Efficacy of nonwoven materials as sound insulator

Transcription

1 Indian Journal of Fibre & Textile Research Vol. 32, June 2007, pp Efficacy of nonwoven materials as sound insulator M D Teli a, A Pal & Dipankar Roy Department of Fibres and Textile Processing Technology, Mumbai University Institute of Chemical Technology, Mumbai , India Revised received 17 May 2006; accepted 10 July 2006 Nonwoven materials of various origins have been studied in terms of their efficacy in reduction of sound using tube setup. Various parameters, such as frequency of sound generated, distance between sound generator and sample, and physical features of samples (air permeability, thickness and GSM), are studied with reference to effectiveness of sound reduction of these fabrics. The sound reduction is also measured using another method resembling real life situation. It is observed that with the increase in frequency and GSM the extent of sound reduction increases while with the increase in air permeability, the extent of sound reduction by the material decreases. Although the extent of sound reduction values slightly differs, the order of efficacy of various nonwoven fabric samples indicated by both the techniques remains the same. This study enables the users to select an efficient material to be used in noise control. Keywords: Air permeability, Cotton, Nonwovens, Polyester, Polypropylene, Sound insulator, Viscose IPC Code: Int. Cl. 8 D04H 1 Introduction Noise is among the most invasive pollutants today. With rapid industrialization and consumerism of our society, we are constantly being cursed with number of pollution hazards. While air and water pollutions are well hyped, noise pollution remained in low profile due to lack of awareness about its ill effects especially in India. Unwanted and uncontrolled noise in work environment can negatively affect privacy or concentration of an individual, thereby reducing the productivity. There is ample evidence showing that the high noise levels interfere with speech and communication, cause sleep disturbance, decrease learning ability and scholastic performance, increase stress-related harmones and blood pressure, etc. Work-related hearing loss continues to be a critical health issue and workplace safety issue. The proper surroundings for working becomes a legitimate right of the workers and thus the modern machines carry special features such as reduced level of noise generation. It is high time, we respond to the need of the hour and take preventive measures. Noise barriers and noise absorbers made up of proper material and design can be effectively used in a To whom all the correspondence should be addressed. drmdteli@udct.org combating noise and realizing ideal ambient sound levels. They also help in enhancing articulation of speech as a result of good privacy. In order to determine the efficacy of such materials, the need of proper instrument has always been there which can measure the sound absorption properly. 1, 2 The theoretical investigation into the sound propagation through flexible porous media is of prime importance for evaluating the noise absorption capacities of foam materials or textiles, such as woven fabrics or nonwoven fibre-webs. Such materials can be considered as noise control elements in a wide range of applications, including wall claddings, acoustic barriers and acoustic ceilings. 3 Study shows that the noise absorption capacity of thermally bonded nonwovens depends primarily on the thickness and surface characteristics of specimens as well as on surface roughness and panel vibration. 4 A numerical method of calculating acoustic performance of nonwovens made from acrylic, cotton and polyester has been proposed and it gives the noise absorption coefficient of fibre-webs as a function of their thickness and porosity. 5 Recycled polyester nonwovens with different fibre types and web properties are studied by different researchers. It has been shown that the absorption coefficient is higher for the nonwoven having more

