Applied Mechanics and Materials Vol. 315 (2013) pp 577-581 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.315.577
Experimental determination of Sound Absorption Coefficients of Four types of Malaysian Wood Elammaran Jayamani1, a, Sinin Hamdan2,b and Nurizahusna binti Suid3,c 1
School of Engineering, Computing and Science, Swinburne University of Technology Sarawak campus, Kuching, Malaysia 2,3
Department of Mechanical and Manufacturing Engineering, Universiti Malaysia Sarawak, Kuching, Malaysia
a
[email protected],
[email protected],
[email protected]
Keywords: Malaysian wood, Sound absorption coefficient, Two microphone method, Noise
Abstract. Currently, one of the important topics in acoustic science is noise control. It is important to control the noise in order to minimize extraneous noise in rooms, buildings, and our environment. Noise control can be achieved by reducing the intensity of sound to the level that is not harmful to human ear. There are four basic principles employed to reduce noise which is absorption, isolation, vibration isolation, and vibration damping. In fact, the most recognized technique to reduce noise is sound absorption on the materials itself. Sound absorption on material such as wood and porous material have been developed and studied by few researchers. Materials that reduce the sound intensity as the sound wave passes through it by the phenomenon of absorption are called sound absorptive materials. There are lot of methods can be used on determining the sound absorption coefficient of materials. In this paper, a preliminary work has been carried out experimentally to determine the sound absorption coefficient of four types of Malaysian wood. They are Tapang (Koompassia excels), Pulai (Alstonai angustiloba), Selunsor merah (Tristianiopsis beccariana) and Jelutong (Dyera polyphylla). The test was performed using the ASTM E1050-98/ISO 10534-2 (American Society for Testing and Material) standards for the sound absorption coefficient testing. This method is known as impedance tube method (TwoMicrophone Method). The absorption coefficient depends on the frequencies. In this study the values of the frequencies used was in the range from 350 Hz to 1000 Hz. Introduction Now a day much importance is given to the acoustical environment. Noise control and its principles play an important role in creating an acoustically pleasing environment. This can be achieved when the intensity of sound is brought down to a level that is not harmful to human ears. Achieving a pleasing environment can be obtained by using various techniques that employ different materials. One such technique is by absorbing the sound. Fibrous, porous and other kinds of materials have been widely accepted as sound absorptive materials. Sound absorbing materials are commonly used to soften the acoustic environment of a closed volume by reducing the amplitude of the reflected waves. Absorptive materials are generally resistive in nature, either fibrous, porous or in rather special cases reactive resonators [1]. Acoustical material plays a number of roles that are important in acoustic engineering such as the control of room acoustics, industrial noise control, studio acoustics and automotive acoustics. A wide range of sound-absorbing materials exist; they provide absorption properties dependent upon frequency, composition, thickness, surface finish, and method of mounting. However, materials that have a high value of sound absorption coefficient are usually porous. A porous absorbing material is a solid that contains cavities, channels or interstices so that sound waves are able to enter through them. It is possible to classify porous materials according to their availability to an external fluid such as air. Those pores that are totally isolated from their neighbours are called “closed” pores. They have an effect on some macroscopic properties of the material such as its bulk density, mechanical strength and thermal conductivity. However, closed pores are substantially less efficient than open pores in All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 58.26.207.170, Swinburne University of Technology, Kuching, Malaysia-14/03/13,04:56:45)
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absorbing sound energy. On the other hand, “open” pores have a continuous channel of communication with the external surface of the body, and they have great influence on the absorption of sound. Open pores can also be “blind” (open only at one end) or “through” (open at two ends). Acoustic studies play an important role in architectural, the design of studios, automobile interiors and others. Nearly all of the study focuses on absorbing materials and how noise can be control by placing the materials between the sound source and the sound receiver. These days, materials like wood, carpet, ceiling, leather seat, and others are widely used to soften the acoustic environment of a close system. Therefore further studies are needed in order to understand the acoustic properties of tropical wood species that might be suitable for noise control system. This paper determines the sound absorption coefficients of Malaysian wood using an Impedance tube. Malaysian Woods Wood is an extremely versatile material with a wide range of physical and mechanical properties among the many species of wood. It is also a renewable resource with an exceptional strength-to-weight ratio. Wood is a desirable construction material because the energy requirements of wood for producing a usable end-product are much lower than those of competitive materials, such as steel, concrete or plastic. As for energy absorption or shock resistance which is functional of the ability of a material to quickly