Acoustical absorptive properties of meltblown nonwovens for textile machinery

Abstract

Noise pollution is one of the harmful physical sources in the textile industry, which is among those industries that are faced with noise exposure problems. The results of environmental sound measurements at modern textile mills have shown that the sound pressure level varied from 95 to 130 dB, where the highest sound pressure level was at weaving machines. Textile insulation materials can be fitted in order to decrease sound pollution at a low cost. The objective of this work is to design a sound absorber that can be fixed to the body of the machines, at the point of the noise generation, to reduce noise pollution. Poly(lactic acid) (PLA), which is an environmentally friendly material, was used to produce different samples of meltblown nonwoven absorbers to be used for damping the noise of textile machinery. PLA meltblown nonwoven fabric with the areal density of 16.7 g/m², average fiber diameter of 1.1 µm, mean pore diameter of 9.8 µm and thickness of 0.27 mm exhibited significant sound absorption. The sample with the smallest average fiber diameter among those investigated had the highest damping effect: 23.95, 41.29 and 29.32 dBA at frequencies of 400, 1000 and 1500 Hz, respectively. Our goal is to have a practical tool that accurately evaluates the absorber sound damping under the actual running conditions of the textile machinery. The design of the absorber from one layer of the PLA meltblown nonwoven over a rigid polyurethane foam sheet had an excellent sound absorption property.

Keywords

poly(lactic acid) fiber multi-layer structure meltblown nonwoven textile machinery sound absorption property

Noise control plays a major role in creating an acoustically pleasing environment, which according to the Occupational Safety and Health Administration (OSHA) standards should be satisfied in a workplace. Workers should not be exposed to a time-weighted average (TWA) noise level of 85 dB over an 8-hour work shift.¹ Textile mills and apparel manufacturing facilities are considered to be noisy; workers are exposed to excessive noise more frequently than in other industries. The duration of exposure per day depends on permissible noise exposures,² which range between 8 and 0.25 h at sound pressure levels of 90 and 115 dBA, respectively (OSHA 1910.95), When sounds have a similar frequency distribution of energy, dBA measurements may be used for ranking subjective responses to the sounds. When the sounds have very different energy distributions, or when there is a frequency dependence involved, such as with sound insulation or other noise control, then an analysis of the frequency distribution of the sound energy is required. Any noise rating above 80 dBA produces physiological effects, and a long exposure at above 90–100 decibels will produce permanent damage to a person’s hearing.^3,4 The sound pressure level of textile machinery varies according to the machine design, speed, lay out, different parts of the vibration level and the manufacturing accuracy of the different parts. Consequently, different values were measured by several researchers,⁵ and surveys of the noise levels in different departments of textile mills varied between 80 and 130 dBA. The spectrum analysis shows that the peak noise of the loom occurs at frequencies between 1400 and 5000 Hz. The weaving machine’s noise level depends on the loom speed and varies between 85 and 100 dBA.^3,5–7 At the cotton opening unit, the sound pressure level ranges between 79 and 87 dBA, where the highest sound pressure level was noticed at the adjacent stations of the carding machine. The average sound pressure level of the carding machine was 86 dBA and the maximum sound pressure level was at the frequency of 1000 Hz.^5–7 At the mixer unit, the highest sound pressure level and the dominant frequencies were 80 dBA and 250 Hz, respectively. Sometimes we use different versions of decibels. A-weighted decibels, or “dBA,” are often used when describing sound level recommendations for healthy listening. While the dB scale is based only on sound intensity, the dBA scale is based on intensity and on how the human ear responds. The results of the environmental sound measurement at a cotton spinning unit have shown that the sound pressure level was from 86 to 95 dBA, where the highest sound pressure level was at Cone Winder stations and ring machines.⁸ A weighted frequency analysis of the ring machine, studied in the textile industry, was 4000 Hz; however, no results of linear frequency analysis were reported for comparison.⁹ The highest and lowest sound pressure levels in the A-weighted network were 89 and 82 dBA and the dominant frequency was 500 Hz.⁹ In another study, the sound pressure level measurements ranged between 75 and 99 dB.¹⁰ Additional mill measurements showed that the mean value was around 93.5 dBA and the dominant frequency varied between 500 and 4000 Hz.

