10.8 NOISE IN A SMALL MEETING ROOM

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In a small meeting room in a university building, the noise level was considered

to be somewhat high; however, the main complaint of people using

the room was that there was an ‘‘echo’’ in the room when someone was

speaking. The floor plan for the meeting room is shown in Fig. 10-12. The

sound pressure level spectrumfor the background noise and for the roomin

the original condition with eight people present in the roomis shown in Fig.

10-13. The overall sound pressure level (measured on the C-scale of the

498 Chapter 10

FIGURE 10-12 Meeting room floor plan. The walls are 3.10m (10 ft 2 in) high. The

floor is carpet on concrete, the ceiling is acoustic tile, the walls are plaster on metal

lath, and the cabinet is constructed of 3

4-in (20-mm) thick plywood. The top cabinet

height is 800mm (311

2 in), and the lower cabinet height is 940mm (37 in). The door is

1:20m _ 2:20m high (3 ft 11 in _ 7 ft 2 1

2 in high), with a thickness of 45mm (13

4 in).

The door has a glass insert, 640mm _ 910mm (25 1

8 in _ 35 5

8 in). There are 16 wooden

chairs in the room.

Copyright © 2003 Marcel Dekker, Inc.

sound level meter) and the A-weighted sound levels are presented in Table

10-2.

10.8.1 Analysis

The speech interference level for the meeting roombefore acoustic treatment

was applied may be calculated from the data in Fig. 10-12:

рLSILЮo ј 1

4 р60ю59ю54ю45Ю ј 54:5 or рLSILЮo ј 55dB

According to the data presented in Table 6-4, a SIL of 55dB corresponds to

face-to-face communication with a ‘‘raised voice’’ for both men and women.

Case Studies in Noise Control 499

FIGURE 10-13 Sound pressure level spectrum for the small meeting room:

(1) before installation of the acoustic material on the walls and (2) after installation

of the acoustic material. The background noise level spectrum (3) is also shown

Copyright © 2003 Marcel Dekker, Inc.

there was a need for reduction of the noise level in the room, because the

room was used for meetings of small groups of people.

Because the reverberation noise seemed to be the major problem with

the acoustic environment of the meeting room, the reverberation time was

estimated by using the Fitzroy expression, Eq. (7-35). The total surface area

of the room was So ј 147:1m2 (1583.6 ft2). The total surface area for the

individual surfaces was as follows: (a) side walls, Sx ј 37:20m2 (400.4 ft2);

end walls, Sy ј 38:44m2 (413.8ft2); and floor–ceiling combination, Sz ј 71:48m2 (769.4ft2). The room volume was V ј 110:8m3 (3913 ft3).

The average surface absorption coefficient for the three sets of interior

surfaces in the 500Hz octave band was estimated as follows:

Side walls: _x ј 0:058; Sx=So ј 0:2529

End walls: _y ј 0:060; Sy=So ј 0:2612

Floor–ceiling: _z ј 0:445; Sz=So ј 0:4859

The number of absorption units for 16 wooden chairs in the 500Hz octave

band was estimated as _р_SЮ ј 0:32m2. It was observed that the surface

absorption of the walls of the roomwas much smaller than that for the floor

and ceiling. This characteristic would allow sound waves traveling horizontally

(fromwall to wall) to decay at a slower rate than those waves traveling

vertically (floor to ceiling and back). It was assumed that this phenomenon

was the source of the ‘‘echoes’’ about which people had complained.

Using Eq. (7-35), the number of absorption units, exclusive of the

chairs, was calculated:

1

ao ј _

1

р147:1Ю

р0:2549Ю

lnр1_0:058Ю ю р0:2612Ю

lnр1_0:060Ю ю р0:4859Ю

lnр1_0:445Ю

_ _

1

ao ј _р_4:233_4:221_0:825Ю

р147:1Ю ј 0:06307m_2

ao ј 15:85m2

500 Chapter 10

TABLE 10-2 Sound Pressure Levels for the Background Noise

and the Noise Before and after Acoustic Treatment of the Meeting

Room Shown in Figure 10-12

LA, dBA Lp, dB LSIL, dB

Background noise 35 48 27

Before acoustic treatment of walls 63 65 55

After acoustic treatment of walls 57 62 48

Copyright © 2003 Marcel Dekker, Inc.

The total number of absorption units, including the 16 chairs, was as follows,

according to Eq. (7-36):

a ј 15:85ю0:32 ј 16:17m2

The reverberation time in the 500Hz octave band before acoustic

treatment was applied was found from Eq. (7-34), using the speed of

sound in air at 228C (728F) as 344.4 m/s (1130 ft/sec):

Tr;o ј р55:26Юр110:8Ю

р344:4Юр16:17Ю ј 1:10seconds

This value of reverberation time was in agreement with measurements for

the empty room.

The optimum reverberation time for a conference room was found

from Eq. (7-38), with a ј _0:101 and b ј 0:3070 from Table 7-3.

