Flanking sound insulation of wood frame assemblies with high axial and lateral load bearing capacity

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Inter-Noise 2013: 42nd International Congress and Exposition on Noise Control

Engineering, 2013-09-18

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Flanking sound insulation of wood frame assemblies with high axial

and lateral load bearing capacity

Zeitler, Berndt; Schoenwald, Stefan; King, Frances

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FLANKING SOUND INSULATION OF WOOD FRAME ASSEMBLIES

WITH HIGH AXIAL AND LATERAL LOAD BEARING CAPACITY

Berndt Zeitler1, Stefan Schoenwald2, and Frances King3

National Research Council Canada 1200 Montreal Rd, Ottawa ON K1A 0R6, Canada

ABSTRACT

As part of a multidisciplinary research project to develop design solutions for mid-rise wooden buildings for the Canadian market, a comprehensive study was conducted on the flanking sound insulation performance of wood frame assemblies that have high axial and lateral load bearing capacity. The axial load bearing capacity depends mainly on framing details and can be increased by strengthening the wall framing by using either more studs at a smaller spacing or by using studs with greater dimension (2 or 3 studs joined together). The lateral load bearing capacity or racking resistance of walls depends on the wall membrane and can be increased by attaching wood board materials (e.g. plywood) in different configurations to the framing. In this paper the effect of shear membranes on the flanking sound insulation of wall-floor systems are presented.

1. INTRODUCTION

The population density of Canadian cities is increasing and taller buildings are needed to cope with that. With the increased height, the lateral loads due to wind and seismic forces also increase . Therefore, it is necessary to add shear layers to walls in order to improve the structural integrity of the building. In this paper we assess whether adding shear layers also improves the acoustical performance of the walls.

The specimen and sound transmission paths of main interest are introduced first, followed by a description of the direct and flanking facilities in which they are tested and measur ed. The acoustical improvement achieved by adding a shear layer is investigated mainly for airborne sound insulation, but also for impact sound insulation.

Throughout this paper sound reduction index, R, is used to refer to both sound reduction index and flanking sound reduction index.

2. SPECIMEN AND PATHS

In this study we are looking at direct and flanking sound transmission through combinations of wall and floor elements with two different non-load bearing (NLB) staggered stud walls – one without shear layer (Wall 1) and one with shear layer (Wall 2).

1

berndt.zeitler@nrc.ca

2

stefan.schoenwald@empa.ch, currently at EMPA, Dübendorf, Switzerland

3

Wall 1 is a conventional 2x4 staggered stud wall with studs (38 mm x 89 mm) attached in two rows to a single 2x6 (38 mm x 139 mm) head plate with a total mass per area of 52.8 kg/m2. The two stud rows were staggered, i.e. one row was flush with the one side of the footer- and header plate and one row flush with the other side. The spacing between the studs in each row was 406 mm as for a typical North American load bearing wall. Two layers of 13 mm thick fire rated gypsum board were directly attached to each side of the wall and the cavity was filled with 90 mm glass fiber insulation.

Wall 2 is the same as the first but with a shear layer between the gypsum board and the studs on one side of the wall. The shear layer of 16 mm thick plywood was nailed at 75 mm o.c. around the perimeter and at intermediate studs. The sheets of plywood were oriented horizontally with blocking along horizontal joint of the shear membranes at mid-wall height. Throughout this paper when referring to shear layers, the plywood including the blocking is meant. The total mass per unit area of plywood and blocking is 8.5 kg/m2.

Below, in Figure 1, the walls are displayed in combination with each other as well as with load bearing (LB) walls, as wall-wall junctions (upper sketches in plan view) and with wood joist floors, as floor-wall junctions (lower sketches in side view).

Figure 1 - Sketches of specimen details and sound transmission paths. Top sketches are plan view of the wall-wall junctions, bottom sketches are side view of non-load bearing floor-wall junctions. Path cases with fewer elements containing shear layers (orange lines) are identified with a 1, mainly in left sketches (except

for case E with both on the right side), and cases with more are identified with a 2.