2 TELI et al.: EFFICACY OF NONWOVEN MATERIALS AS SOUND INSULATOR 203 fine fibres, as they have more chances to contact the sound wave. 6 The films are good reflective covers for sound waves. This fact leads to sound absorption and reduction particularly at high frequencies. Hence, in the design of such absorbers, it is necessary to take into account the film s influence on sound absorption. 7 There is an increasing penetration of nonwovens in automotive sector for their suitability for certain applications (e.g. sound insulation). Textile composite used in cars refers to combination of one or more textile and/or non-textile materials, e.g. foam and warp knitted fabrics used as upholstery or interior trim materials. One of the oldest uses of nonwovens in automotives is in noise damping, keeping things quiet. Impregnated jute or shoddy mat materials have been used almost from the beginning. 3,8,11 Two other ways to improve interior sound absorption are with seating and carpeting. Carpeting contributes to sound absorption in a number of ways. Cut-pile carpeting provides more absorption than a loop-pile of similar thickness, and increased pile height and thickness further increase the sound absorption The present investigation deals with the measurement of various nonwoven materials for sound reduction in a closed tube and the study of the effect of their physical properties like thickness, GSM and air permeability on the sound reduction. After studying the various fabric materials in the experimental set-up, a real life situation has been taken into consideration and the efficacy of the same nonwoven fabric in terms of sound reduction measured. The results of the two techniques have been compared. 2 Materials and Methods 2.1 Materials Nonwoven samples of nine different categories made up of polyester, cotton, viscose and polypropylene were used for the study. They were provided by Tata Mills, Mumbai, India. The specifications of the samples are given in Table Methods Measurement of Transmitted Sound A speaker (Philips speaker, 50 W, 8 Ω impedance) emitting predefined frequencies (125, 250, 500, 1000, 2000 and 4000 Hz) at a fixed intensity was set at one end of a PVC tube of the diameter 10 cm and a microphone (Shure, WL 50, and Omni directional) Nonwoven fabric Table 1 Physical properties of samples Sample code GSM Thickness cm GSM/cm Air permeability cc/cm 2 /s Polyester P P P Cotton C C C C Polypropylene PP Viscose V Fig. 1 Schematic diagram of tube set-up was attached on the other end coaxially with the speaker (Fig. 1). The sample, expected to reduce sound, was kept in between the sound source and the microphone. Samples were kept at 2.54 cm in front of the microphone to study the extent of sound reduction by the material. Length of the tube was kept at 50, 100 and 150 cm and the readings were taken in the absence and presence of the sample at each distance for all the six frequencies. The instrument described above enables to get root mean square voltage (Vrms), when sample is present or absent. Respective db with reference to 0.1 µv was calculated considering the presence and absence of sample, as shown below: Sound level (db) = 20 log 10 (V 1 /V 2 ) where V 1 is the microphone output; and V 2, the db reference (0.1 µv).

3 204 INDIAN J. FIBRE TEXT. RES., JUNE 2007 The percentage sound reduction 1 was calculated using the following equation: dbwos dbws Sound reduction (%) = 100 dbwos where db wos is the sound level without the sample; and db ws, the sound level with sample position. Average sound reduction is measured by averaging out sound reduction levels over the distances from 50 cm to 150 cm (total of three distances) at a fixed frequency, as shown below: Average sound reduction ( X ) = 1/ 3 3 X i i= 1 where X i = X 1, X 2, X 3 are the sound reduction levels at distance of 50 cm, 100 cm and 150 cm respectively. Mean sound reduction is measured by averaging sound reduction over a distance of cm and then over a frequency range of Hz (total of six), as shown below: Mean sound reduction = 1/ 6 6 j= 1 where X j = X 1, X 2, X 3, X 4, X 5, X 6 are the average sound reduction at the frequency of 125, 250, 500, 1000, 2000 and 4000 Hz respectively Evaluation of Efficacy of Sound Reduction of Samples in Real life Situation A motor of 4000 rpm, 0.6 amp was set inside a cylindrical horizontal frame as shown in Fig. 2. The frame was raised above the table surface by wooden support. Microphone was attached with the help of a holder perpendicular to the axis of motor and maintained at 30 cm distance from the motor. Measurement was taken without sample and then after wrapping the motor with the fabric sample to be tested. X j polypropylene(pp) are shown in Fig. 3. The extent of average reduction in sound (%), which is taken over three distances, increases with the increase in frequency, irrespective of the distances between the speaker and the sample for almost all the samples studied. However, the percentage reduction in sound is found to be maximum at 4000 Hz frequency and also at the maximum distance (150 cm). In general, the lower reduction is observed in the frequency range of Hz and at a minimum distance of 50 cm. Results indicate that the average sound reduction values are in the range of 1-8% when the frequency varies from 125 Hz to 4000 Hz. However, at 2000 Hz and 4000 Hz frequencies, the variations in sound reduction due to samples variety are quite different and the average sound reduction varies in the following order of fabric samples: P1>C2>C4>C1>C3>PP>P2>V>P3 Similarly when the mean sound reduction is calculated averaging over the frequencies, results show the following order: P1>C1>C2>C3>P2>C4>PP>V>P3 Figure 4 clearly indicates that the mean sound reduction (%) observed in the case of P1 is maximum Fig. 2 Experimental set-up for real life situation Measurement of Air Permeability Using KarlFrank Permeometer type-843 Number 61, the air permeability of the samples was measured in terms of cubic centimeter of air passing per second per unit area of the sample. 3 Results and Discussion 3.1 Efficacy of Nonwoven Fabrics for Sound Reduction in Tube Set-up The results with respect to these nine nonwoven samples of polyester(p), cotton(c), viscose(v) and Fig. 3 Relation between frequency and average sound reduction of different nonwoven fabrics

