> Subjective Reaction to Changes in Noise Levels of Similar Sources <br /> Change in Level,dBA Subjective Reaction Factor Change in Acoustical Energy <br /> 1 Imperceptible(Except for Tones) 1.3 <br /> 3 Just Barely Perceptible 2.0 <br /> 6 Clearly Noticeable 4.0 <br /> 10 About Twice(or Half)as Loud 10.0 <br /> Note:dBA=A-weighted decibels <br /> Source:Egan 1988 <br /> Sound Propagation and Attenuation <br /> As sound(noise)propagates from the source to the receptor,the attenuation, or manner of noise reduction in <br /> relation to distance, is dependent on surface characteristics, atmospheric conditions,and the presence of physical <br /> barriers. The inverse-square law describes the attenuation caused by the pattern in which sound travels from the <br /> source to receptor. Sound travels uniformly outward from a point source in a spherical pattern with an attenuation <br /> rate of 6 dBA per doubling of distance(dBA/DD). However, from a line source(e.g., a road), sound travels <br /> uniformly outward in a cylindrical pattern with an attenuation rate of 3 dBA/DD. The surface characteristics <br /> EDAW Manteca WQCF and Collection System Master Plans EIR <br /> Noise 4.4-2 City of Manteca <br />