The LB walls have the same details as the NLB walls, but to increase the vertical load bearing capacity, each stud was replaced by a column of three studs. The floors are constructed with a 16 mm subfloor, wood I-joists, cavity insulation, and 2 layers of 13 mm fire rated gypsum board attached to resilient channels. B1 C1 B2 C2 A1 D1 A2 E1 E2 D2 Wall 1 Wall 2 Wall 2 Wall 2 Wall 1 Wall 2 Wall 1 _{Wall 2}

The sound transmission paths of interest are in pairs A1&A2 through E1&E2. In the pairs, path 2 has more shear layers to pass through than path 1. For example for path C2 sound travels through one more shear layer than for C1 and for path E2 sound travels through two more shear layers than for E1. Some of these paths were measured more than once as summarized in Table 1 below. The direct wall-wall sound transmission case A was measured once in the direct sound transmission facility, Alab

and once in the flanking facility, A. The vertical diagonal floor-wall flanking sound transmission case was measured once for airborne sound, C, and once for impact sound, Cimpact.

Table 1 - describing paths of interest

Name Facility Path type Path Orientation Added shear Alab Direct direct wall-wall horizontal one

A Flanking direct wall-wall horizontal one B Flanking flanking wall-wall vertical two C Flanking flanking floor-wall vertical (diag.) one

Cimpact Flanking flanking floor-wall vertical (diag.) one

D Flanking flanking wall-wall horizontal one E Flanking flanking wall-wall horizontal two

There are a total of 2 direct cases (A, Alab) and 5 flanking cases (B-E). The direct cases are both

horizontal paths (rooms side-by-side), and of the flanking cases three are vertical paths (two diagonal room pairs, one room pair one-above-another) and two are horizontal paths (rooms side-by-side).

3. FACILITIES AND MEASUREMENTS

All measurements were conducted at the NRC Direct and Flanking Sound Transmission Facilities that are both equipped with automated sound and measurement systems for data acquisition and post processing. A robot controlled microphone positioning system is used to improve the repeatability. Airborne sound reduction index is measured in both directions between room pairs and results are averaged to reduce measurement uncertainty due to calibration errors.

3.1 Direct Wall Sound Transmission Facility

NRC Construction’s Direct Sound Transmission Facility (see Figure 2) consists of two decoupled rooms with 250 m3 and 140 m3 room volumes, separated by a movable test frame that has an opening 3.66 m wide and 2.44 m high. The non-load bearing staggered stud walls were constructed within the test frame and tested for direct sound reduction index according to ISO 10140-2 [1].

Figure 2 - NRC Construction’s Direct Wall Sound Transmission Facility with movable test frame between two decoupled rooms (room volume: 250 m3 and 140 m3)

3.2 Flanking Sound Transmission Facility

NRC Construction’s Eight-Room Flanking Sound Transmission Facility [2] shown in Figure 3 is comprised of eight rooms (four upper and four lower rooms). The specimen comprises eight walls, four floors, and six junctions, enabling the evaluation of flanking transmission throu gh load bearing and non-load bearing wall-floor junctions, as well as wall-wall paths with a single specimen.

The permanent part of the facility (upper ceiling/roof, perimeter walls, and foundation floor) is constructed of high sound insulating elements that are resiliently isolated from each other and from structural support members, with vibration breaks in the permanent surfaces where the specimens are installed. Upper rooms have a ceiling height of 2.7 m while the lower rooms have a height of 2.4 m. The volume of the rooms used to assess flanking transmission in this study ranged from 35 to 45 m3.

The sound reduction index, R, and normalized flanking impact sound pressure level, Ln,f, were

measured in the flanking facility according to ISO 10848 -3 [3]. The impact sound pressure level utilized the tapping machine described in ISO 10140-5. Shielding was applied according to 10848-1 to ensure that only the path of interest was being measured. More on shielding approaches can also be obtained in the several other publications [4, 5, 6].

Figure 3 - Cut-away sketch and photo of NRC Construction’s Eight-Room Flanking Sound Transmission Facility. Sketch shows the bearing walls of the specimen to which a static load can be applied. Photo shows

specimen under construction.

4. RESULTS

Result in this section will be presented in four steps: (1) the sound reduction index measured in the lab, (2) the improvement due to adding a shear layer for all the direct and vertical flanking cases, (3) the improvement for the horizontal flanking cases, and (4) the impact case.

