6.5 NOISE CRITERIA FOR INTERIOR SPACES

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We have all experienced the problem of attempting to work in an environment

with a high background noise level. The noise may interfere with

conversation with another worker, or the background noise may simply

238 Chapter 6

TABLE 6-4 Expected Voice Level for Face-to-Face

Communication Corresponding to Various Background

SIL Values

Vocal effort

Background speech interference level, LSIL

Women Men

Normal voice 45–49 dB 44–48 dB

Raised voice 53–60 dB 53–62 dB

Loud voice 61–71 dB 63–77 dB

Very loud voice 69–81 dB 71–91 dB

Shouting 77–91 dB 80–99 dB

Source: Lazarus (1987).

TABLE 6-5 Speech Interference Level Limits for Telephone

Communication

Speech interference level,

LSIL, dB Telephone conversation category

<63 dB Satisfactory

63–78 dB Difficult to understand conversation

78–83 dB Unsatisfactory

>83 dB Impossible to understand conversation

Source: Peterson and Gross (1972).

Copyright © 2003 Marcel Dekker, Inc.

‘‘get on our nerves’’ and interfere with effective concentration on the task at

hand. The levels of noise may not be sufficiently high to produce damage to

the person’s hearing; however, work is degraded by the background noise.

Noise criteria (NC) curves were first introduced (Beranek, 1957) to

evaluate existing noise problems in interior spaces such as offices, conference

rooms, and homes. It was found that a background noise that fitted the

original NC curves was not completely neutral. The noise had components

that sounded both ‘‘hissy’’ and ‘‘rumbly.’’ The original NC curves were also

based on the ‘‘old’’ octave bands.

The NC curves were revised (Beranek, 1971) to produce a more nearly

neutral background noise spectrum. These curves, called the preferred noise

criterion (PNC) curves to distinguish them from the older NC curves, were

also based on the present-day octave bands. Finally, the PNC curves were

revised to make equal the perceived loudness for the octave bands that

contain the same number of critical bands (Stevens, 1972). The rating

number on the NCB curves is the average of the NCB values in the

500 Hz, 1000 Hz, 2000 Hz, and 4000Hz octave bands, corresponding to

the octave bands used in calculating the SIL.

The NCB curves specify the maximumnoise levels in each octave band

for a specified noise criterion rating. The NCB rating of a given noise spectrum

is the highest penetration of the noise spectrum into the NCB curves.

The numerical values for the NCB curves are given in Table 6-7 (Beranek,

1989).

The suggested NCB ratings for various activities and different interior

spaces are shown in Table 6-8. The table values may be used to determine if

an existing acoustic situation is satisfactory for its anticipated usage, and to

determine the acoustic treatment required to make the background noise

acceptable if the noise level is too high. The values given in Table 6-8 apply

for background noise consisting of both equipment noise (air conditioning

systems, machinery, etc.) and activity noise due to the activity of the people

in the room.

The NCB curves may also be used to determine the acceptability of the

space for speech communication and whether annoying ‘‘rumbles’’ or

Acoustic Criteria 239

TABLE 6-6 Sound Pressure Level Spectrum for

Example 6-2

Octave band center frequency, Hz

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

Lp(OB), dB 59 65 70 73 69 65 59 50

Copyright © 2003 Marcel Dekker, Inc.

‘‘hisses’’ are present in the background noise spectrum. These terms are

more subjective than precise technical terms. If the LSIL for the background

noise is equal to or less than the NCB rating, the space generally will be

acceptable for speech communication.

To determine whether there may be an annoying ‘‘rumble’’ sound, a

value of

NCB(rumble) ј LSIL ю 3dB

is calculated. The values for this NCB curve are compared with the octave

band sound pressure levels for the background noise for the octave bands

of 500 Hz or lower. If any octave band sound pressure level exceeds the

NCB(rumble) curve, then there is a high probability that a ‘‘rumble’’ will

be perceived in the background noise. Noise control procedures could be

implemented to reduce the sound pressure level in the offending octave

bands to acceptable values. This procedure is illustrated in the following

example. If the octave band sound pressure level in the 63 Hz octave band

exceeds about 75 dB, or if the octave band sound pressure level in the

31.5 Hz octave band exceeds about 70 dB, there will be a good chance

that noticeable vibrations will occur in gypsum board structures, if any

are present in the room. This condition should also be checked.

240 Chapter 6

TABLE 6-7 Octave Band Sound Pressure Levels Associated

with the 1989 Balanced Noise Criterion (NCB) Curves

NCB, dB

Octave band center frequency, Hz

31.5 63 125 250 500 1,000 2,000 4,000 8,000

10 59 43 30 21 15 12 8 5 2

15 61 46 34 26 20 17 13 10 7

20 63 49 38 30 25 22 18 15 12

25 65 52 42 35 30 27 23 20 17

30 68 55 46 40 35 32 28 25 22

35 71 59 50 44 40 37 33 30 27

40 73 62 54 49 45 42 38 35 32

45 76 65 58 53 50 47 43 40 37

50 79 69 62 58 55 52 48 45 42

55 82 72 67 63 60 56 54 51 48

60 85 75 71 67 64 62 59 56 53

65 88 79 75 71 69 66 64 61 58

70 91 82 79 76 74 71 69 66 63

75 94 85 83 80 78 76 74 71 69

Source: Beranek (1989).

Copyright © 2003 Marcel Dekker, Inc.