Eco-materials have the potential to be used as high-performance sound-absorbing noise isolators in several applications in areas such as transportation, architecture, industry and construction.¹¹ Several structures of textile fibers are tested as acoustic absorbers. Samples with various densities and yarn twists were used. The effect of fabric thickness was analyzed using three- and six-layered samples.¹² Multi-layer fibrous materials consist of different fibrous material layers, where both the intrinsic characteristics and layering sequence affect the noise reduction efficiency.^13,14 Research on composite absorbers has received as much attention as acoustical materials.¹¹ Therefore, multi-layer structures, consisting of knitted and woven fabrics, also shed light on noise reduction. Recycled material residues, either from industrial plants or processes, have received much consideration. It is well known that recovered materials and the use of environmentally friendly materials for noise control will be increased in the future.¹¹ Recently, the nanofibrous membrane has gradually become utilized as an acoustic absorption component in multi-layer structures.¹⁵ Another trend is based on the concept of using biobased plastics as the reinforced matrices for natural fiber composites. Several approaches were investigated to manufacture a sound absorber from textile materials and their composites.^15,16 The raw materials used to prepare these composite materials are generated from textile, maize and newspaper wastes. These raw materials were bonded using poly vinyl acetate (PVA) adhesives. The maximum sound absorption coefficient at the highest frequency and extreme noise reduction coefficient (NRC) were found in the sample having 75% maize and 25% textile wastes as reinforcements.¹⁶ Sound-absorbing composites have also been made from known natural fibrous acoustic textile materials or agricultural waste.¹⁷ The composite boards were made up of rice hull sawdust, ramie, flax and jute fibers, sunflower stalk/stubble and cotton fiber waste/textile waste into urea–formaldehyde adhesive resins with plaster.^18–20 Flexible polyurethane foam (FPUF) may also be used as solid as composite. Poly(lactic acid) (PLA)²¹ is a typical aliphatic polyester, which is made of a microbial fermentation product, lactic acid, being non-toxic, completely biodegradable, highly transparent and strong. Meltblown nonwovens can be produced economically; they are lighter nonwoven fabrics with less thickness and can be a good substitute to control sound absorption compared with commercially available heavy needle-punched nonwoven sound absorbers.²² Therefore, this work aimed to prepare a sound absorber to be used as a lining in the textile machinery frame for noise damping. In this study, the sound absorption performance of multi-layer of PLA meltblown nonwovens applied on a sheet of a layer of rigid polyurethane foam (PUF) has been investigated. The effect of the fiber diameter, air permeability, pore diameter, volume density, thickness of the materials and structure of the PLA meltblown on the sound absorption properties was studied.

