Abstract
This study aimed to investigate the effects of an expanded polystyrene (EPS) layer and varying precast concrete thickness on the stiffness and acoustic performance of cross-laminated timber (CLT) slabs. Six different concrete thicknesses and EPS layers were applied to larch and pine CLT slabs for testing. Airborne sound transmission loss (Dw) was measured using speakers, light impact sound (LnT,w) using a tapping machine, and heavy impact sound (LiA,Fmax) using rubber balls, in accordance with KS F ISO717-2. The results indicated that the EPS layer significantly improved light impact sound insulation (by 8 dB) and airborne noise insulation (by 5 dB), but had a minimal effect on heavy impact sound (0.5 dB). Both stiffness and sound insulation increased with concrete thickness, although improvement plateaued beyond 100 mm for larch CLT and 150 mm for pine CLT. The flexural and impact stiffness of larch CLT slabs were 24.3% and 19.2% higher than those of pine CLT slabs, respectively. Moreover, impact stiffness demonstrated a stronger correlation with acoustic performance than the previously established relationship with area density.
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Effect of Concrete Thickness and Expanded Polystyrene Layer on Stiffness and Floor Impact Sound Insulation Performance of Cross-Laminated Timber Slabs
Sung-Jun Pang 
,a Hyo-Jin Lee,b Yeon-Su Ha ,c Chul-Ki Kim ,d Ji-Ho Chang ,e
Ho-Jeong Cho ,a and Sang-Joon Lee d,*
Effects of an expanded polystyrene (EPS) layer and varying precast concrete thickness were investigated relative to the stiffness and acoustic performance of cross-laminated timber (CLT) slabs. Six different concrete thicknesses and EPS layers were applied to larch and pine CLT slabs for testing. Airborne sound transmission loss (Dw) was measured using speakers, light impact sound (LnT,w) using a tapping machine, and heavy impact sound (LiA,Fmax) using rubber balls, in accordance with KS F ISO717-2. The results indicated that the EPS layer significantly improved light impact sound insulation (by 8 dB) and airborne noise insulation (by 5 dB), but had a minimal effect on heavy impact sound (0.5 dB). Both stiffness and sound insulation increased with concrete thickness, although improvement plateaued beyond 100 mm for larch CLT and 150 mm for pine CLT. The flexural and impact stiffness of larch CLT slabs were 24.3% and 19.2% higher than those of pine CLT slabs, respectively. Moreover, impact stiffness had a stronger correlation with acoustic performance than the previously established relationship with area density.
DOI: 10.15376/biores.21.1.2101-2122
Keywords: Cross-laminated timber; Expanded polystyrene; Floor impact sound; Airborne sound; Stiffness
Contact information: a: Department of Wood Science and Engineering, Chonnam National University, Gwangju, Republic of Korea; b: Fire Insurers Laboratories of Korea, Yeoju-si, Republic of Korea; c: Korea Institute of Civil Engineering and Building Technology, Goyang-si, Republic of Korea; d: National Institute of Forest Science, Seoul, Republic of Korea; e: VizWave Inc., Seongnam-si, Gyeonggi-do, Republic of Korea; *Corresponding author: lsjoon@korea.kr
INTRODUCTION
Timber, a renewable construction material, can store carbon, and the Intergovernmental Panel on Climate Change (IPCC) recognizes it as an effective carbon sink (Arehart et al. 2021; Bjånesøy et al. 2023). According to the IPCC guidelines (Penman et al. 2006), harvested wood products produced and used within a country can play a crucial role in delaying carbon emissions. These products can store carbon for extended periods, particularly when used in construction, where they may retain carbon for over 80 years (Lippke et al. 2010). Moreover, even after their service life, wood products can continue to offset emissions through recycling and energy recovery (Pajchrowski et al. 2014; Berger et al. 2020; Kromoser et al. 2022). Consequently, using timber in construction not only reduces carbon emissions but also contributes to global climate change mitigation.
Cross-laminated timber (CLT) has emerged as a leading mass timber product in the construction industry, offering significant advantages, including improved structural stability and efficient prefabrication (Harte 2017; Loss et al. 2016a,b). With active research and industrial application in Europe and North America, CLT has been widely used in high-rise residential buildings. In Korea, CLT manufacturing methods were standardized in 2021 (Eom et al. 2019; KS F 2081 2021; Lee et al. 2023), raising expectations for broader use in the construction sector and creating opportunities for innovation and sustainable building practices that support carbon neutrality and efficient forest resource utilization (Ha et al. 2023).
