On reducing low frequency impact sound transmission in wood framed construction

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41st International Congress and Exposition on Noise Control Engineering 2012 (INTER-NOISE 2012), 8, pp. 6653-6662, 2012

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On reducing low frequency impact sound transmission in wood framed construction

Zeitler, Berndt; Sabourin, Ivan; Schoenwald, Stefan; Wenzke, Erik

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On reducing low frequency impact sound transmission in wood

framed construction

Berndt Zeitlera) Ivan Sabourinb) Stefan Schoenwaldc) Erik Wenzke

National Research Council Canada

1200 Montreal Rd, Ottawa, ON K1A 0R6, Canada

In an effort to generate design details for wood framed construction that meet the very stringent Korean Code requirements, solutions to reduce low frequency impact sound were developed. This paper presents these design details and the approach taken to develop them, which involved a parametric study of direct impact sound insulation followed by a flanking study. Results of the direct sound insulation study focus on the effects of topping and ceiling designs, whereas the flanking study focuses on the significance of flanking on the overall impact sound transmission between two rooms, one above the other, and on how a topping affects direct and flanking impact sound transmission differently.

1 INTRODUCTION

Low frequency impact noise is often a problem between units separated both vertically and horizontally. This is especially the case in lightweight framed construction. The Korean society is extremely sensitive to impact noise, which is why they have arguably the highest insulation sound insulation requirements in the world. The outcomes of this study are solutions for effective reduction of low frequency impact sound insulation in wood framed construction (WFC) through selected topping and ceiling variants. The paper starts with discussion on the measurement procedures used and then compares results obtained for direct and for flanking impact sound transmission.

2 MEASURMENT PROCEDURE

This section introduces the sources and metrics used in this study, followed by a brief description of the impact sound insulation facilities, as well as the specimen under test.

a) email: berndt.zeitler@nrc.ca b) email: ivan.sabourin@nrc.ca c) email: stefan.schoenwald@nrc.ca

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2.1 Sources

The two standardized impact sources used for the sound insulation measurements in these studies are depicted in Figure 1. The transient signals of the heavy impact source (Ball), developed to simulate the low frequency walking and jumping of children, are measured as maximum fast weighted levels LiFmax according to ISO 10140-3. The quasi-stationary signals of the ISO

Tapping Machine (Hammer) are measured according to ISO 10140-3.

2.2 Ratings

Two different single number ratings are presented for each source in this paper. One rating used for the Ball is the Korean Li,Fmax,AW metric, chosen because the goal of this study was to develop

a wood joist floor that meets the very stringent Korean Code requirements. Although the rating curve ranges from the 63 Hz octave band to the 500 Hz octave band, in WFC the Li,Fmax,AW rating

is almost always controlled only by the 63 Hz octave band level, meaning any change in level at that band, changes the single number rating accordingly. The other metric, developed by Ryu (1) and validated by Gover (2) in a Canadian context, is the average of the fast weighted peak levels from the 63 Hz to 1k Hz octave bands (LiFavg,Fmax(63-1k Hz)). It was chosen, because it correlates

better with the subjective annoyance response to walkers than the standardized metrics. The two metrics presented for the Hammer are Ln,W and Ln,W + CI,50-2500 both defined in ISO 717-2. The

former extending from 100 Hz to 3150 Hz, whereas the latter puts more emphasis on the low frequency range (50-2500 Hz). For all of the above impact ratings a smaller number indicates better performance.

2.3 Volume and Absorption Normalization

The standard light impact ratings are normalized for receiving room volume and absorption. However, the standardized ratings of heavy impact sound insulation (KS F 2810-2) are not. Therefore, these measures are highly dependent on the receiving room conditions and it is not possible to directly compare results from one study to another without normalizing to receiving room conditions. A normalization term, developed at NRC (3), has been applied to the single number ratings of the Ball (Li,Fmax,AW) normalizing to a room volume of 40 m3 with a

reverberation time of 0.5 s.

The following generally holds: Reported un-normalized performance of the nominally identical system will be better when measured with

• Increased volume – a doubling of room volume will “apparently” increase performance by 3 dB for heavy impact sources;

• Increased absorption (due to furnishings such as beds, upholstered settees and chairs, carpeting, drapes, etc.) – increases the measured insulation performance.

2.4 Facilities

The effect of varying the topping and ceiling parameters was first studied in the NRC-Direct Floor Sound Transmission Facility (M59) having volumes of source (upper) and receiving (lower) rooms of approximately 175 m3. A more detailed description of the facilities can be found in a previous study (4).

