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European Round Robin Test for sound insulation

Measurements of lightweight partition.

Cyrille Demanet1, Maria Jose De Rozas2, Jean Baptiste Chene3 and Remy Foret4

1 Lafarge gypsum, Avignon FRANCE 2Tecnalia & Basque Government, Laboratory for Quality Control in Dwellings,Vitoria, SPAIN

3-4 CSTB, 84 Avenue Jean Jaurès, 77447 Champ sur Marne FRANCE

ABSTRACT This paper presented a part of the work initiated to conduct a Round Robin in 19 European Laboratories. This objective is to prepare a test code for drywall system of plasterboard with steel studs.CENTC126 decided to set up a working group to first realize a new Round Robin test that has been performed during winter 2010-2011. The Working Group, WG9, is now in phase to analyze data collected : measurement on element, product characterization and laboratory detailed information. Two partitions have been selected and more than 120 data have been asked to the laboratory. Repeatability and reproducibility are presented and compared to the tentative value indicated in annex A of ISO140-2. Afterwards the working group will prepare a test code that will give the uncertainty level of the acoustic measurement on laboratory of a plasterboard drywall system with steel stud. Keywords: Noise, Vibration, Measurement

1. Round Robin Test on lightweight partition In 2010, an inter laboratory test (I.L.T) dealed with an inter comparison of laboratory

measurements of according to EN-ISO 140-1 [3] and EN-ISO 140-3[4] partition airborne sound insulation . 19 laboratories have been participated to the ILT with 9 European countries. An European Working Group has been especially created under the technical building acoustic committees (WG9 TC126).

The scope of the WG9 was to develop an acoustic test code for plasterboard drywall systems and steel studs. WG9 has the objectives to identify the laboratory test configuration details that will have an influence on the acoustic performance. The first part was to quantify the uncertainty in order to prepare a test code that will give the acoustic measurement uncertainty level in the laboratory of a plasterboard drywall system and steel stud.

1.1 Definition of the partitions to be tested in ILT Two partitions have been proposed: the partition 1 that is constructed from a twin framework (two parallel frameworks) of 70 mm metal cannels and studs 25 mm apart. The 165 mm wide cavity is filled with 3 layers of light weight glass wool. Two layers of 12.5 mm plasterboards are screw fixed to both sides of the framework. The total width of the partition will be 215 mm. See figure 1. And partition 2 that is constructed from a single framework of 70 mm metal cannels and studs. The cavity is filled with a single layer of light weight glass wool. One layer of 12.5 mm plasterboards is screw fixed to both sides of the framework. The total width of the partition will be 95 mm. See figure 2. Figure 1 – Partition 1 Figure 2 – Partition 2

1 Cyrille.demanet@lafarge.com 2 mjose.derozas@tecnalia.com 3 jean-baptiste.chene@cstb.fr4 remy.foret@cstb.fr

For financial reasons it was decided to let the laboratory install the partitions and WG9 had to deliver a set of documents to define details on delivery and storage; questionnaire on laboratory details, the procedure of installation under word and power point format, the procedure to characterize the product.

1.2 Comparison of uncertainties between 1994 ILT & 2010 ILT : During 1994 to 1998 an ILT [6] has been coordinated by the Ferrara University (Italy) Engineering

department. Two amendments have been produced. The amendments have been fully integrated under new EN-IS0 10140-2 (2010) and en ISO 10140-5(2010).

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Figure 3 - Reproducibility comparison from ILT 2010 1 1998 on left – Relation between reproducibility and unique single quantities Rw on right

Reproducibility has not been reduced between ILT 1998 and 2010 (left graphic of figure 3).

Correlation between uncertainties and the level of performance is high enough to conclude that reproducibility is depending on the performance of the tested system (right graphic of figure 3). Same laboratory design, same installation procedures and measurements have higher impact if the system tested has a higher performance. Repeatability was progressing with 2010 ILT. Both specimens have achieved a repeatability of maximum 0.6 dB for the Rw. That is under ISO 140-2 requirements. Progress of quality management and measurement procedure are enough optimized to not represent levelers to reduce the reproducibility.