One of the major challenges in high-rise apartments is inter-floor noise, which is particularly problematic in densely populated urban areas (Gibson et al. 2022; Kang et al. 2023). South Korea enforces some of the strictest inter-floor noise regulations globally, requiring impact sound levels in apartment buildings with over 30 units to remain below 49 dB (Ministry of Land, Infrastructure and Transport 2022). For timber residential buildings to gain wider acceptance in South Korea, it is critical to address not only structural and fire safety but also effective sound insulation. CLT, which can be produced in large panels, is expected to provide excellent airtightness and sound insulation. However, research into the vibration and noise characteristics of CLT slabs is necessary to support the construction of high-quality and noise-efficient timber residential buildings.
Noise transmission in buildings—commonly caused by activities such as footsteps, moving furniture, and other vibrations—remains a major challenge in modern construction. This type of noise, known as impact sound, easily propagates through the structural elements. Recent research has highlighted that the sound insulation performance of CLT depends on factors such as wood species and flooring materials. Hirota et al. (2020) investigated the sound insulation properties of CLT slabs made from Japanese larch and fir, reporting that larch performed approximately 4 dB better insulation against rubber ball impacts compared to fir. Additionally, the research demonstrated that the installation of a floating floor over CLT slabs improved sound insulation by 7 to 13 dB.
Similarly, Zhao et al. (2021) reported that the impact sound insulation performance improved with increasing concrete topping thickness, with significant improvement observed at 100 mm. Zeitler et al. (2014) found that combining rubber cushioning with a concrete topping reduced light impact sound by approximately 21 dB, while Hiramitsu and Hirakawa (2022) observed that concrete toppings were particularly effective against heavy impact sound. Furthermore, finishing materials placed on top of concrete significantly enhanced light impact sound insulation but had a minimal effect on heavy impact sound. These findings underscore the potential for optimizing CLT-based floor systems by selecting appropriate material combinations and configurations.
The structural performance of Korean CLT manufactured from local wood species has been evaluated (Pang and Jeong 2018, 2019; Pang et al. 2021), along with its shrinkage and expansion characteristics regarding changes in moisture content (Pang and Jeong 2020). Other studies have explored hybrid CLT using plywood (Choi et al. 2015, 2018; Pang et al. 2019) and composite slabs that combine CLT and concrete (Quang Mai et al. 2018; Pang et al. 2022), with an emphasis on structural safety. Recently, Pang et al. (2024) investigated the correlation between the vibration and noise performance of CLT slabs, attempting to estimate noise performance based on the vibration characteristics.
Overall, the sound insulation performance of CLT slabs is influenced by the concrete and cushioning materials placed over the CLT panels, which affect the stiffness of the CLT floor. To facilitate the broader adoption of CLT in residential buildings, it is essential to gain a deeper understanding of its sound insulation performance through experimental testing. Therefore, this study investigated the effects of adding precast concrete (PC) and resilient material layers to CLT slabs, particularly focusing on their impact on both sound insulation performance and stiffness.
EXPERIMENTAL
Specimens
To examine the relationship between CLT floor stiffness and improvements in floor impact sound insulation performance, 18 test specimens were prepared using two wood species, six different thicknesses of PC, and an expanded polystyrene (EPS) resilient layer (Won-Hak Lee and Chan-Hoon Haan 2021) (Table 1). The specimen identification (ID) format consisted of three parts: the CLT wood species (first term), the thickness of the PC layer (second term), and the presence or absence of EPS (third term).
Figure 1 depicts the composition of the test specimen, consisting of a CLT panel at the bottom, an EPS layer in the middle, and a PC layer on top. The CLT panels were manufactured from either larch (Larix kaempferi) or pine (Pinus densiflora) laminae. Each lamina measured 30 mm in thickness, 130 mm in width, and 4,200 mm in length. The laminae were graded according to the KS F 3020 standard using a machine grader (MGFE-251, IIDA Kogyo, Komaki, Japan).
