on the paradox of brick influence on vertical sound ...

ON THE PARADOX OF BRICK INFLUENCE ON VERTICAL SOUND INSULATION A. Esteban, A. Cortés, M. Fuente and S. Arines LABEIN Technogical Centre Parque Tecnologico de Bizkaia, edif. 700 48160 Derio, Spain [email protected]

ABSTRACT In Spanish buildings, sound transmission between two vertically adjacent rooms is often dominated by flanking paths involving the inner walls. These walls are mainly built with ceramic bricks and there is a rigid junction between floor and walls, though elastic layers at the bottom are more and more used since a new build regulation asks for higher insulation levels. Different brick thickness are used, from 4 to 9 cm, with increasing masses and sound insulation. These walls are usually lined with 1cm of plaster. Common floors are beam and pot style, covered with some elastic element to improve impact sound insulation. This paper shows some surprising results, as overall sound insulation decrease using a 7cm thick brick instead of a 4cm one, and raises again using the 9cm brick. This result is however explained because of the effect of the brick mass on both sound insulation and vibration attenuation through the joint.

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Euronoise 2006, Tampere, Finland

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A. Esteban, A. Cortés, M. Fuente and S. Arines

INTRODUCTION

With the approval of the New Building Regulation (Código Técnico de la Edificación CTE) [1], sound quality of Spanish dwellings will be improved. The aim of this regulation is to consider the whole building as a product itself. Therefore, the acoustical requirements must be fulfilled by the whole building instead of by each element. Because of that, CTE considers in situ measurements to guarantee the established comfort standards. In addition, answering to the claim of improving noise protection, CTE demands higher sound insulation level requirements. This is going to mean an important change in the way that dwellings are built in Spain. Regarding this situation, tools developed to help designers to fulfil these requirements are needed. One simple tool is just to list combinations of building elements suitable to achieve the insulation requirement (DnT,A= 50dB). These combinations of elements can be obtained, for example, from calculations performed using EN12354 [2]. However, one can find some surprises looking at the results. For example, the sound insulation between two vertically adjacent rooms decrease when the inner walls use a 7cm thick brick instead of a lighter 4cm one, and raises again using a heavier 9cm brick. 2

SOUND TRANSMISSION BETWEEN TWO ROOMS SEPARATED BY A FLOOR

Buildings in Spain are usually built with pillars and beams structures, with no structural supporting walls. All the walls (façade, internal walls…) are built when the whole structure is lift. Spanish dwellings are mainly built with ‘beam and block’ floors. These block’s function is to lighten the floor’s structure. The blocks are put in between the beams and then, the concrete is spilled over the set, reaching a level of ~5cm over the block top. This system is highly orthotropic. Floor’s weight goes from 220 up to 400 kg/m2. Floors are covered with some elastic element to improve both impact and airborne sound insulation. Figure 1 shows the sound insulation index obtained with a bare floor (built with ceramic blocks and surface mass 305 kg/m2). With a floating floor of 2cm of elastified EPS with 6cm of mortar over it, the sound insulation rises up to 68 dB, as can be seen on the same figure. Many different kinds of masonry walls are used, but all of them are based on hollow bricks glued with mortar or a mixture of gypsum and glue. Later, walls are covered with a gypsum layer on both sides. The surface mass of the walls used for inner distribution goes from 55 to 135 kg/m2, with thickness from 4 to 10cm, and the following behavior:

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90

65 60

80

4 cm (Rw=36) 7 cm (Rw=38)

55 70

9 cm (Rw=43) 50

R (dB)

R (dB)

60

50

45 40 35

40

30

Floor without covering (Rw=54) 30

25

Floor with covering (Rw=68)

40 00

25 00

16 00

10 00

63 0

40 0

25 0

10 0

16 0

20

5000

4000

3150

2500

2000

1600

1250

800

freq. (Hz)

1000

630

500

400

315

250

200

160

125

100

20

freq. (Hz)

Fig. 1 Floor with and without covering and sound insulation for three brick thickness

With these building elements, the expected sound insulation DnT,W+C , for a given geometry will be:

Sound insulation DnT,w + C

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4cm brick

7cm brick

9cm brick

54.1

52.3

53.6

DISCUSSION OF THE RESULTS

In this situation, sound transmission from one room to the vertically adjacent one is often dominated by the flanking paths involving the inner distribution walls. When façade are double leaf walls with thermal insulation in between, façade path is dominated by the inner wall, being negligible the contribution of the outer wall (fig.2).