The flanking study that followed was conducted in the NRC-Flanking Sound Transmission Facility (5), comprised of eight rooms (four upstairs and four down stairs). More specifically, eight walls, four floors, and six junctions, enabling the evaluation of flanking transmission through load bearing and non-load bearing wall-floor junctions, as well as wall-wall paths with a

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single specimen. A robot controlled microphone positioning system was used to improve the repeatability.

3 SPECIMEN

In earlier analytical and experimental studies (6) and (7) it was found beneficial for low frequency impact insulation to design a stiff subfloor, with a heavy topping and resiliently mounted heavy ceiling. This resulted in the following base reference assembly (NRC-K01, see sketch in Table 1) consisting of two layers of 19 mm OSB subfloor, 2x10 scabbed joists at 406 mm o.c., with 150 mm of insulation in the cavity, and three layers of 12.5 mm fire rated gypsum board attached on resilient channels (RCs) spaced 610 mm o.c.

Topping variations of the reference floor included two toppings, a 100 mm concrete topping (Con100) and a 70 mm concrete topping (Con70), with three different closed cell foam resilient interlayers installed under the toppings: 9 mm foam (Res9), 20 mm foam (Res20), and 42 mm foam (Res42). In one case 50 mm of sand (Sand50) was added below the Con70 topping. The elements of the ceiling study varied were, the RCs, spaced at 406 mm (RC406) and 610 mm (RC610), and the different number of gypsum board layers attached to the ceiling, one (1G), two (2G), and three (3G) layers.

The floors that showed the highest performance for direct sound insulation (in M59), were then built and characterized in the flanking facility, namely Con100 and Con70, the latter on Sand50, both combined with the 3G ceiling on RC610. The walls of the flanking specimen were standard 2x6 wood stud walls with two layers of 12.5 mm fire rated gypsum board directly attached on each side. The floor-wall and floor-floor junctions were designed very rigid, in order to block much of the sound and reduce the relevance of flanking sound transmission.

4 RESULTS

This section is split into two parts, the first talking about direct sound transmission looking at the effect of the topping (including interlayer) and ceiling change, the second talking about flanking sound transmission and how it is affected differently by the toppings.

4.1 Direct Sound Transmission

The modifications made focus on two main areas of the floor; the topping (thickness and interlayer) and the ceiling (layers and attachment), while keeping the joists and subfloor unchanged. All results shown in this section stem from measurements conducted in the NRC-IRC Direct Floor Sound Transmission Facility (M59). Firstly, an appropriate interlayer was selected.

4.1.1 Interlayer

The three interlayers (Res9, Res20, and Res42) were evaluated installed under the two toppings (Con100 and Con70). The maximum difference between their performances relative to Res20 is listed in Table 2. Only the Ln,W metric shows some effect due to interlayer thickness. Assuming

all three resilient interlayers have the same material properties per thickness, the stiffness varies between kRes9 (Res9), ½ kRes9 (Res20), and ¼ kRes9 (Res42). Seeing only little effect after having

quartered the overall stiffness suggests that for this type of wood frame assembly, the interlayer thickness has insignificant effect on the low frequency performance of the floor assembly. This is probably due to the fact that the floor is not rigid enough to cause the interlayer to compress, and decouple the toping from the subfloor, as theoretically on rigid floor, quartering the stiffness

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of the interlayer would ideally shift the resonance down by one octave not seen here. Throughout the rest of the paper investigations will be referring to assemblies utilizing the Res20 interlayer.

4.1.2 Topping

The effects of the two different toppings on the Res20 interlayer are shown in Table 3 relative to the bare reference floor. The improvements are over all very high, reaching from 10 to 18 points. The 100Con topping, with a mass per area of approximately 250 kg/m2 gives 2 points more improvement than the lighter Con70 topping (~150 kg/m2), except for the Ln,W metric where it is

only one point better. Both toppings show a higher improvement for the Ball than for the Hammer. This is probably due to the fact that the Hammer in the mid to high frequency range more easily injects power into the hard concrete toppings than the Ball does, due to the impedance match of source and topping in this range, which pulls the single number ratings down. This effect of the impedance matching it is seen less strongly for the Hammer when using the low frequency adaption term. Also the single number ratings of the Ball only include frequencies up to 500 Hz.

Next is to investigate if the better performance of the Con100 topping is only due to the additional mass it brings, by adding mass (through sand) to the Con70 assembly between the subfloor and interlayer. The benefit of using sand is that it reduces construction time and costs. The down side is that the total floor depth increases by 20 mm. Table 4 shows results for the topping Con100, the Con70, and Con70+Sand50 (230 kg/m2).

