Sound fields characteristics of a box-type CLT building for heavy- weight impact sources Yong-Hee Kim 1 Dae-Gwan Won 2 Gu-In Oh 3 Bon-Su Koo 4 Jang-Won Lee 5 Y’sU Youngsan University 288 Junam-ro, Yangsan, Kyeongnam, South Korea Se-Jong Kim 6 National Institute of Forest Science 57 Hoegi-ro, Dongdaemun-gu, Seoul, South Korea

ABSTRACT This study investigated the effects of interior finishing materials on sound fields characteristics of a box-type CLT building to minimize room responses for heavy-weight impact sources. According to the requirements on ISO 10140-5, the sound fields should have reverberation time (RT) range between 1 s to 2 s with minimized deviation. Therefore, grid-type sound absorbers, reflectors and diffusers with size of 0.6 m by 0.6 m were designed and applied to the box-type building. The building has a volume of 53.8 m 3 , and RT of the current condition with no absorbers or diffusers was 2.56 s at 63 Hz, 1.26 s at 4 kHz. The experiments were carried out with various combination of absorbers and diffusers on lateral walls. As results, it was derived at the case of 44 reflectors with additional 4 bass traps as the optimum sound fields condition. In addition, sound fields variation as functions of interior finishing materials was discussed for driving heavy-weight impact sources such as bang machine.

1. INTRODUCTION

In South Korea, the proportion of apartment type housing is more than 70%, and most of them are constructed as wall-type concrete structure [1]. However, it is strongly needed to develop eco- friendly sustainable apartments using low-carbon materials after the Paris Agreement [2]. The excellent role of wood materials for greenhouse gas mitigation is widely known [3-4]. In Europe and Japan, apartment houses of light-weight wooden structure have been used for a long time, but wooden light-weight structure for apartment buildings had been rarely studied in South Korea [5-8].

Recently, with the introduction of cross-laminated timber (CLT), several research were carried out whether wooden structured apartment housing satisfies the structural and environmental requirements under the Housing Act [9-13]. Regarding the acoustic performance, impact noise

1 yhkim@ysu.ac.kr 2 dgwon@gm.ysu.ac.kr 3 gioh@gm.ysu.ac.kr 4 kbs@gm.ysu.ac.kr 5 jwlee@gm.ysu.ac.kr 6 woodnol2@korea.kr

reduction performances of the CLT structures were investigated through experiments in units of components or field measurements as an early stage [14-15]. Apartment houses in Korea must be recognized in accordance with legal regulations as to whether they can sufficiently reduce floor impact sounds [16]. Although the applicable regulations require performance evaluation in the standard test building, only wall-type concrete structures are defined, but there is no regulation on lightweight wooden structures. Therefore, it is necessary to study the requirements for standard test building of wooden structures to examine floor impact noise performances.

In this study, a box-type simple building made of CLT panels was proposed as a test building. Then, sound fields characteristics of the building were investigated for heavy-weight impact sources. To satisfy the sound field regulations according to ISO 10140-5 [17], sound absorption and reflective materials were additionally installed as indoor finishing materials, and the variables of sound field changes were considered accordingly.

2. MATERIALS AND METHOD

2.1. Related Requirements on ISO 10140-5 ISO 10140-5 specifies the requirements for laboratory test facilities for impact sound insulation measurements in the chapter 5. Among them, items on sound diffusion and reverberation time are mainly related to sound fields condition of the receiving room. Large variations of the sound pressure level in the room should be suppressed to avoid dominating strong standing waves through installing diffusing elements. As for reverberation time, range of 1 s to 2 s above 100 Hz is recommended since it should not be excessively long or short. Figure 1 shows some examples of the test facilities in accordance with ISO 10140-5.

(a) (b) (c) Figure 1: Examples of interior view of receiving room for the test facilities in accordance with ISO

10140-5. (a) Building Research Institute (BRI), Japan; (b) General Building Research Corporation

of Japan (GBRC); (c) Korea Conformity Laboratories (KCL) in South Korea.

