Investigation of Sound Absorption Ability of Acanthopanax senticosus Wastes

Fig. 1 shows a schematic of the preparation of the one-year-old A. senticosus samples used in this study. The samples were supplied by a Korean herbal medicine shop (Jecheon, Korea). Their diameter and length were approximately 5–10 mm and 50–60 mm, respectively. The samples were boiled at 100°C for 2 h and then cut into 10–15 mm lengths followed by drying in an oven at 90°C for three days.

Fig. 1. Preparation of Acanthopanax senticosus wastes.

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The shorter lengths of the A. senticosus wastes enhance the exposed cross-sectional area and number of open pores to the incident sound wave. The samples were then stored in a laboratory at a temperature of 20°C and a humidity of 50% for approximately two weeks. Then, 100 g of wastes was dried at 105°C for approximately seven days to measure the moisture content (MC) in the samples (Korean Standards Association, 2016). The final weight of the dried wastes was measured as 91 g, which indicates that the MC was approximately 9%.

The pore structure of the A. senticosus wastes was observed by SEM (Genesis-1000, Emcraft, Gwangju, Korea). Prior to SEM imaging, the samples were dried in a laboratory oven at 50°C for approximately 2 h to remove the surface moisture. Then, a thin layer of gold was deposited on the samples to prevent the surface charging during SEM imaging. Then, the cross-sections and thorn surfaces of the wastes samples were observed at magnifications of 200 × and 500 ×.

The sound absorption coefficients of A. senticosus wastes were evaluated using an impedance tube (type 4206, Brüel & Kjær, Nærum, Denmark) designed in accordance with the ISO 10534-2 (International Organization for Standardization, 2001) same as the author’s previous studies (Jang, 2022b, 2022c, 2022d). A. senticosus wastes were filled to a height of 40, 60, 80, and 100 mm in two different impedance tubes with diameters of 29 and 99 mm (Fig. 2). The absorption coefficient was measured in the frequency ranges of 500–6,400 Hz and 100–1,600 Hz for the small diameter (29 mm) and large diameter (99 mm) impedance tubes, respectively.

Fig. 2. Schematic of the experimental setup for sound absorption coefficient measurement using impedance tubes of 29 mm and 99 mm diameter.

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The sound absorption coefficient of a material varies with different frequencies. In the industrial setting, the sound absorption ability of a material is evaluated as a single-number index using noise reduction coefficient (NRC). In this study, NRC was computed using the following Equation (1).

where, α₂₅₀, α₅₀₀, α₁₀₀₀, and α₂₀₀₀ are the sound absorption coefficients at 250 Hz, 500 Hz, 1,000 Hz, and 2,000 Hz, respectively.

Originally, NRC should be calculated using the reverberation chamber method (Korean Standards Association, 2012). However, many researchers also apply NRC to the sound absorption coefficient through the impedance tube (Gokulkumar et al., 2019; Jang and Kang, 2021b; Voronina and Horoshenkov, 2004). Accordingly, this study calculated NRC as the sound absorption coefficient value from the impedance tube.

The values of the sound absorption coefficients at 250, 500, and 1,000 Hz were collected from the 99 mm diameter impedance tube, and those at 2,000 Hz were collected from the small-diameter impedance tube (Jang, 2022b, 2022c, 2022d).

Fig. 3 shows SEM images of the cross-section and thorn surface of the A. senticosus wastes. Fig. 3(a) shows the sapwood and pith together. The SEM images indicated that the cross-section of A. senticosus was significantly different from that of the sapwood and pith. The distribution of vessels in the sapwood [Fig. 3(b)] indicates the characteristics of ring-porous wood, and vessel clusters are abundant. This structure is advantageous for absorbing sound (Jang and Kang, 2021e). Fig. 3(c) shows a pith. The core has more large cells and less developed through-pores compared with the sapwood. The surfaces of the pith are found to be rough.

Fig. 3. SEM images of Acanthopanax senticosus cross-section and thorn. (a) Sapwood and pith (200 ×), (b) sapwood (500 ×), (c) pith (500 ×), (d) the tip of thorn (500 ×), (e) the middle of thorn (500 ×), (f) the thorn attached to the bark (500 ×). SEM: scanning electron microscopy.

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Fig. 3(d–f) shows the surface of the thorn of A. senticosus. Fig. 3(d) and (e) show the tip and the middle part, respectively. The thorn surfaces were found to be smooth. Fig. 3(e) shows a thorn connected to the bark of A. senticosus, showing a rougher texture than that of the middle and tip. The rough surfaces of the material can contribute to improved sound absorption at low frequencies (Chung et al., 2017).

