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4.10 TRANSMISSION LOSS FOR COMPOSITE WALLS

Back

The material presented in the previous sections applies for transmission of

sound through homogeneous, single-component panels, such as a plate of

120 Chapter 4

TABLE 4-2 Transmission Loss Values for

Example 4-6.

f, Hz TL, dB Explanation

63 15.9 Down by 6 dB from 125 Hz value

125 21.9 Value calculated

250 27.9 Up by 6 dB from 125 Hz value

500 33.9 Up by 6 dB

1,000 39.9 Up by 6 dB

1,003 40.0 Plateau begins

2,000 40.0 Plateau

4,000 40.0 Plateau

8,000 40.0 Plateau

11,040 40.0 Plateau ends

16,000 45.4 Value calculated

glass. In this section, we will consider some more complex constructions that

can be analyzed analytically.

4.10.1 Elements in Parallel

One common form of construction consists of elements in parallel in a

composite wall, such as a window or door in the wall. The total power

transmitted through the wall is the sum of the power transmitted through

each element, because the incident acoustic intensity is the same for all

elements:

Wtr ј _Wtr;j ј atWin ј atSIin ј Iin_at;j Sj (4-172)

The quantity S ј _Sj is the total surface area, and at;j is the sound power

transmission coefficient for each individual element. The overall sound

power transmission coefficient for elements in parallel in a composite wall

is given by the following:

Transmission of Sound 121

FIGURE 4-13 Solution for Example 4-6.

at ј

_at;j Sj

S ј

at;1S1 юat;2S2 ю_ _ _

S1 юS2 ю_ _ _

(4-173)

The effect of openings in a panel is generally significant, because the

sound power transmission coefficient for an opening is unity (all energy is

transmitted through the opening). This effect is illustrated in the following

example.

Example 4-7. A wall has a transmission loss of 20dB with no opening in

the wall. If an opening having an area equal to 10%of the total wall area is

added in the wall, determine the overall transmission loss for the wall with

the opening included.

The sound power transmission coefficient for the wall material is:

1=at;1 ј 1020=10 ј 100

at;1 ј 0:010

The sound power transmission coefficient for the opening is at;2 ј 1. Using

Eq. (4-173), we may evaluate the overall sound power transmission coefficient:

at ј р0:900SЮр0:010Ю ю р0:10SЮр1:00Ю

S ј 0:1090

The transmission loss for the wall with the opening included is:

TL ј 10log10р1=0:1090Ю ј 9:6dB

We observe that an opening of only 10%of the total wall area reduces

the transmission loss from 20dB to a value slightly less than 10 dB. If the

noise reduction for a wall is to be effective, any openings must be as small as

possible or completely eliminated, if practical.

4.10.2 Composite Wall with Air Space

The double-wall construction, consisting of two panels separated by an air

space, is often used as a barrier to reduce noise transmission. For this

construction, shown in Fig. 4-14, the overall transmission loss is influenced

by the air mass in the space, in addition to the effect of the transmission loss

for each separate panel. The behavior of the TL curve for the composite wall

may be divided into three regimes (Beranek, 1971).

Regime A, the low-frequency regime, occurs for closely spaced panels.

When the two panels are placed very close together, the panels act as one

unit, as far as the sound transmission is concerned. The air space between

122 Chapter 4

the panels has a negligible effect. This behavior occurs for the frequency

range, as follows:

_рMS1 юMS2Ю

< f < fo (4-174)

The density and speed of sound for Eq. (4-174) are the values for the air

around the panel. The frequency fo is the resonant frequency of the two

panels coupled by the air space. This frequency is given by the following:

fo ј

MS1 ю

MS2

_ _ __1=2

(4-175)

The quantities MS1 and MS2 are the specific mass for panels 1 and 2, respectively.

The quantity d is the spacing between the panels.

The transmission loss for Regime A is given by the following:

TL ј 20 log10рMS1 юMS2Ю ю 20 log10р fЮ _ 47:3 (4-176)

As the panels are moved farther apart, standing waves are set up in the

air space between the panels, and Regime B behavior is observed. This

regime occurs for the frequency range, as follows:

fo < f < рc=2_dЮ (4-177)

Transmission of Sound 123

FIGURE 4-14 Composite wall with an air space between the two panels.

