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Journal of Industrial Textiles

Sound absorption properties for multi-layer of composite materials using nonwoven fabrics with kapok

XuetingLiu https://orcid.org/0000-0003-4708-470X,XiaoningTang https://orcid.org/0000-0002-8133-0882, andZhongminDeng [email protected]View all authors and affiliations

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https://doi.org/10.1177/1528083720904926

Abstract

Kapok fiber with high degree of hollow and small fiber diameter has excellent sound absorption. To optimize the sound absorption characters at the low frequency, sound absorption coefficients and specific surface impedance of two-layer, three-layer, and four-layer materials as polyethylene film-reinforced kapok fiber nonwoven fabrics have been studied by using the impedance tube method in the frequency range of 100–2500 Hz. The layering sequence, the polyethylene films thickness, and the gradient thickness structure of nonwoven fabrics affected the sound absorption properties at the low frequency for multi-layer materials that have been analyzed. The results show that the multi-layer composite materials have better sound absorption than a single-layer nonwoven fabrics at the low frequency. The elastic vibration of polyethylene film contributes to sound absorption improvement at the low frequency. The sound absorption at the low frequency of multi-layer materials can be further improved by changing polyethylene film position or increasing polyethylene film thickness. In addition, the gradient thickness structure of nonwoven fabrics can effectively improve sound absorption at the low frequency, and the gradient change direction also exhibits remarkable effects on the sound absorption of multi-layer materials.

Introduction

Nonwoven fabrics have excellent sound absorption at medium and high frequency, but very poor at low frequency. As a natural fiber with high degree of hollow and small fiber diameter, kapok has excellent sound absorption [1,2]. Our previous studies have shown that nonwoven fabrics with kapok fiber have effective sound absorption properties at the low frequency [3,4]. Recently, multi-layer materials with porous mediums are applied to absorb broadband noise. The layer-by-layer attenuation of interior sound wave leads to the better sound absorption of multi-layer materials. In order to further meet the needs of sound absorption at low frequency of kapok fiber nonwoven fabrics, research on acoustical properties of multi-layer materials using nonwoven fabrics with kapok fiber was carried out and results are presented in this paper.

The materials that are commonly used for system of sound absorbing using multi-layer structures with porous materials as perforated panels [5–7] or micro-perforated panels [8–15], as they are resistant to humidity, can be designed to provide wide-band sound absorption. Research on sound absorption of multi-layer absorbers using fiber porous layers and perforated panel showed that the composed materials had good acoustic properties at low and high frequencies [5,6]. A model predicting sound absorption of systems with perforated panel, porous material, and air layers indicated that addition of the perforated panel can improve sound absorption coefficients around the resonant frequency, but ones in other frequency ranges decrease [7]. In many applications, the sound-absorbing bandwidth of the perforated panel is limited. To further widen the bandwidth, many scholars have done a lot of research on sound absorption properties of micro-perforated panel [8–10]. The perforations of micro-perforated panel are reduced to submillimeter size so that they themselves will provide enough acoustic resistance and also sufficiently low acoustic mass reactance necessary for a wide-band sound absorber [11,12]. Micro-perforated panel multi-layer structures have been explored to achieve high sound absorption, such as multi-layered elastic micro-perforated panel [13] and multi-layered micro-perforated panel/porous absorbers [14,15]. The sound-absorbing frequency band of the micro-perforated panel is wide, but its structure is complex, processing is difficult, and cost is high.

The thin membrane has the features of lightweight and easy processing. Sound in multi-layer materials with membrane is absorbed due to repeated reflection of sound waves caused by vibration of membrane. The sound absorption of the layered system was dependent on the acoustical resistance of each layer. Relevant researches indicated that the membrane materials have contributed to improve sound-absorbing properties at the low frequency of layered materials [16–18]. Gaulon et al. [19] compared the acoustic absorption of open-cell polyurethane foams and closed-cell ones with membranes. The study had shown that closed-cell polyurethane foams with membranes had better acoustic properties than the open-cell ones with similar parameters in the 500–6000 Hz frequency range. In addition, Tang et al. [20] improved sound absorption of layered materials composed of nonwoven fabrics and polymeric membrane by adjusting the layering sequence. A research found that the characteristic absorption peaks shifted to the lower frequency by tuning the layer thickness and thickness ratio of the microlayer foam/film sheet materials [21].