absorb and form to another energy via deformation. Malaysian woods are divided into four categories [2]. They are heavy hardwoods (HHW), medium hardwoods (MHW), light hardwoods (LHW) and softwoods (SW). Hardwood trees have broad leaves and are deciduous – they lose their leaves at the end of the growing season. Hardwoods are angiosperms, which mean that they are using flowers to pollinate for seed reproduction. While the softwood trees are conifers (evergreens), have needles or scale-like foliage and are not deciduous. Softwoods are gymnosperms, meaning they do not have flowers and use cones for seed reproduction In Malaysia, there are more hardwoods compare to softwoods. Previous work has also shown that a considerable amount of work has been carried out to determine the sound absorption coefficient of wood or wood-based related materials [3,4]. Wassilieff [3] studied the sound absorption coefficients of New Zealand pine (Pinus radiate) wood fibres and wood shavings by using a simple Rayleigh model which requires the airflow resistivity, porosity and tortuosity of the material as input. He also showed a reasonable agreement for wood fibres samples when a comparison to a single-parameter empirical model of Delany and Bazley [4] method is used. Kang [5] found that the sound absorption coefficient of beech wood (Fagus grandifolia) has no difference in measurement when using the standing wave method compared to the two microphone method. Yoshikawa [6] suggested new way to classify suitable woods that can be used for string instruments: transmission parameter and the antivibration parameter. This scheme can show distinctly the difference between the soundboards and frame boards that are usually the two substantial elements in string instruments. Kidner and Hansen [7] showed that using the Delany and Bazley’s approximation method is adequate when modelling sound absorption in porous materials in comparison to other complex method such as the Biot’s model [8,9]. Bucur [10] has authored a comprehensive book on the wood acoustics. However, there is still lacking on the Malaysian wood properties in terms of the sound absorption coefficient. Therefore, this paper reports some preliminary results on the sound absorption coefficients for four types of Malaysian woods. Experimental Setup Impedance tube method is used to indicate the Normal incidence sound Absorption Coefficient (NAC) by using plane sound waves that strike the material. The Noise Reduction Coefficient (commonly abbreviated NRC) is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. It is the arithmetic average, rounded to the nearest multiple of 0.05, of the sound absorption coefficients for a specific material and mounting condition determined at the one-third
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octave band center frequencies of 250, 500, 1000 and 2000 Hz. This method has the advantage of small size, very handy device for quickly and accurately determining sound absorption coefficient of material, and it only requires small sample size [11]. Theoretically, it is easy to reproduce environmental condition when sound is trapped in a one dimensional continuum [12]. In this method there are two techniques to determine the normal sound absorption coefficient. They are standing wave tube method and two microphone tube method. The experiment was set up according to ASTM E1050/ISO 10534-2 test Method for Impedance and Absorption of acoustical Materials by Two Microphone Impedance Tube. This test method is similar to C384 test method which used an impedance tube with a sound source connected to one end and the test sample was mounted to the other end. However, the measurements techniques for these two methods were different. ASTM E1050/ISO 10534-2[13] standard was use as a research screening tool, useful for manufactures and researchers in evaluating the absorption of materials. This method been used in order to determined sound absorption coefficient of absorptive materials at normal incident, that is 0 degree by using the reflector of a surface on material. The reflector of a surface on material can be used to determine the absorption coefficient of a surface. The absorption coefficient of a surface is a very useful parameter as it states the fraction of that energy that is absorbed when sound is incident to a surface.
Fig. 1: Schematic diagram of Sound absorption coefficient measuring system Preparation of Samples There are four samples used in this experiment which are Jelutong (Dyera polyphylla), Selunsor merah (Tristianiopsis beccariana), Tapang (koompassia excelsa), and Pulai (Alstonia angustiloba). All these samples were fabricated using lathe machine by cutting it into a round shape. The sample was prepared according to the size of the impedance tube. The sample was cut into 15.5cm diameter with the thickness of 2.2cm. It is important to ensure that the sample was fit slung into the tube which is not so loosely in order to avoid the space between its edge and the holder. To achieve a very slight interference fit between the sample and the holder, materials to be tested were sealed around the edges using Vaseline to eliminate the air gap between the sample and the tube. A sound wave traveling down a tube is reflected back by the test specimen, producing a standing wave that can be explored with probe microphone. The normal absorption coefficient is determined from the standing wave ratio. In addition, an impedance ratio at any one frequency can be determined using the position of the standing wave with reference to the face of the specimen. The frequencies used in this experiment is 1/3 octave band frequencies ranging from 350 to 1000 hertz. Results and Discussion The data obtained in this experiment are given in the Table 1 as well as in the form of plots between the frequency (Hz) and sound absorption coefficients (α) in Fig. 2. Measurement of the transfer function between the two microphone signals along with the microphone spacing, the distance from the reference plane to the nearest microphone and the air temperature are required for the evaluation of the normal incidence acoustical properties of the specimen. Sound absorption
