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Recycled materials as a potential replacement to synthetic sound absorbers: A study on denim shoddy and waste jute fibers
Applied Acoustics 159 (2020) 107070
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Recycled materials as a potential replacement to synthetic sound absorbers: A study on denim shoddy and waste jute fibers Manish Raj, Shahab Fatima, Naresh Tandon ⇑ ITMME Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Received 18 April 2019 Received in revised form 23 September 2019 Accepted 27 September 2019
Keywords: Sound absorption coefficient Full factorial design Miki model Environment-friendly sound absorber Sustainable materials Noise control
a b s t r a c t Sound absorbing materials used for noise control is desired by the practitioners to be environment friendly and of low cost. This has motivated the researchers to look for alternate materials from natural sources. One step in this direction is exploring the use of recycled materials for potential application as sound absorbers. This paper identifies two materials, denim shoddy and waste jute fibers. They are characterized by their acoustical and physical properties. Experiments have been conducted as per 2-level 2factor full factorial design of experiments. A comprehensive comparison of the sound absorption coefficients of denim shoddy and waste jute fibers, measured in an impedance tube and predicted by using empirical relations are shown. The absorption spectrum of these materials are compared with commercial glasswool with similar process parameters, and the results obtained show denim shoddy to be a better sound absorber than that of glasswool. A mathematical equation for Noise Reduction Coefficient is formed using the experimental results obtained and validated with a fresh set of experiments. Finally, economic comparison of these materials is conducted. The results obtained from technical and commercial research clearly states that the application of these materials for noise control will fetch better results apart from being cost-effective and environment-friendly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Noise pollution is one of the outcomes of the developments taking place in the twentieth and twenty-first century owing to the revolution across industries to ease human life. It used to be ignored earlier, but gradually researchers figured out its harmful effects on human beings [1,2] which led to law enforcement agencies to bring out new regulations [3–5] to keep it under the prescribed limit. One of the ways to achieve this is to use sound absorbers, and for this purpose the material that has been used is glasswool. Glasswool serves its purpose well but has a severe effect on the environment and the health of the people associated with its production or application [6,7]. This motivated the researchers to look for other materials, and one of the steps in this direction is studies on natural materials like jute, coir, cotton, etc. Just like glasswool, these natural materials are porous in nature and sound energy is dissipated in their pores, they are light in weight and have an additional advantage of being environment friendly and without any health hazard.
⇑ Corresponding author. E-mail address: [email protected] (N. Tandon). https://doi.org/10.1016/j.apacoust.2019.107070 0003-682X/Ó 2019 Elsevier Ltd. All rights reserved.
Many researchers around the globe have reported about the use of natural materials for acoustic applications. One of the first preliminary studies was reported around the year 2000, when sound absorption and sound insulation properties of various materials were compared with an intention to select the best material for automotive NVH applications [8] and it was found that the performance jute and cotton are very much comparable to that of commercial glasswool. This was followed by research on bamboo fiber felt and the effect of variation in its thickness, density, and air gap was reported [9]. These preliminary results were very promising which motivated other researchers to report the natural materials locally available to them. Coir panels prepared from coconut husk and latex were studied for their acoustical behavior in conjunction with air gap and perforated plates [10]. Later, the same authors characterized and studied the acoustical properties of fresh coconut husk in fiber form and its performance was compared with the commercially available coir [11]. Many other materials like cotton, wool, jute, sisal, ramie, flax, bagasse were also studied sub sequentially and the results obtained were very promising [12–14]. The performance of natural materials in laboratory was very promising, and then a study was conducted for its actual implementation. A study of Jute in fiber form, felt form and composite with latex rubber as a binder was reported in which
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Noise Reduction Coefficient (NRC) of up to 0.77 was obtained [15]. Further, these authors extended their study and demonstrated to the world that it was possible to actually reduce the noise by using these materials [16,17]. All these natural materials already have some primary application as in coir is used to make ropes, jute for making bags, cotton for clothes, etc. This motivated the researchers to further cut down the cost of sound absorbers by exploring materials which do not have any primary application and are a waste of some processes. One of the first recycled material studied for this was industrial tea-leaf-fiber and its samples of different thickness backed by woven cotton cloth were studied in which an NRC values up to 0.5 was reported [18]. Similar research extended towards recycled rubber particle [19], oil palm empty fruit bunch fiber [20], orange tree pruning pulp fiber [21], cigarette butt [22,23], oil palm kernel shell and cockle shell [24], by varying their process parameters. These studies inferred that recycled materials have a good amount of potential for its potential application as a sound-absorbing material. This work is an addition in exploring some industrial waste material for potential application as a sound absorber. It highlights the performance of waste materials obtained from the fabric industry of denim and jute. They are characterized for their physical properties and an experiment has been designed as per a twolevel two-factor design of experiments. With relevant parameters, their samples have been fabricated and characterized for their acoustical properties. Further, their normal Sound Absorption Coefficient (SAC) is measured in a 2-microphone impedance tube as per transfer function method. The experimentally measured SACs are compared with the ones obtained by using commercial glasswool with same physical parameters. The experimentally measured SAC is also compared with the one estimated empirically by using Delany-Bazley and Miki models. The experimental result to measure the SACs is then used to develop a mathematical equation to estimate the NRC from experimentally obtained results and
Table 1 List of commonly used abbreviations. SAC NRC Zc
c j
Co f
Sound absorption coefficient Noise reduction coefficient Characteristics impedance Propagation constant pffiffiffiffiffiffiffi 1 Speed of sound in air frequency
is validated experimentally. Finally, a basic economic comparison of denim shoddy, waste jute fibers, and commercial glasswool is then shown to obtain the same amount of noise reduction in a room. Table 1 shows the commonly used symbols in this study.
2. Materials and their physical characterization Denim is the widely used fabric across the world which is mainly used in the form of jeans. The yarn used to make it usually low count which makes it very cost-effective material. The finishing operation of this fabric to jeans involves a lot of tailoring operation which generates waste denim fabric. This waste fabric is collected from the industries and shredded through rag machine and teaser machine which opens up the material back into threaded chunks, known as denim shoddy. Jute is one of the primary agricultural products of eastern parts of India and Bangladesh. Its fibers are obtained from the bast of jute plants and used to make many products such as bags which mainly require long fibers. The fibers which breaks out from other fibers while processing is mainly of shorter length (20–50 mm) which does not remain of much significant application. Both these materials were procured locally (from ESKAY INTERNATIONAL, Kolkata, West Bengal, India) and are shown in bulk form in Fig. 1, after drying them in the sun for around 8 h to remove any moisture trapped within them. Diameters of these fibers are measured on a Leica 205 FA fluorescence stereo microscope with different magnifications varying between 40 and 100. The diameter measured on 30 different fibers and the average fiber diameter of denim shoddy is found out to be 9.8 ± 0.3 mm and waste jute fiber is 24.60 ± 1.8 mm. The scanning electron microscope (SEM) images are taken on the ZEISS EVO MA10 scanning electron microscope. The fibers are first gold coated for 2 min to avoid the charging effect due to electrons. These microscopic and SEM images of both fibers are shown in Fig. 2. Micro fibrils, split strands, disruption in the physical structure of the fibers along the length and surface textures are observed in the SEM image of waste jute fibers (Fig. 2(d)). The SEM image of denim shoddy fibers (Fig. 2(c)) is free of such disruptions, and thus it is concluded that denim shoddy fibers appear to be smoother as compared to the waste jute fibers. The strands on the waste jute fibers boost up the sound absorption. The twists present in denim shoddy fibers make it curlier than the waste jute fibers. It implies that the sound absorber made by this material will
Fig. 1. Materials in bulk form a) Denim shoddy fibers b) Waste jute fibers.
M. Raj et al. / Applied Acoustics 159 (2020) 107070
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Fig. 2. Microscopic and SEM image of fibers a) Denim shoddy microscopic image b) Waste jute fiber microscopic image c) Denim shoddy fiber SEM image d) Waste jute fiber SEM image.
have a greater pore length because of the curliness of the pores; this will increase the sound absorption. 2.1. Density measurement One of the most important properties of acoustic materials is their density. Its direct estimation by dividing mass with volume is not a feasible option because of the very lightweight of a single fiber, plus their porous structure which makes it difficult to estimate their exact volume. So, the alternate method is to try by buoyancy method where the volume of the fiber chunk is estimated by amount of liquid displaced. This method works quite well with organic liquids like toluene but is pretty timeconsuming. Hence, in this study the robust method of density estimation by floatation method was adopted. In this method, a liquid column is prepared by mixing two chemicals in such a way that, in the resultant column the density linearly varies with the depth of the column. The two liquids are selected in such a way that density of the fiber is in between these two liquids. Thus, n-Heptane (density = 685 kg/m3) and Carbon tetrachloride (density = 1590 kg/m3) is selected to cover a wide region of interest Table 2. The next step is to prepare a liquid column of uniformly varying density. It has been found from works of literature that density of most of the natural fibers is higher than 1300 kg/m3 [25,26]. For Table 2 Volume of chemicals to be mixed to achieve the desired density. Solution A
Solution B
q = 1300 kg/m3
q = 1580 kg/m3
V1 = 277 ml V2 = 583 ml
V1 = 10 ml V2 = 850 ml
this reason, the column is designed for a minimum density of 1300 kg/m3 at the top and a maximum density of 1580 kg/m3 at the bottom. This is achieved by mixing two solutions, solution A and solution B formed by mixing the two chemicals (with volumes V1 and V2) in such a way that, V1 q1 þ V2 q2 ¼ Vq. where, V1 = volume of chemical with low density (n-Heptane) V2 = volume of chemical with high density (Carbon tetrachloride) q1 = density of chemical with low density (685 kg/m3) q2 = density of chemical with high density (1590 kg/m3) V = Half the volume of the column to be prepared (860 ml) q = Resultant density of the solution achieved by mixing two chemicals (1300 kg/m3 for solution A, and 1580 kg/m3 for solution B) Table 2 summaries the volume V1 and V2 taken to form solution A and solution B. Solution A and B are steered magnetically in a flask for around 2 h to achieve uniform mixing. They are poured in a vertical column slowly in such a way that a column with linearly varying density is formed. Fig. 3 shows the vertical column, which is calibrated by standard floats by dropping them in the column. The float sinks at a level where the density of liquid surrounding it is equal to the density of the float. Thus, the column is calibrated by measuring the position of the floats measured by the microscope mounted. Small samples of both the fibers are placed slowly in the prepared column. The sample starts drowning and sinks at position proportional to its density. Measurement is conducted on ten samples, and the average density of the denim shoddy and waste jute fibers are found to be 1501 ± 13 kg/m3 and 1452 ± 8 kg/m3 respectively.
