23 rd International Congress on Sound & Vibration Athens, Greece -4 July 26 ICSV23 SOUND ABSORPTION PERFORMANCE OF OIL PALM EMPTY FRUIT BUNCH FIBERS Or Khai Hee, Azma Putra, Mohd Jailani Mohd Nor, Mohd Zulkefli Selamat and Lim Zhi Ying Universiti Teknikal Malaysia Melaka UTeM, Centre for Advanced Research on Energy (CARe), Hang Tuah Jaya, Durian Tunggal, 76 Melaka, Malaysia email: jai@utem.edu.my The conventional acoustic absorbers are made from synthetic materials which are not only causing global warming and pollutions during its production, but also harmful to human. Thus, scientists have turned their attention to find sustainable and green materials which have the potential to replace synthetic materials. Oil Palm Empty Fruit Bunch (OPEFB) fiber is a biodegradable material and is available in abundance quantity as agricultural waste in Malaysia. In this study, sound absorption performance of OPEFB fiber is investigated. The impedance tube testing according to ISO 534-2 was conducted to measure the sound absorption coefficient. Samples of raw OPEFB fibers with different masses and thicknesses are tested. In the experimental work, the effects of fiber density (or fiber mass), thickness and air gap on sound absorption coefficient are explored. Increment in fiber density showed improvement of sound absorption performance in high frequency region. The increase in thickness and application of air gap enhanced the sound absorption performance, especially at lower frequency region. At thickness of 5 mm, OPEFB fibers with density of 292 kg/m 3 showed good sound absorption performance where the sound absorption coefficient achieved in average above khz. Comparison with industrial acoustic rock wool showed that OPEFB fibers possess similar sound absorption performance at the same thickness of 5 mm. Delany-Bazley model is used to validate the experimental work. It is found that the mathematical model is able to predict the trend of the sound absorption performance curve with minor deviation.. Introduction Acoustic absorber is a material used to absorb sound energy. Recently, the utilization of green and sustainable materials as acoustic absorber are expanding in the research sector due to its benefits over synthetic materials. The conventional acoustic absorbers are made of synthetic materials such as rock wool, glass wool and foam glass, which are harmful to the environment and human s health []. Numerous studies have been done on the natural fibers to explore its potential as sound absorbing materials. Those studies include research on bamboo fibers [2], tea-leaf-fibers [3], Arenga Pinnata fibers [4], coir fibers [5, 6], sugarcane fibers [7], paddy fibers [8], date palm fibers [9] and kenaf fiber sheets []. The findings showed that the sound absorption performance can be improved by increasing the fiber density. Enhancement of sound absorption performance, especially at lower frequency region can be done by increasing the thickness of the sample and by providing air gap behind the sample. Ersoy and Küçük [3] and Putra et al. [8] discovered that the addition of fabric on the surface of the sample can enhance the sound absorption bandwidth and increases the level of absorption (α). Comparison with the synthetic material such as glass wool showed that the fibrous natural material has comparable sound absorption performance [8].
Delany and Bazley [] had developed empirical formulas to predict the sound absorption coefficient of fibrous absorbent materials. The empirical formulas only depend on flow resistance, which is affected by the bulk density and size of the fiber. Thus, the measurement of sound absorption coefficient can be done with simple lab equipments and requires a short amount of time. Fouladi et al. [6] analyzed the acoustical properties of coir fiber by using two analytical models, namely Delany-Bazley and Biot-Allard. They found out that Delany-Bazley model is easier to use compared to Biot-Allard model, which will be beneficial for the application of natural fibers in the industry. Conventionally, oil palm empty fruit bunch (OPEFB) fibers are used as fertilizers, mulching material, reinforcement materials in polymer composites and burning fuel in boilers to supply electricity for the mill [2, 3]. As the agricultural by-product, the quantity of the OPEFB is still abundant and causing environmental problems in Malaysia. Thus, due to its abundant quantity and the availability of technology to extract the fibers [2], OPEFB fibers can be a good alternative to act as green and sustainable acoustic absorber. The purposes of this study are to determine the sound absorption performance of OPEFB fibers and to discuss the application of Delany-Bazley model in predicting the sound absorption performance of OPEFB fibers, which according to the author s knowledge has not been investigated by other researchers. 2. Methodology 2. Experimental Works 2.. Preparation of The Samples The samples of OPEFB fiber sound absorber were fabricated with variation in density and thickness. The raw OPEFB fibers were weighted for, 3, 5 and 7 grams. The weighted raw fibers were fitted into a 33 mm-diameter aluminium mold to form cylindrical shape sample. The OPEFB fibers were compressed into samples of mm, 2 mm and 5 mm-thick by using hot compression technique under temperature of C and 5 minutes of compression time. The fabrication of the OPEFB fiber samples were done without using any chemical binder as the supplied heat was sufficient to form the samples. The example of the samples are shown in Fig.. (a) (b) (c) Figure : OPEFB fibers sound absorber: (a) 5 grams of 2 mm-thick sample, (b) 5 grams of mm-thick sample and (c) raw fiber. 2 ICSV23, Athens (Greece), -4 July 26