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coefficient of Malaysian woods was measured by the impedance tube method (ASTM E1050/ISO 10534-2). The first peak in the impulse response is the direct wave component, and the second peak is the reflected wave component. Frequency response function for both incident and reflected wave for this data are given in a form of sound pressure level (dB). Calculations of the normal incident absorption coefficient for acoustical materials were performed by processing an array of complex data from the measured transfer function. The data from incident and reflected component are used to determine sound absorption coefficient of tested material. Table 1: Summary of the Sound Absorption Coefficient of the Four Types of Wood in Study Frequency [Hz]
absorption coefficient
350 400 450 500 550 600 650 700 750 800 850 900 950 1000
Medium hardwood Light hardwood Sound Absorption Coefficients Tapang Selunsur Pulai Jelutong 0.024 0.022 0.023 0.023 0.027 0.024 0.025 0.025 0.029 0.029 0.029 0.030 0.033 0.032 0.032 0.033 0.038 0.034 0.036 0.036 0.039 0.035 0.037 0.036 0.044 0.039 0.041 0.038 0.046 0.044 0.041 0.045 0.048 0.046 0.045 0.045 0.038 0.034 0.039 0.036 0.047 0.043 0.049 0.045 0.058 0.053 0.054 0.049 0.066 0.059 0.063 0.058 0.063 0.061 0.064 0.062
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
α tapang α selunsur α pulai α jelutong Hz
Fig. 2, Summary of Normal Incident Absorption Coefficient for Tapang, Selunsor Merah, Pulai, and Jelutong Summary of the result obtained are shown in Fig. 2, the values for these four species of woods were determined from the resonant frequency of 350 Hz to 1000 Hz. This frequency is simply an average of all frequencies within the third-octave band which is frequencies of human hearing (because the frequency range for human hearing is between 0 Hz and 20 KHz). These range of frequency also been selected to satisfy the fundamental constraint that had been highlight by ASTM. According to the standard, the usable frequency range depends on the diameter of the tube and the spacing between the microphone positions. It is important to know the limits of the
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working frequency in order to maintain the plane wave propagation inside the tube. Thus, this low frequency zone was selected as the test frequency range. As indicated in Fig. 2 it can be seen that the sound absorption coefficients of medium hardwood Tapang and Selunsor Merah, light hardwood Pulai, and Jelutong increased with the frequency. However, the sound absorption coefficient decreased at the frequency 800 Hz and then increased again. This decreased and increased behavior was due to the specific characteristic of the material itself in reflecting sound at 800 Hz. This is because the absorption coefficient of a material depends on the material and frequency of the sound which strikes the surface of the materials. Conclusions The normal absorption coefficients of four types of Malaysian woods were studied, it increased with frequency from frequencies 350 Hz to 1000 Hz, the absorption coefficient of these woods lies within the range of 0.022 to 0.064 with the positive gradient. Medium hard wood Tapang has high absorption coefficient values for most of the frequencies compared to three other species. The sound absorption coefficient of medium hardwood Selunsor Merah, light hardwood Pulai, and Jelutong were slightly jumbled together i.e. increased with frequency. In conclusion, the experiments on determining the sound absorption coefficient of four species of Malaysian woods were successful conducted. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13]
Lewis, H. Bell, Industrial noise control, Fundamentals and applications, 2nd edition, New York: M. Dekker, 1994. Malaysian Timber Industry Board (MTIB), 100 Malaysian Timbers MTIB, Malaysia, 2008. C. Wassilieff, Sound absorption of wood-based materials, Applied Acoustics. 48 (1996) 339356. M.A. Delany and E.N. Bazley, Acoustic properties of fibrous absorbent materials, Applied Acoustics, 3 (1970) 105-116. C. Kang, J. Matsumura and K. Oda, A comparison of the standing wave and two microphone methods in measuring the sound absorption coefficient of wood, J. Fac. Agr. Kyushu Univ., 51 (2006) 1-4. S. Yoshikawa, Acoustical classification of woods for string instruments, J. Acoust. Soc. Am. 122 (2007) 568-573. M.R.F. Klidner and C.H. Hansen, A comparison and review of theories of the acoustics of porous materials, Int. J. Acoust. and Vib. 13 (2008) 112-119. M. Biot, Theory of propagation if elastic waves in a fluid saturated porous solid. 1. lowfrequency range, J.Acoust. Soc. Am. 28 (1956) 168-178. M. Biot, Theory of propagation if elastic waves in a fluid saturated porous solid. 2. higherfrequency range. J.Acoust. Soc. Am., 28 (1956) 179-191. V. Bucur, Acoustics of Wood, Springer, Germany, 2006. F. Alton Everest, Fundamental of sound, absorption of sound: “The Master Handbook of Acoustics, third ed., United states, McGraw-hill, 1994. M. moser, Sound absorbers: Engineering Acoustics an Introduction to Noise control, Berlin Heidelberg: Springer, 119-153 (2004) Robert Oldfield, Measuring Absorption Coefficient: “Improved Membrane Absorbers, University of Salford, Salford: UK, 41-56 (2006)