The transmission loss in Regime B is given by the following:

TL ј TL1 юTL2 ю20log10р4_f d=cЮ (4-178)

The quantities TL1 and TL2 are the transmission loss values for each of the

panels acting alone.

When the panels are moved sufficiently far apart, the two panels act

independently, and Regime C behavior is observed. The air space between

the panels acts as a small ‘‘room.’’ This behavior occurs for the frequency

range, f > рc=2_dЮ. The transmission loss in Regime C is given by:

TL ј TL1 юTL2 ю10log10

1юр2=_Ю

_ _

(4-179)

The quantity _ is the surface absorption coefficient for the panels.

The transmission loss expressions given in this section apply for the

sound transmitted through the airspace only. There is a second path that the

sound may take, called the structureborne flanking path, which involves

sound transmission through mechanical links between the panels.

Prediction methods for this contribution to the transmission loss are given

by Sharp (1973).

Example 4-8. Two panels of glass, each having a thickness of 6mm

(0.24in), are to be used to reduce the sound transmission through an opening

1.00m (39.4 in) high and 2.00m (78.7 in) wide. The panels are spaced

75mm (2.95in) apart, and the air around the panels is at 248C (758F), for

which _ ј 1:188 kg=m3 (0.0742 lbm=ft3) and c ј 345:6m/s (1134 ft/sec). The

surface absorption coefficient for the glass is _ ј 0:03. Determine the transmission

loss at the following frequencies: (a) 250 Hz, (b) 1000 Hz, and (c)

4kHz.

The properties of glass are found in Appendix C:

Longitudinal sound wave speed cL ј 5450 m/s (17,880 ft/sec)

Density _w ј 2500 kg=m3 (156 lbm=ft3Ю Critical frequency product

MS fc ј р30,300 Hz-kg/m2)(345.6/346.1)2

ј 30,210 Hz-kg/m2 (6190 Hz-lbm=ft2Ю Damping factor _ ј 0:002

Young’s modulus E ј 71:0 GPa р10:3 _ 106 psi)

Poisson’s ratio ј 0:21

First, let us determine the transmission loss for a single glass panel.

The first resonant frequency is found from Eq. (4-157):

124 Chapter 4

f11 ј 0:4534cLhЅр1=aЮ2 ю р1=bЮ2_

f11 ј р0:4534Юр5450Юр0:006ЮЅр1=1:00Ю2 ю р1=2:00Ю2_ ј 18:5Hz

The surface mass for one panel is:

MS ј _wh ј р2500Юр0:006Ю ј 15:0kg=m2 р3:07 lbm=ft2Ю

The critical frequency is:

fc ј рMS fcЮ=MS ј р30,210Ю=р15:00Ю ј 2014 Hz

For the frequencies of 250 Hz and 1000 Hz, the single panel operates in

Region II, the mass-controlled region. The sound power transmission coefficient

for normal incidence is found from Eq. (4-160) for a frequency of

250 Hz:

atn ј 1 ю р_Юр250Юр15:0Ю

р1:188Юр345:6Ю

_ _2

ј 1 ю 823:3 ј 824:3

The transmission loss for normal incidence is:

TLn ј 10 log10р824:3Ю ј 29:2dB

The transmission loss for a single panel of glass at 250 Hz is found from Eq.

(4-163):

TL ј 29:2 _ 5 ј 24:2dB

Repeating the calculations for a frequency of 1000 Hz, we find the

following values:

1=atn ј 1 ю 13,174 ј 13,175

TLn ј 41:2dB

TL ј 41:2 _ 5 ј 36:2dB

The frequency of 4 kHz lies in Region III, the damping-controlled

region. The transmission loss for normal incidence at the critical frequency

is found from Eq. (4-167):

TLnр fcЮ ј 10 log10 1 ю р_Юр30,210Ю

р410:6Ю

_ _2

( )

ј 47:3dB

The transmission loss for a single panel at 4000 Hz is found from Eq.

(4-166):

TL ј 47:3 ю 10 log10р0:002Ю ю 33:22 log10р4000=2014Ю _ 5:7

TL ј 47:3 _ 27:0 ю 9:9 _ 5:7 ј 24:5dB

Transmission of Sound 125

The complete transmission loss curve for a single panel of glass is given in

Table 4-3.