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be observed that the failure plane is at an angle to the crosssection of denim shoddy fiber indicating it to be a ductile material, the waste jute fiber failed normal to its cross-section which means it is a brittle material. This is further confirmed by looking at Table 3 which summarizes the results, and it is observed that the strain at maximum load and strain at break in denim shoddy is greater than 10% while for waste jute fibers is less than 5%. The maximum load borne by denim shoddy is greater than that of waste jute fibers which makes denim shoddy a stronger fiber. 2.3. Thermo-gravimetric analysis Sound absorbing materials are often used in mufflers, heating, ventilation, and air-conditioning systems, which has very harsh temperature conditions. Thus thermal stability of these materials becomes an important parameter, which needs to be studied. It was studied on Linseis TGA PT 1000. Chopped fibers of 10 mg and size less than 100 mm was heated from 70 °C to 700 °C with a heating rate of 10 °C/min. The results are shown in Fig. 5 in which it is observed that the materials are thermally very stable till 250 °C beyond which they start decomposing. The decomposition rate increases rapidly after 300 °C and till the temperature is increased to 400 °C, 60% and 80% reduction in mass is achieved for denim shoddy and waste jute fibers respectively. Thus denim shoddy is thermally superior to that of denim shoddy. 3. Design of experiments and sample fabrication Fig. 3. The prepared column to measure the density.
2.2. Physical strength of fibers The physical strength of these fibers is measured on the Instron 5960 tensile testing machine as per standard ASTM-D-3822 [27]. The sample length of denim shoddy fibers was kept at 10 mm and of waste jute fibers was 25 mm. The crosshead speed was kept at 1.5 mm/min in such a way that the specimen fails within 20 ± 3 s. The failed fiber samples are shown in Fig. 4. The failure plane provides a fair idea of the ductility or brittleness and it can
The outcome of the literature review suggests that the thickness and density of the sound absorbers are the two major parameters governing the absorption behavior. Providing air gap behind the sound absorber just boosts up the low-frequency response. Hence this study proceeds by varying the thickness (a) and density (b) of the sound absorbers. Sound absorption performance of a material is frequency-dependent term, and the most efficient performance is obtained at a quarter wavelength thickness, but this criterion fetches non-viable thickness particularly below 1000 Hz. This study takes the primary thickness of around one-tenth of wavelength at 500 Hz, i.e. 70 mm. To fabricate the test samples of this
Fig. 4. Tensile failure of fibers a) Denim shoddy b) Waste jute fibers.
M. Raj et al. / Applied Acoustics 159 (2020) 107070 Table 3 Results obtained from tensile test. Parameter
Denim shoddy
Waste jute fiber
Maximum Load [N] Strain at maximum load [%] Strain at break [%]
2.889 10.527 12.902
1.295 2.048 2.052
5
The required amount of mass as per the sample number of experiment point is weighed and filled in the mould. The deadweight of 10 kg is kept on the mould to bring the test samples in shape. The mould used and specimen test samples prepared of both the fibers are shown in Fig. 7. After conducting the experiment on a prepared sample, it is destroyed by opening the fibers by hand, and the test samples are made again with the same fibers. The same material is used just to avoid any nuisance factor in the experiment. In this way, 2 runs are conducted at each experiment point for each test samples. 4. Acoustical measurements This section discusses measurements and prediction of various acoustical properties of the denim shoddy and waste jute fibers as per the designed experiments. 4.1. Acoustical characterization The major parameter to characterize a sound-absorbing material is its porosity, tortuosity, airflow resistivity, viscous characteristics length and thermal characteristics length [28–30].
Fig. 5. TGA curves of Denim Shoddy and Waste jute fibers.
thickness, the most suitable density was estimated by hit and trial in a sample making mould which came out to be around 45 kg/m3. To show the variation of these parameters, the second thickness of 50 mm is selected with an intention of further reducing the thickness and hence reducing the material consumption for noise control applications. The compressibility of the fibers plays a significant role in determining the density for a given thickness, which makes it very difficult to achieve two different thickness with the same density manually. Though large difference in thickness with the same density may be achieved by using hydraulic press, but this process will severely reduce the porosity of the material. Due to these limitations, only a slight change in density was made and was kept at 42 kg/m3. Thus, 2 factor-2 levels full factorial experiment is designed. As per the standard procedure in design of experiments, the minimum level of the parameters is coded with (1), and maximum level of the parameter is coded with (+1). This is mainly done to avoid complications due to variation in dimensions of the process parameters. Fig. 6 shows the designed experiment in thickness-density plane where each corner point of the square represents an experiment points which are four in number. The parameters of the test samples associated with these experiment points are tabulated in Table 4 .
4.1.1. Porosity (/) This is the most important property of a sound absorber as the sound is mainly dissipated in its pores. It gives a measure of how much mass of the material is occupied by air. For high dissipation inside the material, higher porosity is desired so that the air trapped can be exited. It is empirically estimated by the Eq. (1)
/¼
ma ms
ð1Þ
where, ma = mass of air trapped in the material (gm), ms = total mass of the sample (gm) As measuring the mass of air in a sample is not always feasible, an alternate and much more convenient equation to estimate the same is given by Eq. (2):
/¼1
qs qf
ð2Þ
where, qs = Bulk density of the sound-absorbing sample, qf = Density of the fibers. 4.1.2. Tortuosity (a1) If the pores are straight, the length of the pore will be approximately equal to the thickness of the material. This parameter gives a measure of the complexity of the network of pores. Higher tortuosity implies the pores are very curly which implies higher interaction between sound and fibers of the materials which result in higher dissipation. It is estimated by the porosity dependent relation as shown in Eq. (3),
a1 ¼ 1 þ
1/ 2/
ð3Þ
where / = Porosity of the sound absorber. 4.1.3. Airflow resistivity (r) This parameter gives a measure of the potential of the sound to be dissipated inside a sound-absorbing material. The flow of air is generated in a tube and pressure drop across the test sample and volume flow rate of the air in the tube is measured by a pressure indicator and flowmeter respectively. Finally, airflow resistivity is estimated by Eq. (4),
r¼ Fig. 6. Design of experiment as per 2 level- 2 factor full factorial design.
DPA Qvd
ð4Þ
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Table 4 Parameters of the test samples at experiment points. No. Sample Sample Sample Sample
No. No. No. No.
1 2 3 4
Coded coordinate
Coded values
Thickness (mm)
Density (kg/m3)
(1,1) (1,1) (1,1) (1,1)
min b a ab
50 50 70 70
42 45 42 45
(SAC). For this reason, new samples have been fabricated as per the required density and having a thickness of 10 mm. 4.1.4. Characteristics length Thermal characteristics length (TCL,K0 ) and viscous characteristics (VCL,K) length are very common during the characterization of acoustic materials. Thermal characteristic length gives a measure of the ratio of surface area of the pore to its volume, and the extent of interaction between the fibers of the materials and sound can be inferred by using this parameter. Viscous characteristics length gives a measure of the interconnection between two adjacent pores giving us an idea of the ease by which sound can travel inside a porous medium. These are estimated empirically by Eq. (5), Eq. (6)
K¼
1 2Prl
K0 ¼ 2 K Fig. 7. Mould and specimen test samples of denim shoddy and waste jute fibers.
ð5Þ ð6Þ
where, r = diameter of the fiber, and Eq. (7)
l¼
1
Pr 2 qqfs
ð7Þ
where, DP= pressure difference across the test sample, Pa; 3
Q v = volume flow rate of the air approaching the test sample, ms A = cross-sectional area of the test sample, m2; d= thickness of the test sample, m. An experimental set up developed and validated as per ISO9053 [31] is shown in Fig. 8. It uses a blower with variable speed to generate the flow of air which is measured by a flowmeter. Finally, the pressure drop across the test sample in the sample holder is measured by a differential pressure indicator with a resolution of 1 Pa. Airflow resistivity of both the materials, denim shoddy and waste jute fibers are measured on this set up after preparing test samples which are 10 mm thick and has the density of 42 kg/m3 and 45 kg/m3. There is a slight variation in the diameter of the tubes used in airflow resistivity (AFR) set up and the impedance tube, which measures the sound absorption coefficients
4.2. SAC estimation by empirical relations There are many empirical relations to predict the SAC, and many of them are developed using the experimental results. These models use the acoustical parameter of the material discussed in section 4.1 as input and predict the characteristics impedance ðZ c Þ and propagation constant ðcÞfor a material. This is ultimately used estimate the surface impedance ðZ s Þ to predict the SAC spectrum ðaÞ. Delany-Bazley model is the most basic model which uses only the airflow resistivity of the material for SAC estimation [33]. Later, Miki modified this model using the same experimental data as that of Delany-Bazley and suggested some changes in coefficients for better predictions even at a low frequency which was an issue in the earlier one [34]. Later on, Champaux-Allard came up with their model which included all the five parameters dis-
Fig. 8. Experimental set up to measure the airflow resistivity.
