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IBut hearing impairment is only one of the harmful effects of <br />noise on people. Table 4 -1 summarizes the potentially harmful <br />effects of noise on sensitive noise receptors. The Appendix <br />provides additional details on physical and psychological re- <br />sponses of humans to noise (see page 13). <br />TABLE 4 -1 <br />IHARMFUL EFFECTS OF NOISE <br />------------------------------------------------------------------ <br />Effect Noise Levels At Which <br />Harmful Effects Occur <br />Prevention Or Interruption Of Sleep 35 - 45 dB(A) <br />I Speech Interference 5o - 6o dB(A) <br />Extra Auditory Physiological Effects 65 - 75 dB(A) <br />Hearing Loss 75 -- 85 dB(A) <br />----------------------------------------------------------------- <br />Source: Calif. Dept. of Public Health Report to 1971 Legislature <br />Figure 9 illustrates how excessive background noises can reduce <br />the amount and quality of verbal exchange and thereby impact <br />education, family lifestyles, occupational efficiency and the <br />quality of recreation and leisure time. Speech interference <br />begins to occur at about 40 to 45 decibels and becomes severe at <br />about 60 decibels. <br />Community Response To Noise <br />Approximately 10 percent of the population has such a low toler- <br />ance for noise that they object to any noise not of their own <br />making. Consequently, even in the quietest environment, some <br />complaints will occur. Another 25 percent of the popul ?tion will <br />not complain even in very severe noise environments. Thus, a <br />variety of reactions can be expected from people exposed to any <br />given noise environment. <br />Despite this, the population as a whole can be expected to ex- <br />hibit the following responses to changes in noise levels: an <br />increase or decrease of 1.0 dBA cannot be perceived except in <br />carefully controlled laboratory experiments; a 3.0 dBA increase <br />is considered just noticeable outside of the laboratory; an <br />increase of 5.0 dBA is often necessary before any noticeable <br />1. "Literature Survey for the FHA Contract on Urban Noise ", <br />iii Report No.1460, BB&N, January, 1967. <br />4 -2 <br />75C -458 <br />