Next, let us examine the case for two glass panels, each having a surface

mass of MS1 ј MS2 ј 15:0kg=m2. The various frequencies which

divide the different regimes of behavior for the double panel may be evaluated:

_рMS1 юMS2Ю ј р410:6Ю

р_Юр15:0 ю 15:0Ю ј 4:4Hz

Using Eq. (4-175), we find the resonant frequency for the panel:

fo ј р345:6Ю

р2_Ю

р1:188Ю

р0:075Ю

15:0 ю

15:0

_ _ __1=2

ј 79:9Hz

2_d ј р345:6Ю

р2_Юр0:075Ю ј 733 Hz

We note that the frequency f ј 63 Hz lies in Regime A.

(a) For f ј 250 Hz, we find that 79:9Hz < f ј 250 Hz < 733 Hz. This case

lies in Regime B. The transmission loss may be calculated from Eq. (4-178):

TL ј 24:2 ю 24:2 ю 20 log10 р4_Юр250Юр0:075Ю

р345:6Ю

_ _

ј 48:4 ю р_3:3Ю dB

TL ј 45:1dB

(b) For f ј 1000 Hz, we find that 733 Hz < f ј 1000 Hz. This case lies in

Regime C. The transmission loss may be calculated from Eq. (4-179):

TL ј 36:2 ю 36:2 ю 10 log10

1 ю р2=0:03Ю

_ _

ј 72:4 _ 12:3 ј 60:1dB

126 Chapter 4

TABLE 4-3 Tabular Results for Example 4-8.

Frequency, Hz

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

Single panel:

Region II II II II II II III III

TL, dB 12.3 18.2 24.2 30.2 36.2 42.2 24.5 34.5

Double panel:

Regime A B B B C C C C

TL, dB 18.2 27.1 45.1 63.1 60.1 72.1 36.7 56.7

the regime is Regime C; however, the individual panels are operating in

Region III, the damping-controlled region. The transmission loss may be

determined from Eq. (4-179), using the single panel transmission loss values

calculated for Region III at 4000 Hz:

TL ј 24:5ю24:5юр_12:3Ю ј 36:7dB

The complete transmission loss curve is tabulated in Table 4-3 and

plotted in Fig. 4-15.

4.10.3 Two-Layer Laminate

Panels composed of two or more solid layers are often used as partitions for

enclosures and other acoustic structures. If the layers are bonded at the

interface with no air space, as shown in Fig. 4-16, then the composite

panel bends about an overall neutral axis. If we let _ be the distance from

the interface to the overall neutral axis, positive toward material 1 side, we

may find the quantity in terms of the properties of the individual layers:

Transmission of Sound 127

FIGURE 4-15 Solution for Example 4-8.

_ ј

E1h21

_ E2h22

2рE1h1 ю E2h2Ю

(4-180)

The transmission loss for Region II, the mass-controlled region, may

be determined from Eq. (4-163):

TL ј 10 log10 1 ю

_fMS

_oc

_ _2

" #

_ 5 (4-181)

The specific mass for the layered panel is given by the following:

MS ј _1h1 ю _2h2 (4-182)

The critical or wave coincidence frequency for the layered panel may

be found from the following expression:

fc ј

_ _1=2

(4-183)

The quantity c is the speed of sound in the air around the panel, and B is the

flexural rigidity of the panel, given by the following expression:

B ј

E1h31

12р1 _ 2

1 Ю Ѕ1 ю 3р1 _ 2_=h1Ю2_ ю

E2h32

12р1 _ 2

2 Ю Ѕ1 ю 3р1 ю 2_=h2Ю2_

(4-184)

Note that the algebraic sign for _ must be maintained in Eq. (4-184). The

quantity _ is positive when the overall neutral axis is on the material 1 side

of the interface.

The transmission loss for a layered panel may be determined from Eq.

(4-166) with the overall damping coefficient _ calculated from the following:

_ ј р_1E1h1 ю _2E2h2Юрh1 ю h2Ю2

E1h31

Ѕ1 ю 3р1 _ 2_=h1Ю2_ ю E2h32

Ѕ1 ю 3р1 ю 2_=h2Ю2_

(4-185)

128 Chapter 4

FIGURE 4-16 Two-ply laminated panel: _ is the distance from the interface of the

two materials to the overall neutral axis of the composite panel in bending, with

positive values measured toward material 1.

