Changes in Sound Absorption Capability and Air Permeability of Malas (Homalium foetidum) Specimens after High Temperature Heat Treatment

Malas (Homalium foetidum) lumbers imported from Papua New Guinea were used in this study. The green malas lumbers with 120 mm in width, 30 mm in thickness and 1800 mm in length were heated under the temperature of 190°C for 3 hours. From green malas and heat treated malas, five cylindrical log cross sections with 28.9 mm in diameter and 10 mm in thickness were prepared. The total quantity of 10 pieces log cross sections were oven dried under the temperature of 100°C. The average moisture content (MC) of control and heat treated specimens were 6.08% and 4.12%, respectively. The average specific gravity of control and heat treated specimens were 0.76 g/cm³ and 0.65 g/cm³, respectively.

The permeability of specimens was measured by membrane pore size analyzer (Porous Material Incorporated, CFP-1200AEL) according to capillary flow porometry as precious research (kang et al., 2017). The principle of this method was described in ASTM 316-03. The pore structure and permeability were evaluated using liquid and gas flow as a function and the pressure difference. When a wood specimen was soaked in the wet solution, the specimen with different diameter of holes was easily wet because the solution had the low surface tension of 15.9 N/m. After thoroughly soaked specimen was put into sample chamber, the chamber was sealed as shown in Fig. 1, and then non-reactive nitrogen gas was circulated. The bubble point was first measured at the pore with largest diameter inside which the pressure was the minimum value compared to other pores, that is, at the place to overcome the capillary action of the fluid from gas pressure, as the solution was forcing out. After that, the flow was measuring until all pores was empty as the pressure being increasing. The pressure and flow rate of the gas through the dried specimen were measured although the liquid in all pores was not existence. The gas permeability and pore structure of wood specimen were evaluated from the results. The gas permeability of wood specimen was calculated according to Equations (1) and (2).

Fig. 1. Diagram Of Sample Chamber.

Download Original Figure

where, v = flow rate, k = permeability, μ = viscosity, p = pressure, x = location

where, F = flow, A = cross sectional areas

The sound absorption rate of specimen was measured by two microphone transfer function method using impedance tube (B&K company, kit type 4706), pulse analysis equipment and a spectrum analyzer (B&K company), as precious research (kang et al., 2012). In this study, the variation of sound absorption rate was measured as change of frequency in the range of 50~6,400 Hz. During measurement, temperature, relative humidity and atmospheric pressure were 25°C, 52% and 1008.0 hPa for control specimens and 24.6°C, 48% and 1008.0 hPa for treated specimens, respectively. Moreover, the velocity of sound, air density and acoustic impedance were 346.15 m/s, 1.176 kg/m³ and 407.0 Pa/ (m/s) for control specimens and 345.92 m/s, 1.177 kg/m³ and 407.2 Pa/(m/s) for treated specimens, respectively.

Capillary flow porometry estimates the constricted pore diameter of a porous material, which is the smallest pore diameter of one flow pathway. This constricted pore may be enlarged by heat treatment as shown in Fig. 2. The pore size distribution as a function of average pore diameter by CFP represent that the pore diameter enlarged and the number of permeable pores in treated wood increased, as compared to control wood.

Fig. 2. Typical plots of pore size distribution as a function of average pore diameter of 10 mm thick malas wood (top: control, bottom: heat treated).

Download Original Figure

The change of permeability for control and treated specimens is presented in Fig. 3. Average permeability of control specimens was 3.11 darcy while that of treated specimens was 23.48 darcy, which is about 8 times higher than that of control specimens. This trend could be attributed to the flow passageway inside wood after treatment being more and larger than before treatment.

Fig. 3. Typical plots of air flow rate in longitudinal direction vs. pressure for 10 mm thick malas wood (top: control, bottom: heat treated).

Download Original Figure

The sound absorption rates of specimens in the frequency range of 50~6,400 Hz are shown in Fig. 4.

Fig. 4. Relationship between sound absorption coefficient and frequency for 10 mm thick malas wood (top : control, bottom : heat treated).

Download Original Figure

As showed in Fig. 4, the sound absorption rates of treated specimens showed higher values than those of control specimens in almost all frequency range. The difference in sound absorption rates between treated and control specimens became larger with increasing of frequency beginning from 1 kHz, reached 2 times in frequency range of nearby 2 kHz and nearby 6 kHz. It indicates that the sound absorption rate of treated specimens is increasing with increasing of frequency, which is a typical characteristic of porous sound absorber. This trend could be accounted for there being more pores and larger flow passageway in wood due to the inclusions inside wood such as gum, carbohydrate, etc. being melted and discomposed out during treatment. It can be inferred that the sound absorption rate of treated specimens was significantly increased because the flow of the air as the medium transferring the energy of sound wave was increased due to improved permeability during heat treatment.

The NRC (noise reduction coefficient, mean value of sound absorption coefficient at the frequency of 250, 500, 1,000 and 2,000 Hz) showed a value of 0.17 for treated specimens and 0.10 for control specimens, which indicated that the NRC of treated specimens is improved by 70 percent compared to that of control specimens. It is advised that to apply the heated malas wood as environmentally friendly sound absorption materials is possible.