EXHIBIT 4-2_55A_BRISTOL WIDENING TECHNICAL STUDIES

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But hearing impairment is only one of the harmful effects of noise on people. Table 4 -1 summarizes the potentially harmful effects of noise on sensitive noise receptors. The Appendix provides additional details on physical and psychological re- sponses of humans to noise (see page 13). TABLE 4 -1 IHARMFUL EFFECTS OF NOISE Effect Noise Levels At Which Harmful Effects occur Prevention Or Interruption Of Sleep 35 - Speech Interference 50 - Extra Auditory Physiological Effects 65 - Hearing Loss 75 -- Source: Calif. Dept. of Public Health Report to 45 dB (A) 60 dB (A) 75 dB (A) 65 dB(A) 1971 Legislature Figure 9 illustrates how excessive background noises can reduce the amount and quality of verbal exchange and thereby impact education, family lifestyles, occupational efficiency and the quality of recreation and leisure time. Speech interference begins to occur at about 40 to 45 decibels and becomes severe at about 60 decibels. Community Response To Noise Approximately 10 percent of the population has such a low toler- ance for noise that they object to any noise not of their own I making. Consequently, even in the quietest environment, some complaints will occur. Another 25 percent of the population will not complain even in very severe noise environments. Thus, a variety of reactions can be expected from people exposed to any given noise environment. Despite this, the population as a whole can be expected to ex- hibit the following responses to changes in noise levels: an increase or decrease of 1.0 dBA cannot be perceived except in carefully controlled laboratory experiments; a 3.0 dBA increase is considered just noticeable outside of the laboratory; an increase of 5.0 dBA is often necessary before any noticeable 1.^ "Literature Survey for the FHA Contract on Urban Noise ", Report No-1460, BB &N, January, 1967. 4 -2