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Figure 4 – Repeatability comparison from ILT 2010 and 1998

The second objective of the WG9 was to analyze the ILT in order to propose solutions to decrease

laboratory uncertainties. The work is possible by understanding that reproducibility in the convergence of a laboratory design, a measurement procedure and an installation procedure adapted to the laboratory configuration. WG9 has collected information around these three aspects.

1.3 Analyse Results of both partitions. The results of airborne sound insulation of all the laboratories in third octave bands frequencies for

both partitions (single and double one) are shown in the Figure 5. Laboratories data according to the groups of results have been analyzed: geometry of the rooms and

of test aperture, frame type, reverberation time, dry time of test specimen, Rmax, etc. According to this analysis it has been observed that there is any trend that relates the characteristics of the laboratories with the result families

For the single partition some result families have been found: the ones with very high insulation

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from 800 Hz (I) ; the ones with insulation similar to the average(II) ; some laboratories that from 800 Hz to 2000 Hz give insulations a bit lower than the rest (III). There is a laboratory that presents a very low result in medium frequencies (IV) and other laboratories that have a fall at 250 Hz, but for the rest of frequencies that is similar to the average (V).

For the twin partitions there exist more variations in the results; therefore it is more difficult to find trends in the results. The results can be grouped in laboratories that give results higher than the average.(A), laboratories that give results similar to the average (B), laboratories that give results lower than the average (C) and ‘special cases’: result that do not present any resonance frequency, result that cross the average in middle frequencies and the one with very low insulation. In this case has been detected that in the family C the laboratories have the test aperture smaller than the separating wall.

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Figure 5 – Airborne sound insulation (R) measured in each laboratory for both test partition of plasterboard:

single frame (left) and twin frame (right).

2. Installation details In order to have relevant acoustic conception of building, using for example EN 12354 method, it is

necessary to have “good” (representative) input data and not only on the single number. That is why one requirement of the test standard EN 10140-2 [1] is: “The test element should be installed in a similar manner to the actual construction with careful simulation of typical connections and sealing conditions at the perimeter and at all joints.” (§6.2 of [1]). WG9 decided to use compound at the periphery of the two partitions even if habits in many European countries, laboratory are pushing to use acrylic or silicone sealant. WG9 procedures for installation didn’t set any drying times and no awareness on cracks could appear because paper tapes were not used as required in reality. The two next parts are a discussion on the effect of those two oversights.

The apparition of cracks in some laboratories on the perimeters of the partition yields air tightness problems. The contribution of such problem on the transmission loss of element is a filter, impacting a large range of frequency depending on the “size” of the cracks. Figure 6 shows the effect of small cracks on the single frame partition of the ILT: the slope of the curve is broken at 630Hz and seems to be filtered at 50dB.

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Figure 6 – Influence of cracks on the transmission loss of a single frame partition ( - with cracks;

- without cracks)

The drying time of the peripheral compound has a different impact on single and double frame partition. This factor influences the structural shortcut between the two leafs. For the single frame partition, as you also have the short cut of the studs and the channels, the influence of the short cut through the compound is less important than for the double frame partition where it is the main one.

Figure 6 presents two examples, the first one (on the left) is the double frame partition of the ILT in wet and dry conditions and the second one (on the right) is comparing peripheral conditions : quick set compound at different drying times and Perenator sealant (referent sealant for glazing measurement).

Those two examples show the same effect in high frequency range with a diminution of the TL when the compound becomes dry and rigid. If this effect has few influences on the single number, we can see that we can measure more than 10dB difference around the critical frequency.

