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Niresh et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945 Research Paper DEVELOPMENT OF AN INSTRUMENT TO MEASURE SOUND ABSORPTION Niresh J1, Neelakrishnan. S2, Subha Rani. S3 Address for Correspondence 1,2 Department of Automobile Engineering, 3Department of Electronics and communication Engineering, PSG College of Technology, Coimbatore, Tamilnadu, India – 641004. ABSTRACT: Impedance tube is an important device used to measure sound related properties of a material, for measuring the sound absorption characteristics of a material. The most commonly used methods are the standing wave ratio method and transfer function method. In this study a modified impedance tube with single microphone method is implemented. This type is commonly used for determining the normal incidence sound absorption coefficient. The aim of this study is to determine the accuracy of the sound absorption coefficient of various textile materials by comparing measurements from a commercially available impedance tube with the developed tube using various test samples. The developed tube can measure sound absorption coefficient in the frequency range within 80 Hz-1600 Hz. In this study various parameters such as frequency of sound generated, distance between the sound generated and physical features of samples are studied, Two different samples, Polypropylene and Coir fibers are tested in this tube and results are analyzed. The samples show significant amount of absorption at frequencies around 1KHz. The modified circuitry provides better results at higher frequencies when compared with the standard values. KEY WORDS: Impedance Tube, Sound Absorption, Standing Wave, coir fibers, polypropylene. 1. INTRODUCTION With the increase in public awareness and concern for noise pollution, there is a need to control noise by absorbing sound in various applications involving sound producing elements and sound absorbing materials. Many kinds of sound absorbing or isolating materials, such as glass wool, polymeric fibrous materials, and various types of foams are currently used to absorb airborne noise and optimize the transmission loss (TL). The factors that mainly influence acoustic performance of sound absorptive materials are fibre dimension, material thickness, density, air flow resistance and porosity which will be changing the sound absorbing behavior[1],[2],[3],[4]. Various studies have proved such as polyster[3], coir[4], wool[2], cotton[4]. Various research have proven that sound absorption coefficient is more for the nonwoven have more fine fabric as they have more chances to contact with the sound waves[5]. Nonwovens materials are used in the automobile sector for certain application eg foam and warp knitted fabrics are used as upholstery or interior trim materials. The present investigation deals with the measuring sound absorption coefficient of various materials in the closed tube. Automobiles come with different noise producing elements whose frequency ranges from 200 Hz to 3 kHz[6], and hence require noise control materials with selective acoustic properties for different frequency range of interest. Measuring the acoustical properties and predicting the noise control impact of these materials at the design stage is important. The properties of various textile materials can be determined either theoretically using the empirical prediction[7] based on regression analysis[8],[9] or experimentally using impedance tube. The quantification of acoustical impedance is an essential task in the design of acoustical systems such as horns, acoustical pipes, absorbent materials and silencers. The mechanical impedance is obtained by dividing the applied force to the particle velocity. In addition acoustical impedance is the applied pressure divided by volume velocity[10]. Therefore, the relationship between mechanical and acoustical impedances is derived from the force-pressure and volume-velocity. However, most of the present acoustical impedance Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267 quantification methods used today such as ISO10534-1[11] and ISO-10534-2[12] do not provide a direct quantification of volume velocity. There are two standardized methods for measuring the normal sound absorption coefficient for small samples, one using standing wave ratio standardized in ISO10534-1 and the transfer function method standardized in ISO-10534-2. The standing wave ratio (SWR) method is named as "classical kundt duct" in ISO-10534-1. In this method, one end of the kundt duct is closed while a speaker is placed at the other end to generate a standing wave within the duct. A microphone is moved along the duct to quantify and locate the maximum and minimum pressure amplitude points along the duct. The standing wave ratio and the acoustical impedance are then calculated accordingly[13]. 