4 TELI et al.: EFFICACY OF NONWOVEN MATERIALS AS SOUND INSULATOR 205 Fig. 4 Comparison of sound reduction between tube set-up and actual conduction and that of P3 is minimum. This may be attributed to the highest GSM (999.5) of P1 which may be responsible for absorbing the sound energy during the maximum interaction of sound waves with the substrate over 0.3 cm thickness. The air permeability of the P1 sample is much reduced (Table 1) and hence such a dissipation of energy is maximum, giving the highest extent of reduction in sound. The least of air permeability (68.28 cc/cm 2 /s) was displayed by cotton sample C2 giving second order in mean sound reduction. In an average sound reduction at 4000 Hz frequency, C1 exhibits such a second order efficacy. Both these samples C1 and C2 have very low air permeability with equivalent thickness, although C1 possesses slightly higher GSM than that of C2. The samples P1, C1, C2 give the GSM per unit cm of the thickness (Table 1) much higher than 3000 and at the same time their air permeability values are somewhere in the range of cc/cm 2 /s and hence P1, C1 and C2 samples show the highest mean sound reduction as well as highest extent of average sound reduction level. The mean sound reduction level of C4 (cotton) is also found to be higher mainly because its GSM per unit thickness value is relatively high ( ) although its air permeability is relatively low. The P3 sample shows the least of extent of mean sound reduction, basically because the GSM per unit thickness is almost minimum and at the same time air permeability is second most highest. This is followed by the viscose whose GSM per unit thickness is second last and the air permeability is also next to P3. It is clear from the Table1 that the nonwoven fabric samples because of their differing air permeability and GSM values give varied response with regard to mean sound reduction. The sample P2 gives middle order sound reduction even though it has lowest GSM per unit thickness and highest air permeability and is found to be typically different than any other fabric samples. Its thickness is almost 3-10 times of the other samples. This sample displays enhanced level of absorption of sound, may be attributed to its open structure and subsequent dissipation of sound energy which is quite moderate. Polypropylene sample shows a low order mean sound reduction, which may be attributed to its low order GSM per unit thickness and relatively higher air permeability. In other words, air permeability, GSM, thickness and fabric structure significantly affect the extent of sound reduction. 3.2 Efficacy of Nonwoven Fabrics as Sound Insulator in Actual Condition Nine nonwoven samples tested earlier in tube setup were again tested for their efficacy in this set-up resembling real life situation. The results of mean sound reduction (%), measured at 30 cm distance, are shown in Fig. 3. The order in which the efficacy of these samples acting as a sound barrier decreases from left to right (Fig. 4) is shown below: P1>C1>C2>C3>C4>PP>P2>V>P3 This order is found to be good in comparison to earlier findings using tube set-up, in terms of mean sound reduction (Fig. 4). It is interesting to note that the measurements, done earlier using tube set-up on the fabric samples to estimate their effectiveness as sound barriers, are found in conformity with the results obtained in the real life situation and hence this method is successful in ascertaining the efficacy of various samples to act as sound barrier. However, the comparison of the fabric sample is not critically discussed, simply because the nonwoven samples used are randomly selected based on the availability and our aim was to test the applicability of the measurement of sound reduction method vis-a-vis the one based on testing of random samples in real life situation. 4 Conclusions The efficacy of a material as a sound (noise) barrier depends on frequency of the sound wave to which material is exposed to, GSM and air permeability of the substrate, thickness and construction of the material, etc. In general, with the increase in frequency, GSM and the distance from the source, the extent of sound reduction increases while with the increase in air permeability, the extent of sound reduction by the

5 206 INDIAN J. FIBRE TEXT. RES., JUNE 2007 material is decreased. The method based on tube setup for sound reduction is found to be in agreement with that representing real life situation. Since the order of performance of the materials predicted in this study goes hand in hand with real life experiments, this technique of testing various sound insulators could be considered as a reliable one. Acknowledgement One of the authors (MDT) is grateful to Mr D V Karandikar of Electrotech Enterprises, for his keen interest and precious assistance in fabricating the assembly for measurement of sound absorption. References 1 Porges G, Applied Acoustics (Edward Arnold Publications Ltd, London), 1977, Practical Building Acoustics (John Wiley and Sons Inc., New York), 1976, Shoshani Y & Yakubov Y, Appl Acoust, 59 (2000) Lee Y E & Joo C W, J Appl Polym Sci, 92 (2004) Shoshani Y & Yakubov Y, Text Res J, 69 (7) (1999) Lee Y & Joo C, Autex Res J, 3 (2) (2003) Voronina N, Appl Acoust, 49 (2) (1996) Sasile S & Langenhove L, J Text Apparel Technol Management, 3 (4) (2004) (16/08/2004). 10 Shoshani Y, Text Res J, 60 (8) (1990) Shoshani Y & Wilding M, Text Res J, 61 (12) (1991) 736.