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Inner wall

Façade

Negligible path Main paths

Fig. 2 Main sound transmission paths

Façade (T junction)

Internal walls

Assuming the three inner walls are built with the same brick than the façade inner wall (which is a very common situation), the contribution of different paths can be analysed. Next table shows the apparent sound reduction index R´ of each sound transmission path. 4cm brick

7cm brick

9cm brick

Direct path

68

68

68

Flanking path FF Flanking path FD Flanking path DF FF + FD + DF (FF + FD + DF) x 3 inner walls Flanking path FF Flanking path FD Flanking path DF FF + FD + DF (façade) Overall sound insulation ( R´ )

63,4 78,3 92,3 63,2

61,1 79,8 93,8 61,0

62,2 83,2 97,2 62,2

58,5

56,3

57,4

58,3 75,3 89,3

56,6 76,8 90,8

58,2 80,2 94,2

58,2

56,6

58,1

55,1

53,3

54,6

This behaviour can be explained regarding the calculations on EN12354. Equation (1) is used to calculate each transmission path Rij: R + R j ,W S Rij = i ,W + ΔRij ,W + K ij + 10 log sep (1) 2 l0 ·l f

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Where: Ri,W , Rj,W are the laboratory sound reduction indexes of the elements included in the transmission path ΔRij,W is the sound reduction index improvement by an additional layer on elements i and j. Kij is the vibration reduction index over a junction Ssep is surface area of separating element l0 reference length (=1m) lf common coupling length between elements. For a given geometry and fixed the floor’s characteristics, the only parameters that may change are the vibration reduction index Kij and the sound reduction index of the wall Rwall, W. Kij depends on the junction kind (+, T, L...) and on the ratio between surface masses of floor and wall. As walls are quite lighter than floors, to increase the mass of the wall will mean to decrease the atenuation trough the junction Kij. These walls have a quite low critical frequency and so damping and other parameters are of great importance, although sound insulation will be also improved when increasing the surface mass of the walls. Therefore, the sound transmision depends on two parameters with opposite variation when increasing the surface mass of the walls, as can be seen on figure 3: 45,0

40,0

35,0

30,0

dB

Rw 25,0

Kij FF (+ junction) Kij FD and DF (+ junction)

20,0

15,0

10,0

20 0

19 0

18 0

17 0

16 0

15 0

14 0

13 0

12 0

11 0

10 0

90

80

70

60

50

40

5,0

surface mass of the wall (kg/m2)

Fig. 3 Rwall and Kij vs wall surface mass When the surface mass is low, although sound insulation is not too good, vibration attenuation through the joint keeps the flanking transmission in a lower value than other heavier bricks.

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Using elastic layers at both bottom and top of the walls, provides an improvement of the vibration attenuation index ΔKij which minimices flanking transmission, allowing the use of lightweight floors with brick inner walls and yet achieveing a good performance of the whole building.

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CONCLUSIONS

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-

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Sound transmission between two vertically adjacent rooms is often dominated by flanking paths involving the inner walls and the inner leaf of the facade when those are masonry walls. Due to the opposite effect of increase surface mass of the wall (improved sound insulation but worse vibration attenuation at the joint), using heavier bricks supposed to be better may result in a worse in situ sound insulation. Using elastic layers at top and bottom of masonry walls is the easiest solution to achieve good levels of insulation using masonry walls.

REFERENCES

[1] [2]

Drafts of the new Spanish Building Regulations. www.codigotecnico.org EN 12354-1,2: Building acoustics - Estimation of acoustic performance of buildings from the performance of elements. Part 1: Airborne sound insulation between rooms; Part 2: Impact sound insulation between rooms (2000).

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