3.2.1. Sound Absorption
On the uncoated particle board, the sound absorption coefficients of particle boards with varying particle sizes, densities, and coating thicknesses were determined. For both 0.4 g/cm
3 and 0.6 g/cm
3 coarse particle board, sound absorption at frequencies over 2000 Hz was more superior to that of fine particle board of the same density (
Figure 9). The effect of panel type (fine or coarse particle board) on absorption at 500–1000 Hz was demonstrated by the fact that the fine particle board performed better at the same density. At frequencies over 1600 Hz, the lower densities had superior sound absorption versus the higher densities and were unaffected by panel type (fine or coarse particle board). At low frequencies up to 500 Hz, values reached approximately 0.1 for all boards and continued to increase to approximately 0.4 at 2000 Hz, with a peak of approximately 0.9 obtained by an uncoated coarse particle board with a density of 0.4 g/cm
3 at frequency 3150 Hz, followed by a fine particle size at the same density for approximately 0.7 (f = 2500 Hz). While at a density of 0.6 g/cm
3, a maximum value of around 0.5 is achieved at a frequency of 2000 Hz. This is consistent with the findings of Iswanto et al. [
27], Wong et al. [
42], Khair et al. [
43], and Karlinasari et al. [
12]. Thus, bamboo boards with a density of 0.4 g/cm
3 are found to be effective sound absorbers, with sound absorption greater than half (>0.5).
The effect of coating investigated in this study was on particle board with an acoustic absorption value more than 0.3 at frequencies greater than 1600 Hz. The study differed from prior work on acoustic qualities conducted by Xu et al. [
29] and Ivanova et al. [
30], which employed solid wood with a sound absorption coefficient of less than 0.3. Coatings can improve the performance and the look of products while also extending their service life and providing qualities specific to their intended application [
21]. Both Xu et al.’s [
29] and Ivanova et al.’s [
30] research concluded that the addition of a polyurethane layer enhances absorption performance, particularly at high frequencies. However, it was shown that the improvement in performance could not surpass 0.3 in terms of sound absorption. Our findings indicate that the addition of a coating significantly lowered high-frequency sound absorption performance (
Figure 10). This is because the coating has the ability to seal the pores that absorb sound at high frequencies. These findings offer an overview of the influence of PU coating on porous materials with a high capacity for sound absorption at high frequencies and are novel in comparison to previous research.
The presence of a 0.3 mm-thick PU coating improved performance at frequencies of 1600 Hz (0.05 points) and 2000 Hz (0.14 points) in a fine bamboo particle board with a density of 0.4 g/cm
3 (
Figure 10a). Performance decline occurred above 2000 Hz, with the highest drop of 0.35 points occurring at 5000 Hz. At frequencies greater than 2000 Hz, the absorption performance declined as the coating thickness increased. The addition of a 0.6 mm thick layer diminishes the absorption performance between 500 and 5000 Hz, dropping to 0.52 points at 2500 Hz.
In coarse board settings with a density of 0.4 g/cm
3 (
Figure 10b), it was demonstrated that adding a PU coating of 0.6 mm increased sound absorption capacity by 0.01–0.24 points between 1600–2500 Hz. At frequencies greater than 2000 Hz, the coating performance deteriorated, with the highest drop of 0.39 points happening at 3150 Hz. The addition of a 0.3 mm thick coating reduced the absorption performance between 500 and 5000 Hz. These results were comparable to those obtained when 0.6 mm of coating was applied to fine particle boards.
In general, at a density of 0.4 g/cm3, the presence of a coating can increases the absorption performance at a frequency of 1600–2000 Hz with coating conditions adjusting to the pore size. When the pore size is small (fine), a coating with a thickness of 0.3 mm is sufficient to provide improved performance, but when the pore size is large (coarse), a thicker coating (0.6 mm) is necessary to provide higher performance. Inadequate coating thickness results in a loss of performance between 500–5000 Hz, with the greatest loss occurs at high frequencies.
For bamboo particle board with a density of 0.6 g/cm
3, the performance was generally below that of particle board with a density of 0.4 g/cm
3, especially at high frequencies. On the fine particle board (
Figure 10c), the presence of a 0.3 mm PU coating thickness gave an increase in absorption at a frequency of 250–630 Hz by 0.02–0.07 points. On the coarse particle board (
Figure 10d), the increased same performance occurred with the addition of a coating with a thickness of 0.3 mm, even though it occurred at different frequencies. On panels with higher densities, the performance improvement occurred at a lower frequency than on boards with lower densities.
On the coarse particle board, the presence of a 0.3 mm PU coating thickness improved performance at a frequency of 250–1250 Hz with an increase of 0.01–0.11 points. The decrease occurred at frequencies above 1250 Hz with a decrease of 0.04–0.23 compared to the condition without coating. The presence of a 0.6 mm thick coating could improve absorption performance at a frequency of 200–1000 Hz with an increase of 0.01–0.07 points compared to the condition without coating. The decrease in sound absorption occurred at frequencies above 1000 Hz, varying from 0.06 to 0.33 points.