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
cussed in section 4.1 as input [35]. Using multiple parameters certainly improves the result but has severe computation requirement. A comparison shows that the variation in the result is only about 3% [36]. Due to this reason, this study only discusses Delany-Bazley and Miki model. The format of equations used in both, Delany-Bazley and Miki model is the same which is shown from Eqs. (8)–(11) in terms of unknown coefficients a, b, c, d, E, g, i and k which are tabulated in Table 5. Here f represents frepffiffiffiffiffiffiffi quency and j ¼ 1, C0=speed of sound in air = 348 m/s, Z 0 =impedance of air = q0 c0 , q0 = density of air = 1.225 kg/m3, l = Thickness of the sample
(
b d ) f f 1þa jc
Zc ¼ Z0
2p f c¼ C0
r
ð8Þ
r
( g k !) f f E þj 1þi
r
ð9Þ
r
Zs ¼ Z c cothðclÞ
ð10Þ
Zs q0C02 Zs þ q0C0
a ¼ 1
ð11Þ
4.3. Measurement of the sound absorption coefficient The ability of material as a sound absorber is estimated by sound absorption coefficient (SAC). An experimental set up for its measurement is developed as per ASTM-E-1050 [32] is shown in Fig. 9. Two separate tubes of diameter 100 mm and 29 mm have been designed to work as impedance tubes. The larger tube operates in the frequency range of 100–1800 Hz and the smaller one operates in the range of 1600–5000 Hz. SAC is estimated by measuring the sound pressure at two locations, and after estimating the transfer function (H) between these signals at two locations, Reflection coefficient (R) is estimated by Eq. (12),
R¼
H ejKS j2KðlþSÞ e ejKS H
ð12Þ
where, K = complex wavenumber, e = exponential, S = spacing between the microphones, l = distance between the sample and nearest microphone. Finally, the sound absorption coefficient ðaÞ is estimated by Eq. (13),
a ¼ 1 R2
ð13Þ
In order to compare the SAC of the materials in this paper with commercially available material, glasswool samples having the same density and thickness is prepared, and experiments are repeated with them at all the four experiment points. Fig. 9 also shows all three materials kept side by side.
Table 5 Coefficients of empirical models. Coefficients
Delany-Bazley
Miki
a b c d E g i k
9.08 0.75 11.9 0.73 10.3 0.59 10.8 0.59
0.070 0.632 0.107 0.632 0.160 0.618 0.109 0.618
4.4. Discussions about the measured acoustical properties The properties obtained from acoustical characterization is presented in Table 6. It can be observed that the porosity of sample no. 2 and sample no. 4 is less than that of the sample no.1 and sample no. 3, because of a slight increase in their density. But at the same time, their tortuosity and AFR has increased slightly. The porosity of samples of denim shoddy is greater than of corresponding samples of waste jute fibers, but their tortuosity is slightly lesser because of higher fiber density. It can also be observed that change in density of the samples has a direct effect on its AFR which increases with increase in density of the sample. A further observation which can be made is VCL decreases with increase in density and so does TCL. These characteristics lengths are also a measure of the size of the pores, and it can be seen that pore size of denim shoddy samples is small than that of waste jute fiber, although their porosity is higher. It implies the pores in denim shoddy fibers are more compact and hence the number of pores in denim shoddy samples must be higher and hence denim shoddy must be a better sound absorber than waste jute fibers. The SAC of all the four samples for both the fibers is measured and presented in Figs. 10 and 11. Sample no. 1 and sample no. 3 have the same density, and they differ in thickness. Similar is the case for sample no. 2 and sample no. 4. Increase in thickness increases their SAC by a large amount. As for denim shoddy samples, the SAC at 500 Hz is around 0.4 for sample 1, which increases to 0.7 for sample 3. Similarly, SAC of sample no. 2 at 500 Hz is 0.5 which increases to up to around 0.75. The performance of waste jute fibers is not very good in the 500 Hz band and below but they perform very good at 1000 Hz, and for this frequency, increase in thickness increased the SAC at 1000 Hz from 0.6 to 0.9. It is also inferred from Figs. 10 and 11 that increase in density increased the SAC for less thick material as can be observed in the results of sample no 1 and 2, but when the thickness is already high, the effect is not very significant as is observed from the results of sample no. 3 and sample no 4. These results also suggest that denim shoddy can be used everywhere for noise control, even in lowfrequency sound source like diesel generators while waste jute fibers can perform well in building acoustics where the main concern is sound above 1000 Hz. The SAC for denim shoddy fibers remains mostly greater than 0.9 beyond 1000 Hz, which for waste jute fibers remains greater than 0.8. The performance of denim shoddy fibers, waste jute fibers and commercial glasswool having the same thickness and density is compared and shown in Fig. 12. For sample no.1, glasswool performs much better than waste jute fibers but just slightly better than the denim shoddy fibers. A marginal increase in density made the performance of denim shoddy becomes very much comparable with that of glasswool, and it is observed in Fig. 12b. Further increase in density and thickness made the performance of denim shoddy slightly better than that of commercial glasswool which is observed in Fig. 12c and Fig. 12d. The performance of glasswool is mostly better than that of waste jute, and gradually with increase in thickness and density, their performance becomes very much close to each other. Noise reduction coefficient (NRC) is used to rate a sound absorber and is calculated by Eq. (14),
NRC ¼
a250 þ a500 þ a1000 þ a2000 4
ð14Þ
The calculations to estimate NRC values are performed by referring to ASTM C 423 [37] and ASTM C 634 [38]. Though this standard is for NRC rating of materials by reverberation chamber method (random incidence of sound), it serves well for a comparative study of different materials in an impedance tube (normal incidence of sound). The NRCs thus obtained are not rounded off
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Fig. 9. Experimental set up to estimate the sound absorption coefficient.
Table 6 Acoustical characterization of the fibers. Property
Denim shoddy
Waste jute fibers
Sample No.
1
2
3
4
1
2
3
4
Porosity (/) Tortuosity (a1) AFR (r), Pas m2 VCL (K), mm 0 TCL (K ), mm
0.9720 1.0144 18,000
0.9700 1.0154 18,700
0.9720 1.0144 18,000
0.9700 1.0154 18,700
0.9710 1.0149 12,290
0.9690 1.0159 14,500
0.9710 1.0149 12,290
0.9690 1.0159 14,500
0.1371 0.2742
0.1149 0.2298
0.131 0.2742
0.1149 0.2298
0.3558 0.7116
0.3812 0.7624
0.3558 0.7116
0.3812 0.7624
to nearest 0.05 purposely to notice the exact difference between the NRCs of different materials tested with different process parameters. Again, it is worth stating that these NRC values represent the mean SAC of the entire band and not just that of the central frequency. A plot comparing the NRCs of denim shoddy, waste jute fibers and commercial glasswool is shown in Fig. 13. It can be inferred that, for a thickness of 70 mm, and density of 42 kg/m3 and 45 kg/m3 (Sample no. 3, and 4), denim shoddy has a higher rating than the commercial glasswool. The rating of waste jute is only slightly lesser than that of commercial glasswool. These results clearly state that these recycled material have very good potential for real-life acoustic applications.
4.5. Comparison of theoretically estimated and experimentally measured sound absorption coefficients
Fig. 10. Measured SAC for denim shoddy fibers.
Fig. 11. Measured SAC for waste jute fibers.
A comparison of the SACs measured with the experimental setup, predicted by the Delany-Bazley model and predicted by the Miki model are compared in Figs. 14 and 15. Fig. 14 is for denim shoddy, and for which the two models agree quite well with each other. Extrapolating the SAC obtained by Delany-Bazley model at low frequencies gives a negative value which is physically invalid. This issue was resolved by Miki model. The materials in this study are very good sound absorbers as seen in Figs. 10 and 11, and this is the reason both the model agrees quite well with each other. Though there is a slight deviation between measured and empirically estimated SAC for sample no. 01 of denim shoddy fibers, the experimental and theoretical results nearly overlap for sample no. 02, 03 and 04. For waste jute fibers, there is a significant difference between theoretically estimated and experimentally measured SAC for sample no. 01. The difference between experimental and theoretical results gradually converges on moving from sample no. 02, 03 and 04. The predicted and measured values of Fig. 15(c) and (d) match fairly well. There is variation in Fig. 15 (a) and (b), which are for a thickness of 50 mm. The deviation can be accounted for various reasons like these empirical models mentioned were originally developed for very fine fibers (typically less than 10 mm). As waste jute fibers in this study have a diameter
M. Raj et al. / Applied Acoustics 159 (2020) 107070
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Fig. 12. Comparison of denim shoddy, waste jute fibers, and commercial glasswool for the same density and thickness a) sample no.01 b) sample no.02 c) sample no.03 d) sample no.04.