Five layers of laminae were laminated to produce CLT grades C-E10-E8 (larch CLT) and C-E8-E6 (pine CLT), in accordance with the KS F 2081 standard. In this nomenclature, the first letter represents CLT, the second denotes the grade of the outer layers, and the third denotes the grade of the inner layer. Both outer layers used laminae of the same grade, and the three inner layers also used laminae of a uniform grade.
Table 1. Materials and Combinations of Test Specimens
Fig. 1. Composition of materials for test specimens
A phenol–resorcinol–formaldehyde adhesive was used for bonding. It was applied at a level of 200 g/m2 per layer. The panels were pressured at 1 MPa for 20 h, followed by one week of curing. The mean moisture content of the CLT was 12% ± 2%. The specific gravity of the panels was 0.587 for larch CLT and 0.476 for pine CLT. The overall dimensions of the CLT panel were 150 mm in thickness, 1,000 mm in width, and 4,200 mm in length. For testing, the CLT panels were connected using spline joints to form assemblies measuring 3000 × 4200 mm (Fig. 2). The spline joints were reinforced with self-tapping screws (Ø 6 × 80 mm, HBS model, Rothoblass).
Fig. 2. Joint types used to connect the CLT panels (Pang et al. 2024)
Figure 3 depicts the EPS and PC plates used. Flat-type EPS, commonly used in floating floor structures at construction sites, was used as a resilient material, with a thickness of 30 mm. Six types of PC plate were prepared with thicknesses of 50, 75, 100, 125, 150, and 210 mm. Each plate measured 3,000 × 4,200 mm.
Methods
Figure 4 presents the schematic layout of the experimental laboratory. The test slab was installed between an upper reverberation chamber and a lower sound-receiving chamber. In this study, three types of sound insulation performance were evaluated: airborne sound transmission loss (Dw), heavy-weight floor impact sound (LiA,Fmax), and light-weight floor impact sound (LnT,w). Airborne sound insulation was measured according to KS F ISO 10140-3 2021. Wideband random noise was generated by an omnidirectional loudspeaker (B&K, type 4292L) installed in the upper reverberation chamber and recorded by five omnidirectional microphones (B&K, type 4189) located in the lower sound-receiving chamber. The recorded signals were analyzed using a multichannel analyzer (B&K, type 3099) to calculate 1/3-octave band sound pressure levels from 50 to 5,000 Hz. From these, the 1/3 octave band sound transmission loss and the single-number rating (Dw) were determined according to KS F 2862 2002. A higher Dw value indicates better airborne sound insulation.
Heavy- and light-weight floor impact sounds were measured following KS F ISO 10140-3 2021 and evaluated as single-number ratings according to KS F ISO 717-2 2020. The microphone-recorded sound pressure levels were divided into 1/3-octave bands within the frequency range of 50 to 630 Hz. Lower LiA,Fmax and LnT,w values represent better sound insulation performance.
a.) Expanded polystyrene
b.) Precast concrete
Fig. 3. Materials used to improve the sound insulation performance of CLT slabs
All tests were conducted at the Fire Insurers Laboratories of Korea, an internationally certified testing agency specializing in floor impact sound insulation. Each configuration was tested with a single specimen. To ensure reliability despite the one-specimen design, instrument repeatability was verified, and measurements were taken at multiple positions on the slab (five drop positions for heavy impact and multiple microphone positions for airborne sound). This approach provided consistent results across repetitions, supporting the robustness of the dataset.
Fig. 4. Schematic diagram of the laboratory for the experiment (Hyo-Jin Lee et al. 2023)
Figure 5 illustrates the CLT specimen setup and designated impact source positions (red circle). Heavy-weight floor impact sounds were generated using a standard rubber ball (Rion, YI-01) dropped from a height of 1 m at five points: the center of the slab and four corners positioned 750 mm from the borders of the slab. Each drop was repeated nine times at each specified position. Light-weight floor impact sounds were generated using a tapping machine that simulates hard, repetitive impacts, such as heel tapping. The tapping machine consisted of five 500 g hammers arranged in a line, sequentially dropping at a rate of 10 impacts per second.