Figure 4 shows Case Alab, where the direct sound reduction index through the NLB wall was

measured in the laboratory without and with a shear layer. The addition of a shear layer improves the sound insulation properties of the wall throughout the whole frequency range on average by about 1-2 dB. The improvement cannot be attributed to a shift in the mass -spring-mass resonance as that only shifts by a few Hertz. However, assuming a homogeneous wall and using mass law, the expected improvement would be 1.3 dB = 20 log (m2/m1), which is very similar to what was measured. Although

repeatability errors due to removing and reapplying a layer of gypsum board are similar to these improvements, the results are trusted to be physical, based on two considerations. First, the improvement is quite consistent over the whole frequency range and second, the improvement for other cases that will be discussed later shows the same trend.

Although the plywood has a much higher dynamic stiffness than gypsum board along the long axis, the coincidence frequency of the combined layers does not change due to adding the shear layer. It is known that when coupling two plates, the vibration of the stiffer board is forced upon the less stiff one, which should result in a lower coincidence frequency. Yet, since the boards are not connected everywhere, but only with screws every 305 mm, they act as one only when more than half a bending wavelength fits between the screw spacing. This happens far below the coincidence frequency,

between around 150 and 300 Hz, so the vibration of the outer layer of gypsum board still controls the radiation at the coincidence frequency.

Figure 4 - Sound reduction index R of non-load bearing wall without shear layer (A1-solid line) and with shear layer (A2-dashed line)

The difference between A2lab and A1lab, or improvement due to adding the shear layer, is compared

in Figure 5 to the other direct and vertical flanking cases (A -C). Throughout the paper a positive number means an improvement of the sound insulation properties due to adding a shear layer. The x’s denote frequencies at which the signal to noise ratio was below 7 dB for Case C below 80 Hz. All curves have a very similar trend, with improvement around 1 -2 dB on average. Below 125 Hz most cases show an even greater improvement, except Case B which shows a slight worsening. Although that shear layers were added on two walls for Case B, the improvement is the same as for the other cases where a shear layer was only added to one wall. Stopping analysis here would suggest that adding a shear layer improves the sound insulation performance.

Figure 5 - Improvement of sound reduction index or flanking sound reduction index due to adding shear layer for direct (Alab & A), vertical (B), and diagonal (C) sound transmission paths.

Although, as shown below in Figure 6, the horizontal cases (D&E) show improvement similar to the direct and vertical flanking cases at low frequencies, above approximately 125 Hz adding a shear layer gives a worsening of approximately 2 dB per octave. Using a shear layer as an acoustic insulation measure doesn’t look very beneficial when these cases are included. The black vertical lines identify

63 125 250 500 1k 2k 4k 20 30 40 50 60 70 freq in Hz R in d B R w 50 : Case A1lab R w 52 : Case A2lab 63 125 250 500 1k 2k 4k -6 -4 -2 0 2 4 6 freq in Hz  R in d B Case A lab Case A Case B Case C

the frequencies at which the wavelength along the short and long axis of the gypsum (GS & GL) and

plywood (PS& PL) boards are equal to double the stud spacing.

Figure 6 - Improvement of sound reduction index or flanking sound reduction index due to adding shear layer, for direct (Alab) and horizontal (D&E) sound transmission paths.

In Figure 7 the theoretical bending wavelengths calculated from measured Young’s Modulus of the board in different direction are plotted. Here one can see that the bending wavelength in gypsum board is very similar in its both directions to that of plywood along the short axis. This applies for the vertical transmission cases (rooms one-above-another) because the plywood was installed with the long axis horizontally. The plywood along the long axis has much longer bending wavelengths. The dashed line shows wavelength in air that is equal to that of the gypsum board at around 2k Hz where the coincidence dip can be seen in Figure 4.

Figure 7 - Wavenumber of 16 mm plywood and 13 mm gypsum board calculated from Young’s modulus measured along short and long axis of board. Black horizontal line indicates where

half a wavelength equals the stud spacing.