5. Equation development in terms of thickness and density to approximate the NRC The primary factors of the experiments conducted are thickness (a) and density (b) of the sound absorbers. This section aims to inspect which of these factors profoundly affect the NRC, statistically. The detailed statistical analysis beyond the present scope of the work presented here. As per the conventions used in the design of experiments [39], the experiment points are coded and shown in Table 7. The main effect of factor ‘a’ (thickness) on NRC, (M.E)a is calculated by Eq. (15):
ðM:EÞa ¼
Fig. 13. A comparison of NRC values of denim shoddy, waste jute fibers and glasswool.
ð15Þ
The main effect of factor ‘b’ (density) on NRC, (M.E)b is calculated by Eq. (16)
ðM:EÞb ¼ of 25 mm, it is coarser than those fibers; thus there is some deviation between theoretical and measured results. This is one of the reasons behind the good match achieved in the results of denim shoddy fibers (Fig. 14) as its diameter is less than 10 mm. With an increase in thickness, the effect of fiber coarseness of waste jute fibers has minimized and the experimental and theoretical results have come closer. This can be observed in Fig. 15(c) and (d) which are for 70 mm thickness and in Fig. 21 which is for a thickness of 60 mm. Besides, these models have considered only the airflow resistivity (AFR) and thickness as input parameters. Whereas, there are various other acoustical parameters too which might have affected the sound absorption coefficient. Such non-conformation has also been reported in many works of literature, like in reference [11] and [14]. These results once again reclaim our faith in empirical models which fetches fairly accurate results by measuring only airflow resistivity and thickness of the sample.
ða þ abÞ ðb þ minÞ 2n
ðb þ abÞ ða þ minÞ 2n
ð16Þ
The interaction between the variables thickness and density, on NRC, known as interaction effect (I.E)ab is calculated by Eq. (17):
ðI:EÞab ¼
ðmin þ abÞ ða þ bÞ 2n
ð17Þ
n = number of repetitions of experiment t = 2. These effects for denim shoddy and waste jute fibers are calculated and tabulated in Table 8. It can be observed from Table 8 that the effect of thickness is more than the effect of density, which implies NRC is primarily controlled by thickness. A similar observation was made from experimental results too. Change in the density of waste jute fibers effects the NRC more than the similar change in density of the denim shoddy fibers. These two conclusions are in accordance with the observations made from Figs. 10 and 11. The interaction effect
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Fig. 14. SAC measured in an impedance tube and predicted by empirical models for denim shoddy a) sample no. 01 b) sample no. 02 c) sample no. 03 d) sample no. 04.
Fig. 15. SAC measured in an impedance tube and predicted by empirical models for waste jute fibers a) sample no. 01 b) sample no. 02 c) sample no. 03 d) sample no. 04.
Table 7 Coded experiment points. level
Sample no.
NRC at these points coded as
(1,1) (1, 1) (1, 1) (1,1)
01 02 03 04
min b a ab
is very small as compared to the main effects, and its negative sign states that the effect of thickness on NRC will decrease as density increases. This is true as significant increase in density will lead to a large reduction in porosity.
Table 8 Effects of factors. Effect
Denim shoddy
Waste jute fiber
Thickness, (M.E)a Density, (M.E)b interaction (I.E)ab
0.0739 0.0341 0.0269
0.1178 0.0153 0.0042
A mathematical equation can be developed using these two factors [39], which estimates the response (NRC) as a function of two governing variables (thickness and density) and is given as Eq. (18):
M. Raj et al. / Applied Acoustics 159 (2020) 107070 Table 9 Coefficients of response surface. Variable
Denim shoddy
Waste jute fiber
b0 b1 b2 b12
0.8443 0.0369 0.0171 0.0135
0.7278 0.0589 0.0076 0.0021
zz ¼ b0 þ b1 X þ b2 Y þ b12 XY
ð18Þ
where: zz = response, in this case NRC; X = Coded value of thickness {1, 1}, Y = Coded value of density {1,1}. The coefficients of Eq. (18) is estimated by Eqs. (19–22):
b0 ¼
sumðminÞ þ sumðaÞ þ sumðbÞ þ sumðabÞ 4n
ð19Þ
Fig. 16. Response surface for denim shoddy.
where, {Sum(min), Sum(a), Sum(b), Sum(ab)} represents the sum of response (NRC) at the coded point-min, a, b and ab as mentioned in Table 7.
b1 ¼
ðM:EÞa 2
ð20Þ
b2 ¼
ðM:EÞb 2
ð21Þ
ðI:EÞab 2
ð22Þ
b12 ¼
The coefficients estimated for denim shoddy and waste jute fibers are tabulated in Table 9. The response surface plotted by using Eq. (18) and coefficients of Table 9 are shown in Figs. 16 and 17. The NRC of denim shoddy is observed to be very high, and waste jute too performs fairly well. The effect of the process parameters can be visualized by observing the slope along them. The slope along thickness appears to be very high, which boost up the claim of their high contribution towards increasing NRC. The NRC values can be approximated by putting the coefficients of Table 8 in Eq. (18). There will be some difference between actually measured NRC and the one predicted by the equation. This difference, known as residual, is calculated by Eq. (23)
Residual ¼ ðNRCÞmeasured ðNRCÞapproximated
Fig. 17. Response surface for waste jute.
ð23Þ
A plot for the normalized residual probability for the regression equations of both the fibers are shown in Figs. 18 and 19 and the trend is linear in nature. This validates the regression equation, which can be assumed to correctly predict the NRC values at intermediate points of the experiments. With an intent to validate equation (18) with coefficients of Table 9, an experiment is conducted by preparing a test sample of both the fibers at an intermediate point of the process parameters (thickness and density). The details at this point are tabulated in Table 10. The measured and predicted spectrum of SAC is shown in Figs. 20 and 21 where it is observed that the result obtained are similar to the ones shown in Figs. 10 and 11. Further, it can be observed in Table 10 that the predicted NRC is very close to the one experimentally measured. This validates the equations, which can be used to approximate the NRCs at any intermediate point of the experiments.
Fig. 18. Normalized residual probability plot for denim shoddy.
6. Cost comparison Almost all sound absorbers perform well at frequencies greater than 2000 Hz. Owing to the limitations by the laws of physics, none of them perform well below 250 Hz under normal circumstances.
Fig. 19. Normalized residual probability plot for waste jute fibers.
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Table 10 Parameters and Results of experimental results at the midpoint. Material
Coded coordinate
Thickness (mm)
Density (kg/m3)
Airflow Resistivity (Measured)
NRC (Measured)
NRC (Predicted by Eq. (15))
Denim Shoddy Waste jute fiber
(0,0) (0,0)
60 60
43.5 43.5
18,690 14,091
0.8343 0.7237
0.8443 0.7278
7. Conclusions
Fig. 20. SAC measured in an impedance tube and predicted by empirical models for Denim shoddy fibers at the midpoint of the process parameters.
This is systematic study to explore the potential of some recycled materials to be used as sound-absorbing materials. Denim shoddy and waste jute fibers have been identified as the materials with practically very low utility, and hence they are focused. They have been thoroughly characterized for their physical and acoustical properties. Experiments have been conducted by varying the process parameters to measure and predict their sound absorption coefficient. These materials have performed in really well and have fetched a very high Noise Reduction Coefficients. Denim shoddy performed better than the commercial glasswool. The performance of waste jute fiber with increased thickness was also comparable to that of glasswool. A mathematical equation is developed to visualize the effect of process parameters which observed that the change in thickness had a direct effect on the material’s NRC. This equation can be successfully used to predict the NRC values. Finally, a cost analysis is conducted for the same amount of sound absorption in which denim shoddy turned out to be 10 times; the waste jute fiber turned out to be 5 times, economical than the commercial glasswool. In this way, some environment friendly economical sound absorbing materials are the outcome of this work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgement The authors are thankful to the Department of Science and Technology-Government of India for funding this project.
Fig. 21. SAC measured in an impedance tube and predicted by empirical models for Waste jute fibers at the midpoint of the process parameters.
Table 11 Cost comparison to achieve the same amount of sound absorption. Material
Selected sample number
Mass of material required (kg) = thickness*area*density
Rate ($/kg)
Total cost ($)
Commercial glasswool Denim shoddy
2 (Fig. 12b) 2 (Fig. 12b) 4 (Fig. 12d)
22.5
0.714
16.065
22.5
0.072
1.620
31.5
0.115
3.622
Waste jute fibers
Hence, for a fair comparison, this study focusses on 1000 Hz band. It is desired to provide a sound absorption of 0.9 at 1000 Hz band, over an area of 10 m2. Looking at the result of Fig. 12, the amount of required material is calculated as per the current rate (April 2019) in New Delhi and presented in Table 11. This is clearly inferred from Table 11 that using denim shoddy will be very economical as compared to commercial glasswool which is 10 times less expensive than the commercial material. Even though more amount of waste jute is needed than the commercial glasswool, still it comes out to be 5 times more economical.
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[26] Shanmugasundaram N, Rajendran I, Ramkumar T. Characterization of untreated and alkali treated new cellulosic fiber from an areca palm leaf stalk as potential reinforcement in polymer composites. Carbohydr Polym 2018;195:566–75. [27] ASTM D 3822. Standard Test Method for Tensile Properties of Single Textile Fibers. [28] Cox TJ, D’antonio P. Acoustic absorbers and diffusers: theory, design and application. CRC Press; 2009. [29] Bansod PV, Mohanty AR. Inverse acoustical characterization of natural jute sound absorbing material by the particle swarm optimization. Appl Acoust 2016;112:41–52. [30] Allard JF. Propogation of sound in porous media. Elsevier Applied Science; 1993. [31] ISO 9053: Acoustics-Materials for acoustical applications-Determination of airflow resistance. [32] ASTM E 1050-12. Standard test method for impedance and absorption of acoustic materials using a tube, two microphones, and a digital frequency analysis system. [33] Delany ME, Bazley EN. Acoustical properties of fibrous absorbent materials. Appl Acoust 1969;3:105–16. [34] Miki Y. Acoustical properties of porous materials-modifications of DelanyBazley models. J Acoust Soc Japan 1990;11:19–24. [35] Allard JF, Champoux Y. New empirical equations for sound propagation in rigid frame fibrous materials. J Acoust Soc Am 1992;91:3346–53. [36] Manning J, Panneton R. Acoustical model for Shoddy-based fiber sound absorbers. Text Res J 2013;83:1356–70. [37] ASTM Standard C423. Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. [38] ASTM Standard C634. Standard Terminology Relating to Building and Environmental Acoustics. [39] Montgomery DC. Design and analysis of experiments. John Wiley & Sons; 2001.