The results show that for the Ball, rated only in the low frequency range by Li,Fmax,AW, adding the

sand gives no improvement, and the floor assembly is still 2 points below that of the Con100 topping. For the other metrics that were seen to better correlate with subjective responses, an improvement of 2 to 3 points were gained by adding the sand. This is because in the higher frequency range the sand has a dampening effect, reducing the sound transmission through the floor-ceiling assembly. The performance of the Con70 floor with sand even surpasses that of the Con100 floor for Ball LiFavg,Fmax and Hammer Ln,W by 1 and 3 points respectively.

4.1.3 Ceiling

First the effect of increasing the spacing of the resilient ceiling channels is investigated, followed by the effect of increasing the number of gypsum board layers.

The effect of increasing the spacing of RCs from 406 mm to 610 mm is shown in Table 5. The improvement is quite large and ranges from 4 to 6 points in all cases. This large increase occurs, because by reducing the number of RCs, the overall stiffness of the connection decreases, meaning the resonance frequency shifts downwards also. This means that the improvement due to adding RCs starts earlier.

The second investigation in the ceiling study is the effect of adding gypsum board layers to the ceiling. Increasing the number of layers from one to two layers of gypsum board on the ceiling, both times attached via RC at 610 mm o.c., gives a 3 to 4 point improvement for all single number ratings. This improvement is also cause by lowering the resonance frequency, however, this time, by adding mass.

As shown in Table 7 the improvement due to adding one layer of gypsum board, increasing from two layers to three, is smaller than the initial change from one to two layers. This is because

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when going from one to two layers that mass is being doubled, whereas in the other case the mass is only being increased by 3/2.

4.2 Flanking Sound Transmission

The two toppings described in Section 3, were applied to the bare reference specimen (NRC-FF01) built in the flanking facility. The specimens were nominally the same as those built in M59. Table 8 shows the absolute single number ratings for the Ball and Hammer for the two toppings applied, as well as the improvements gained relative to the bare floor.

As seen in the direct sound transmission investigation, the LiFmax,AW ratings for the Ball are

slightly better for the Con100 topping whereas the Ln,W ratings for the Hammer are slightly better

for the Con70 topping. Note that the bare reference floors also have slightly different ratings, due to having been measured in rooms with different dimensions. Although the data is normalized to the same room volume, the effects of the room’s modal characteristics (in the very low frequency range) are not necessarily captured by this normalization, as it is based on diffuse field theory. The table also shows that flanking only contributes slightly for the Ball on both toppings, resulting in an apparent sound transmission only one point higher than the direct sound transmission. The same is the case for the Hammer on the Con70 floor with sand, however on the Con100 floor, flanking contributes more to the apparent sound transmission, which is three points higher than the direct sound insulation. The different effect the Hammer shows on the two toppings is caused by the damping from the sand in the higher frequency range. Whereas the Con100 topping transports sound energy towards the junction without much attenuation, the sand in the Con70 floor attenuates the sound propagating to the junction (see flanking improvements for Hammer). The difference in improvements between direct and flanking for the Ball is very similar for both toppings (2-3 points), because number are driven in the low frequency range.

5 SUMMARY AND CONCLUSSION

Large improvements of low frequency impact insulation can be obtained through proper choice of topping and ceiling details. Admittedly, much mass is needed to achieve this. The 100 mm concrete topping gives the greatest improvement, of approximately 20 points for the Ball and 15 points for the Hammer for both direct and flanking sound insulation. The 70 mm concrete floor with 50 mm of sand also shows large improvements for direct sound insulation of approximately 15 points for the Ball and Hammer, and even larger improvments of approximately 25 points for the Hammer on flanking sound insulation, as the sand seems to cause high damping in the higher frequencies captured by the single number rating. The three foam interlayers investigated did not show significant differences in the low frequency impact sound ratings.

More low frequency improvement can be obtained through the increase of resilient channel spacing. With two layers of 12.5 mm fire rated gypsum board attached, changing the channel spacing from 406 mm o.c. to 610 mm o.c. increased the single number ratings by approximately 5 points. Another few points can be gained by adding a third layer of gypsum board to the ceiling. Both of these improvements are caused by shifting the resonance frequency of the ceiling attachment.

With these measures the extremely stringent Korean minimum Code requirement could be met. Note, that the Code requirements are actually for the Bang Machine (not discussed here), a small

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car tire that is dropped on the ground from 85 cm height, the first heavy impactor designed to simulate children walking and jumping.

6 ACKNOWLEDGMENTS

We would like to thank the Canada Wood Group (CWG) and the Council of Forest Industries (COFI) for funding this project and for their great support.

7 REFERENCES

1. Subjective ratings of heavy-weight floor impact sounds in wood frame construction. Ryu, J. et al. 5, s.l. : Acoust. Sci. & Tech, 2010, Vol. 31.

2. Objective and subjective assessment of lightweight wood-framed floor assemblies in response

to footstep and low-frequency impact sounds. Gover, B. et al. Osaka : InterNoise, 2011.