Fig. 5. Installation of CLT specimen and locations for impact source hitting (Pang et al. 2024)
Fig. 6. Installation of accelerometers (left) and microphones (right) (Pang et al. 2024)
Figure 6 illustrates the placement of four accelerometers and a microphone beneath the CLT slab. To analyze deflection and the fundamental natural frequency during impact testing, four accelerometers (B&K, type 4527) were mounted at 400 mm intervals under the slab (blue stars in Fig. 4). For sound insulation measurements, five microphones (B&K, type 4189) were installed at a height of 1.2 m within the receiving chamber.
In actual buildings, CLT slabs are commonly supported by beams or by walls along one or more edges. However, the objective of the present experiment was to isolate and evaluate the effects of the concrete and EPS layers on the CLT slab itself. For this reason, as shown in Figs. 5 and 6, the test slabs were installed without any supporting beams or additional vertical supports beneath them. Consequently, in the experimental setup, the CLT slab carried the entire load except for the edges supported by the testing structure.
The slab exhibited multiple natural frequencies corresponding to vibration modes, including bending and torsion. In building design, the first vibration mode is most critical, as low-frequency noise is difficult to isolate, while high-frequency noise can be reduced by floating floor systems or sound-absorbing materials. Accordingly, this study focused on the first vibration mode (largest amplitude), defined as the fundamental natural frequency. The fundamental frequency of each CLT slab was derived by converting the time-domain displacement response into the frequency domain using the Fast Fourier Transform (Xie et al. 2020; He et al. 2023; Pang et al. 2024).
Stiffness of Test Specimens
Three types of stiffness related to slab behavior were evaluated: bending stiffness (EI), dynamic stiffness, and impact stiffness. Bending stiffness represents the resistance of a structural member to deformation under bending. It depends on the material’s elastic modulus (E) and its geometric properties, particularly the moment of inertia (I). The EI was calculated using Eq. 1, following Eurocode 5(BS EN 1995-1-1 2004; Wallner-Novak et al. 2014),
(1)
where represents the effective bending stiffness of the test slab (N·mm2), denotes the modulus of elasticity of ith layer (MPa), indicates the moment of inertia of the ith layer (mm4), is the connection efficiency factor of the ith layer (0 < γ ≤ 1), represents the cross-sectional area of the ith layer (mm2), and denotes the distance between the centroid of the ith layer and the neutral axis of the composite section (mm).
Dynamic stiffness is associated with the slab vibration and indicates its behavior at specific frequencies. It was determined using the mass of the slab and the vibration frequency measured by accelerometers. Assuming simple harmonic motion and approximating the slab system as a single degree-of-freedom vibration system, Eq. 2 describes the relationship among vibration frequency, stiffness, and mass. From this, Eq. 3 was derived as follows,
(2)
(3)
where f represents the fundamental natural frequency obtained from the acceleration response spectrum (Hz), kdynamic denotes the dynamic stiffness of the CLT slab (N/m), and indicates the mass of the CLT slab (kg).
The impact stiffness is associated with the slab deflection under out-of-plane loads. It was calculated as the ratio of the applied loads to the maximum deflection, as described in Eq. 4. The applied loads included the impact force from the test ball, the self-weight of the slab, and the weight of the experimenter. The impact force was generated using a hollow silicone rubber ball (185 mm diameter, 30 mm thickness, 2.5 ± 0.2 kg) dropped from a height of 1 m, which produced an impact load of approximately 1,500 N (KS F ISO 10140-3 2021; Yazbec et al. 2022; Ha et al. 2023; Lee et al. 2023).
(4)
where kimpact represents the impact stiffness of the CLT slab (N/m), Wimpact denotes the impact load from the rubber ball (1,500 N), WCLT indicates the self-weight of the CLT panel (N), Wexperimenter represents the weight of the experimenter (N), and 
denotes the maximum slab deflection (mm). Although this quantity is based on static assumptions while the impact load is dynamic, it remains a meaningful indicator in structural analysis, as it is intuitive and indicates a strong correlation with noise levels.
Since it is difficult to measure slab deflection experimentally at all points, the maximum deflection was simulated using the finite element method (FEM) with Midas NFX software (Midas I.T. 2024). The input parameters (material properties) are presented in Table 2. Lamina properties were assumed to be orthotropic.