Contents lists available at ScienceDirect
Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust
Recycled materials as a potential replacement to synthetic sound absorbers: A study on denim shoddy and waste jute fibers Manish Raj, Shahab Fatima, Naresh Tandon ⇑ ITMME Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
a r t i c l e
i n f o
Article history: Received 18 April 2019 Received in revised form 23 September 2019 Accepted 27 September 2019
Keywords: Sound absorption coefficient Full factorial design Miki model Environment-friendly sound absorber Sustainable materials Noise control
a b s t r a c t Sound absorbing materials used for noise control is desired by the practitioners to be environment friendly and of low cost. This has motivated the researchers to look for alternate materials from natural sources. One step in this direction is exploring the use of recycled materials for potential application as sound absorbers. This paper identifies two materials, denim shoddy and waste jute fibers. They are characterized by their acoustical and physical properties. Experiments have been conducted as per 2-level 2factor full factorial design of experiments. A comprehensive comparison of the sound absorption coefficients of denim shoddy and waste jute fibers, measured in an impedance tube and predicted by using empirical relations are shown. The absorption spectrum of these materials are compared with commercial glasswool with similar process parameters, and the results obtained show denim shoddy to be a better sound absorber than that of glasswool. A mathematical equation for Noise Reduction Coefficient is formed using the experimental results obtained and validated with a fresh set of experiments. Finally, economic comparison of these materials is conducted. The results obtained from technical and commercial research clearly states that the application of these materials for noise control will fetch better results apart from being cost-effective and environment-friendly. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Noise pollution is one of the outcomes of the developments taking place in the twentieth and twenty-first century owing to the revolution across industries to ease human life. It used to be ignored earlier, but gradually researchers figured out its harmful effects on human beings [1,2] which led to law enforcement agencies to bring out new regulations [3–5] to keep it under the prescribed limit. One of the ways to achieve this is to use sound absorbers, and for this purpose the material that has been used is glasswool. Glasswool serves its purpose well but has a severe effect on the environment and the health of the people associated with its production or application [6,7]. This motivated the researchers to look for other materials, and one of the steps in this direction is studies on natural materials like jute, coir, cotton, etc. Just like glasswool, these natural materials are porous in nature and sound energy is dissipated in their pores, they are light in weight and have an additional advantage of being environment friendly and without any health hazard.
⇑ Corresponding author. E-mail address: [email protected] (N. Tandon). https://doi.org/10.1016/j.apacoust.2019.107070 0003-682X/Ó 2019 Elsevier Ltd. All rights reserved.
Many researchers around the globe have reported about the use of natural materials for acoustic applications. One of the first preliminary studies was reported around the year 2000, when sound absorption and sound insulation properties of various materials were compared with an intention to select the best material for automotive NVH applications [8] and it was found that the performance jute and cotton are very much comparable to that of commercial glasswool. This was followed by research on bamboo fiber felt and the effect of variation in its thickness, density, and air gap was reported [9]. These preliminary results were very promising which motivated other researchers to report the natural materials locally available to them. Coir panels prepared from coconut husk and latex were studied for their acoustical behavior in conjunction with air gap and perforated plates [10]. Later, the same authors characterized and studied the acoustical properties of fresh coconut husk in fiber form and its performance was compared with the commercially available coir [11]. Many other materials like cotton, wool, jute, sisal, ramie, flax, bagasse were also studied sub sequentially and the results obtained were very promising [12–14]. The performance of natural materials in laboratory was very promising, and then a study was conducted for its actual implementation. A study of Jute in fiber form, felt form and composite with latex rubber as a binder was reported in which
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Noise Reduction Coefficient (NRC) of up to 0.77 was obtained [15]. Further, these authors extended their study and demonstrated to the world that it was possible to actually reduce the noise by using these materials [16,17]. All these natural materials already have some primary application as in coir is used to make ropes, jute for making bags, cotton for clothes, etc. This motivated the researchers to further cut down the cost of sound absorbers by exploring materials which do not have any primary application and are a waste of some processes. One of the first recycled material studied for this was industrial tea-leaf-fiber and its samples of different thickness backed by woven cotton cloth were studied in which an NRC values up to 0.5 was reported [18]. Similar research extended towards recycled rubber particle [19], oil palm empty fruit bunch fiber [20], orange tree pruning pulp fiber [21], cigarette butt [22,23], oil palm kernel shell and cockle shell [24], by varying their process parameters. These studies inferred that recycled materials have a good amount of potential for its potential application as a sound-absorbing material. This work is an addition in exploring some industrial waste material for potential application as a sound absorber. It highlights the performance of waste materials obtained from the fabric industry of denim and jute. They are characterized for their physical properties and an experiment has been designed as per a twolevel two-factor design of experiments. With relevant parameters, their samples have been fabricated and characterized for their acoustical properties. Further, their normal Sound Absorption Coefficient (SAC) is measured in a 2-microphone impedance tube as per transfer function method. The experimentally measured SACs are compared with the ones obtained by using commercial glasswool with same physical parameters. The experimentally measured SAC is also compared with the one estimated empirically by using Delany-Bazley and Miki models. The experimental result to measure the SACs is then used to develop a mathematical equation to estimate the NRC from experimentally obtained results and
Table 1 List of commonly used abbreviations. SAC NRC Zc
c j
Co f
Sound absorption coefficient Noise reduction coefficient Characteristics impedance Propagation constant pffiffiffiffiffiffiffi 1 Speed of sound in air frequency
is validated experimentally. Finally, a basic economic comparison of denim shoddy, waste jute fibers, and commercial glasswool is then shown to obtain the same amount of noise reduction in a room. Table 1 shows the commonly used symbols in this study.
2. Materials and their physical characterization Denim is the widely used fabric across the world which is mainly used in the form of jeans. The yarn used to make it usually low count which makes it very cost-effective material. The finishing operation of this fabric to jeans involves a lot of tailoring operation which generates waste denim fabric. This waste fabric is collected from the industries and shredded through rag machine and teaser machine which opens up the material back into threaded chunks, known as denim shoddy. Jute is one of the primary agricultural products of eastern parts of India and Bangladesh. Its fibers are obtained from the bast of jute plants and used to make many products such as bags which mainly require long fibers. The fibers which breaks out from other fibers while processing is mainly of shorter length (20–50 mm) which does not remain of much significant application. Both these materials were procured locally (from ESKAY INTERNATIONAL, Kolkata, West Bengal, India) and are shown in bulk form in Fig. 1, after drying them in the sun for around 8 h to remove any moisture trapped within them. Diameters of these fibers are measured on a Leica 205 FA fluorescence stereo microscope with different magnifications varying between 40 and 100. The diameter measured on 30 different fibers and the average fiber diameter of denim shoddy is found out to be 9.8 ± 0.3 mm and waste jute fiber is 24.60 ± 1.8 mm. The scanning electron microscope (SEM) images are taken on the ZEISS EVO MA10 scanning electron microscope. The fibers are first gold coated for 2 min to avoid the charging effect due to electrons. These microscopic and SEM images of both fibers are shown in Fig. 2. Micro fibrils, split strands, disruption in the physical structure of the fibers along the length and surface textures are observed in the SEM image of waste jute fibers (Fig. 2(d)). The SEM image of denim shoddy fibers (Fig. 2(c)) is free of such disruptions, and thus it is concluded that denim shoddy fibers appear to be smoother as compared to the waste jute fibers. The strands on the waste jute fibers boost up the sound absorption. The twists present in denim shoddy fibers make it curlier than the waste jute fibers. It implies that the sound absorber made by this material will
Fig. 1. Materials in bulk form a) Denim shoddy fibers b) Waste jute fibers.