Example 4-9. An aluminumplate (material 1) having a thickness of 1.6mm

(0.063 in) is bonded to a rubber sheet (material 2) having a thickness of

4.8mm (0.189 in). The panel dimensions are 400mm (15.75in) by 750mm

(29.53 in). The air around the panel is at 218C (708F), for which the density

and speed of sound are _o ј 1:200 kg=m3 (0.0749 lbm=ft3) and c ј 343:8m/s

(1128 ft/sec), respectively. Determine the transmission loss for the panel at

(a) 500Hz and (b) 8kHz.

The properties of the aluminum (subscript 1) and rubber (subscript 2)

are found from Appendix C:

density _1 ј 2800 kg=m3 (174.5 lbm=ft3Ю;

_2 ј 950 kg=m3 (59.3 lbm=ft3)

Young’s modulus E1 ј 73:1 GPa (10.6_106 psi);

E2 ј 2:30 GPa р0:334 _ 106 psi)

Poisson’s ratio 1 ј 0:33; 2 ј 0:400

damping factor _1 ј 0:001; _2 ј 0:080

The specific mass for the composite panel is found from Eq. (4-182):

MS ј р2800Юр0:0016Ю ю р950Юр0:0048Ю MS ј 4:48 ю 4:56 ј 9:04 kg=m2 р1:852 lbm=ft2Ю The location of the neutral axis for the composite panel is found from Eq.

(4-180):

_ ј р73:1Юр0:0016Ю2 _ р2:30Юр0:0048Ю2

р2ЮЅр73:1Юр0:0016Ю ю р2:30Юр0:0048Ю_ ј 0:000524m ј 0:524mm

Let us calculate the following parameters for Eq. (4-184):

1 ю 3р1 _ 2_=h1Ю2 ј 1 ю р3ЮЅ1 _ р2Юр0:524Ю=р1:60Ю_2 ј 1:357

1 ю 3р1 ю 2_=h2Ю2 ј 1 ю р3ЮЅ1 ю р2Юр0:524Ю=р4:80Ю_2 ј 5:453

The flexural rigidity for the composite panel is found from Eq. (4-184):

B ј р73:1Юр109Юр0:0016Ю3р1:357Ю

р12Юр1 _ 0:332Ю ю р2:30Юр109Юр0:0048Ю3р5:453Ю

р12Юр1 _ 0:402Ю

B ј 38:0 ю 137:6 ј 175:6 Pa-m3 ј 175:6 N-m р129:5 lbf -ftЮ

The critical or wave-coincidence frequency for the composite panel is

found from Eq. (4.183):

fc ј р343:8Ю2

9:04

175:6

_ _2

ј 4268 Hz

If the panel were constructed of aluminum only, the critical frequency would

be found from Eq. (4-164):

Transmission of Sound 129

fcр1Ю ј

ffiffiffi3p р343:8Ю2

р_Юр5420Юр0:0016Ю ј 7515 Hz

(a) For a frequency of 500 Hz. This frequency is less than the critical frequency,

so the panel behavior falls in Region II, the mass-controlled region.

The transmission loss may be found from Eq. (4-181) for the composite

panel:

atn ј 1 ю р_Юр500Юр9:04Ю

р1:20Юр343:8Ю

_ _2

ј 1186

TL ј 10 log10р1186Ю _ 5 ј 30:7 _ 5 ј 25:7dB

For a single aluminum sheet, the transmission loss is as follows:

1=atn ј 1 ю 291 ј 292

TL ј 24:7 _ 5 ј 19:7dB

The addition of the mass of the rubber sheet increases the transmission loss

by 6.0 dB.

(b) For a frequency of 8kHz. This frequency is greater than the critical

frequency, so the panel behavior falls in Region III, the damping-controlled

region. The composite panel damping factor is found from Eq. (4-185):

_ ј Ѕр0:001Юр73:1Юр0:0016Ю ю р0:008Юр2:30Юр0:0048Ю_р0:0016 ю 0:0048Ю2

р73:1Юр0:0016Ю3р1:357Ю ю р2:30Юр0:0048Ю3р5:453Ю

_ ј 0:00469

Using Eq. (4-167), we find the transmission loss at the critical frequency for

normal incidence:

TLnр fcЮ ј 10 log10 1 ю р_Юр9:04Юр4268Ю

р1:20Юр343:8Ю

_ _2

( )

ј 10 log10р86,320Ю

ј 49:4dB

The transmission loss for the composite panel is found from Eq. (4-166):

TL ј 49:4 ю 10 log10р0:00469Ю ю 33:22 log10р8000=4268Ю _ 5:7

TL ј 49:4 ю р_23:3Ю ю 9:1 _ 5:7 ј 29:5dB

For a single layer of aluminum, we find the following value, using Eq.