Materials and methods

Physical characteristics of PLA meltblown nonwoven fabric

In the choice of materials for the sound absorber, the high capability of biodegradability, recyclability, reuse capability and the use of corrosion-free materials was considered to ensure the longer life span of the product.²² In this work, the PLA meltblown was used to produce a nonwoven fabric that fulfills the following three main criteria: the highest sound absorber performance; the lightest nonwoven structure; and the most environmentally friendly material. PLA is a sustainable material because it is produced from natural renewable resources.²³ In this work, PLA was meltblown to produce nonwoven fabrics. The raw PLA pellets used in this study are Ingeo biopolymer 6252D with a relative viscosity of 2.5 and a melting index of 115, as determined at 230 ℃, supplied by Nature Works LLC (Minnetoka, MN, USA). The meltblown webs are comprised of randomly oriented fibers with diameters ranging from microns to sub-microns. The meltblown webs have a high degree of fiber entanglement and one of the smallest average pore diameters of any fabric. The porous structure of the web plays an important role in improving performance in sound absorption.²⁴ This unique structure with a large number of micropores increases the damping effect when using the meltblown nonwovens as absorber materials compared with needle-punched or spunbonded nonwovens.¹⁵ Meltblown samples have average fiber diameters less than 2.5 µm and low air permeability of less than 26 (cm³/s/cm²), which means a low average pore diameter value. The meltblown nonwovens were produced at the University of Tennessee (Knoxville, TN, USA). Extrusion of the well dried PLA was performed using a twin-screw extruder under N₂ gas flow to prevent thermo-oxidative degradation of the polymer during processing. Meltblowing was conducted on a pilot line with 15 cm wide Exxon dies with 10 holes per centimeter. Nonwoven samples were collected on a moving belt at a speed of 15 m/min with the die-to-collector distance set at 30 cm. In this process, the polymer is fed to the extruder through the hopper. The motor-driven screw pushes the material through the extruder while five independently controlled heaters help melt the polymer and maintain the desired temperatures. A pressure transducer measures the melt pressure between zones 4 and 5, and the pressure was used to monitor and ensure that a constant polymer flow rate (throughput) was being supplied to the die while the samples were being collected. Samples were produced with or without a processing aid, 0.5% by weight of zinc stearate, airflows varying between 55 and 83 kPa and different screw speeds of 10 and 16 rpm to achieve the desired fiber diameter. For all samples, the melt temperature was set to 230℃ and air temperature to 265℃. The thickness of the nonwoven composite was determined using a thickness gauge, according to the ASTM D 5736 standard. Weight per unit area of the meltblown nonwoven fabric was determined according to the ASTM D 3776-96 standard. Air permeability was measured according to the ASTM D73796 standard with an area specimen of 38 cm² and under the constant pressure of 125 Pa. A TEXTEST FX3330 air permeability tester was used and the average of five measurements was recorded for each sample. The results are expressed in SI units as cm³/s/cm².¹⁵ Density was calculated from the area weight and thickness measurements of the samples. The testing apparatus of the sample’s mean pore diameter is a Capillary Flow Analyzer CFP-1100-A (PMI Company, USA). The standard of the measurement was ASTM F316-03. Galwick with a surface tension of 16 dynes/cm was used as the wetting agent. The obtained data for the mean flow pore diameter (MFPD) and pore diameter range (µm) were reported. The data, shown in Table 1, confirm that decreasing fiber diameter is followed by decreases in the average pore size. The microstructure of the meltblown web depends on the polymer density, air pressure and the heating temperatures of the different zones of the production line.²⁴ For composite samples, rigid PUF in 1 cm thickness was used as a supporting material, with GSM 200 g/m², thickness 0.5 cm, density 40 kg/m³ and NRC 0.7. Figure 1 shows the meltblown nonwoven samples and foam.

Figure 1.

Meltblown nonwoven and foam samples.

Table 1.

Properties of poly(lactic acid) (PLA) meltblown nonwoven sample numbers and their mean value (standard deviation)

	Material: PLA
Sample number	_ID1	_ID2	_ID3	_ID4
Weight (GSM)	29.6 (0.15)	49.3 (0.2)	36.4 (0.01)	16.7 (0.01)
Thickness (mm)	0.32 (0.023)	0.38 (0.73)	0.31 (0.015)	0.27 (0.151)
Average fiber diameter (µm)	2.3 (0.03)	2 (0.03)	1.8 (0.03)	1.1 (0.03)
Air permeability (cm³/s/cm²)	80.1 (0.6)	20.2 (0.17)	15.3 (0.07)	25.6 (0.13)
Mean flow pore diameter (µm)	26.8 (2.97)	11.2 (1.8)	9.2 (1.66)	9.8 (0.73)
Pore diameter range (µm)	5.6–55.1	2.4–17.7	2.4–18	3.2–14
Density g/cm³	0.0925	0.1298	0.1103	0.06185

GSM: grams per square meter.

To measure the acoustical properties of the fabric materials, the impedance tube method (ASTM C 384-98) and the acoustical chamber method were used.^25,26

Although these methods do not represent the real transmission loss in a diffuse sound field, they are quite useful for testing small material prototypes.²⁶ The developed sound insulation measuring apparatus is based on the acoustical chamber method and is illustrated schematically in Figure 2. The dimension of each chamber was length × width × height = 0.3 m ×0.3 m × 0.6 m. The material used for the test chamber was plywood and the wall of the chambers was a double wall filled with sound isolating material to prevent the outside noise affecting the recorded sound inside the receiver room. Circular samples of 10 cm in diameter were used for the acoustical measurement. Noise with sound frequencies ranging from 0 to 4000 Hz was used for testing the sound insulation of the material. The sound signals are fed to the laptop either through a signal generator or are recorded by a digital voice recorder with the actual measured sound from the running machinery in the textile mills. Afterward, the signals are amplified and sent to the loudspeaker. The test specimen holder position is placed at an equal distance from the speaker and the microphone and is capable of holding a circular or rectangular specimen. The specimen is fixed in the holder to ensure the position and flatness of the front and back surface of the specimen as well as the axis of the specimen being exactly on the centerline of the loudspeaker and microphone. The sound source should have a uniform power response over the frequency range of interest, 100–6000 Hz. The walls of the box are sound-isolated from the outside and the structure-brought sound excitation inside the box is minimized. The chamber is insulated by a top cover.²⁵ The mean sound absorption coefficient values at frequencies of 250, 500, 1000, 1600 and 3500 Hz were used to calculate the sample’s NRC.²⁶

Figure 2.