M. Raj et al. / Applied Acoustics 159 (2020) 107070
3
Fig. 2. Microscopic and SEM image of fibers a) Denim shoddy microscopic image b) Waste jute fiber microscopic image c) Denim shoddy fiber SEM image d) Waste jute fiber SEM image.
have a greater pore length because of the curliness of the pores; this will increase the sound absorption. 2.1. Density measurement One of the most important properties of acoustic materials is their density. Its direct estimation by dividing mass with volume is not a feasible option because of the very lightweight of a single fiber, plus their porous structure which makes it difficult to estimate their exact volume. So, the alternate method is to try by buoyancy method where the volume of the fiber chunk is estimated by amount of liquid displaced. This method works quite well with organic liquids like toluene but is pretty timeconsuming. Hence, in this study the robust method of density estimation by floatation method was adopted. In this method, a liquid column is prepared by mixing two chemicals in such a way that, in the resultant column the density linearly varies with the depth of the column. The two liquids are selected in such a way that density of the fiber is in between these two liquids. Thus, n-Heptane (density = 685 kg/m3) and Carbon tetrachloride (density = 1590 kg/m3) is selected to cover a wide region of interest Table 2. The next step is to prepare a liquid column of uniformly varying density. It has been found from works of literature that density of most of the natural fibers is higher than 1300 kg/m3 [25,26]. For Table 2 Volume of chemicals to be mixed to achieve the desired density. Solution A
Solution B
q = 1300 kg/m3
q = 1580 kg/m3
V1 = 277 ml V2 = 583 ml
V1 = 10 ml V2 = 850 ml
this reason, the column is designed for a minimum density of 1300 kg/m3 at the top and a maximum density of 1580 kg/m3 at the bottom. This is achieved by mixing two solutions, solution A and solution B formed by mixing the two chemicals (with volumes V1 and V2) in such a way that, V1 q1 þ V2 q2 ¼ Vq. where, V1 = volume of chemical with low density (n-Heptane) V2 = volume of chemical with high density (Carbon tetrachloride) q1 = density of chemical with low density (685 kg/m3) q2 = density of chemical with high density (1590 kg/m3) V = Half the volume of the column to be prepared (860 ml) q = Resultant density of the solution achieved by mixing two chemicals (1300 kg/m3 for solution A, and 1580 kg/m3 for solution B) Table 2 summaries the volume V1 and V2 taken to form solution A and solution B. Solution A and B are steered magnetically in a flask for around 2 h to achieve uniform mixing. They are poured in a vertical column slowly in such a way that a column with linearly varying density is formed. Fig. 3 shows the vertical column, which is calibrated by standard floats by dropping them in the column. The float sinks at a level where the density of liquid surrounding it is equal to the density of the float. Thus, the column is calibrated by measuring the position of the floats measured by the microscope mounted. Small samples of both the fibers are placed slowly in the prepared column. The sample starts drowning and sinks at position proportional to its density. Measurement is conducted on ten samples, and the average density of the denim shoddy and waste jute fibers are found to be 1501 ± 13 kg/m3 and 1452 ± 8 kg/m3 respectively.
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
be observed that the failure plane is at an angle to the crosssection of denim shoddy fiber indicating it to be a ductile material, the waste jute fiber failed normal to its cross-section which means it is a brittle material. This is further confirmed by looking at Table 3 which summarizes the results, and it is observed that the strain at maximum load and strain at break in denim shoddy is greater than 10% while for waste jute fibers is less than 5%. The maximum load borne by denim shoddy is greater than that of waste jute fibers which makes denim shoddy a stronger fiber. 2.3. Thermo-gravimetric analysis Sound absorbing materials are often used in mufflers, heating, ventilation, and air-conditioning systems, which has very harsh temperature conditions. Thus thermal stability of these materials becomes an important parameter, which needs to be studied. It was studied on Linseis TGA PT 1000. Chopped fibers of 10 mg and size less than 100 mm was heated from 70 °C to 700 °C with a heating rate of 10 °C/min. The results are shown in Fig. 5 in which it is observed that the materials are thermally very stable till 250 °C beyond which they start decomposing. The decomposition rate increases rapidly after 300 °C and till the temperature is increased to 400 °C, 60% and 80% reduction in mass is achieved for denim shoddy and waste jute fibers respectively. Thus denim shoddy is thermally superior to that of denim shoddy. 3. Design of experiments and sample fabrication Fig. 3. The prepared column to measure the density.
2.2. Physical strength of fibers The physical strength of these fibers is measured on the Instron 5960 tensile testing machine as per standard ASTM-D-3822 [27]. The sample length of denim shoddy fibers was kept at 10 mm and of waste jute fibers was 25 mm. The crosshead speed was kept at 1.5 mm/min in such a way that the specimen fails within 20 ± 3 s. The failed fiber samples are shown in Fig. 4. The failure plane provides a fair idea of the ductility or brittleness and it can
The outcome of the literature review suggests that the thickness and density of the sound absorbers are the two major parameters governing the absorption behavior. Providing air gap behind the sound absorber just boosts up the low-frequency response. Hence this study proceeds by varying the thickness (a) and density (b) of the sound absorbers. Sound absorption performance of a material is frequency-dependent term, and the most efficient performance is obtained at a quarter wavelength thickness, but this criterion fetches non-viable thickness particularly below 1000 Hz. This study takes the primary thickness of around one-tenth of wavelength at 500 Hz, i.e. 70 mm. To fabricate the test samples of this
Fig. 4. Tensile failure of fibers a) Denim shoddy b) Waste jute fibers.
M. Raj et al. / Applied Acoustics 159 (2020) 107070 Table 3 Results obtained from tensile test. Parameter
Denim shoddy
Waste jute fiber
Maximum Load [N] Strain at maximum load [%] Strain at break [%]
2.889 10.527 12.902
1.295 2.048 2.052
5
The required amount of mass as per the sample number of experiment point is weighed and filled in the mould. The deadweight of 10 kg is kept on the mould to bring the test samples in shape. The mould used and specimen test samples prepared of both the fibers are shown in Fig. 7. After conducting the experiment on a prepared sample, it is destroyed by opening the fibers by hand, and the test samples are made again with the same fibers. The same material is used just to avoid any nuisance factor in the experiment. In this way, 2 runs are conducted at each experiment point for each test samples. 4. Acoustical measurements This section discusses measurements and prediction of various acoustical properties of the denim shoddy and waste jute fibers as per the designed experiments. 4.1. Acoustical characterization The major parameter to characterize a sound-absorbing material is its porosity, tortuosity, airflow resistivity, viscous characteristics length and thermal characteristics length [28–30].
Fig. 5. TGA curves of Denim Shoddy and Waste jute fibers.
thickness, the most suitable density was estimated by hit and trial in a sample making mould which came out to be around 45 kg/m3. To show the variation of these parameters, the second thickness of 50 mm is selected with an intention of further reducing the thickness and hence reducing the material consumption for noise control applications. The compressibility of the fibers plays a significant role in determining the density for a given thickness, which makes it very difficult to achieve two different thickness with the same density manually. Though large difference in thickness with the same density may be achieved by using hydraulic press, but this process will severely reduce the porosity of the material. Due to these limitations, only a slight change in density was made and was kept at 42 kg/m3. Thus, 2 factor-2 levels full factorial experiment is designed. As per the standard procedure in design of experiments, the minimum level of the parameters is coded with (1), and maximum level of the parameter is coded with (+1). This is mainly done to avoid complications due to variation in dimensions of the process parameters. Fig. 6 shows the designed experiment in thickness-density plane where each corner point of the square represents an experiment points which are four in number. The parameters of the test samples associated with these experiment points are tabulated in Table 4 .
4.1.1. Porosity (/) This is the most important property of a sound absorber as the sound is mainly dissipated in its pores. It gives a measure of how much mass of the material is occupied by air. For high dissipation inside the material, higher porosity is desired so that the air trapped can be exited. It is empirically estimated by the Eq. (1)
/¼
ma ms
ð1Þ
where, ma = mass of air trapped in the material (gm), ms = total mass of the sample (gm) As measuring the mass of air in a sample is not always feasible, an alternate and much more convenient equation to estimate the same is given by Eq. (2):
/¼1
qs qf
ð2Þ
where, qs = Bulk density of the sound-absorbing sample, qf = Density of the fibers. 4.1.2. Tortuosity (a1) If the pores are straight, the length of the pore will be approximately equal to the thickness of the material. This parameter gives a measure of the complexity of the network of pores. Higher tortuosity implies the pores are very curly which implies higher interaction between sound and fibers of the materials which result in higher dissipation. It is estimated by the porosity dependent relation as shown in Eq. (3),
a1 ¼ 1 þ
1/ 2/
ð3Þ
where / = Porosity of the sound absorber. 4.1.3. Airflow resistivity (r) This parameter gives a measure of the potential of the sound to be dissipated inside a sound-absorbing material. The flow of air is generated in a tube and pressure drop across the test sample and volume flow rate of the air in the tube is measured by a pressure indicator and flowmeter respectively. Finally, airflow resistivity is estimated by Eq. (4),
r¼ Fig. 6. Design of experiment as per 2 level- 2 factor full factorial design.
DPA Qvd
ð4Þ
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Table 4 Parameters of the test samples at experiment points. No. Sample Sample Sample Sample
No. No. No. No.
1 2 3 4
Coded coordinate
Coded values
Thickness (mm)
Density (kg/m3)
(1,1) (1,1) (1,1) (1,1)
min b a ab
50 50 70 70
42 45 42 45
(SAC). For this reason, new samples have been fabricated as per the required density and having a thickness of 10 mm. 4.1.4. Characteristics length Thermal characteristics length (TCL,K0 ) and viscous characteristics (VCL,K) length are very common during the characterization of acoustic materials. Thermal characteristic length gives a measure of the ratio of surface area of the pore to its volume, and the extent of interaction between the fibers of the materials and sound can be inferred by using this parameter. Viscous characteristics length gives a measure of the interconnection between two adjacent pores giving us an idea of the ease by which sound can travel inside a porous medium. These are estimated empirically by Eq. (5), Eq. (6)
K¼
1 2Prl
K0 ¼ 2 K Fig. 7. Mould and specimen test samples of denim shoddy and waste jute fibers.