Table 2. Material Properties for Finite Element Analysis
For each grade, elasticity was set to the median value defined in KS F 3020 (Table 2). Transverse elasticity was defined as 1/30 of the longitudinal elasticity, according to the CLT handbook(Innovations 2014). The shear modulus in the longitudinal direction was assumed to be 1/16 of longitudinal elasticity, and the shear modulus in the transverse direction was 1/10 of the longitudinal shear modulus.
Poisson’s ratios were based on the Wood Handbook (Ross 2021). In this notation, the first subscript denotes the stress direction and the second represents the lateral deformation direction. For stress applied longitudinally, radial deformation is minimal; therefore, no specific value for is provided. Following a previous study, was assumed to be 0.01 in this work (Pang et al. 2024).
RESULTS AND DISCUSSION
Effects of the Concrete Layer
Table 3 presents the area density and experimental results (fundamental natural frequency, stiffness, and acoustic performance) of the specimen slabs. The area density was calculated as slab weight divided by area, while the fundamental natural frequency was measured using an accelerometer. The bending stiffness (Eq. 2) was derived from the cross-sectional properties and elastic modulus, dynamic stiffness (Eq. 4) from the fundamental natural frequency and the mass of the slab, and impact stiffness (Eq. 5) from the applied load and measured deflection. Figure 7 depicts the predicted slab deflection under load, with the maximum deflection used to determine impact stiffness.
a.) Larch CLT-based test slabs
b) Pine CLT-based test slabs
Fig. 7. Deflection of test slabs
Table 3. Experimental Results
The relationship between the PC thickness and the CLT slab stiffness is depicted in Fig. 8, indicating that all types of stiffness increased with greater PC thickness. Bending stiffness, determined from the cross-sectional properties, increased gradually with added concrete (Fig. 8a).
a) Larch CLT-based test slabs
b) Pine CLT-based test slabs
Fig. 8. Effect of precast concrete on the stiffness of test slabs
Conversely, the dynamic stiffness increased with PC thickness but plateaued after 100 mm. This trend was also consistent across both larch and pine CLT specimens (Fig. 8b). Bending and impact stiffness primarily reflect cross-sectional geometry and flexural modulus, while dynamic stiffness reflects vibration frequency. Because the vibrator was attached to the bottom of the CLT layer, the additional mass and stiffness from the concrete had less influence on CLT frequency. For larch CLT, the effect of added mass on vibration frequency diminished at PC thicknesses above 100 mm, and for pine CLT, above 150 mm. This suggests there is a threshold concrete thickness beyond which further increases have a limited influence on vibration behavior.
Deflection values were greater when EPS was present at the same concrete thickness. This can be attributed to the relatively low stiffness of EPS and variations in the load transfer mechanisms. As a soft material, EPS caused the load from the upper concrete layer to be transferred unevenly, concentrating stress at the center of the CLT. Without EPS, the concrete was bonded directly to the CLT, contributing to flexural resistance. With the implementation of EPS, the indirect connection altered stress transfer, thereby reducing bending stiffness.
Line figure showing acoustic performance versus thickness of precast concrete
a) Larch CLT-based test slabs
b) Pine CLT-based test slabs
Fig. 9. Effect of precast concrete on the acoustic performance of test slabs
Figure 9 illustrates the relationship between the thickness of PC and acoustic performance. The presence of a concrete layer significantly improved the acoustic performance compared with specimens lacking a layer. Further increases in concrete thickness produced gradual improvements. The sound insulation index (Dw) increased with the thickness of the PC, indicating that thicker concrete enhanced sound insulation (Fig. 9a). In contrast, heavy-weight (LiA,Fmax) and light-weight (LnT,w) impact noise levels decreased as the PC thickness increased. Similar trends were observed in specimens made from pine CLT (Fig. 9b).
The plateau in performance, even with increasing concrete thickness, may have been influenced by flanking transmission. Previous studies on CLT and timber–concrete floors have also shown that once a slab reaches a certain mass–stiffness level, the acoustic benefit of additional topping mass becomes limited, and further improvements may be masked by non-direct transmission paths such as flanking (Bao et al. 2025; Martins et al. 2015; Schluessel et al. 2014).