To determine if there may be an annoying ‘‘hiss’’ sound, an average of

the NCB values for the 125 Hz, 250 Hz, and 500 Hz octave bands is calculated:

NCBрhissЮ ј ЅNCBр125 HzЮ ю NCBр250 HzЮ ю NCBр500 HzЮ_=3

The values for this NCB curve are compared with the octave band sound

pressure levels for the background noise for the octave bands of 1000 Hz or

higher. If any octave band sound pressure level exceeds the NCB(hiss) curve,

Acoustic Criteria 241

TABLE 6-8 Recommended Values of Noise Criteria (NCB) Ratings for Steady

Background Noise in Various Indoor Spaces

Activity and type of space NCB rating

Broadcast and recording studio:

Distant microphone pickup used 10

Close microphone pickup used only Not to exceed 25

Sleeping, resting, relaxing:

Suburban and rural homes, apartments, hospitals 25–35

Urban homes, hotels, hospitals 30–40

Excellent listening conditions required:

Concert halls, opera houses, recital halls 10–15

Very good listening conditions required:

Large auditoriums, drama theaters, large churches 15–20

Small auditoriums, music rehearsal rooms, large

conference rooms

25–30

Good listening conditions required:

Private offices, school classrooms, small conference rooms

libraries

30–40

Moderately good listening conditions required:

Large offices, reception areas, retail stores, restaurants 35–45

Fair listening conditions required:

Living rooms in dwellings (conversation and listening to

television)

30–40

Lobbies, laboratory work spaces, general secretarial areas 40–50

Moderately fair listening conditions required:

Light maintenance shops, industrial plant control rooms,

kitchens, and laundries

45–55

Acceptable speech and telephone communication areas:

Shops, garages 50–60

Speech communication not required:

Factory and shop areas 55–70

Source: From L. L. Beranek and I. L. Veґ r, Noise and Vibration Control Engineering, 1992.

By permission of John Wiley and Sons, Inc.

Copyright © 2003 Marcel Dekker, Inc.

then there is a high probability that a ‘‘hiss’’ will be perceived in the background

noise. Noise control procedures could be implemented to reduce the

sound pressure level in the offending octave bands to acceptable values. This

procedure is also illustrated in the following example.

Example 6-3. The sound pressure level spectrum for the air distribution

system noise from Example 5-7 is given in Table 6-9. Suppose the room to

which the air is distributed is a living room in a residence. Determine the

NCB rating for the room.

The NCB values are found fromTable 6-7 at the corresponding values

of the octave band sound pressure level. The largest value of NCB is 43,

which occurs for the 500Hz and 1000Hz octave bands. The noise criteria

rating for the room is NCB-43.

It is noted from Table 6-8 for living rooms in dwellings (fair listening

conditions required) that the recommended NCB rating is from NCB-30 to

NCB-40. The calculated NCB rating exceeds this value by 3 dB. From Table

6-9, we see that sound pressure levels in the 500 Hz, 1000 Hz, and 2000 Hz

octave bands produce a NCB rating above 40 dB. To reduce the NCB rating

to NCB-40, we would need to reduce the octave band sound pressure levels

to 45 dB (500 Hz), 42 dB (1000 Hz), and 38 dB (2000 Hz). This could be

achieved by (a) increasing the room constant by adding acoustic treatment

on the walls and ceiling or (b) decreasing the acoustic power input to the

room by adding a plenum chamber after the fan outlet.

Let us check the speech interference level. Taking the average of the

sound pressure levels in the octave bands from 500 Hz to 4000 Hz, we find

the SIL:

LSIL ј 1

4 р48 ю 45 ю 39 ю 35Ю ј 41:75 dB ! Use 42 dB

In this case, LSIL < NCB-43, so background noise would allow satisfactory

speech communication in the room.

242 Chapter 6

TABLE 6-9 Solution for Example 6-3

Octave band center frequency, Hz

31.5 63 125 250 500 1,000 2,000 4,000 8,000

Lp,dB 63 55 51 49 48 45 39 35 31

NCB 20 30 36 40 43a 43a 41 40 39

NCB-45 76 65 58 53 50

NCB-40 45 42 38 35 32

aLargest values.

Copyright © 2003 Marcel Dekker, Inc.

Next, let us check for annoyance due to ‘‘rumble’’ noise:

NCBрrumbleЮ ј 42ю3 ј 45 dB

The pertinent portion of the NCB-45 curve is given in Table 6-9. It is

observed that all of the sound pressure level values in the range from 31.5

to 500 Hz are less than the NCB-45 values, so there will be no annoyance

due to low-pitched rumble noise.

Finally, let us check for annoyance due to ‘‘hiss’’ noise. Averaging the

NCB values for the 125 Hz, 250 Hz, and 500 Hz octave bands, we obtain the

following value:

NCBрhissЮ ј р36 ю 40 ю 43Ю=3 ј 39:7dB ! Use 40 dB

The pertinent portion of the NCB-40 curve is given in Table 6-9. It is

observed that the sound pressure levels in the 1000 Hz and 2000 Hz octave

bands exceed the NCB-40 values for those octave bands. The occupants of

the room would probably experience some annoyance due to the perceived

‘‘hissing’’ noise of the air distribution system. This problem could be alleviated

by reducing the sound pressure level in the 1000 Hz octave band from

45 dB to 42 dB, and by reducing the sound pressure level in the 2000 Hz

octave band from 39 dB to 38 dB.

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