ð5Þ ð6Þ
where, r = diameter of the fiber, and Eq. (7)
l¼
1
Pr 2 qqfs
ð7Þ
where, DP= pressure difference across the test sample, Pa; 3
Q v = volume flow rate of the air approaching the test sample, ms A = cross-sectional area of the test sample, m2; d= thickness of the test sample, m. An experimental set up developed and validated as per ISO9053 [31] is shown in Fig. 8. It uses a blower with variable speed to generate the flow of air which is measured by a flowmeter. Finally, the pressure drop across the test sample in the sample holder is measured by a differential pressure indicator with a resolution of 1 Pa. Airflow resistivity of both the materials, denim shoddy and waste jute fibers are measured on this set up after preparing test samples which are 10 mm thick and has the density of 42 kg/m3 and 45 kg/m3. There is a slight variation in the diameter of the tubes used in airflow resistivity (AFR) set up and the impedance tube, which measures the sound absorption coefficients
4.2. SAC estimation by empirical relations There are many empirical relations to predict the SAC, and many of them are developed using the experimental results. These models use the acoustical parameter of the material discussed in section 4.1 as input and predict the characteristics impedance ðZ c Þ and propagation constant ðcÞfor a material. This is ultimately used estimate the surface impedance ðZ s Þ to predict the SAC spectrum ðaÞ. Delany-Bazley model is the most basic model which uses only the airflow resistivity of the material for SAC estimation [33]. Later, Miki modified this model using the same experimental data as that of Delany-Bazley and suggested some changes in coefficients for better predictions even at a low frequency which was an issue in the earlier one [34]. Later on, Champaux-Allard came up with their model which included all the five parameters dis-
Fig. 8. Experimental set up to measure the airflow resistivity.
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
cussed in section 4.1 as input [35]. Using multiple parameters certainly improves the result but has severe computation requirement. A comparison shows that the variation in the result is only about 3% [36]. Due to this reason, this study only discusses Delany-Bazley and Miki model. The format of equations used in both, Delany-Bazley and Miki model is the same which is shown from Eqs. (8)–(11) in terms of unknown coefficients a, b, c, d, E, g, i and k which are tabulated in Table 5. Here f represents frepffiffiffiffiffiffiffi quency and j ¼ 1, C0=speed of sound in air = 348 m/s, Z 0 =impedance of air = q0 c0 , q0 = density of air = 1.225 kg/m3, l = Thickness of the sample
(
b d ) f f 1þa jc
Zc ¼ Z0
2p f c¼ C0
r
ð8Þ
r
( g k !) f f E þj 1þi
r
ð9Þ
r
Zs ¼ Z c cothðclÞ
ð10Þ
Zs q0C02 Zs þ q0C0
a ¼ 1
ð11Þ
4.3. Measurement of the sound absorption coefficient The ability of material as a sound absorber is estimated by sound absorption coefficient (SAC). An experimental set up for its measurement is developed as per ASTM-E-1050 [32] is shown in Fig. 9. Two separate tubes of diameter 100 mm and 29 mm have been designed to work as impedance tubes. The larger tube operates in the frequency range of 100–1800 Hz and the smaller one operates in the range of 1600–5000 Hz. SAC is estimated by measuring the sound pressure at two locations, and after estimating the transfer function (H) between these signals at two locations, Reflection coefficient (R) is estimated by Eq. (12),
R¼
H ejKS j2KðlþSÞ e ejKS H
ð12Þ
where, K = complex wavenumber, e = exponential, S = spacing between the microphones, l = distance between the sample and nearest microphone. Finally, the sound absorption coefficient ðaÞ is estimated by Eq. (13),
a ¼ 1 R2
ð13Þ
In order to compare the SAC of the materials in this paper with commercially available material, glasswool samples having the same density and thickness is prepared, and experiments are repeated with them at all the four experiment points. Fig. 9 also shows all three materials kept side by side.
Table 5 Coefficients of empirical models. Coefficients
Delany-Bazley
Miki
a b c d E g i k
9.08 0.75 11.9 0.73 10.3 0.59 10.8 0.59
0.070 0.632 0.107 0.632 0.160 0.618 0.109 0.618
4.4. Discussions about the measured acoustical properties The properties obtained from acoustical characterization is presented in Table 6. It can be observed that the porosity of sample no. 2 and sample no. 4 is less than that of the sample no.1 and sample no. 3, because of a slight increase in their density. But at the same time, their tortuosity and AFR has increased slightly. The porosity of samples of denim shoddy is greater than of corresponding samples of waste jute fibers, but their tortuosity is slightly lesser because of higher fiber density. It can also be observed that change in density of the samples has a direct effect on its AFR which increases with increase in density of the sample. A further observation which can be made is VCL decreases with increase in density and so does TCL. These characteristics lengths are also a measure of the size of the pores, and it can be seen that pore size of denim shoddy samples is small than that of waste jute fiber, although their porosity is higher. It implies the pores in denim shoddy fibers are more compact and hence the number of pores in denim shoddy samples must be higher and hence denim shoddy must be a better sound absorber than waste jute fibers. The SAC of all the four samples for both the fibers is measured and presented in Figs. 10 and 11. Sample no. 1 and sample no. 3 have the same density, and they differ in thickness. Similar is the case for sample no. 2 and sample no. 4. Increase in thickness increases their SAC by a large amount. As for denim shoddy samples, the SAC at 500 Hz is around 0.4 for sample 1, which increases to 0.7 for sample 3. Similarly, SAC of sample no. 2 at 500 Hz is 0.5 which increases to up to around 0.75. The performance of waste jute fibers is not very good in the 500 Hz band and below but they perform very good at 1000 Hz, and for this frequency, increase in thickness increased the SAC at 1000 Hz from 0.6 to 0.9. It is also inferred from Figs. 10 and 11 that increase in density increased the SAC for less thick material as can be observed in the results of sample no 1 and 2, but when the thickness is already high, the effect is not very significant as is observed from the results of sample no. 3 and sample no 4. These results also suggest that denim shoddy can be used everywhere for noise control, even in lowfrequency sound source like diesel generators while waste jute fibers can perform well in building acoustics where the main concern is sound above 1000 Hz. The SAC for denim shoddy fibers remains mostly greater than 0.9 beyond 1000 Hz, which for waste jute fibers remains greater than 0.8. The performance of denim shoddy fibers, waste jute fibers and commercial glasswool having the same thickness and density is compared and shown in Fig. 12. For sample no.1, glasswool performs much better than waste jute fibers but just slightly better than the denim shoddy fibers. A marginal increase in density made the performance of denim shoddy becomes very much comparable with that of glasswool, and it is observed in Fig. 12b. Further increase in density and thickness made the performance of denim shoddy slightly better than that of commercial glasswool which is observed in Fig. 12c and Fig. 12d. The performance of glasswool is mostly better than that of waste jute, and gradually with increase in thickness and density, their performance becomes very much close to each other. Noise reduction coefficient (NRC) is used to rate a sound absorber and is calculated by Eq. (14),
NRC ¼
a250 þ a500 þ a1000 þ a2000 4
ð14Þ
The calculations to estimate NRC values are performed by referring to ASTM C 423 [37] and ASTM C 634 [38]. Though this standard is for NRC rating of materials by reverberation chamber method (random incidence of sound), it serves well for a comparative study of different materials in an impedance tube (normal incidence of sound). The NRCs thus obtained are not rounded off
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Fig. 9. Experimental set up to estimate the sound absorption coefficient.
Table 6 Acoustical characterization of the fibers. Property
Denim shoddy
Waste jute fibers
Sample No.
1
2
3
4
1
2
3
4
Porosity (/) Tortuosity (a1) AFR (r), Pas m2 VCL (K), mm 0 TCL (K ), mm
0.9720 1.0144 18,000
0.9700 1.0154 18,700
0.9720 1.0144 18,000
0.9700 1.0154 18,700
0.9710 1.0149 12,290
0.9690 1.0159 14,500
0.9710 1.0149 12,290
0.9690 1.0159 14,500
0.1371 0.2742
0.1149 0.2298
0.131 0.2742
0.1149 0.2298
0.3558 0.7116
0.3812 0.7624
0.3558 0.7116
0.3812 0.7624
to nearest 0.05 purposely to notice the exact difference between the NRCs of different materials tested with different process parameters. Again, it is worth stating that these NRC values represent the mean SAC of the entire band and not just that of the central frequency. A plot comparing the NRCs of denim shoddy, waste jute fibers and commercial glasswool is shown in Fig. 13. It can be inferred that, for a thickness of 70 mm, and density of 42 kg/m3 and 45 kg/m3 (Sample no. 3, and 4), denim shoddy has a higher rating than the commercial glasswool. The rating of waste jute is only slightly lesser than that of commercial glasswool. These results clearly state that these recycled material have very good potential for real-life acoustic applications.
4.5. Comparison of theoretically estimated and experimentally measured sound absorption coefficients
Fig. 10. Measured SAC for denim shoddy fibers.
Fig. 11. Measured SAC for waste jute fibers.
A comparison of the SACs measured with the experimental setup, predicted by the Delany-Bazley model and predicted by the Miki model are compared in Figs. 14 and 15. Fig. 14 is for denim shoddy, and for which the two models agree quite well with each other. Extrapolating the SAC obtained by Delany-Bazley model at low frequencies gives a negative value which is physically invalid. This issue was resolved by Miki model. The materials in this study are very good sound absorbers as seen in Figs. 10 and 11, and this is the reason both the model agrees quite well with each other. Though there is a slight deviation between measured and empirically estimated SAC for sample no. 01 of denim shoddy fibers, the experimental and theoretical results nearly overlap for sample no. 02, 03 and 04. For waste jute fibers, there is a significant difference between theoretically estimated and experimentally measured SAC for sample no. 01. The difference between experimental and theoretical results gradually converges on moving from sample no. 02, 03 and 04. The predicted and measured values of Fig. 15(c) and (d) match fairly well. There is variation in Fig. 15 (a) and (b), which are for a thickness of 50 mm. The deviation can be accounted for various reasons like these empirical models mentioned were originally developed for very fine fibers (typically less than 10 mm). As waste jute fibers in this study have a diameter
M. Raj et al. / Applied Acoustics 159 (2020) 107070
9
Fig. 12. Comparison of denim shoddy, waste jute fibers, and commercial glasswool for the same density and thickness a) sample no.01 b) sample no.02 c) sample no.03 d) sample no.04.