The aim of this paper is to investigate sound absorption of multi-layer lightweight absorbers composed of nonwoven fabrics with kapok fiber and polyethylene (PE) film. This paper had studied the effects of the layering sequence, the thickness of PE film, and the gradient thickness of nonwoven fabrics on sound absorption coefficients and specific surface impedance for two-layer, three-layer, and four-layer materials.

Experimental

Sample preparation

In this paper, the kapok fiber diameter is 23.4 µm, the average fineness is 1.25 dtex, and the length is 8–32 mm. The hollow polyester fineness is 6.67 dtex, and the length is 64 mm. The layered materials are composed of the nonwoven fabrics (a mixture of kapok–polyester hollow fabrics produced via carding and needle punching) reinforced by PE films. The punching density, depth, and frequency are 1400 needle/m, 3 cm, and 278 times/min, respectively. Based on our previous studies, the content percentage of kapok fibers to the hollow polyester fibers is (90%:10%) for all the samples. The thicknesses of PE films are 0.16, 0.2, 0.3, and 0.45 mm. The melting temperature of the PE film is 120°C. PE film and kapok fiber nonwoven fabrics were compounded at high temperature and pressure-free in the oven. The compound temperature is 110–130°C. Finally, samples were cooled in the air. The details of multi-layer composite materials are listed inTable 1. The mass density and porosity of all nonwoven fabrics with kapok fiber are 17.25 kg/m³ and 94%, respectively.

Table 1. The details of multi-layer material samples.

Sample ID	Layer number	first layer material/thickness (mm)	second layer material/thickness (mm)	third layer material/thickness (mm)	fourth layer material/thickness (mm)
P	Single layer	PE/0.45
K		Kapok/20
KP1	Two layer	Kapok/20	PE/0.45
KP2		Kapok/20	PE/0.16
PK1		PE/0.45	Kapok/20
PK2		PE/0.2	Kapok/20
KPP	Three layer	Kapok/20	PE/0.16	PE/0.16
PKP1		PE/0.16	Kapok/20	PE/0.16
PKP2		PE/0.3	Kapok/20	PE/0.3
PKP3		PE/0.45	Kapok/20	PE/0.45
PPK		PE/0.16	PE/0.16	Kapok/20
KKPP	Four layer	Kapok/10	Kapok/10	PE/0.16	PE/0.16
KPKP1		Kapok/10	PE/0.16	Kapok/10	PE/0.16
KPKP2		Kapok/15	PE/0.16	Kapok/15	PE/0.16
KPKP3		Kapok/10	PE/0.16	Kapok/20	PE/0.16
KPKP4		Kapok/10	PE/0.3	Kapok/20	PE/0.3
KPKP5		Kapok/20	PE/0.16	Kapok/10	PE/0.16
PKKP		PE/0.16	Kapok/10	Kapok/10	PE/0.16
PKPK1		PE/0.16	Kapok/10	PE/0.16	Kapok/10
PKPK2		PE/0.16	Kapok/15	PE/0.16	Kapok/15
PKPK3		PE/0.16	Kapok/10	PE/0.16	Kapok/20
PKPK4		PE/0.3	Kapok/10	PE/0.3	Kapok/20
PKPK5		PE/0.16	Kapok/20	PE/0.16	Kapok/10
PPKK		PE/0.16	PE/0.16	Kapok/10	Kapok/10

In the later study of this paper, kapok fiber nonwoven fabric is called kapok for short and PE plastic film is called PE for short. In addition, the first layer represents one that faces the sound source. Schematic diagrams of multi-layer sound absorption structures are shown inFigure 1.

Figure 1. Schematic diagrams of multi-layer sound absorption structures.

Measurement of material parameters

The normal incidence sound absorption coefficients

The normal incidence sound absorption coefficients were assessed by using a two-microphone transfer-function method, according to ISO 10534-2 standards. The testing apparatus was a part of a complete acoustic system SW260 (BSWA Technique Company, China), as illustrated inFigure 2. Diameter 100 and 30 mm round samples were used for measurement of sound absorption in the frequency range of 100–800 and 400–2500 Hz, respectively. Each value represents the average value of three measurements of the same sample.

Figure 2. Measurement setup for the normal incidence sound absorption coefficient.