3. Dependency between standardized heavy impact sound pressure level and receiving room

properties. Schoenwald, S. et al. Osaka, Japan : Proceedings of InterNoise 2011, 2011.

4. Warnock, A.C.C. et al. Detailed Report for Consortium on Fire Resistance and Sound

Insulation of Floors: Sound Transmission and Impact Insulation Data in 1/3 Octave Bands.

Ottawa, Canada : NRC Canada, 2000. IR-811.

5. NRC-IRC Flanking Facility. Estabrooks, T., et al. Niagara-on-the-Lake, Canada : Canadian Acoustical Association, 2009.

6. Two modeling approaches for periodic rib-stiffened plates typical of floor assemblies. Nightingale, T. and Bosman, I. Cairns, Australia : Proceedings of 14th International Congress on Sound and Vibration, 2007.

7. Parametric study of sound transmission through lightweight floors Internoise. Zeitler, B. et al. Lisbon, Portugal : Proceedings of InterNoise 2010, 2010.

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Table 1: Single number ratings of bare reference floor (NRC-K01) measured in M59 (direct sound transmission)

Floor

ID Sketch (not to scale)

Heavy impact (Ball) Light Impact (Hammer) LiFavg,Fmax (63-1k Hz) Li,Fmax,AW Ln,W+CI (50-2500 Hz) Ln,W NRC-K01

Bare reference floor

59 57 56 54

Table 2: Maximum difference between results with differently thick foam interlayers relative to 20 mm interlayer.

Sketch (not to scale)

Heavy impact (Ball) Light Impact (Hammer) LiFavg,Fmax (63-1k Hz) Li,Fmax,AW Ln,W+CI (50-2500 Hz) Ln,W Con100 0 0 0 2 Con70 1 1 1 2

Table 3: Improvement of songle number ratins for Ball and Hammer due to adding topping of different thickness to the bare reference floor (NRC-K01)

Floor

Heavy impact (Ball) Light Impact (Hammer) LiFavg,Fmax (63-1k Hz) Li,Fmax,AW Ln,W+CI (50-2500 Hz) Ln,W NRC-K12 Con100 17 18 14 11 NRC-K11 Con70 15 16 12 10

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Table 4: Comparison of single number ratings due to changing mass of topping.

Floor

Heavy impact (Ball) Light Impact (Hammer) LiFavg,Fmax (63-1k Hz) Li,Fmax,AW Ln,W+CI (50-2500 Hz) Ln,W NRC-K11 Con70 + Res20 44 41 44 44 NRC-K14

Con70 + Res20 + Sand50

41 41 42 40

NRC-K12

Con100 + Res20

42 39 42 43

Table 5: Comparison of single number ratings due to increasing spacing of resilient channels from 406 mm o.c. to 610 mm o.c..

Floor

Heavy impact (Ball) Light Impact (Hammer) LiFavg,Fmax (63-1k Hz) Li,Fmax,AW Ln,W+CI (50-2500 Hz) Ln,W NRC-K23 2G + RC406 46 49 50 47 NRC-K15 2G + RC610 42 44 44 43 NRC-K23 – NRC-K15 Difference 4 5 6 4

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Table 6: Comparison of single number ratings due to adding second layer of gypsum board to ceiling attached using resilient channels (from one to two layers).

Floor

Table 7: Comparison of single number ratings due to adding third layer of gypsum board to ceiling attached on resilient channels.

Floor

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Table 8: Single number ratings and improvement of Direct (D), Flanking (F), and Apparent (A) Li,Fmax,AW for Ball and Ln,W for Hammer of two floors with toppings tested in NRC-

Flanking Sound Transmission Facility. Improvements are relative to bare floor (NRC-FF01)

Floor ID Sketch (not to scale)

Heavy Impact (Ball) Li,Fmax,AW Light Impact (Hammer) Ln,W Absolute Improve-ment Absolute Improve-ment NRC-K15 Con100 + Res20 _D ₃₇ ₂₂ ₄₂ ₁₄ F 33 19 41 15 A ₃₈ ₂₁ ₄₅ ₁₄ NRC-K14

Con70 + Res20 Sand50 _D ₄₀ ₁₆ ₃₈ ₁₆

F 38 14 32 24

A ₄₁ ₁₆ ₃₉ ₁₉

Figure 1 - Standardized impact sources. Left: Heavy Impactor (Ball), Right: ISO Tapping Machine (Hammer). Detailed descriptions of sources can be found in the following standards: Ball: ISO 10140-3, JIS A 1418-2, KS F 2810-2. Hammer: ISO 10140-3, JIS A 1418-1, KS F 2810-1.