(4-167):

TLnр fcЮ ј 10 log10 1 ю р_Юр4:48Юр7515Ю

р1:20Юр343:8Ю

_ _2

( )

ј 48:2dB

130 Chapter 4

The transmission loss for the aluminum alone at 8000 Hz is as follows:

TLр1Ю ј 48:2 ю 10 log10р0:001Ю ю 33:22 log10р8000=7515Ю _ 5:7

TLр1Ю ј 48:2 ю р_30:0Ю ю 0:9 _ 5:7 ј 13:4dB

The addition of the rubber layer increases the transmission loss at 8kHz by

about 16 dB.

4.10.4 Rib-Sti!ened Panels

Panels may have ribs attached to increase the stiffness of the panel and to

reduce stress levels for a given applied load. The rib-stiffened panel shown in

Fig. 4-17 has a stiffness that is different in the direction parallel to the ribs

(the more stiff direction) than in the direction perpendicular to the ribs. This

difference in stiffness has an influence on the transmission loss for the panel

(Maidanik, 1962).

In the mass-controlled region (for f < fc1), the transmission loss may

be calculated from Eq. (4-163), using the following expression for the surface

mass or mass per unit surface area:

MS ј _whЅ1 ю рhr=hЮрt=dЮ_ (4-186)

There are two different wave coincidence or critical frequencies for an

orthotropic plate, such as a rib-stiffened panel, corresponding to the different

stiffness of the panel. The two critical frequencies are given by expressions

similar to Eq. (4-183):

fc1 ј

_ _1=2

(4-187)

fc2 ј

_ _1=2

(4-188)

Transmission of Sound 131

FIGURE 4-17 Dimensions for a rib-stiffened panel.

The flexural rigidity in the two perpendicular directions is given by the

following expressions (Ugural, 1999):

B1 ј EI=d (4-189)

where I is the moment of inertia about the neutral axis of the T-section

shown shaded in Fig. 4-17 and d is the center-to-center spacing of the ribs.

B2 ј

Eh3

12f1 _ рt=dЮ ю рt=dЮ=Ѕ1 ю рhr=hЮ_3g

(4-190)

For the intermediate frequency range, fc1 < f < fc2, the transmission

loss may be calculated from the following expression (Beranek and Veґ r,

1992):

TL ј TLnр fc1Ю ю 10 log10р_Ю ю 30 log10р f =fc1Ю _ 40 log10Ѕlnр4 f =fc1Ю_

ю 10 log10Ѕ2_3р fc2=fc1Ю1=2_ (4-191)

The quantity TLnр fc1Ю is the transmission loss from Eq. (4-167) evaluated at

the first critical frequency fc1.

For the high-frequency range, f > fc2, the transmission loss may be

found from the following expression:

TL ј TLnр fc2Ю ю 10 log10р_Ю ю 30 log10р f =fc2Ю _ 2 (4-192)

Example 4-10. A pine wood sheet, 1.22m (48 in) by 2.44m (96 in) with a

thickness of 12.7mm (0.500 in), has pine wood ribs attached. The dimensions

of the ribs are 25.4mm (1.000 in) high and 19.1mm (0.750 in) thick.

The ribs are spaced 101.6mm (4.000 in) apart on centers and are oriented

parallel to the long dimension of the sheet. Air at 258C (778F) and 101.3 kPa

(14.7 psia) is on both sides of the panel. Determine the transmission loss for

the panel for a frequency of 500 Hz.