The sound absorption coefficient curves are shown in Fig. 4. When the filling height of the A. senticosus wastes inside the impedance tube was 40 mm, the maximum sound absorption coefficient was 0.387 at 1,540 Hz; when the filling height was increased to 100 mm, the maximum sound absorption coefficient increased to 0.535 at 714 Hz [Fig. 4(a)]. As the filling height of A. senticosus wastes increased, the maximum sound absorption coefficient shifted towards a lower frequency.

Fig. 4. Sound absorption curves of Acanthopanax senticosus. (a) Measured sound absorption coefficients for different filling heights of A. senticosus wastes using an impedance tube of dimeter 99 mm. (b) Measured sound absorption coefficients for different filling heights of A. senticosus wastes using an impedance tube of dimeter 29 mm.

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The sound wave incident on the porous material is converted into thermal energy as it collides with the material’s pores or void space between materials, and the sound energy is lost. As the thickness of the sound absorber increases, the sound absorption performance improves in the low-frequency region, as it provides more space for sound waves to convert thermal energy (Bhingare et al., 2019). However, the thickness of the sound-absorber needs to regulate within the optimum range. The efficiency of transferring sound energy into thermal energy significantly may decline as the sound absorber’s thickness exceeds the ideal values (Taban et al., 2021).

As shown in Fig. 4(b), the sound absorption coefficient’s curves fluctuated. As the hollow space between the granules acts as a resonator, the sound absorption curve has oscillated in the high-frequency region above 1,000 Hz (Voronina and Horoshenkov, 2004). This phenomenon is observed in most of granular type sound-absorbing materials (Jang, 2022b, 2022c, 2022d; Maderuelo-Sanz et al., 2014; Vašina et al., 2006).

In addition, the number of sound absorption peaks increased as the filling height was increased in the high-frequency region. There were only two sound absorption peaks at a filling height of 40 mm. The number of sound absorption peaks increased by one, three, four and five as the filling height was increased by 40, 60, 80, and 100 mm, respectively.

Table 1 depicts the sound absorption coefficient at 250, 500, 1,000, and 2,000 Hz and the NRC of A. senticosus wastes. As the filling height of A. senticosus wastes inside the impedance tube was increased from 40 to 100 mm, the sound absorption coefficient at 250 Hz increased by 1.56 times, at 250 Hz by 2.97 times, at 500 Hz by 1.83 times, and at 2,000 Hz by 2.09 times. The NRC also increased by approximately 2.1 times, from 0.214 to 0.446, as the filling height was increased from 40 to 100 mm. The improvement of NRC is due to improving the sound absorption effect depending on the increase in thickness. This trend is typical of porous or granular sound-absorbing materials (Jang, 2022b, 2022c, 2022d; Jang and Kang, 2021c).

The KS F 3503 (Korean Standards Association, 2012) classifies the sound absorption performance into four grades (0.3M grade: 0.21–0.40, 0.5M grade: 0.41–0.60, 0.7M grade: 0.61–0.80, and 0.9M grade: Above 0.81) depending on the NRC of the sound absorption material. The sound absorption rating of A. senticosus wastes investigated in this study is 0.3M and 0.5M grade for filling heights of 80 and 100 mm, respectively. Therefore, our results confirm that A. senticosus wastes can be considered as an eco-friendly sound-absorbing material. The rough surfaces of the pith and thorn of A. senticosus and the vessel structure of the cross-section have a favorable effect on sound absorption. Furthermore, a large number of thorns complicated the paths of empty spaces between A. senticosus wastes, which would have created a practical path for sound absorption.

According to the previous study, the NRC of boards made of natural materials such as coir, corn, banana, bamboo, and bagasse was 0.21–0.40 (Sim et al., 2014). In addition, the NRC of peanut shells investigated in the author’s previous study was 0.23–0.53 (Jang, 2022c), and that of pine pollen corns was 0.30–0.52 (Jang, 2022d). The sound absorption ability investigated in this study was approximately similar to these. In the future, if the A. senticosus wastes is compressed more to increase the density and the path of the void space between materials is made more complicated, it is estimated that the sound absorption performance can be further improved.

This study is meaningful because it is the first approach to investigate the sound absorption ability of A. senticosus wastes. However, there is a limit to the continuous supply of materials to produce commercial sound-absorbing materials. Therefore, it is necessary to investigate whether all medicinal plants (e.g., herbal medicines), including A. senticosus wastes, can be used as sound-absorbing materials. In the future, if their excellent sound absorption performance is revealed, it will be a more meaningful study from a practical point of view.