Tr;opt ј _0:101 ю 0:3070 log10р110:8Ю ј 0:53 seconds

The optimum reverberation time was approximately half that of the room

before acoustic treatment was applied.

To achieve a reverberation time of 0.53 s for the empty room, including

the 16 chairs, the required number of absorption units was calculated as

follows:

a ј р55:26Юр110:8Ю

р344:4Юр0:53Ю ј 33:544m2

The average surface absorption coefficients for the side walls and the

end walls were approximately the same; therefore, the required surface

absorption coefficient was estimated by using the same value for both sets

of surfaces:

So

a _ 0:32 ј р147:1Ю

р33:544 _ 0:32Ю ј 4:428 ј _рSx=SoЮ ю рSy=SoЮ

lnр1 _ _x;yЮ ю 0:825

_lnр1 _ _x;yЮ ј р0:2529 ю 0:2612Ю

р4:428 _ 0:825Ю ј 0:1426

_x;y ј 0:133

If we let x ј fraction of the side-wall surface area covered with acoustic

material and _m ј surface absorption coefficient for the acoustic material,

the following relationship is valid for the absorption coefficient with the

acoustic material applied:

_x;y ј

_xSxр1 _ xЮ ю _mxSx

Sx

(10-12)

Case Studies in Noise Control 501

Copyright © 2003 Marcel Dekker, Inc.

The acoustic material selected was an acoustic foam, 25-mm (1-in) thick,

with a decorative surface coating. The surface absorption coefficient for the

acoustic material in the 500Hz octave band was _m ј 0:51. The fraction of

the side-wall surface area that should be covered with acoustic material was

determined as follows:

x ј

0:133_0:058

0:51_0:058 ј 0:1657

The surface area for both side walls that was to be covered with the

acoustic material was as follows:

Sm;x ј р0:1657Юр37:20Ю ј 6:164m2 р66:35ft2Ю

For each side wall, the covered surface area was р1

2Юр6:164Ю ј 3:082m2

(33.17ft2).

The fraction of the end walls y to be covered with the acoustic material

was calculated in a similar manner:

y ј

0:133_0:060

0:51_0:060 ј 0:1620

Sm;y ј р0:1620Юр38:44Ю ј 6:223m2 р67:03ft2Ю

By using this amount of wall coverage, the reverberation time for the empty

room should be reduced to approximately 0.5 second.

10.8.2 Control Approach Chosen

Because the walls of the roomhad a fairly small value of surface absorption

coefficient, it was decided to cover the side walls with a total of 7.15m2

(77 ft2) of acoustic material, and the end walls were covered with a total of

7.71m2 (83 ft2) of acoustic material.

The acoustic material selected was an acoustic foam, 25-mm (1-in)

thick, with a decorative surface coating. The acoustic material was available

in the form of 305-mm(12-in) squares, and the material was attached to the

plaster wall using an adhesive compatible with the acoustic material. The

squares were distributed over the wall surface in an aesthetically pleasing

pattern (at least, pleasing to the engineers who used the room).

The overall sound pressure level (measured on the C-scale of the sound

level meter) was 62dB (a 3dB reduction) with eight people in the roomafter

the acoustic treatment was applied, as given in Table 10-2. The A-weighted

sound level after the acoustic material was applied was 57 dBA (a reduction

of 6dBA) with eight people in the room. The speech interference level after

the acoustic material was applied was found from the octave band sound

pressure level measurements given in Fig. 10-13. The SIL after acoustic

502 Chapter 10

Copyright © 2003 Marcel Dekker, Inc.

treatment of the room was LSIL ј 48 dB, which corresponded to the situation

in which conversation in a ‘‘normal’’ voice should be possible in the

room.

The reverberation time for the room after the acoustic treatment was

applied was 0.49 seconds, which is slightly lower than the calculated optimum

value. The problem with annoying ‘‘echoes’’ in the conference room

was eliminated by the use of the acoustic treatment, however.

10.8.3 Cost

The cost for the acoustic material was $140 for one carton containing

1.49m2 (16 ft2) of material. Ten cartons were used, for a total cost of

$1400 for the acoustic material. The cost of the adhesive required to attach

the material to the wall was $26, and shipping and handling costs were $49.

The total material cost for the wall treatment of the conference room was

$1475. University maintenance personnel were used to install the acoustic

material, and the resulting labor cost was approximately $385. The corresponding

total cost for the acoustic project was $1860.

10.8.4 Pitfalls

For the type of acoustic treatment used in this application to be most

effective, it was important to distribute the acoustic material over the

walls. The acoustic material was not concentrated in one area on the walls.

It was noted that the steady-state sound pressure level reduction was

modest (about 6 dB total). If a reduction in steady-state sound pressure level

much more than about 8–10 dB were required, noise control measures other

than acoustic treatment of the walls should be considered. In addition, if the

direct sound field were predominant, the acoustic treament of the room

surface would be ineffective in reducing the noise received directly from

the source.

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