The location of the centroid axis for the T-section shown in Fig. 4-17

may be found, as follows. The cross-sectional areas of the sheet portion (1)

and the rib (2) are first determined, along with the distances from the interface

to the individual centers of the areas, with the positive direction toward

the sheet:

A1 ј р101:6Юр12:7Ю ј 1290mm2 and y1 ј р1

2Юр12:7Ю ј 6:35mm

A2 ј р19:1Юр25:4Ю ј 485mm2 and y2 ј _р1

2Юр25:4Ю ј _12:7mm

The distance from the interface between the sheet and the rib to the overall

centroid axis is as follows:

132 Chapter 4

_ ј р1290Юр6:35Ююр485Юр_12:7Ю

р1290ю485Ю ј 1:145mm р0:045 inЮ

The area moments of inertia of the individual areas are as follows:

I1 ј р101:6Юр12:7Ю3=р12Ю ј 17,343mm4 р0:0417 in4Ю

I2 ј р19:1Юр25:4Ю3=р12Ю ј 26,083mm4 р0:0627 in4Ю

The distances from the individual centroids to the overall centroid axis are

as follows:

r1 ј 6:35_1:145 ј 5:205mm and

r2 ј _12:7_1:145 ј _13:845mm

The area moment of inertia of the T-section about the overall centroid

axis may be calculated from the following expression:

I ј _рIj юAjr2

j Ю

I ј 17,343юр1290Юр5:205Ю2 ю26,083юр485Юр_13:845Ю2

I ј 171,380mm4 ј 17:138 cm4 р0:4117 in4Ю

The flexural rigidity of the stiffened panel in the direction parallel to

the ribs is found fromEq. (4-189). The properties of pine wood are found in

Appendix C.

B1 ј р13:7Юр109Юр17:138Юр10_8Ю=р0:1016Ю ј 23,109 Pa-m3

B1 ј 23,109 N-m ј 23:109 kN-m р17,040 lbf -ftЮ

The flexural rigidity in the direction perpendicular to the ribs is found from

Eq. (4-190):

B2 ј р13:7Юр109Юр0:0127Ю3

р12Юf1 _ р19:1=101:6Ю ю р19:1=101:6Ю=Ѕ1 ю р25:4=12:7Ю_3g

B2 ј 2856 Pa-m3 ј 2856 N-m ј 2:856 kN-m р2110 lbf -ftЮ

The surface mass or mass per unit surface area for the rib-stiffened

panel may be found using Eq. (4-186):

MS ј р640Юр0:0127ЮЅ1 ю р25:4=12:7Юр19:1=101:6Ю_ ј 11:184 kg=m2

The two critical frequencies may be determined from Eqs (4-187) and

(4-188):

Transmission of Sound 133

fc1 ј р346:1Ю2

р2_Ю

11:184

23,109

_ _1=2

ј 419:4Hz

fc2 ј р346:1Ю2

р2_Ю

11:184

2856

_ _1=2

ј 1193:1Hz

For the panel without the ribs, the surface mass is as follows:

S ј _wh ј р640Юр0:0127Ю ј 8:128 kg=m2

The critical frequency for the panel without stiffening ribs is determined as

follows:

f o

c ј рMS fcЮ=Mo

S ј р8160Ю=р8:128Ю ј 1004 Hz

For the rib-stiffened panel, the frequency of 500 Hz falls in the intermediate

region, so the transmission loss may be found from Eq. (4-191). The

transmission loss for normal incidence at the lower critical frequency is

calculated from Eq. (4-167):

TLnр fc1Ю ј 10 log10 1 ю р_Юр11:184Юр419:4Ю

р409:8Ю

_ _2

( )

TLnр fc1Ю ј 10 log10р1294Ю ј 31:1dB

The transmission loss for the rib-stiffened panel at 500 Hz is as follows:

TL ј 31:1 ю 10 log10р0:020Ю ю 30 log10р500=419:4Ю

_ 40 log10flnЅр4Юр500Ю=419:4_g ю 10 log10Ѕр2_3Юр1193:1=419:4Ю1=2_

TL ј 31:1 ю р_17:0Ю ю 2:3 _ 7:7 ю 20:2 ј 28:9dB

For the pine wood panel without stiffening ribs, the frequency of

500 Hz falls in the transmission loss Region II, the mass-controlled region.

The sound power coefficient for normal incidence may be found from Eq.

(4-160):

1=atn ј 1 ю Ѕр_Юр500Юр8:128Ю=р409:8Ю_2 ј 971:7

The transmission loss for the panel without stiffening ribs is calculated from

Eq. (4-163):

TL ј 10 log10р971:7Ю _ 5 ј 29:9 _ 5 ј 24:9dB

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4.10 TRANSMISSION LOSS FOR COMPOSITE WALLS

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