Schematic drawing of the setup for measuring the sound transmission loss.

The NRC can be calculated by the following equation

NRC = (α_{250} + α_{500} + α_{1000} + α_{1600} + α_{3500}) / 5

(1)

where α_p is the coefficient of sound absorption at the sound pressure level p (Hz).

For each sample, the mean of five sound absorption coefficient readings (α_p) at the sound frequency (p) Hz were recorded, and the mean of the measured data was considered.

Mill measurement procedure

In the working area, where workers are subjected to different levels of sound depending on the machine design, the noise was measured according to the ISO standard method (ISO 9612, 2009). Measurements were carried out using a portable sound pressure level meter, PCE-428, which is a device with a measuring range from 25 to 136 dB(A) at a frequency of 20 Hz–12.5 kHz. Based on this standard method, the direction of the microphone was at the height of 1 m above the ground,⁸ in the area of the normal existence of the workers performing their duties. Several records of the noise level at different machine locations were executed, and the average was considered. Digital sound records were fed to a laptop to be amplified and sent to the loudspeaker. The sound pressure level at each machine speed was recorded and fed to the laptop of the setup to be broadcast by the loudspeaker, so the sound was transferred through the sample of the absorber material, which was fixed in the sample holder. In the beginning, the sound pressure value H₀ dB was recorded without the sound absorber sample and was taken as a reference. Then the sample of the sound absorber was fixed to the sample holder, the test was done again, and the sound pressure value H₁ dB was recorded. In both cases, the sound broadcast was that recorded from the machine at a particular speed. To get the effect of the sound absorber, it is necessary to subtract one noise from another; machine noise alone must be subtracted from total noise to obtain the sound recorded when using the absorber. Finally, the sound transmission difference between two records, that is, the sample damping effect, was calculated²⁷

Sampledampingeffect = 10 {log}_{10} (1 0^{(H_{1} / 10)} - 1 0^{(H_{0} / 10)})

(2)

Fast Fourier transform (FFT) transforms a function of the time domain into a function of the frequency domain, that is, from amplitude versus time to magnitude versus frequency. Fourier transformation of the data from the test without the sample served as the baseline. The results are given in the form of plots between the frequency (Hz) and the transfer function magnitude (dBA) for the samples. The data from the microphone of the acoustical chamber is appropriate for obtaining the acoustical insulation information of the absorber material installed in the sample holder.

Results and discussion

With gradually increased noise pollution due to the increase in the production speed of the textile machinery, the application of fibrous materials in sound absorption has received great attention. The textile machinery sound pressure level is used to determine the sound emission model around machines, and measurements were performed by the grid method. The height of the measurements as well as the distance from the source of the noise depend on the type of the machine and the worker’s route (0.5 m from the machine and 1 m height) . All mechanisms of the machine are sources of noise; moreover, the motor and the ventilation installation all cause an increase in the noise level. Measurements were carried out at several points around the sources.

The specifications of the processed PLA meltblown nonwovens are given in Table 1.

Textile machinery sound pressure level

Ring spinning machine

The measurements of the sound pressure level of the ring spinning machine was done at three positions: at both ends of the machine and along the path of the labor around the machine. The results of the measurements are illustrated in Figure 3. The values of the sound measurements were done at different spindle speeds. The noise level increased along the spinning machine and at the ends of the side. It will increase with the increase of the spindle speed.

Figure 3.

Level of sound intensity of the ring spinning machine versus spindle speed (rpm).

This indicates that, according to the OSHA, these sound pressure levels are higher than those recommended for the working place; hence, workplace noise risk occurs, and sound absorption solutions are essential. The noise level with the increase of spindle speed of 1000 rpm of the ring spinning machine leads to an increase in the noise level by 2 dB.