2. MATERIALS AND METHODS: Sample of different categories are made up of coir fibre, polyester samples are provided by the PSG Tech Centre of Excellence, Coimbatore, INDIA, the specifications are listed in Table1. Table1. Physical properties of samples Physical properties of sample Polyester Coir Fiber Straight cross section Coir Fiber Diagonal cross section Sample code GSM Thickness cm P1 C1 224 263 2.0 0.8 Air permeability Cc/cm2/s 2396.74 900.36 C2 271 0.9 898.76 2.1 System Implementation The impedance tube set up contains a signal source along with power amplifier section. The impedance tube signal characteristics is improved by modifying the signal source from generating monotone signals into one that generates white noise signals The main purpose of adding white noise is to provide a uniform reference frame in the time-frequency space[14]. The noise circuitry is designed by adding the signals generated from two similar signal generators, through a resistive network. The signal thus generated, is amplified using a power amplifier circuit and fed into the impedance tube through a loud speaker. Niresh et al., International Journal of Advanced Engineering Technology Microphones that are placed inside the tube capture the incident signal and reflected signal from the sample. These signals are too weak to be transmitted to recording devices and hence preamplifiers are used to increase a microphone signal to line-level by providing stable gain while preventing induced noise that would otherwise distort the signal. A microphone preamplifier increases the 0 to 100 microvolt range level up by 40 dB, to approximately 0 to 10 volts. The amplitude of the incident and reflected signals vary according to the sound absorption of the sample. Therefore by monitoring the amplitude variations of both signals and taking the ratio of it, the sound absorption coefficient of material is calculated. A digital storage oscilloscope, spectrum analyzer or a PC is used for storing and analyzing these signals. 2.2. Electronic circuit design The power amplifier circuit is to have high gain and signal filtering characteristics to achieve high signal fidelity. The microphone probes are made of double core shielded wires to eliminate noise interference. The signal from the microphone is amplified by a newly designed pre amplifier circuit. The circuit takes the audio signal from the condenser microphone and amplifies it, so that the microphone output can be used as input to some device which wouldn’t normally accept microphone level signals (which are very low). The output of the microphone is fed to a two stage amplifier. There is a current series feedback and a voltage shunt feedback configuration configured to pick up smaller sounds. The output can be varied with a 10 kΩ potentiometer. Since the microphone circuit is biased, the output is a sinusoidal signal. 2.3 Measurement of Air permeability This test method covers the measurement of the air permeability the rate of air flow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a material of textile fabrics and is applicable to most fabrics including woven fabrics, air bag fabrics, blankets, napped fabrics, knitted fabrics, layered fabrics, and pile fabrics. The fabrics may be untreated, heavily sized, coated, resin-treated or otherwise treated. It is measured by using Air Permeability Tester FX 3300 Lab Air IV. 3. SYSTEM ANALYSIS The complete experimental set up to measure the acoustical characteristics in given in Fig.1, which contains a power supply, amplifier and sound source, microphone and pre-amplifier circuit, impedance tube and data acquisition system (spectrum analyzer). Fig 1 Schematic measuring setup Two different materials are tested in the designed impedance tube. The sample holder is removed from Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267 E-ISSN 0976-3945 the impedance tube set up. The samples with diameter equal to that of impedance tube are pasted above the surface of the movable termination which runs along the length of the sample holder. The signal generator excites the loud speaker and the movable termination is moved to a position in impedance tube where maximum peak voltage captured from the microphone occurs. The absorption coefficient is calculated by taking average of repeated measurements at the same frequency. The normal and average absorption coefficient is calculated as in equation (1) and (2). = (1) αavg= ( α1+ α2+ α3+ α4+ α5+ α6+ α7) /7 (2) Three different materials Polyester and coir fibers, are tested in impedance tube. The coir fiber materials are tested with two different configurations. The sample holder is removed out from the impedance tube set up. The samples with the diameter equal to that of impedance tube are pasted above the surface of the movable termination which runs along the length of the sample holder. The signal generator is made to excite the loud speaker and the movable termination is moved to a position in impedance tube where maximum peak voltage captured from the microphone occurs. 