2.2. Box-type CLT Building As shown in Figure 2, a box-type test building was prepared using larch CLT panel with thickness of 150 mm. Inner room dimension was 4.5 m for length, 3.69 m for width, and 3.24 m for ceiling height. Room volume was about 53.8 m 3 . The whole test building was laid on anti-vibration steel structure. Floor in the receiving room was covered by thin rubber sheet.

(a) (b) Figure 2: Pictures of the tested box-type CLT buildings. (a) Exterior view; (b) Interior view.

2.3. Measurement Method and Equipment Measurement methods are specified in KS F 2810-1 for light-weight impact sounds and KS F 2810- 2 for heavy-weight impact sounds. As shown in Figure 3(a), a position of 0.75 m off from the source room’s corner was selected for impacting bang machine. In the receiving room, twenty-four positions were selected as grid type of 4 by 6 with spacing of 0.6 m as shown in Figure 3(b). In this study, bang machine was employed as a standard heavy-weight impact source as shown in Figure 4(a). Four microphones were employed at the same time with height of 1.2 m as shown in Figure 4(b). Frequency bands of 50 Hz to 200 Hz in 1/3 octave bands were considered for analysis of floor impact sound, whereas 63 Hz to 4,000 Hz in 1/1 octave bands were considered for analysis of reverberation time. Regarding reverberation time measurement, omni-directional loudspeaker was employed with swept-sine signal.

(a) (b) Figure 3: Diagram of the measurement positions. (a) Impact position in the source room; (b)

Receiver and loudspeaker positions in the receiving room.

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(a) (b) Figure 4: Pictures of the sound source and receivers. (a) Bang machine; (b) Four microphones.

2.4. Sound Absorbers and Reflectors To control sound fields characteristics of the receiving room, rectangle shaped modular sound absorbers and reflectors made of medium density fiber (MDF) boards with thickness of 9 mm were installed. Table 1 shows the estimated sound absorption coefficient values in 1/1 octave bands for the selected materials based on literature [18]. Size of each panel was 0.6 m by 0.6 m with basic backing air gap of 50 mm. Table 1: Estimated sound absorption coefficients of the selected sound absorbers and reflectors.

Materials 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz (A) Multi-perforated MDF of 9 mm thick

with backing air cavity 0.03 0.03 0.09 0.46 0.31 0.18 0.15

(B) Multi-perforated MDF of 9 mm thick with backing air cavity and PE foam of 25

0.26 0.26 0.63 1.00 0.79 0.36 0.27

mm thick and 24 kg/m 3 density

(C) Non-perforated MDF of 9 mm thick 0.15 0.15 0.11 0.10 0.07 0.06 0.06

A

Sound absorbers and reflectors were installed only on lateral wall surfaces as shown in Figure 5(a). In the experiments, effect of sound reflectors was firstly investigated with maximum number of 44 modules as shown in Figure 5(b). Then, effect of sound absorbers was investigated with combination of optimized number of sound reflectors. In addition, effects of presence of backing porous material, increased air cavity, and corner bass trap were investigated as shown in Figure 5(c). Table 2 shows the experimental configuration with combination of sound absorbers and reflectors.

(a)

Surface D Surface C

(b) (c) Figure 5: Installation plan of sound absorber and reflectors. (a) Wall diagram for module application; (b) Interior view with sound reflector modules; (c) Interior view with bass trap. Table 2: Experimental configuration with combination of sound absorbers and reflectors.

Number of sound reflectors

Number of sound

with air cavity of x mm

absorbers

Case

No. Descriptions

50 mm 80 mm 80 mm

60 mm

90 mm

Perforated

MDF Bass trap

w/ PE

w/ PE

w/ PE

0 Current condition without any treatments 0 0 0 0 0 0 0 1

4 0 0 0 0 0 0 2 10 0 0 0 0 0 0 3 14 0 0 0 0 0 0 4 18 0 0 0 0 0 0 5 22 0 0 0 0 0 0 6 28 0 0 0 0 0 0 7 32 0 0 0 0 0 0 8 36 0 0 0 0 0 0 9 44 0 0 0 0 0 0 10

Ca.