Sulzer weaving machine (rapier weft insertion system)

Not only does the noise level in spinning machines need to be controlled, but also that in weaving machines.²⁸ Figure 4 illustrates the values of the sound pressure level at the different positions: (I) at the driving head; (II) along the cloth beam; (III) at the warp beam; (IV) at the area of the heald shafts.

Figure 4.

Level of sound intensity of the weaving machine rapier weft insertion system versus loom speed (ppm).

The noise level increases by 2.67 dB with every increase in the weft insertion rate of 100 ppm of the rapier weaving machine.

Picanol Optimax weaving machine (with a jacquard system)

The noise level was tested on another type of weaving machine. In Figure 5, the analysis of the sound map of the jacquard loom indicates the values of the sound pressure level at different positions.

Figure 5.

Level of sound intensity of the weaving machine with a jacquard system versus loom speed (ppm).

The maximum sound pressure level was located at the area of the heald shafts, while at the working area the sound pressure level reached 104–106, depending on the (ppm) of the loom. The noise level with an increase of 100 ppm of the weaving machine jacquard system increases the noise level by 3.75 dB.

Con Winder

In a mill, the sound pressure level map will be affected by the speed of the machines and their layout. The sonic map results for cotton spinning showed the sound pressure level levels at a Con Winder 95 – 108 dBA at a winding speed of 600 m/min.

Analysis of the sound absorption coefficient of one layer of meltblown nonwoven samples

Table 2 gives the coefficient of sound absorption of the meltblown nonwoven fabric samples. The sound absorption coefficient of the samples is determined based on various PLA fiber properties and the PLA meltblown nonwoven fabric specifications. The results are graphically depicted in Figure 6. As illustrated, the sound absorption coefficient is frequency-dependent. The results show a pattern in which the absorption coefficient value increased at the low-range frequency (at almost 0.9–0.96) and dropped slightly at the mid-range frequencies. At high frequencies, it gradually increased back and reached the absorption coefficient of 0.91–0.95 at 3500 Hz, depending on the sample specifications.

Figure 6.

Coefficient of sound absorption versus sound frequency (Hz).

Table 2.

The coefficient of sound absorption of one layer meltblown nonwoven samples

Frequency, Hz	_ID1	_ID2	_ID3	_ID4
250	0.4	0.48	0.49	0.5
500	0.74	0.71	0.81	0.8
1000	0.92	0.96	0.97	0.98
1600	0.92	0.92	0.88	0.87
3500	0.95	0.9	0.88	0.9

Effect of the microstructure of PLA meltblown nonwoven samples

From the scanning electron microscopy (SEM) photographs shown in Figure 7, it is apparent that the meltblown webs are comprised of randomly oriented fibers with diameters ranging from microns to sub-microns. The meltblown webs with a high degree of fiber entanglement or a small average fiber diameter are expected to have smaller pore sizes with higher total porosity, as shown in the SEM images of meltblown fibers in Figure 7.

Figure 7.

Scanning electron microscopy images of meltblown web samples.

The porous structure of the web plays an important role in improving the performance of sound absorption of the meltblown webs. The porosity affects the capability of the meltblown web to damp the noise; hence, when sound enters the pores of the fibrous web, owing to sound pressure, air molecules oscillate in the interstices of the porous material with the frequency of the exciting sound wave. This oscillation results in frictional losses and a change in the flow direction of sound waves, which together with the expansion and contraction phenomenon of flow through irregular pores, results in a loss of momentum. These materials with an interconnected microporous structure become good absorber materials of different lengths that contain cavities, channels or spaces so that sound waves can enter through them.

In fibrous materials, much of the energy can also be absorbed by scattering from the fibers, and by the vibration caused by the individual fibers, as the fibers of the material rub together under the influence of the sound waves.²⁹ The finer the fiber, the more easily it is vibrated and the higher the sound absorbed energy.