The performance of bamboo particle board with a density of 0.6 g/cm
3 was generally inferior to particle board with a density of 0.4 g/cm
3, particularly at high frequencies. On the fine particle board (
Figure 10c), the inclusion of a 0.3 mm PU coating increased absorption by 0.02–0.07 points between 250 and 630 Hz. On coarse particle board (
Figure 10d), the same performance improvement occurred with the addition of a 0.3 mm thick layer, but at a different frequency. The performance enhancement occurred at the lower frequency on panels with higher densities than on boards with lower densities.
On coarse particle board, the presence of a 0.3 mm PU coating increased performance by 0.01–0.11 points between 250–1250 Hz. The reduction occurred at frequencies greater than 1250 Hz, with a reduction of 0.04–0.23 points compared to the uncoated condition. A 0.6 mm thick coating can improve absorption performance by 0.01–0.07 points between 200–1000 Hz as compared to the uncoated state. At frequencies greater than 1000 Hz, the sound absorption decreased by 0.06 to 0.33 points.
In general, the particle size type, density, and coating thickness of bamboo particle board influence the sound absorption ability. The presence of a coating reduces the performance of the material at high frequencies and increases it at lower frequencies. The performance decline at high frequencies is bigger (0.02–0.52 points) than the performance increase at low frequencies (0.01–0.24 points decrease). The occurrence of a frequency range with enhanced performance at lower frequencies and decreased performance at higher frequencies is similar with the findings of Ivanova et al. [
30], researching scotch pine coating. In comparison to the uncoated samples, those coated with PU demonstrated an increase in performance at 100 Hz and a loss in performance between 630–1400 Hz. The decrease in performance at higher frequencies is greater than the improvement in performance at lower frequencies, which is consistent with the conclusion obtained by Ivanova et al. [
30].
The variation in density influences the frequency in which the sound absorption performance increases or decreases. At 0.4 g/cm
3, performance was improved at a greater frequency than at 0.6 g/cm
3, which was 1600–2000 Hz and 1600–2500 Hz for the fine and coarse particle board, respectively. At a density of 0.6 g/cm
3, the fine bamboo particle board performed better at frequencies of 250–1250 Hz, whereas the coarse bamboo panel performed better at frequencies of 200–1000 Hz. Other research has not examined the influence of board density on coating performance in terms of sound absorption. Xu et al. [
29] varied the specific gravity, panel thickness, and coating thickness. While Ivanova et al. [
30] experimented with coating thickness and the type of coating.
Bamboo panels with an acoustic absorption value greater than 0.3 at frequencies greater than 1600 Hz were used in this study. This is in contrast to the work of Xu et al. [
29] and Ivanova et al. [
30], who employed solid wood with a sound absorption coefficient of less than 0.3. Both investigations concluded that the inclusion of a PU layer enhanced sound absorption, particularly at high frequencies. However, it turns out that the improvement in performance cannot surpass the absorption coefficient of 0.3.
In our study, we discovered that adding a coating significantly reduces sound absorption performance at high frequencies. This is because of the coating’s ability to seal the pores that absorb sound at high frequencies. The construction of the product, the natural properties of the material, the type of surface, density, and porosity characteristics all had an effect on sound absorption [
36]. These parameters are associated with sound dissipation through material friction, which allows sound to travel through and be dampened [
44]. Our investigation discovers that the thickness of the coating has a substantial effect on the surface roughness, CA, and K value (
Table 2). This is most likely due to the board’s inadequate capacity to dissipate sound at low and very high frequencies but adequate performance at medium frequencies of 1000–2000 Hz (
Figure 9).
3.2.2. Noise Reduction Coefficient
In practice, the sound absorption coefficient is typically expressed as a single value refered to as the noise reduction coefficient (NRC). This is the average sound absorption coefficient at frequencies of 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz following ASTM C243 ASTM 2017). These numbers enable the assumption that the sound absorption coefficient is determined by the material properties and not by the sound field. As a result, the sound absorption coefficient of any material is angle dependent and frequency dependent [
36].
The results of this investigation indicate that as particle board density increases, the noise reduction coefficient decreases. At 0.4 g/cm
3 board density, the NRC rose as the PU-coating thickness increased for both fine and coarse particle board, except for fine particle board with 0.6 mm coating thickness. On the other hand, the NRC decreased with increasing thickness of the PU-coating samples at a board density of 0.6 g/cm
3. The lowering value may reach a maximum of 0.07 point (
Figure 11). While one material may absorb mostly at low frequencies and another at higher frequencies, both may have comparable NRC values [
36]. The ANOVA revealed a substantial difference in the NRC values for each variation (
Table 6). This shows that the difference in treatment resulted in a statistically significant change in the NRC value. While there is a significant difference, the treatment for variation in the NRC value was not as extensive as indicated in
Figure 11. Additionally, the maximum NRC value in this data set is 0.26 and the minimum is 0.11, indicating that the shift in the NRC value is often small, around 0.15.
In this context, the application of a surface PU coating is one of the most promising treatments for improving the sound absorption qualities of low-density particle board. However, the layer thickness should be optimized to ensure the desired performance of the surface coating. Coating significantly minimizes the dissipation of results, most likely due to its more homogenous surface.