Number of sound reflectors with backing air

cavity of 50 mm

36 0 0 0 0 2 0 11 36 0 0 0 0 5 0 12 36 0 0 0 0 7 0 13 36 0 0 0 0 10 0 14

Number of sound absorbers using multi- perforated MDF based with sound reflectors

36 8 0 0 0 0 0 15 36 0 8 0 0 0 0 16 36 0 0 8 0 0 0 17 36 0 0 0 8 0 0 18 Effect of corner bass trap as low frequency

Effects of presence of backing porous material,

increased air cavity

36 0 0 0 8 0 2 19 36 0 0 0 8 0 4

sound absorber

3. RESULTS

3.1. Reverberation Time

Table 3 shows the measurement results of reverberation time (RT) with its standard deviation values. The current condition (Case 0) showed RT values over 2 s for frequency bands below 1 kHz. From the results of Cases 1 to 9, sound reflectors of non-perforated MDF boards with air cavity of 50 mm contributed to decrease low frequency RT as shown in Figure 6(a). However, there was a limit to control RT at 125 Hz only with sound reflectors with the fixed air cavity depth. From the results of Cases 10 to 13, application of sound absorbers with multi-perforated MDF boards could decrease RT for all frequency bands, but high frequency RT decreased significantly to less than 1 s as shown in Figure 6(b). Approach with backing porous materials and increased air cavity was helpful to have RT distribution between 1 s to 2 s as shown in the results of Cases 14 to 17. By additionally installing a base trap as shown in the results of Cases 18 to 19, it was possible to reduce RT only at the low frequency bands. In addition, Case 19 showed the lowest values of standard deviation of RT all over the frequency bands. Therefore, it can be judged that Case 19 is the optimized results. Figure 7 shows the spatial characteristics of the receiving room’s sound fields in terms of reverberation time at 63, 125, and 250 Hz in 1/1 octave frequency bands before and after installation of sound absorbers and reflectors (comparison between Case 0 and Case 19).

Table 3: Measurement results of averaged reverberation time with its standard deviation. Case

Averaged reverberation time [s] Standard deviation of reverberation time [s] 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 63 Hz 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz

No.