Although the experiment was conducted in an internationally certified testing facility using a standardized concrete base, inherent limitations were identified in isolating the transmission behavior of timber slabs, particularly regarding connections and wall-related flanking paths. These findings highlight the need for a testing environment specifically designed for timber slabs to minimize such influences and improve the reliability of future measurements.
Effects of EPS Layer and CLT Species
The stiffness and acoustic performance of slabs with four different PC thicknesses (L-PC75, P-PC150, L-PC150, and L-PC-210) were analyzed with and without the EPS layer. Figure 10(a) depicts the difference in impact stiffness, dynamic stiffness, and bending stiffness depending on the presence of the EPS layer.
As the concrete thickness increased, the overall stiffness of the slab also increased. The slabs with EPS exhibited greater impact stiffness and dynamic stiffness compared to those without EPS, and the difference became more pronounced with thicker concrete layers. The increase in impact stiffness can be attributed to the reduced elasticity of EPS, which altered the way impact loads were transferred to the CLT layer. When EPS was used, loads from the concrete were more centralized rather than evenly distributed, resulting in greater deflection at the center of the CLT slab.
Dynamic stiffness also decreased with the addition of the EPS layer. Acting as a buffer, the EPS layer absorbed part of the vibration energy, lowering the natural frequency (Hz) of the CLT layer. Consequently, despite the added mass of the EPS layer, the slabs with EPS indicated lower dynamic stiffness than those without EPS.
The bending stiffness, which depends on the elastic modulus and cross-sectional area of the material, exhibited only a small difference between the slabs with and without the EPS layer. This is because EPS has an extremely low elastic modulus—approximately 0.0008% of that of structural materials such as concrete and timber (larch and pine)—and its thickness (30 mm) was small compared with that of the CLT and concrete layers.
Figure 10(b) visualizes the acoustic performance of slabs with and without EPS layers. When the EPS layer was installed, LiA,Fmax improved by approximately 5 dB. Meanwhile, increasing concrete thickness by 135 mm (from L-PC75 to L-PC-210) improved LiA,Fmax by approximately 2 dB. Therefore, installing a 30 mm EPS layer was more effective in blocking airborne noise than increasing the concrete thickness.
For impact noise, the light-weight impact noise was reduced by approximately 8 dB with EPS, while the heavy-weight impact noise was reduced by approximately 0.5 dB. This indicates that the EPS layer was highly effective in reducing light-weight impact noise but has minimal impact on heavy-weight impact noise. Overall, the EPS layer was found to be particularly effective in reducing airborne and light-weight impact noise, but less effective against heavy impact noise.
a) Stiffness of test slabs
b) Acoustic performance of test slabs
Fig. 10. Effect of EPS layer on the stiffness and acoustic performance of test slabs
Figure 11 compares the stiffness and acoustic performance of slabs made of larch and pine, under the same cross-sectional areas of CLT, concrete, and EPS layers. When larch CLT was used, the bending stiffness was consistently approximately 24.3% higher and the impact stiffness about 19.2% higher than with pine CLT (Figure 11a). This can be explained by the higher density (approximately 23.4%) and an elastic modulus (approximately 23.5%) of larch compared with pine. In contrast, dynamic stiffness indicated no clear difference between larch and pine CLT, as it reflects overall mass and vibration characteristics. This suggests that the high mass of concrete and the vibration absorption properties of EPS exerted a greater effect on dynamic stiffness than the CLT species.
a) Stiffness of test slabs
b) Acoustic performance of test slabs
Fig. 11. Effect of CLT species on the stiffness and acoustic performance of test slabs
Figure 11(b) depicts the acoustic performance comparison between larch and pine slabs. The Dw exhibited little difference with CLT species or increasing concrete thickness, but it improved substantially with EPS. Similarly, LiA,Fmax and LnT,w varied little with CLT species but decreased significantly with EPS, and continued to decrease gradually with thicker concrete. Therefore, these results indicate that the effects of CLT species on acoustic performance were minimal compared with the dominant influences of concrete and EPS.
Relationship between Stiffness and Acoustic Performance
Figure 12 depicts the relationship between stiffness and acoustic performance of test slabs with the EPS layer.