2..2 Experimental Setup The measurement of sound absorption coefficient was conducted by using the impedance tube testing according to ISO 534-2 [4]. The experiment setup is shown in Fig. 2. The sample was fitted tightly into the 33 mm-diameter sample holder as shown in Fig. 3. The sample holder was located at one end of the tube, while at the other end of the tube is the location of a loudspeaker. A white noise signal was generated from the computer and supplied into the tube through the loudspeaker. Two /2 pre-polarized free field microphones (GRAS 4AE) with /2 CCP pre-amplifier (GRAS 26CA) were used to record the built-up sound pressure inside the impedance tube. The RT Pro Photon+ analyzer was used as the data acquisition system to process the recorded signals and to obtain the transfer function between the two microphones. MATLAB software was used to calculate the sound absorption coefficient and to generate the results of the experiment. In this experiment, the validity of the result is between frequencies of 5 to 45 Hz due to the tube diameter of 33 mm. Computer (Noise generator, data acquisition system and display) Signal analyzer Removable cap (Rigid) Amplifier Microphone Microphone 2 Loudspeaker Impedance tube Sample Air gap plunger Figure 2: Experimental setup for determination of sound absorption coefficient. Figure 3: The OPEFB fibers sample was fitted into the sample holder of impedance tube. The amount of sound energy absorbed by the material with respect to the total incident sound energy is referred as sound absorption coefficient, α, which is in the range of zero to unity. α with value of unity indicates that all sound energy is absorbed, while value of zero shows no absorption of sound energy. The effectiveness of a sound absorber is referred to the level of sound absorption coefficient and the bandwidth of the absorption frequency. Material with absorption coefficient of more than is usually considered as good sound absorber [5]. ICSV23, Athens (Greece), -4 July 26 3
2.2 Mathematical Model In this paper, Delany-Bazley model [6, ] was studied as the predictive approach to obtain the sound absorption coefficient curve for OPEFB fibers. The characteristic impedance Z and propagation constant γ of a layer of homogeneous porous material are shown in Eqs. () and (2) [6]. The Z and γ are dependent on the frequency of the analysis and flow resistivity of the porous material, i.e. OPEFB fibers. {[ ( ) c2 ] [ ( ) c4 ]} fρ fρ Z = ρ c + c i c 3 σ σ {[ ( ) c6 ] [ ( ) c8 ]} fρ fρ γ = k c 5 i + c 7 σ σ where ρ is density of air, c is speed of sound, c -c 8 is Delany-Bazley regression constants [], f is frequency of sound, σ is flow resistivity and k = 2πf/c is wave number of air. The surface acoustic impedance Z of a rigidly-backed material of thickness t is shown in Eq. (3) []. () (2) Z = Z coth γt (3) The normal incidence sound absorption coefficient of OPEFB fibers based on Delany-Bazley model is shown in Eq. (4) [], which is derived from Eq. (3). Z ρ c α = Z + ρ c 2 (4) 3. Results and Discussion Figure 4 shows the sound absorption coefficient for mm-thick samples with different densities. The bulk density of the sample is defined as the ratio of mass m of the fiber, over the total volume V of the cylindrical shape sample given by ρ bulk = m/v. At the same thickness of mm, density of 35 and 585 kg/m 3 shows good sound absorption coefficient where α > above 2 khz, which is a typical frequency range for a fibrous type absorber. From Fig. 4, it can be seen that there is some improvement in the absorption coefficient as the density or mass of the fibers increases. The increase in density resulted in increase of flow resistivity and tortuosity, which enable more sound waves to be absorbed. However, excessive amount of fibers can limit the porosity in the sample and thus impact towards poor sound absorption performance. As shown in Fig. 4, sample with density of 88 kg/m 3 shows α < for the whole tested frequencies. Similar poor absorption performance can also be seen from the curve represented by sample with density of 7 kg/m 3. In this case, insufficient amount of fibers caused the reduction of the total sound energy to be absorbed. The results on the effect of thickness can be seen in Fig. 5. Figure 5 shows that the increment in thickness improves the peak of the absorption coefficient curve and shifts the peak of the curve to the lower frequency region. This is due to more sound energy can be absorbed across the increasing thickness of the sample via increase of flow resistivity and tortuosity. By shifting from Fig. 5(a) to Fig. 5(b), the improvement on the sound absorption performance curves can be seen clearly as wider coverage of absorption coefficient with α >. The application of air gap behind the sample can also be used to improve the sound absorption coefficient at lower frequency region. Figure 6 illustrates the air gap thickness denoted by d, which is implemented behind the sample. Figure 7 shows the utilization of air gaps for OPEFB fibers samples with same thickness of 2 mm but 2 different masses. It can be observed that air gap shifts the sound absorption coefficient curve to the left or lower frequency region. However, improvement of 4 ICSV23, Athens (Greece), -4 July 26