0 2.55 3.24 3.48 2.97 2.09 1.56 1.26 0.48 0.24 0.28 0.27 0.15 0.04 0.06

1 2.36 3.38 3.75 2.77 1.97 1.54 1.27 0.28 0.18 0.21 0.09 0.06 0.04 0.01

2 2.39 3.32 3.53 2.51 1.89 1.50 1.23 0.29 0.20 0.29 0.09 0.05 0.02 0.02

3 2.38 3.33 3.47 2.44 1.80 1.47 1.22 0.27 0.20 0.32 0.09 0.05 0.02 0.02

4 2.36 3.28 3.45 2.39 1.75 1.45 1.21 0.28 0.21 0.25 0.10 0.07 0.03 0.02

5 2.34 3.24 3.46 2.31 1.71 1.44 1.18 0.25 0.18 0.26 0.08 0.04 0.03 0.02

6 2.33 3.21 3.39 2.19 1.66 1.41 1.17 0.24 0.23 0.34 0.10 0.12 0.06 0.10

7 2.34 3.20 3.36 2.16 1.59 1.39 1.14 0.20 0.18 0.25 0.10 0.04 0.03 0.02

8 2.38 3.37 3.25 2.06 1.48 1.35 1.16 0.54 0.48 0.43 0.13 0.19 0.09 0.09

9 2.15 2.96 2.33 1.81 1.46 1.35 1.11 0.26 0.21 0.25 0.08 0.09 0.03 0.02

10 2.30 2.93 2.99 1.68 1.32 1.25 1.08 0.23 0.22 0.31 0.17 0.09 0.08 0.06

11 2.14 2.64 2.43 1.37 1.10 1.18 1.04 0.42 0.44 0.27 0.06 0.11 0.09 0.02

12 2.20 2.43 2.15 1.24 1.01 1.07 1.03 0.39 0.44 0.38 0.05 0.07 0.11 0.03

13 2.11 2.26 1.84 1.05 0.86 1.01 0.98 0.12 0.12 0.16 0.05 0.03 0.02 0.01

14 2.25 3.01 2.77 1.88 1.49 1.35 1.08 0.28 0.19 0.28 0.09 0.05 0.05 0.04

15 2.04 2.36 1.70 1.32 1.18 1.16 1.04 0.28 0.26 0.20 0.16 0.13 0.06 0.07

16 2.01 2.18 1.44 1.11 1.07 1.12 1.00 0.13 0.16 0.12 0.05 0.05 0.03 0.02

17 1.86 1.96 1.40 1.09 1.07 1.10 0.99 0.10 0.17 0.11 0.04 0.04 0.03 0.02

18 1.77 1.85 1.41 1.09 1.06 1.10 0.99 0.08 0.16 0.18 0.06 0.03 0.02 0.01

19 1.68 1.57 1.17 1.09 1.05 1.09 0.97 0.04 0.16 0.08 0.07 0.03 0.03 0.01

(a) (b) Figure 6: RT by frequency bands as a function of sound absorbers and reflectors. (a) Effect of

sound reflectors; (b) Effects of sound absorbers.

(a) (b) (c) (d)

er cam

(i)

(e) (f) (g) (h) Figure 7: Spatial contour map of the measured reverberation time (RT) for low frequency bands before and after installation of sound absorbers and reflectors. (a) Interior view of Case 0; (b) 63 Hz

RT of Case 0; (c) 125 Hz RT of Case 0; (d) 250 Hz RT of Case 0; (e) Interior view of Case 19; (f) 63 Hz RT of Case 19; (g) 125 Hz RT of Case 19; (h) 250 Hz RT of Case 19; (i) Color indicator of

RT scale.

3.2. Maximum Sound Pressure Levels by Bang Machine

Table 4 shows the measurement results of maximum sound pressure levels (SPL) by impacting bang machine with its standard deviation values. From the result of Case 0, SPL at 50 Hz and 63 Hz was very dominant as around 100 dB. Unlike the results of RT, absolute values of SPL were not much varied by sound absorbers and reflectors. Case 19, which was judged by the optimized result in terms of RT distribution, showed slightly lower SPL values than Case 0 due to suppressed room modes. Figure 8 shows the spatial characteristics of the receiving room’s sound fields in terms of maximum sound pressure level by bang machine at 50, 63, and 80 Hz in 1/3 octave frequency bands before and after installation of sound absorbers and reflectors (comparison between Case 0 and Case 19).

Table 4: Measurement results of sound pressure levels by bang machine with its standard deviation. Case

Maximum sound pressure level [dB] Standard deviation of sound pressure level [dB] 50 Hz 63 Hz 80 Hz 100 Hz 125 Hz 160 Hz 200 Hz 50 Hz 63 Hz 80 Hz 100 Hz 125 Hz 160 Hz 200 Hz

No.