Figure 7- On left: Influence of the drying time of the compound on the TL of the ILT double frame

partition - on right: Influence of the drying time of the compound on the TL of another type of double frame partition

3. Laboratory design consequences The 2010 ILT collected 120 data to describe each laboratory, for majorities the main ISO

requirements are respected. ISO 140 series make possible laboratories with very different designs. For instance the emission chamber volume spread is starting from 50 m3 to 135 m3. The highest cut frequency of the test room is at 442 Hz to 220 Hz for the lowest. As a consequence the limit under which the sound is not totally diffused is different for each laboratory that impacts low frequency uncertainties at different frequencies. WG9 reviewed all the data collected by searching how data are correlated to the uncertainties. The differences between laboratories have so many degrees of freedom that most of the laboratories have an unique design. It was not possible to find any correlation or to propose a unique laboratory design

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During the work of ILT preparation, diffusity has been identified to be way of improvements. Indeed , volume is only one of the parameters that participates to have diffused field in test rooms. ISO 140-3 [3] already proposed to evaluate that diffusity is enough to make sound reduction index independent from new diffuser panel. WG9 proposed to laboratories to do an extra measurement by putting two more diffusers (single plasterboard). For few laboratories a non negligible difference has been observed with and without diffusers.

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- Figure 8 graphic that compares sound reduction index without extra diffusers (blue curve) and standard measurement (Red curve) with in dash line ISO 140-2 uncertainties envelop, on right

a photo (not same laboratory) with the two extra diffusers –

Craiks [7] gave a numerical model to predict the behaviors of laboratories. Craiks showed that laboratory design influenced the sound reduction index through flanking transmission phenomena with lightweight construction. WG9 has not launched a work to simulate how laboratory design could influence sound reduction index. But some conclusions appear evident to the group: laboratories that measured the highest performance have same construction type: box within box, no reduction of aperture and independent frame with a thickness close to the partition thickness tested. One laboratory found high performance without having a real box within a box. This laboratory used parallel wooden timber separated with apparent mineral wool that cut a part of flanking transmission and added absorption close to the sample. This example explains why WG9 has difficulties to impose one laboratory design: laboratories are versatile, that imposed some design adapted for a construction type but created weakness for another one. Laboratories found a solution to compensate negative design effect and are able to achieve high results without having a perfect box within the box. That makes design difficult to expertise.

4. Method with reference systems: WG9 has shared experiences to analyze uncertainties. Fulfill the standard requirements is the

minimum that most of laboratories achieved, but that can‘t guarantee that measurement is accurate. Laboratories participated to ILT to be in conformity with national accreditation requirements. Some used results to calibrate new facilities. WG9 studies proposed to set the two partitions as reference of drywall partitions. Average performance will be considered as the “ideal values”. To reduce the uncertainty, an envelope curve at +/- one standard deviation from the average was used to select laboratories. For most of the laboratories outside envelop, some hypothesis have been identified like air tightness of the perimeters, problem of diffusity, flanking transmission (partition 1) and unusual laboratory design (depth of the mission room and/or too high reduction of the aperture).

For partition 1, it was not possible to apply directly the proposed method. The flanking transmission effect is much more difficult to identify because that could impact all the frequency range. A solution was to not select results identified as “special case” in the previous paragraph (no resonance; curve crossing the average and the laboratory with the unique design: big reduction of aperture and small depth of test room). It was afterwards possible to apply the same method than for partition 2.

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- Figure 8 Partition 2 example with graphic on the left represents all the laboratories with in red

the average and dash line are the envelop of one standard deviation – on the right with the laboratory selected –

The method proposed makes possible to reduce reproducibility if a work is done by the laboratory to deeper adjust facilities configuration, measurements and installation procedure to drywall partition cases.

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Figure 9 – Repeatability comparison from ILT 2010 and 1998 and Drywall Test code method proposition -

5 Laboratory average values and partition modelizations. The here above presented averages are compared, in this section, to calculated sound reduction

indices for both partitions (single and twin). AcouSYS software (developed by CSTB) and additional methods are used to compute the sound reduction index R of such lightweight double partitions.