3.1 Evaluation of samples The sample of polyester sample is shown in fig.2. Which is tested in the modified impedance tube and the existing impedance tube. Fig. 2. Polyester sample The absorption coefficient is calculated by taking average of all the measurements. The normal and average absorption coefficient is calculated from the equation and the values are tabulated in Table 2 Table 2. Experimental results for Polyester F(Hz) Amplitude Without Sample(v1) Amplitude With sample(v2) Absorption coefficient (α) Existing Impedance tube (α) 411 554 870 1190 1380 1540 0.8 1.8 1.6 0.9 0.84 1 0.72 1.7 1.12 0.4 0.72 0.6 0.10 0.06 0.30 0.56 0.14 0.40 0.16 0.09 0.33 0.57 0.19 0.45 Fig 5 shows the absorption characteristics of Polyester with thickness of 20cm. The average absorption coefficient for Polyester is 0.33. It is seen from the figure that the Polyester material shows higher absorption at 2 KHz region. Because of long time, large surface to volume ratios and high heat conductivity of fibers, heat exchange takes place isothermally at low frequencies. So the absorption is low at low frequencies. At the same time in the high frequency region compression takes place adiabatically. In the frequency region between these isothermal and adiabatic compression, the heat exchange results in loss of sound energy. This loss is high if the sound propagates parallel to the plane of fibers and may account up to 40% of sound attenuation. It is suitable for higher frequencies than Niresh et al., International Journal of Advanced Engineering Technology lower frequencies for noise absorption. The first peak occurs at around 1 KHz and the second peak is observed at same bandwidth. The experimental results are comparable to actual result at the frequency of above 1 KHz. The other sample coir fiber with straight cross section and diagonal section which is used for the testing purpose is shown in fig 3 and fig 4. E-ISSN 0976-3945 Fig. 4. Coir fiber with diagonal cross section The experimental results for coir fiber diagonal cross section is shown in table 3. Table 4. Experimental results for Coir Fiber Diagonal cross section Fig. 3. Coir fiber with straight cross section The average absorption coefficient of coir fiber with diagonal cross section is 0.13 .The coir fiber with diagonal cross section is less absorbing than the coir fiber with straight cross section. The primary factors causing sound attenuation when the sounds contact a sound absorber are frictional losses, momentum losses and temperature fluctuations[15]. Table 3. Experimental results for Coir Fiber Straight cross section F (Hz) Amplitude Without Sample(v1) Amplitude With sample(v2) Absorption coefficient (α) Existing Impedance tube (α) 411 554 870 1190 1380 1540 0.92 1.6 1.55 0.76 0.84 1 0.84 1.55 1.45 0.66 0.8 0.88 0.09 0.03 0.06 0.13 0.05 0.12 0.12 0.06 0.09 0.15 0.05 0.15 F(Hz) Amplitude Without Sample (v) Amplitude With sample (v) Absorption coefficient (α) Existing Impedance tube (α) 411 554 870 1190 0.92 1.6 1.55 0.76 0.84 1.55 1.25 0.68 0.09 0.03 0.19 0.11 0.12 0.06 0.22 0.12 1380 1540 0.84 1 0.82 0.9 0.02 0.10 0.06 0.11 The same samples are tested using the commercial tube available in the market and a comparison is made between both the readings to ensure the efficiency of the designed tube in measuring the absorption coefficient. The difference between the modified tube and the existing tube is projected in the figure which show that for all samples, the developed tube provides a near value of absorption coefficient to that of the existing tube. Fig 6 shows comparatively lower absorption than the Polyester. It is showing lowest absorption for frequencies lower than 500 Hz. The average absorption coefficient of coir fiber with straight cross section is 0.12.The different absorption results at different frequencies are attributed to the frequency dependence of the sample physical material properties like thickness, porosity, and flow resistivity. Fig.5 Relation between Frequency and sound absorption coefficient of materials . 