.3 7 kg/m 3 ( g) 35 kg/m 3 (3 g) 585 kg/m 3 (5 g) 88 kg/m 3 (7 g). 5 5 2 25 3 35 4 45 Figure 4: The sound absorption coefficient of OPEFB fibers with different fiber densities for mm-thick samples. mm 2 mm mm 2 mm.3.3.. 5 5 2 25 3 35 4 45 (a) 3 gram fibers. 5 5 2 25 3 35 4 45 (b) 5 gram fibers. Figure 5: The sound absorption coefficient of OPEFB fibers with different masses and thicknesses. Sample Air gap d Air gap plunger Rigid aluminium surface Figure 6: Illustration of air gap behind the sample in the impedance tube. sound absorption coefficient at lower frequency region is compromised with the degradation of sound absorption at higher frequency region. Sound absorption performance of OPEFB fibers was compared with the industrial synthetic material, i.e. rock wool. The comparison of the sound absorption coefficient is shown in Fig. 8. At the same thickness of 5 mm, the OPEFB fibers have shown similar sound absorption performance ICSV23, Athens (Greece), -4 July 26 5
with rock wool at all different densities. Thus, OPEFB fibers have the potential to replace synthetic material as industrial acoustic material..3.3. d = mm d = 2 mm 5 5 2 25 3 35 4 45 (a) 3 gram fibers.. d = mm d = 2 mm 5 5 2 25 3 35 4 45 (b) 5 gram fibers. Figure 7: The effect of air gap on the sound absorption coefficient of 2 mm-thick OPEFB fibers with different masses..3. 4 kg/m 3 of Rock Wool 8 kg/m 3 of Rock Wool 2 kg/m 3 of Rock Wool 292 kg/m 3 of OPEFB fibers 5 5 2 25 3 35 4 45 Figure 8: Sound absorption coefficient of OPEFB fibers and rock wools at the thickness of 5 mm. A mathematical model was obtained from literature by Fouladi et al. [6] and Delany and Bazley [] to validate the experimental results. Figure 9 shows the comparison of experimental sound absorption performance of OPEFB fibers with Delany-Bazley model. Figures 9(a) and 9(b) show samples with different densities but having the same thickness. It can be seen that the Delany-Bazley model can predict the performance of the curves with minor divergence from the experimental results. Figures 9(b) and 9(c) show the samples with the same density but having the different thicknesses. Again, the Delany-Bazley model can roughly estimate the trend of the sound absorption coefficient curves. However, the mathematical model was unable to predict the dips at.5 to 2.3 khz and 3.8 to 4.5 khz in Fig. 9(c). This might be due to Delany-Bazley model only considered flow resistivity to predict the sound absorption coefficient of the porous material. Despite of the poor prediction at the dips of the sound absorption performance curve, the Delany-Bazley model is still considered easy and convenient to be apply in the industry. 6 ICSV23, Athens (Greece), -4 July 26
Absorption Coefficient, α.3 Absorption Coefficient, α.3. Experiment Delany Bazley model 5 5 2 25 3 35 4 45 Frequency (Hz) (a) 2 mm-thick, 3 gram (75 kg/m 3 ) of fibers.. Experiment Delany Bazley model 5 5 2 25 3 35 4 45 Frequency (Hz) (b) 2 mm-thick, 5 gram (292 kg/m 3 ) of fibers. Absorption Coefficient, α.3. Experiment Delany Bazley model 5 5 2 25 3 35 4 45 Frequency (Hz) (c) 5 mm thick, 2.5 gram (292 kg/m 3 ) of fibers. Figure 9: Comparison of sound absorption coefficient between experimental results and Delany-Bazley model. 4. Conclusion The oil palm empty fruit bunch fibers are found to be good sound absorber. The increment of mass or density improves sound absorption performance in high frequency region. The addition of the value of thickness and application of air gap show enhancement of sound absorption coefficient at lower frequency region. The sample with density of 292 kg/m 3 and thickness of 5 mm shows good results as the sound absorption coefficient is in average above khz. Comparison study of OPEFB fibers with rock wool at the same thickness has shown comparable sound absorption performance. Thus, OPEFB fibers have the potential to replace synthetic material as industrial acoustic material. Validation on the experimental works by using Delany-Bazley model shows good agreement in terms of the trend of the absorption coefficient. 5. Acknowledgement Part of this research is supported under the Exploratory Research Grant Scheme No. ERGS// 23/STG2/UTEM/2/ provided by the Ministry of Higher Education Malaysia (MoHE). Authors would like to acknowledge the MyBrain UTeM scholarship from Universiti Teknikal Malaysia Melaka which also has made this research possible. ICSV23, Athens (Greece), -4 July 26 7
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