5. Equation development in terms of thickness and density to approximate the NRC The primary factors of the experiments conducted are thickness (a) and density (b) of the sound absorbers. This section aims to inspect which of these factors profoundly affect the NRC, statistically. The detailed statistical analysis beyond the present scope of the work presented here. As per the conventions used in the design of experiments [39], the experiment points are coded and shown in Table 7. The main effect of factor ‘a’ (thickness) on NRC, (M.E)a is calculated by Eq. (15):
ðM:EÞa ¼
Fig. 13. A comparison of NRC values of denim shoddy, waste jute fibers and glasswool.
ð15Þ
The main effect of factor ‘b’ (density) on NRC, (M.E)b is calculated by Eq. (16)
ðM:EÞb ¼ of 25 mm, it is coarser than those fibers; thus there is some deviation between theoretical and measured results. This is one of the reasons behind the good match achieved in the results of denim shoddy fibers (Fig. 14) as its diameter is less than 10 mm. With an increase in thickness, the effect of fiber coarseness of waste jute fibers has minimized and the experimental and theoretical results have come closer. This can be observed in Fig. 15(c) and (d) which are for 70 mm thickness and in Fig. 21 which is for a thickness of 60 mm. Besides, these models have considered only the airflow resistivity (AFR) and thickness as input parameters. Whereas, there are various other acoustical parameters too which might have affected the sound absorption coefficient. Such non-conformation has also been reported in many works of literature, like in reference [11] and [14]. These results once again reclaim our faith in empirical models which fetches fairly accurate results by measuring only airflow resistivity and thickness of the sample.
ða þ abÞ ðb þ minÞ 2n
ðb þ abÞ ða þ minÞ 2n
ð16Þ
The interaction between the variables thickness and density, on NRC, known as interaction effect (I.E)ab is calculated by Eq. (17):
ðI:EÞab ¼
ðmin þ abÞ ða þ bÞ 2n
ð17Þ
n = number of repetitions of experiment t = 2. These effects for denim shoddy and waste jute fibers are calculated and tabulated in Table 8. It can be observed from Table 8 that the effect of thickness is more than the effect of density, which implies NRC is primarily controlled by thickness. A similar observation was made from experimental results too. Change in the density of waste jute fibers effects the NRC more than the similar change in density of the denim shoddy fibers. These two conclusions are in accordance with the observations made from Figs. 10 and 11. The interaction effect
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Fig. 14. SAC measured in an impedance tube and predicted by empirical models for denim shoddy a) sample no. 01 b) sample no. 02 c) sample no. 03 d) sample no. 04.
Fig. 15. SAC measured in an impedance tube and predicted by empirical models for waste jute fibers a) sample no. 01 b) sample no. 02 c) sample no. 03 d) sample no. 04.
Table 7 Coded experiment points. level
Sample no.
NRC at these points coded as
(1,1) (1, 1) (1, 1) (1,1)
01 02 03 04
min b a ab
is very small as compared to the main effects, and its negative sign states that the effect of thickness on NRC will decrease as density increases. This is true as significant increase in density will lead to a large reduction in porosity.
Table 8 Effects of factors. Effect
Denim shoddy
Waste jute fiber
Thickness, (M.E)a Density, (M.E)b interaction (I.E)ab
0.0739 0.0341 0.0269
0.1178 0.0153 0.0042
A mathematical equation can be developed using these two factors [39], which estimates the response (NRC) as a function of two governing variables (thickness and density) and is given as Eq. (18):
M. Raj et al. / Applied Acoustics 159 (2020) 107070 Table 9 Coefficients of response surface. Variable
Denim shoddy
Waste jute fiber
b0 b1 b2 b12
0.8443 0.0369 0.0171 0.0135
0.7278 0.0589 0.0076 0.0021
zz ¼ b0 þ b1 X þ b2 Y þ b12 XY
ð18Þ
where: zz = response, in this case NRC; X = Coded value of thickness {1, 1}, Y = Coded value of density {1,1}. The coefficients of Eq. (18) is estimated by Eqs. (19–22):
b0 ¼
sumðminÞ þ sumðaÞ þ sumðbÞ þ sumðabÞ 4n
ð19Þ
Fig. 16. Response surface for denim shoddy.
where, {Sum(min), Sum(a), Sum(b), Sum(ab)} represents the sum of response (NRC) at the coded point-min, a, b and ab as mentioned in Table 7.
b1 ¼
ðM:EÞa 2
ð20Þ
b2 ¼
ðM:EÞb 2
ð21Þ
ðI:EÞab 2
ð22Þ
b12 ¼
The coefficients estimated for denim shoddy and waste jute fibers are tabulated in Table 9. The response surface plotted by using Eq. (18) and coefficients of Table 9 are shown in Figs. 16 and 17. The NRC of denim shoddy is observed to be very high, and waste jute too performs fairly well. The effect of the process parameters can be visualized by observing the slope along them. The slope along thickness appears to be very high, which boost up the claim of their high contribution towards increasing NRC. The NRC values can be approximated by putting the coefficients of Table 8 in Eq. (18). There will be some difference between actually measured NRC and the one predicted by the equation. This difference, known as residual, is calculated by Eq. (23)
Residual ¼ ðNRCÞmeasured ðNRCÞapproximated
Fig. 17. Response surface for waste jute.
ð23Þ
A plot for the normalized residual probability for the regression equations of both the fibers are shown in Figs. 18 and 19 and the trend is linear in nature. This validates the regression equation, which can be assumed to correctly predict the NRC values at intermediate points of the experiments. With an intent to validate equation (18) with coefficients of Table 9, an experiment is conducted by preparing a test sample of both the fibers at an intermediate point of the process parameters (thickness and density). The details at this point are tabulated in Table 10. The measured and predicted spectrum of SAC is shown in Figs. 20 and 21 where it is observed that the result obtained are similar to the ones shown in Figs. 10 and 11. Further, it can be observed in Table 10 that the predicted NRC is very close to the one experimentally measured. This validates the equations, which can be used to approximate the NRCs at any intermediate point of the experiments.
Fig. 18. Normalized residual probability plot for denim shoddy.
6. Cost comparison Almost all sound absorbers perform well at frequencies greater than 2000 Hz. Owing to the limitations by the laws of physics, none of them perform well below 250 Hz under normal circumstances.
Fig. 19. Normalized residual probability plot for waste jute fibers.
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M. Raj et al. / Applied Acoustics 159 (2020) 107070
Table 10 Parameters and Results of experimental results at the midpoint. Material
Coded coordinate
Thickness (mm)
Density (kg/m3)
Airflow Resistivity (Measured)
NRC (Measured)
NRC (Predicted by Eq. (15))
Denim Shoddy Waste jute fiber
(0,0) (0,0)
60 60
43.5 43.5
18,690 14,091
0.8343 0.7237
0.8443 0.7278
7. Conclusions
Fig. 20. SAC measured in an impedance tube and predicted by empirical models for Denim shoddy fibers at the midpoint of the process parameters.
This is systematic study to explore the potential of some recycled materials to be used as sound-absorbing materials. Denim shoddy and waste jute fibers have been identified as the materials with practically very low utility, and hence they are focused. They have been thoroughly characterized for their physical and acoustical properties. Experiments have been conducted by varying the process parameters to measure and predict their sound absorption coefficient. These materials have performed in really well and have fetched a very high Noise Reduction Coefficients. Denim shoddy performed better than the commercial glasswool. The performance of waste jute fiber with increased thickness was also comparable to that of glasswool. A mathematical equation is developed to visualize the effect of process parameters which observed that the change in thickness had a direct effect on the material’s NRC. This equation can be successfully used to predict the NRC values. Finally, a cost analysis is conducted for the same amount of sound absorption in which denim shoddy turned out to be 10 times; the waste jute fiber turned out to be 5 times, economical than the commercial glasswool. In this way, some environment friendly economical sound absorbing materials are the outcome of this work. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgement The authors are thankful to the Department of Science and Technology-Government of India for funding this project.
Fig. 21. SAC measured in an impedance tube and predicted by empirical models for Waste jute fibers at the midpoint of the process parameters.
Table 11 Cost comparison to achieve the same amount of sound absorption. Material
Selected sample number
Mass of material required (kg) = thickness*area*density
Rate ($/kg)
Total cost ($)
Commercial glasswool Denim shoddy
2 (Fig. 12b) 2 (Fig. 12b) 4 (Fig. 12d)
22.5
0.714
16.065
22.5
0.072
1.620
31.5
0.115
3.622
Waste jute fibers
Hence, for a fair comparison, this study focusses on 1000 Hz band. It is desired to provide a sound absorption of 0.9 at 1000 Hz band, over an area of 10 m2. Looking at the result of Fig. 12, the amount of required material is calculated as per the current rate (April 2019) in New Delhi and presented in Table 11. This is clearly inferred from Table 11 that using denim shoddy will be very economical as compared to commercial glasswool which is 10 times less expensive than the commercial material. Even though more amount of waste jute is needed than the commercial glasswool, still it comes out to be 5 times more economical.
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