0 99.2 103.9 83.8 89.0 84.1 80.9 80.8 3.3 5.7 2.6 3.3 2.6 2.3 1.9

1 99.7 102.4 82.5 88.4 80.8 79.0 79.2 4.2 7.4 2.8 3.2 2.5 1.9 2.0

2 97.6 100.9 81.5 89.2 81.1 80.0 79.9 3.9 5.4 3.5 3.6 2.4 2.1 2.0

3 99.3 103.2 80.7 90.2 82.5 80.0 80.1 3.8 5.3 3.0 3.4 2.6 2.1 1.9

4 96.7 102.3 80.1 86.9 79.9 76.0 80.6 4.1 6.2 3.3 3.3 2.6 1.6 2.5

5 99.0 102.6 82.3 88.6 81.9 80.1 79.9 4.3 6.7 3.6 3.6 2.8 2.1 2.0

6 98.4 103.0 81.1 88.8 80.8 80.5 80.1 3.1 5.3 2.8 3.5 2.6 1.8 1.8

7 98.5 100.7 82.2 89.5 81.3 77.3 82.1 3.5 5.5 2.1 3.7 2.9 1.6 2.3

8 98.8 103.0 87.5 89.4 80.8 79.8 80.7 3.5 5.0 3.3 3.0 2.0 1.9 2.1

9 99.7 103.0 83.3 90.0 82.9 79.6 77.6 3.6 5.3 3.7 3.8 2.8 2.5 1.6

10 98.2 101.5 85.8 88.0 80.4 78.0 78.6 5.4 8.0 4.0 3.8 2.3 2.6 1.9

11 98.5 101.6 82.8 89.1 81.0 77.4 78.8 3.9 6.7 3.7 3.5 3.1 1.6 2.2

12 99.5 102.2 86.2 89.0 79.9 81.2 78.2 3.3 5.0 3.0 3.4 2.7 2.5 1.9

13 98.1 101.1 82.7 89.5 81.4 80.9 78.7 3.1 5.0 2.7 3.2 2.1 2.5 1.6

14 98.4 103.1 80.4 88.5 82.1 78.6 77.1 4.1 7.0 2.4 3.4 2.8 2.0 1.6

15 98.5 101.8 85.9 87.8 82.5 78.5 77.3 3.7 5.8 3.8 4.0 2.4 2.3 1.8

16 98.5 103.1 82.6 87.8 80.8 78.2 76.6 4.2 7.0 2.4 3.5 2.9 2.1 1.6

17 98.8 102.5 82.9 88.8 79.3 78.2 76.6 4.3 6.1 2.6 3.7 2.4 2.2 1.6

18 99.4 103.8 85.1 89.5 82.3 81.4 78.3 3.3 5.4 2.7 2.9 2.4 2.2 1.3

19 98.5 103.1 82.7 88.4 81.0 78.1 77.2 3.3 5.3 2.3 2.9 2.1 1.9 1.5

(a) (b) (c) (d)

(i)

(e) (f) (g) (h) Figure 8: Spatial contour map of the measured maximum sound pressure levels (SPL) by bang machine for low frequency bands before and after installation of sound absorbers and reflectors. (a)

Interior view of Case 0; (b) 50 Hz SPL of Case 0; (c) 63 Hz SPL of Case 0; (d) 80 Hz SPL of Case 0; (e) Interior view of Case 19; (f) 50 Hz SPL of Case 19; (g) 63 Hz SPL of Case 19; (h) 80 Hz SPL

of Case 19; (i) Color indicator of SPL scale.

4. CONCLUSIONS

In this study, sound field characteristics of the box-type test building made of CLT panel were investigated in terms of reverberation time and sound pressure level by bang machine. Based on ISO 10140-5, sound field requirements were reviewed. The empty receiving room without any sound absorbers or reflectors showed longer reverberation time more than 2 s except for 2 kHz and 4 kHz. Sound absorbers and reflectors were designed based on non-perforated and multi-perforated MDF boards with and without porous backing materials. Simplified bass trap was additionally applied to the test building for low frequency control. As results, it was derived at the case of 44 reflectors with additional 4 bass traps as the optimum sound fields condition. By controlling sound fields using sound absorbers and reflectors, smooth frequency characteristics and even spatial distribution of reverberation time could be achieved. However, maximum impact sound pressure level by bang machine was slightly decreased by the optimized sound absorbers and reflectors. As a further study, it is needed to develop effective CLT structures and materials with good acoustic performance using the box-type test building. 5. REFERENCES

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