The growing interest for improving the quality of life in recent years, the environmental noise issues have worsened, and there is a considerable increase in the demand for noise control in residential environments. Accordingly, many studies have investigated building materials that offer excellent sound absorption coefficients and the measurement of the sound absorption coefficient.
Sound absorption is a process of absorbing or reducing sound energy using the friction in the pores of porous materials, the vibration of plates or membranes that form layers of air between them and the steel walls, and absorption tools based on resonators (Jang et al., 2018; Kang et al., 2010; Kang et al., 2018a; Kang et al., 2018b) Porous sound absorption offers the sound absorption effect using fiberglass or porous synthetic resin, and fabric and fiberglass are the main examples of porous sound absorption materials. Because of biological characteristics, wood is naturally porous because it contains many pores through which water and nutrients pass; however, its use as a sound absorption material is negligible. Even though wood contains several pores, the direction of these pores is toward the direction of contraction; quartered or block wood are used as the building materials, and wood pores are not exposed to the plane of sound incidence. If the longitudinal wood is used as the plane of sound incidence, wood pores may contribute to sound absorption; even in this case, the tracheid of softwood trees does not have pores at the end of cells, and the pits function as passages so that the penetrating pores effective in sound absorption are narrow. In case of hardwood trees, vessel and perforation plate, through pores effective to sound absorption, and thus, the cross-section of hardwood trees may offer a high sound absorption coefficient. However, there is a large deviation among the species of trees in terms of the size of the diameter of the vessel, the form of the perforation plate between vessel, and the rate of the vessel. Kang et al. (2011) reported that the cross- section of yellow poplar exhibits large-diameter and high-porosity vascular tissues, offering excellent sound absorption performance that there are different sound absorption coefficients in radial direction between heartwood and sapwood (Kang et al., 2011).
Aiming to offer key basic data in the expansion of the usage of the Paulownia tomentosa wood, a fast-growing tree species in Korea that offers high added value has been investigated. Further, to determine the effect of heat treatment on the gas permeability, crosssectional sound absorption coefficient, and sound transmission loss of the Paulownia tomentosa wood, this study measured the longitudinal gas permeability of the Paulownia tomentosa wood discs after being subjected to heat treatments at 100 °C, 160 °C, and 200 °C. The same specimens were used to measure the sound absorption coefficient in longitudinal direction using the transfer function method and the sound transmission loss using the transfer matrix method. The results were subsequently compared with the gas permeability, sound absorption coefficient, and sound transmission loss of the non-treated discs. Thus, the study aimed to evaluate the longitudinal sound absorption performance and sound transmission loss as well as the effect of the heat treatment on the characteristics of the Paulownia tomentosa wood to determine its potential usage as an indoor building material.
2. MATERIALS and METHODS
For this study, 11- to 15-year-old Paulownia woods (Royal paulownia, Paulownia tomentosa (Thunb.) Steud.) that were approximately 30 cm in diameter at breast height were collected from the research forest at Kangwon National University. The dried specimens were heat-treated at 100 °C, 160 °C, and 200 °C for two hours when the rate of temperature increase was 2 °C/min.
Gas permeability is the degree by which a gas fluid passes through a porous material, generally expressed as Darcy’s law. 1 darcy is 1 cm3/s of gas fluid in the viscosity of 1 cp (1 mPa·s) from 1 cm under the pressure of 1 atm.
The test initially measured the change of the flow rate by increasing the pressure using a capillary flow porometer (Model: CFP-1200AEL, Porous material Inc., Ithaca U.S.A) in real time, as shown in Fig. 1, and calculated the specific permeability (K) using Eq. (1) and Eq. (2) (Kang and Lu, 2005).
K = specific permeability (darcy)
k = permeability (cm4/dyne s)
η = viscosity of air (1.81 × 10−4dyne s/cm2)
Q = gas flow rate (cm3/s)
A = cross-sectional area of the specimen (cm2)
ΔP = pressure difference (dyne/cm2)
L = length of the specimen (cm)
The sound absorption coefficient in 50–6400 Hz was measured with the transfer function method using B&K’s impedance tube, pulse analyzer, and spectrum analyzer according to ISO 10534-2. (Kang et al., 2012) A 29-mm diameter impedance pipe was used to measure the change in the sound absorption coefficient based on the change in frequency from 50 to 6400 Hz.
The external conditions at the time of the measurement, i.e., the temperature, relative humidity, and air pressure, were 18.7 °C, 37%, and 1027.50 hPa, respectively; further, the speed of sound, air density, and acoustic impedance were 342.48 m/s, 1.224 kg/m3, 419.3 Pa/(m/s), respectively.
The sound transmission loss in 50–6400 Hz was measured based on the transfer matrix method using B&K’s impedance tube, pulse analyzer, and spectrum analyzer according to ASTM E2611-09. A 29-mm diameter impedance tube was used to measure the sound transmission loss in the frequency from 50 to 6400 Hz at which the temperature and air pressure (external conditions) were 20 °C and 1031 hPa, respectively. Fig. 2.
To observe the porous structure of the cross-section of wood from the specimens of which the sound absorption coefficient, gas permeability, and sound transmission loss were measured, a sample measuring approximately 10 mm (T) × 10 mm (R) × 10 mm (L) was obtained from the wood specimens; it was submerged in water for 48 hours under decompression for softening it, and the cross-section of the sample was cleanly obtained using Microtome (Model: HM400S, MicromGmbH, Germany) and observed using a scanning electron microscope (XL30ESEM, Philips, Netherlands).