For the twin frame partition (Partition 1), the direct sound transmission through the partition is computed using a wave approach applied to an infinite thin plate line connected to periodically spaced beams and excited in flexure by a single or random (diffuse field) incident plane acoustical wave. The reaction forces and moments at the line connections are calculated from the flexural and torsional line impedance of the beams. The structural flanking path at the panel boundaries is taken into account using SEA [9][10].For the single frame partition (Partition 2), the direct sound transmission through the partition, ie, the path panel/cavity/panel is computed using a Transfer Matrix Method (TMM) [11]. The structural transmission path through the studs is computed using a mix approach (wave based approach and SEA) [12]. The studs (or ties) are assumed to behave like springs and correspond to either line in the low frequency range or point connections in the high frequency range. Therefore, below the transition between line and point connections a wave based method is used: the line connections are represented as normal forces and moments acting on the panels. Above this transition a SEA approach is considered to compute the transmission path panel/studs/panel. From a general point of view, the diffuse field excitation is obtained by using random incident plane acoustic waves; a spatial windowing technique to consider the finite size of the system is applied [13]. For these predictions the mechanical properties of the plasterboards, the acoustic properties of the wool and the translational stiffness of the studs (and the boundary rails) are required. The Young moduli of the plasterboards have been characterized using an adaptation of the ISO/PAS 16940 [14], the results and the data used in the modeling are presented in the Table 1.

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Gypsum board

Thickness h (mm) 12,5

Density ρ (kg.m-3) 734

Poisson ratio (-) 0,1

f=48 Hz f=304 Hz f=833 Hz f=1572 Hz f=2479Hz f=3454 Hz f=4468 Hz Young modulus E

(Pa) 3,37e9 3,40e9 3,26e9 3,08e9 2,76e9 2,40e9 2,06e9

f=48 Hz f=304 Hz f=833 Hz f=1572 Hz f=2479Hz f=3454 Hz f=4468 Hz Damping η 2 % 0,8 % 0,9 % 0,95 % 1,63 % 1,42 % 1,28%

Table 1 – Mechanical properties of the gypsum board used in the modelling. The metallic frames (studs and boundary rails) have also been characterized using mobility and

vibration level differences measurements (the method is presented in [15]), results are presented below.

Ktr-punctual

Studs 0,5 MN/m

Boundary Rails 2,6 MN/m

Boundary Studs 2,0 MN/m

Table 2 – Translational stiffness Ktr-punctual used in the predictions.

5.1 Partition 1 - Results and discussion Figure 1(left) compares the predicted sound reduction index and the associated measured average.

One can observe a good agreement between calculation and measurements all over the frequency range. Thus, this average seems realistic of the acoustic behavior and phenomena which occur in such partitions. Figure 1(right) presents the transmission loss accounting for the different effect considered here, i.e., direct sound transmission and flanking path. Due to lack of time, the stiffeners were not taken into account and only the modeling without stiffeners is presented here (usually the stiffeners reduce the sound transmission loss above 200 Hz, it explains the difference between prediction and measurement). The flanking path at the boundaries has an impact above 630 Hz, it explains a part of the large spread of values observed in the Figure 9. This flanking path contribution can be adjusted for the different laboratories; in our case unfortunately the vibrational level difference between the plates has been measured for another CSTB mounting setup (another twin frame) and not for this partition.

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Figure 10 – Comparison between AcouSYS prediction and average measurement for partition 1.

(left) Comparison between predicted and measured sound reduction index. (right) Details of the different transmission paths.

Partition 2 – Results and discussion Figure 2(left) compares the predicted sound reduction index and the associated average for the

Partition 2. A quite good agreement can be observed between both transmission losses all over the frequency range. Figure 2(right) presents the different sound reduction indexes associated to the direct transmission and the flanking paths through the studs and the boundary rails. As expected, the boundary rails and studs limit the sound reduction index R in the mid-high frequency range. At the critical frequency of the gypsum boards (3150 Hz one-third-octave-band) the predicted transmission loss is lower than the average; it could be due to the fact that at this frequency the damping has a major influence (and is very sensitive to mounting etc).

