Malas (Homalium foetidum) timbers were supplied from Jeonil Timber Co., Ltd in Korea. They cut logs into 20 cylindrical specimens with a 2.9 cm diameter and 1 cm cross-sectional thickness (Fig. 1). They were exposed to standard room conditions for one month. Their air-dried specific gravity and MC (moisture content) were 0.8 and 12%, respectively.
We added 200 ml of water into an ultrasonic cleaner (model: SD-80H, SungDong Ultrasonic Co. Ltd.), and 20 cylindrical specimens were treated for about 25 min with a 40 kHz ultrasonic wave frequency and 50 W output power. After ultrasonic treatment, the specimens were dried at 40 ˚C for 8 h in a dryer and then equilibrated to the laboratory environment for 24 h. The MC of cylindrical specimens was similar between untreated and ultrasonic-treated samples.
Gas permeability measurements
Gas permeability of untreated and ultrasonic-treated cylindrical specimens was measured by a capillary flow porometer (model: CFP-1200AEL, Porous Material Inc., Ithaca, NY, U.S.A). It measured the flow rate through the sample while pressing the sample vertically from atmospheric pressure to 1 atm.
Capillary flow porometer calculated automatically the Darcy permeability constant (K) using Eq. (1) and Eq. (2) :
K = specific permeability (Darcy), k = permeability (cm3/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), ∆P = pressure difference (dyne/cm2), L = length of the specimen (cm).
We sealed the samples’ side surfaces with a silicon O-ring to prevent air leakage from the edges and estimated the same gas permeability direction both untreated and ultrasonic-treated cylindrical specimens.
Sound absorption coefficient measurements
Sound absorption coefficients of untreated and ultrasonic-treated cylindrical specimens were measured according to ISO 10534–2 using the two microphone transfer function method with an impedance tube kit (model: type 4706, B&K Company, nærum, Denmark), pulse analysis software, and a spectrum analyzer (model: type 3560, B&K Company, nærum, Denmark) . We used a silicone O-ring to prevent experimental error from the air gap between the test specimen and impedance tube and, using the same direction for both, estimated the gas permeability before and after treatment. We used a frequency range of 50 Hz to 6.4 kHz. Prior to ultrasonic treatment, atmospheric pressure, temperature, relative humidity, sound velocity, air density, and acoustic impedance were 1030 hPa, 20.20 ˚C, 35%, 343.35 m/s, 1.221 kg/m3, and 419.2 Pa/(m/s), respectively. After treatment, conditions were 1021 hPa, 25.4 ˚C, 26%, 346.38 m/s, 1.189 kg/m3 and 411.9 Pa/(m/s), respectively.
Cross-sectional surface observations
To observe microscopic feature changes caused by ultrasonic treatment, we cut untreated and treated specimens into 7 mm (radial) × 7 mm (tangential) × 6 mm (longitudinal) samples using a microtome (model: HM400S, Microm GmbH, Germany). We coated the specimens with gold ions using an ion sputter-coater (model: SCM, Emcrafts, Korea) and observed the samples at a 20 kV acceleration voltage and 200 × magnification using a scanning electron microscope (SEM) (model: Genesis-1000, Emcrafts, Korea).