It was revealed that the meltblown structure is an efficient acoustic absorber, particularly in the low- and middle-frequency regions.^15,30 Air permeability is one of the structural properties of the meltblown nonwoven fabric that affects its sound absorption coefficient; its air permeability is generally understood as the transmission of air through a material. Air passes through the textile, through inter-fiber spaces (pores). Air permeability is one of the properties that defines the internal structure of the material and depends on the number and size of the pores, which enables air to pass through the textile. Air permeability is measured as the rate of the airflow passing perpendicularly through a known area under a prescribed air pressure differential between the two surfaces of a textile material. The effect of air permeability of the meltblown samples and their coefficient of sound absorption was investigated.¹⁵

The value of the NRC is illustrated in Figure 8(a), and was found to reduce as the air permeability increased. Figure 8(b) demonstrates that the NRC depends also on the MFPD and, hence, the sound absorption coefficient increases with an increase in the number of pore openings in the unit area or with a decrease of the diameter of the pore openings.³¹ The fiber diameter is also positively correlated to the pore diameter consequently and, therefore, the NRC is inversely proportional to the fiber diameter (Figure 8(c)).

Figure 8.

Different relations of the sample properties and the noise reduction coefficient (NRC): (a) relationship between the NRC and the air permeability; (b) relationship between the NRC and the mean flow pore diameter; (c) relationship between the NRC and the average fiber diameter.

The maximum value of a specific NRC was noticed for sample ID4, showing better sound insulation than the other test samples (Figure 9(a)). When fine PLA fibers are used, the sound absorption performance improves by up to 1000 Hz. This phenomenon is due to the fact that the MFPD and fabric thickness are small.

Figure 9.

Specific noise reduction coefficient (NRC) of the samples: (a) specific NRC for different samples; (b) specific NRC for different samples versus fabric density.

A comparison of the specific NRC (NRC/g/m² of the sample) for different results from various fabrics (Figure 9(b)) shows fabric density to be the key factor determining sound absorption. This is assumed to be because the low-frequency sound was absorbed less in dense fabric, whereas low-frequency sound was absorbed more in the air path in the fabric pores of small diameters.

Multi-layer fibrous structures for noise reduction

Thin meltblown nonwoven fabric with a low strength can easily be damaged during a machine’s working life, and thus the fabrication of a multi-layer structure as a sound absorber can find a wider application. In order to fix meltblown material on the inside surface of the machine frame covering the mechanical parts, multi-layers of meltblown nonwoven fabrics and a layer of rigid PUF sheet have been investigated. Two configurations were tested: in configuration I, a meltblown layer was positioned on the surface of the foam sheet; in configuration II, meltblown layers were placed on both sides of the foam sheet. The sound absorption coefficients of the multi-layer sound absorber composed of meltblown nonwoven fabric and a foam layer are represented in Figures 10 and Figure 11, indicating the change in the coefficient of sound absorption of the different samples. Results showed that the difference in transmitted sound varied between test samples only at a frequency of less than 1000 Hz, and all samples had a lower coefficient of sound absorption at a higher frequency. The value of the NRC is illustrated in Figure 12.

Figure 10.

Coefficient of sound absorption versus sound frequency dBA (configuration I: one layer of meltblown nonwoven + one layer of foam).

Figure 11.

Coefficient of sound absorption versus sound frequency dBA (configuration II: two layers of meltblown+ one layer of foam).

Figure 12.

Noise reduction coefficient (NRC) of multi-layer meltblown samples and with foam (configurations I and II).

The results showed the difference in transmitted sound between test samples made from different specifications that affect their specific NRC. The sound insulation of meltblown nonwoven material (sample ID4) using one layer on the surface of the foam layer has the highest value of the specific NRC than the other samples, as illustrated in Figure 12. A comparison of NRC results for various fabrics shows fabric density to be the major factor determining sound absorption (Table 2).

The meltblown nonwoven layered structures of a low layer density, fiber diameter and average pore diameter have relatively high acoustic absorption efficiency, particularly in the middle-frequency region. In the case of the multi-layer structure, the coefficient of sound absorption mainly depends on the coefficient of sound absorption of the layer directly facing the sound wave. The increase of the NRC, when using a layer of the meltblown fabric over the foam sheet, is due to the air gap between the two layers; the surface texture of the foam is not smooth, therefore keeping these air gaps. The noise emission of the ring spinning machine was recorded at different spindle speeds and analyzed on the setup without and with the use of the absorber sample ID4 (configuration I).

