6.6 COMMUNITY REACTION TO ENVIRONMENTAL NOISE

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Before a new plant is constructed or new equipment is installed, the effects

of noise produced by the plant or equipment should be estimated. The

response of the surrounding community to the additional environmental

noise may be a factor in the site selection, the acoustic design of the

plant, and public relations for the company. It would be much better to

anticipate and correct noise problems before the plant is built or equipment

is installed than to endure lawsuits because of noise produced by the plant

or equipment.

There are several factors that influence the community tolerance for

environmental noise. Some of these factors are listed as follows.

1. Discrete frequency sounds. A whistle tone is more annoying than

a broadband hissing noise, even when the broadband noise level

is somewhat higher than that of the pure tone noise.

2. Repetitiveness or fluctuation in sound level. An obvious change

in sound level outdoors generally directs one’s attention to the

noise source. On the other hand, people tend to become acclimated

to a steady marginal source of sound.

Acoustic Criteria 243

Copyright © 2003 Marcel Dekker, Inc.

3. Sleep-disturbing noises. People usually become somewhat testy

when their sleep is disturbed.

4. Ambient noise level. When the ambient or background noise

level is very low, even the dripping of water will annoy some

people. For locations in a busy urban area where traffic noise

is present, an additional noise source may not produce a significant

community reaction; whereas, the same additional noise

source in a quiet rural setting would produce many complaints

from the community.

5. Impulsive or startling noises. A sudden intrusion of noise may be

unsettling for many people.

6. Visibility of the noise source. People tend to be more tolerant of

marginal noise sources that are concealed from their view.

7. Noise that conveys unpleasant information. Most people prefer

to hear pleasant music than the sound of shattering glass, for

example.

Community noise rating curves for outdoor (environmental) noise

have been developed (Stevens et al., 1955) based on principles similar to

those used to establish the NCB curves for noise in interior spaces. These

curves allow the estimation of the community response to the noise level

outdoors. The base rating рNoЮ curves are given in Table 6-10. The base

244 Chapter 6

TABLE 6-10 Base Noise Rating Curves (No) for

Environmental Noise Rating

Base noise rating,

No

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

25 55 43 36 29 25 22 18 16

30 59 47 40 34 30 27 24 22

35 63 52 45 39 35 32 30 27

40 67 56 49 44 40 37 35 33

45 71 61 54 49 45 42 40 38

50 74 65 58 53 50 47 45 44

55 78 70 63 58 55 53 50 48

60 82 74 68 63 60 58 55 54

65 86 78 72 68 65 63 60 59

70 90 83 77 73 70 68 66 65

75 94 87 82 78 75 73 71 70

80 97 91 86 83 80 78 76 75

Source: Thumann and Miller (1986).

Copyright © 2003 Marcel Dekker, Inc.

noise rating is given by the highest penetration (largest value of No) of the

measured octave band sound pressure level data into the noise rating curves.

A composite correction factor (CF) must be applied to the base noise

rating value to allow for the various factors influencing a person’s annoyance

to the outdoor noise. The corrected composite noise rating рLCNRЮ is

determined from the following expression:

LCNR ј No юCF (6-6)

Values for the component correction factors are given in Table 6-11. It is

important to note that only one value of correction factor is used from each

of the six categories of influencing factor. The overall correction factor is the

sum of the values for the six individual effects.

The average community response to a noise with a given composite

noise rating is summarized in Table 6-12. One must exercise some caution in

using the data in Table 6-12, because it is somewhat subjective, and there is a

range of responses in any population to the same environmental noise. Some

people are intolerant of noise (and maybe they are naturally grouchy),

whereas others have a high tolerance level for intrusive noise.

Example 6-4. An air vent located outdoors produces the noise spectrum

shown in Table 6-13. The vent noise is broadband, and the sound is not

impulsive. The vent operates about 30 times each hour, 1 minute duration,

during the daytime and the evening, but not during the nighttime. The vent

operates year-round. The vent is located in an area with light industry.

Determine the environmental noise rating and the anticipated community

reaction to the vent noise.

The base noise rating values from Table 6-10 are shown in Table 6-13.

the largest value is 75 dB, which occurs for the 4000 Hz octave band. Thus,

the base noise rating is No ј 75 dB.

The correction factors may be found from Table 6-11:

Noise spectrum ............................................... 0

Repetitiveness (10–60 times/hour) .................. _5

Time of day (during the evening) ................... _5

Season (year-round) ........................................ 0

Area (light industry) ....................................... _10

Peak factor (non-impulsive) ............................ 0

____________

Total ................................................................ CF ј _20 dB

The composite noise rating for the vent noise is found from Eq. (6-6):

LCNR ј No ю CF ј 75 ю р_20Ю ј 55 dB

Acoustic Criteria 245

Copyright © 2003 Marcel Dekker, Inc.

From Table 6-12, we see that the anticipated community response

would be widespread complaints. If it is desired to reduce the community

reaction to only mild annoyance, the composite noise rating would need to

be reduced to LCNR ј 45dB, or the base noise rating would be No ј 65dB.

As noted fromTable 6-13, a reduction in the octave sound pressure levels in

the octave band from 1000Hz to 8000Hz would be required. The required

reduction in octave band sound pressure level is greatest (11 dB and 10 dB,

respectively) in the 4000Hz and 8000Hz octave bands.

246 Chapter 6

TABLE 6-11 Correction Factors (CF) for Various Influencing

Factors for Community Noise Reactiona

Influencing factor Possible condition CF, dB

Noise spectrum Noise with pure-tone components ю5

Broadband noise 0

Repetitiveness Continuous to 1/minute 0

10–60 times/hour _5

1–10 times/hour _10

4–24 times/hour _15

1–4 times/day _20

1 time/day _25

Time of day Daytime only (7:00 a.m. to 6:00 p.m.) _10

Evening (6:00 p.m. to 10:00 p.m.) _5

Nighttime (10:00 p.m. to 7:00 a.m.) 0

Season of the year Winter only _5

Summer (and winter) 0

Type of area Rural ю10

Suburban ю5

Urban residential 0

Residential with some business _5

Area with light industry _10

Area with heavy industry _15

Peak factor Impulsive sounds ю5

Non-impulsive sounds 0

aOnly one correction is applied from each of the six categories.

Source: From A. Thumann and R. K. Miller, Fundamentals of Noise Control

Engineering. By permission of the Fairmont Press, Inc.

Copyright © 2003 Marcel Dekker, Inc.

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