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Figure 11 – Comparison between AcouSYS prediction and average measurement for partition 2. (a)

Comparison between predicted and measured sound reduction.

6 CONCLUSIONS The 2010 ILT have shown that uncertainties were not reduced by the two amendments from

1994 ILT. WG9 proposed to set two reference systems to guide laboratories to adapt their procedure and facilities to the particularity of drywall partition. Control of air tightness, drying times, test room diffusity and flanking transmission are identified to be real levelers to reduce reproducibility. The test code that is being prepared will propose solutions to reduce reproducibility.

ACKNOWLEDGEMENTS Authors Working Group 9 wants to acknowledge 19 participating laboratories for their contribution The sample suppliers are Lafarge Plâtres and Isover with production. Finally, a great acknowledgement goes to EURIMA and EUROGYPSUM for financing delivery and packaging.

REFERENCES [1] EN ISO10140-2 Acoustics - Laboratory measurement of sound insulation of building elements -Part 2:

Measurement of airborne sound insulation (2010). [2] EN ISO10140-5 Acoustique - Mesurage en laboratoire de l'isolation acoustique des éléments de

construction - Partie 5: Exigences relatives aux installations et appareillage d'essai (2010). [3] EN ISO 140-1 Acoustics - Laboratory measurement of sound insulation of building elements -Part 1:

Requirements for laboratory test facilities with suppressed flanking transmissions (ISO 140-1:1997) [4] EN ISO 140-2 Acoustics - Measurement of sound insulation in buildings and of building elements - Part

2: Determination, verification and application of precision data. [5] EN ISO 140-3 Acoustics -- Measurement of sound insulation in buildings and of building elements -

Part 3: Laboratory measurements of airborne sound insulation of building elements. [6] Patrizio Fausti, Roberto Pompoli and R.Sean Smith, “An Intercomparison of Laboratory Measurements

of Airborne Sound Insulation of Lightweight Plasterboard Walls”, Building acoustic Volume 6, 1999. [7]Robert J.M. Craik, “The influence of the laboratory on measurements of wall performance”, Applied

Acoustics 35(1992)25 46 [8]Keith O Ballagh, “Accuracy of Prediction Methods for Sound Transmission Loss”, Internoise 2004, [9] C. Guigou-Carter and M. Villot (2003) Modelling of Sound Transmission Through Lightweight

Elements with Stiffeners, Building Acoustics, Vol. 10, No. 3, pp. 193-209, 2003.. [10] M. Villot and C. Guigou-Carter (2001) Modelling of Sound Transmission Through Lightweight

Elements with Stiffeners, Proceedings of 17th ICA, Rome, Italy, 2001. [11] M.L. Munjal, Response of a multi-layered infinite plate to an oblique plane wave by means of transfer

matrices, Journal of Sound and Vibration, Vol. 162, pp. 333-343, 1993. [12] C. Guigou-Carter and M. Villot (2006) Analytical and experimental study of single frame double wall,

Proceedings of Euronoise 2006, Tampere, Finland, 2006. [13] M. Villot, C. Guigou-Carter and L. Gagliardini (2001) Predicting the acoustical radiation of finite size

multi-layered structures by applying a spatial windowing on infinite structures. Journal of Sound and Vibration, Vol. 245, No. 3, pp. 433-455, 2001.

[14] ISO/PAS 16940, Glass in building - Glazing and airborne sound insulation - Measurement of the mechanical impedance of laminated glass, 2008.

[15] J. Poblet-Puig, A. Rodríguez-Ferran, C. Guigou-Carter and M. Villot (2006) Experimental and numerical characterization of metallic studs, Proceedings of Euronoise 2006, Tampere, Finland, 2006.

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