Frequency analysis of the textile machine acoustic signal using the fast Fourier transformation

Most manufacturers that are producing textile machinery, such as ring spinning machines, weaving machines or winding machines, have a problem with the final product because these machines can make noise and vibrations during their operation. Manufacturers try to decrease this unpleasant noise level. However, firstly, we need to know the frequencies of noise and then can take measures to reduce it. Recording sound to a digital file and transforming the data by the fast Fourier transformation is one of the ways to accomplish that. FFT analysis is a good choice for transforming some digital signals from the time domain to the frequency domain. FFT software was used to convert waveform data from the time domain to the frequency domain. The Fourier series can decompose any periodic signal or function into the sum of simple goniometric functions, namely function, sines and cosines. This decomposition of a complex function to a set of simple functions is the main advantage of the Fourier method. Figure 13 illustrates the Fourier transform from the test without the test sample, which served as the baseline for the comparison with the FFT of the tested samples under the effect of a certain sound frequency. Figures 13 illustrates the results at different frequencies, 400, 1000 and 1500 Hz, for meltblown nonwoven samples. For sample ID4, the damping effect was the highest, 23.95, 41.29 and 29.32 dB at frequencies 400, 1000 and 1500 Hz, respectively.

Figure 13.

Fast Fourier transform (FFT) transmitted sound results for the meltblown nonwoven at different input sound frequencies: (a) FFT transmitted sound results for meltblown nonwoven ID4 (configuration I) at input sound frequency 400 Hz; (b) FFT transmitted sound results for meltblown nonwoven ID4 (configuration I) at input sound frequency 1000 Hz; (c) FFT transmitted sound results for meltblown nonwoven ID4 (configuration I) at input sound frequency 1500 Hz.

The sound record of ring spinning machine, at different spindle speeds, was recorded on the setup for the cases with and without absorber material (configuration I, sample ID4). The FFT graphs in both cases are illustrated in Figure 14. The analysis shows that as the spindle speed increased the frequency of different sound waves increased; however, the dispersion of their range decreased.

Figure 14.

Sound absorption performance of configuration I, sample ID4: (a) fast Fourier transform (FFT) transmitted sound at 6000 rpm; (b) FFT transmitted sound at 7500 rpm; (c) FFT transmitted sound at 10,500 rpm.

Figure 15 shows the damping effect of the absorber (configuration I, sample ID4) at each spindle speed, which is varied according to the sound frequency (Equation (2)). These figures illustrate the damping effect by using an absorber material at different frequencies, which indicates that the absorber is more effective at high frequencies than low ones, in the range between 2000 and 5000 Hz with the maximum value at 4550 Hz. Absorber configuration I had the highest damping effect at frequency varied between 4500 and 5500 Hz, and its maximum value reaches 67 dB.

Figure 15.

The damping effect of sound absorption at different spindle speeds (configuration I, sample ID4): (a) damping effect of the sound absorber at spindle speed 6000 rpm; (b) damping effect of the sound absorber at spindle speed 7500 rpm; (c) damping effect of the sound absorber at spindle speed 10,500 rpm.

Conclusion

The knowledge of material properties and their sound damping effect enables proper selection and design of collective sound absorption material to protect workers from the noise emitted by various textile machines and high-speed devices applied in modern spinning and weaving manufacturing processes. The experimental data signify the following.

A laboratory procedure to test the noise-damping effect at different sound frequencies was developed, as a successful design requires a good estimation of the absorber performance before application. Different PLA meltblown absorbers were investigated and the effect of several structural parameters, including air permeability, thickness, areal density and pore diameter, on sound transmission loss was analyzed.

It was revealed that a thin sample of low fiber diameter, smallest pore diameter, high air permeability and low density exhibited significant sound absorption. The application of PLA meltblown nonwoven fabric with the areal density of 16.7 g/m², average fiber diameter of 1.1 µm, MFPD of 9.8 µm and thickness of 0.27 mm exhibited significant sound absorption.

The tests performed on samples with PLA meltblown nonwoven fabrics of different specifications and a layer of rigid PUF sheets indicated that the configuration I absorber had the highest damping effect at frequencies varied between 4500 and 5500 Hz.

It is recommended for the design of the sound absorber for textile machinery to use a sheet of rigid PUF covered by one layer of PLA meltblown (similar to sample ID4), as the foam sheet makes it possible to fix the absorber in the frame of the machine. Based on these results, more work needs to be done to study the suggested absorber design on an actual running mill.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Magdi El Messiry

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