3. RESULTS and DISCUSSION
Fig. 3 shows the difference in gas permeability between the heat-treated and non-heat-treated Paulownia tomentosa wood specimens. After being subjected to heat treatments at 100 °C, 160 °C, and 200 °C, the specific permeabilities of the specimens in case of 20-mm thick wood was 0.254, 0.279, 0.314, and 0.452 darcy, respectively; a slight increase in gas permeability was observed after the heat treatment. It is expected that the Paulownia tomentosa wood offers a considerably high gas permeability and sound absorption coefficient because of its low specific gravity, large porosity, and big vessels; however, the overall measurement results were small. This indicates that the big vessels contributing to an increase in gas permeability did not function as expected; as shown in Fig. 6, the cross-sectional surface of the Paulownia tomentosa wood vessels showed many tylosis, which lower penetrating pores that would contribute to the passage of fluid, resulting in low gas permeability. Because of the heat treatment, the gas permeability slightly increased when compared with that of the non-treated specimens. Kang et al. (2018c) reported that when mamala was heat-treated at 200 °C for three hours, its gas permeability increased eight times and its sound absorption coefficient increased by 40%. When compared with a previously conducted study, this study showed a smaller effect of heat treatment, which may be attributed to the difference in the structure and size of pores, the content of the penetrating pores, the content of resin in various species of trees, and the difference in treatment conditions such as the treatment temperature and duration; therefore, further research on these aspects is needed.
Fig. 4 shows the sound absorption coefficient of the heat-treated Paulownia tomentosa wood specimens in 50–6400 Hz using the transfer function method. The mean sound absorption coefficient in 50–6400 Hz for the non-treated Paulownia tomentosa wood disc with a thickness of 20 mm was 0.101, whereas those of the discs subjected to heat treatment at 100 °C, 160 °C, and 200 °C were 0.109, 0.096, and 0.106, respectively. The NRC were 0.060, 0.067, 0.062, and 0.071, respectively.
If wood is used as boards, it is difficult for wood pores to contribute to sound absorption when sound hits the radial section or tangential section of wood. However, if sound hits the cross-sectional surface of wood, it is easier for wood pores to contribute to sound absorption; thus, the cross-sectional surface of wood with many penetrating pores would offer a higher sound absorption coefficient. The Paulownia tomentosa wood, one of the fast-growing species in Korea, low specific gravity, high porosity, and big vessels so that it can offer high sound absorption coefficient in a cross-section; however, the overall sound absorption coefficient in all the measured frequency ranges was low at approximately 10%. Generally, the sound absorption coefficient of the porous materials increases in the high-frequency range but for the Paulownia tomentosa wood, despite the high porosity, many tylosis were present that caused many pores closing, resulting in the low sound absorption coefficient in case of high frequencies. Furthermore, even after heat treatment, the increase in sound absorption coefficient was negligible, which implies that the contribution of the penetrating pores to porous sound absorption was minimal and that there was no structural change due to the heat treatment. Meanwhile, Kang et al. (2018c) reported the two-fold increase in the sound absorption coefficient of Malas (Homalium foetidum) after heat treatment at 190 °C for three hours. When compared to that study, this study showed a smaller change due to heat treatment, which may be due to the difference in pores and resin content between tree species as well as the treatment conditions such as temperature and duration; therefore, further research is needed. Meanwhile, the sound absorption property of the porous materials is known to increase in the high-frequency range than in the low-frequency range; however, the Paulownia tomentosa wood showed low sound absorption coefficients in both high- and low-frequency ranges. The low sound absorption coefficient in the high-frequency range can be attributed to the decreased rate of through pores caused by many tylosis in vessels.
Fig. 5 shows the sound transmission loss of the Paulownia tomentosa wood discs in case of a thickness of 20 mm in 50–6400 Hz.
Sound transmission loss shows the sound-blocking effect of materials; the higher the transmission loss, the higher will be the sound-insulation effect, which depends on the frequency (Kim et al., 2015; Kook et al., 2007; Lee et al., 2011). The graphshows the frequency on X-axis and the sound transmission loss on Y-axis, and the transmission loss is expressed in [dB]. Generally, the higher the frequency and the larger the area density, i.e., the higher the thickness, the higher will be the transmission loss. The sound transmission loss of the non-treated Paulownia tomentosa wood disc having a thickness of 20 mm was approximately 30–40 and 36.93 dB on an average in 50–6400 Hz. In case of Paulownia tomentosa wood, the sound transmission loss was high despite its low specific gravity and high porosity.
Fig. 6 shows the anatomical features of the crosssectional surface of the Paulownia tomentosa wood. It is expected that the sound absorption coefficients of the Paulownia tomentosa wood in longitudinal direction is high because its diffuse porous wood and many big vessels as shown, However, it also contains many tylosis in vessels that would contribute to low sound absorption; therefore, the overall sound absorption coefficient is observed to be low. Such anatomical features are believed to also affect the sound transmission loss, resulting in a high sound transmission loss despite the low specific gravity and high porosity.
The measurement of the gas permeability, sound absorption coefficient, and sound transmission loss of the Paulownia tomentosa wood using different degrees of heat treatment resulted in the following conclusions:
The change in gas permeability in the Paulownia tomentosa wood that was heat-treated under the temperature conditions considered in this study was insignificant.
The cross-section surface of the Paulownia tomentosa wood, which is a low specific gravity material and contains big vessels, shows many tylosis and have a low sound absorption coefficient. Further, the change in its sound absorption rate due to heat treatment was insignificant.
The cross-sectional surface of the Paulownia tomentosa wood has a low sound absorption coefficient; therefore, it is believed that further treatment is required to improve its sound absorption coefficient for it to be used as an indoor acoustic material.