0.6 0.5 0.4 0.3 0.2 0.1 0 Polyester Absorption coefficient (α) Polyester Existing Coir Fiber Straight Coir Fiber Straight Coir Fiber Diagonal Coir Fiber Diagonal cross section cross section cross section cross section Impedance tube (α) Absorption Existing Impedance Absorption Existing Impedance coefficient(2) tube(2) coefficient(3) tube (3) Fig. 6 Comparison of sound absorption for materials tested It is observed that the values measured using the proposed tube deviate from the original tube by an average of 4.55% for most of the frequency ranges. The different absorption results at different frequencies are attributed to the frequency dependence of the sample's physical properties like thickness, porosity, and flow resistivity. The amplitude vs frequency for two different samples is plotted in figure 7. The observed error may be due to the lower sensitivity of the microphone and offer Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March.,2016/264-267 electronic components used for the developed tube. Further the error can be considered as minimum when compared to the existing tube. However the low cost tube can be implemented for measuring the sound absorption coefficient of the material by the industries at the initial stage of the material. Also, calibration of the developed tube may lead to decrease in the error. Niresh et al., International Journal of Advanced Engineering Technology [8] [9] [10] [11] [12] Fig. 7. Amplitude vs Frequency for different materials 4. CONCLUSION A low cost impedance tube is designed and developed with modified electronic circuitry for the measurement of absorption characteristics of fifteen sound absorbing materials encountered in vehicles. Standing wave method of measurement is used according to ISO 10534-1 regulation. The results of the absorption characteristics of various materials were compared with the existing tube and the results prove the efficiency of the designed tube as the error rate is only around 4.5% for all the samples. The cost of the impedance tube is reduced to around 35% of the commercially available impedance tube and hence the developed tube is cost effective. This paper discussed about the modified implementation of the electronic circuitry for the measurement of absorption characteristics of the various porous materials. The results of the absorption characteristics of coir fiber and the Polyester are compared. The experimental results for polyester are matching with standard values above 1 KHz. The Polyester shows higher absorption with absorption coefficient of 0.6 than coir fiber material for frequencies around 2 KHz. The coir fiber with straight cross section provides better absorption at 0.6 whereas the coir fiber with diagonal cross section provides lesser absorption at 0.5. Thus it is concluded that the coir fiber with straight cross section is compatible with the Polyester. 5. ACKNOWLEDGMENT The support extended by PSGTECHS Centre of Excellence for Industrial Textiles for carrying out this study is acknowledged. REFERENCES [1] [2] [3] [4] [5] [6] [7] Y. Z. Shoshani and M.A Wilding, "Effect of Pile Parameters on the Noise Absorption Capacity of Tufted Carpets,"Text Res J, Vol no. 61,pp.736,1991. YoungJoo Na, Jeff Lancaster, John Casali and Gilsoo Choo, "Sound Absorption Coefficients of Micro-fiber Fabrics by Reverberation Room Method,"Text Res J, Vol. 77(5),pp.330,2007. M.D. Teli, A. Pal and Dipankar Roy," Efficacy of nonwoven materials as sound insulator," Indian J Fibre Text Res, Vol.32, pp.202,2007. 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Douglas C, Montgomery, Engineering Statistics, 2nd edition(Arizona State University USA), 2001, 293-315. Kinsler Lawrence E, Fundamentals of acoustics (John Wiley & Sons, USA) 2000. ISO standard 10534–1, "Acoustics determination of sound absorption coefficient and impedance in impedance tube" Part 1: Method using standing wave ratio, 1996. ISO standard 10534–2, "Acoustics determination of sound absorption coefficient and impedance in impedance tube" Part 2: Transfer function method, 1998. R.Boonen ,P. Sas, W. Desmet,W. Lauriks and G. Vermeir, "Calibration of the two microphone transfer function method to measure acoustic impedance in a wide frequency range," Proc Int Conf Noise Vibrat Eng (Leuven, Belgium), pp.4502–4512, 2006. Marius Deaconu, "Comparative analysis of transfer function and standing wave methods in determination of acoustic absorption coefficient," in Int Symp Acoust (Romanian Academy, Bucharest) 22-23 May 2014. 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