The vertical diagonal D case was also investigated for impact sound (see Figure 8) and is compared to the airborne improvement. Background noise was only an issue in the very low and very high frequency ranges. Overall, the trend of changes due to adding the shear layer are similar for airborne and impact sound. For the impact source the slight improvement at low frequencies reduces to around zero above 1k Hz. 63 125 250 500 1k 2k 4k -15 -10 -5 0 5 10 freq in Hz  R in d B P_L P_S G_L G_S Case A lab Case D Case E 63 125 250 500 1k 2k 4k 0 0.5 1 1.5 freq in Hz  in m P_L P_S G_L G_S / 2= d plywood long (P L) plywood short (P_S) gypsum long (G L) gypsum short (G S)

Figure 8 - Improvement of sound reduction index or flanking sound reduction index or normalized flanking impact sound insulation level due to adding shear layer for direct

airborne (Alab), diagonal airborne (C) and impact (Cimpact) sound transmission paths.

Finally, the results are presented as single number ratings using the weighted sound reduction index (Rw) for airborne sound and the weighted normalized impact sound pressure level (LnW) for impact

sound (and the corresponding ratings according to ISO 717 for in -situ and flanking paths).

Table 2 – List of single number ratings an improvement due to adding shear membrane for Rw and Ln,fW.

Case A Alab B C Cimpact D E

1 50 49 51 61 51 72 70

2 52 51 51 62 50 68 68

Improvement 2 2 0 1 1 -4 -2

For all direct and vertical paths (A-C) the improvement in single number ratings ranges from zero to two points as expected from the third octave data. For the horizontal flanking cases (D&E) the single number ratings worsened by “only” 2 to 4 points, although the third octave bands worsened by up to 10 dB.

5. CONCLUSIONS AND OUTLOOK

Adding a horizontally applied shear layer is beneficial for direct sound insulation and for vertical flanking sound insulation. Improvements of 1 to 2 points can be expected both the single number ratings and third octave band results. The improvement achieved is similar whether a shear layer is added to one or two of the flanking elements. A conservative estimate would be to assume that the shear layer has no acoustical significance for direct and vertical flanking sound transmission if applied horizontally.

Horizontal flanking insulation however, shows a worsening in 1/3-octave band levels above 125 Hz of up to 10 dB and in the single number ratings between two to four points. This may not be as much of an issue as it first seems, because these horizontal flanking paths provide high sound insulation and the effects only become significant if overall sound insulation in the mid to high 60’s is the goal.

In the future it would be interesting to assess how much of the change in sound transmission is due to the shear membrane and how much due to the blocking, how much the results would change if the shear membrane were installed vertically, and if the sound transmission through the side of the wall to which no shear layer is added, would be affected by adding a shear layer.

63 125 250 500 1k 2k 4k -6 -4 -2 0 2 4 6 freq in Hz  R o r  L n, f in d B Case A lab Case C Case C impact

ACKNOWLEDGEMENTS

The work presented here was conducted as part of a client funded research project on mid -rise wooden construction. The authors acknowledge the support of Canadian Wood Council, FPInnovations and the Provinces of Quebec and Ontario. The authors would also like to thank Curtis Langley for doing an exceptional job at supporting this project.

REFERENCES

[1] International Organization for Standardization, ISO 10140, “Acoustics - Laboratory measurement of sound insulation of building elements,” (2010).

[2] T. Estabrooks, et al. “NRC-IRC Flanking Facility”, Niagara-on-the-Lake, Canada: Canadian Acoustical Association (2009).

[3] International Organization for Standardization, ISO 10848, “Acoustics -- Laboratory measurement of the flanking transmission of airborne and impact sound between adjoining rooms,” (2006).

[4] T.R.T. Nightingale, J.D. Quirt, F. King and R.E. Halliwell, “RR-218, Flanking Transmission in Multi-Family Dwellings Phase IV,” (2006).

[5] B. Zeitler, S. Schoenwald, I. Sabourin, “Impact sound insulation of hybrid wood-concrete masonry assemblies”; Inter-Noise 2013, Innsbruck, Austria (submitted), Sept (2013).

[6] S. Schoenwald; T. Nightingale, B. Zeitler, F. King, “Approaches for estimating flanking transmission for heavy impact sources”, Inter-Noise 2010, Lisbon, Portugal, June (2010)