- Bending stiffness on acoustic performance
- Dynamic stiffness on acoustic performance
c) Impact stiffness on acoustic performance
Fig. 12. Relationship between stiffness and acoustic performance
The bending stiffness was influenced more by the elasticity of CLT than by the concrete or EPS layers. Consequently, when larch CLT was used, the bending stiffness was 24.3% higher than when pine CLT was used. The bending stiffness of specimens with larch CLT was concentrated around 6700 kN·m², while those with pine CLT were concentrated around 5400 kN·m² (Fig. 12a). Because of this large difference between CLT species, a non-significant correlation was found between bending stiffness and acoustic performance.
Figure 12(b) indicates the relationship between dynamic stiffness and acoustic performance of test slabs. The dynamic stiffness was distributed over a wide range, indicating a different pattern from the relationship between bending stiffness and acoustic performance. Since dynamic stiffness was affected by the mass and frequency of the slab, increases in CLT density and concrete thickness improved dynamic stiffness. Consequently, both LiA,Fmax and LnT,w exhibited negative correlations with dynamic stiffness, with R² values of 0.7365 and 0.816, respectively. In contrast, Dw exhibited a weaker positive relationship with dynamic stiffness (R² = 0.5997). These findings indicate that dynamic stiffness was more closely related to impact noise than to airborne noise.
Figure 12(c) depicts the relationship between impact stiffness and acoustic performance. The trend was similar to that of dynamic stiffness, but impact stiffness indicated an even higher correlation (R² value) with acoustic performance. Impact load depends on both the applied load and the maximum deflection of the slab, where the mass of all materials contributed to the load, and the elastic modulus affected deflection. This explains why the physical and mechanical properties of the CLT species were more strongly reflected in impact stiffness than in dynamic stiffness.
Previous researchers have used area density as a predictor of acoustic performance. Figure 13 depicts the correlation between area density and acoustic performance of test slabs. While area density exhibited a strong correlation, the results above demonstrate that impact stiffness has an even stronger relationship with acoustic performance.
Fig. 13. Relationship between area density and acoustic performance
Overall, this study contributes to improving the acoustic performance of CLT structures in timber buildings. The findings can guide the design and material selection of CLT structures aimed at reducing floor impact sound in timber construction.
CONCLUSIONS
- The expanded polystyrene (EPS) layer significantly enhanced light impact noise (by 8 dB) and airborne sound insulation (by 5 dB), but it had only a minor effect on heavy impact noise (0.5 dB). Notably, the EPS layer increased Dw by approximately 5 dB, which is more effective than increasing the PC thickness by 135 mm (from L-PC75 to L-PC210), which improved Dw by approximately 2 dB.
- Increasing the thickness of the precast concrete (PC) layer improved the stiffness and sound insulation performance of cross laminated timber (CLT) slabs. However, improvements in dynamic stiffness slowed once the concrete thickness exceeded 100 mm for larch CLT and 150 mm for pine CLT.
- The bending stiffness of larch CLT slabs was consistently about 24.3% higher than that of pine CLT slabs (concentrated around 6700 kN·m² for larch and around 5400 kN·m² for pine CLT). Similarly, the impact stiffness of larch CLT slabs was about 19.2% higher than that of pine CLT slabs, due to the higher density and elastic modulus of larch.
- The impact stiffness indicated a stronger correlation with acoustic performance than area density, which has traditionally been used as a predictor. This suggests that impact stiffness is a more effective indicator for predicting the acoustic performance of slabs.
ACKNOWLEDGEMENTS
This research was supported by a Research Project (FP0200-2022-01-2025) through the National Institute of Forest Science (NIFoS), Korea.
Competing Interests
The authors declare they have no competing interest.
Authors’ Contributions
Sung-Jun Pang analyzed the test results and drafted the manuscript. Hyo-Jin Lee and Yeon-Su Ha designed the experimental test procedures and analyzed the acoustic performance. Chul-Ki Kim provided the mechanical properties of CLT. Ji-Ho Chang and Ho-Jeong Cho revised the manuscript, including figures and tables. Sang-Joon Lee managed the research project and approved the final manuscript.
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Article submitted: October 15, 2025; Peer review completed: November 22, 2025; Revised version received: November 25, 2025; Accepted: November 30, 2025; Published: January 20, 2026,
DOI: 10.15376/biores.21.1.2101-2122