The specific surface impedance

Z

is defined as [22]

Z = Z_{s} / Z_{0}

(1)

where

Z_{s}

is the surface impedance of composite materials and was measured by using the impedance tube.

Z_{0}

is the characteristic impedance of air,

Z_{0} = ρ_{0} c_{0}

ρ_{0}

is the air density,

c_{0}

is the sound velocity in the air. For air at normal temperature and atmospheric pressure,

ρ_{0} = 1.2 {kg/m}^{3}

c_{0} = 340 m/s

. The specific surface impedance has the real part and the imaginary part.

Thickness

The thickness of the samples was measured by a digital thickness gauge with 0.01 mm accuracy, according to GB/T 1380-1997 standards. The pressing pressure on the samples is 0.5 kPa. The measurements of thickness were made at Wuhan Textile University, Key Laboratory of Textile Fiber & Product, Ministry of Education.

Results and discussion

The sound absorption coefficients and specific surface impedance of two-layer, three-layer, and four-layer absorbers composed of kapok fibers nonwoven fabrics reinforced with PE films are shown inFigures 3 to12. The effects of the layering sequence, the thickness of PE film, and the gradient thickness of nonwoven fabrics are discussed hereafter.

Effects of the layering sequence on sound absorption at the low frequency

Sound absorption coefficients and specific surface impedance of two-layer, three-layer, and four-layer composite materials with different layering sequence are shown inFigures 3 to6. FromFigures 3 to5, it can be seen that the layering sequence has significant influence on sound absorption of multi-layer materials. Sound absorption coefficient curve of sample P inFigure 3 shows that PE film has bad sound absorption at the frequency range of 100–2500 Hz, while sound absorption coefficients of KP1 two-layer material sample are higher than those of K and P single-layer samples at the frequency range of 100–2500 Hz. Sound absorption of sample KPP inFigure 4 and sample KKPP inFigure 5 is all better than that of sample K. These phenomena can be explained by the specific surface impedance curves inFigure 6. FromFigure 6, it can be seen that compared to sample K, the real parts of specific surface impedance of samples KP, KPP, and KKPP are nearer to one and the imaginary parts of specific surface impedance are nearer to zero. This means that the more incident sound waves transmitting into materials due to the less reflection on the materials surface and the more loss of sound waves in the materials. The combination of two conditions leads to the better sound absorption of materials [23]. Gaulon et al. [19] also revealed that the membrane will vibrate under the action of the incident sound waves because soft membrane is sensitive to the sound pressure variation. So sound absorption of porous materials backed by the membrane is better than that of porous materials backed by the rigid wall. The vibration of membrane creates more scattering and energy loss of sound wave when sound waves transmit on the membrane surface from the porous materials.

Figure 3. The effect of the layering sequence on sound absorption coefficients of two-layer materials.

Figure 4. The effect of the layering sequence on sound absorption coefficients of three-layer materials.

Figure 5. The effect of the layering sequence on sound absorption coefficients of four-layer materials.

Figure 6. The real part and imaginary part of the specific surface impedance of two-layer (a-1), (a-2), three-layer (b-1), (b-2), and four-layer (c-1), (c-2) materials with different layering sequence.

Figures 3 to5 also show that PE film position change has the important effect to sound absorption of multi-layer materials. FromFigure 3, it can be seen that the sound absorption coefficients of sample PK1 are higher than those of KP1 below 1117 Hz, but lower above 1117 Hz. That could be because that in the low frequency range, PE film and porous layer constitute a resonant cavity structure which generates a sound absorption peak at 780 Hz, but greatly reduces sound absorption at the medium and high frequency [23–25]. FromFigure 4, it can be seen that compared to sample PKP1, the maximum sound absorption coefficient of sample PPK shifts to the lower frequency, but sound absorption at the medium frequency further decreases. The same phenomenon has been found by comparing sound absorption curves of samples KPKP1, PKKP, and PPKK inFigure 5. In addition, compared to sample PKKP, the maximum sound absorption coefficient of sample PKPK1 shifts to the lower frequency. It means that the sound absorption peak of the sample can be tuned to the lower frequency by changing PE film position.

Effects of the thickness of PE film on sound absorption at the low frequency

Sound absorption coefficients and specific surface impedance of two-layer, three-layer, and four-layer composite materials with different thickness of PE films are shown inFigures 7 to10. FromFigure 7(a), it can be seen that sound absorption of kapok/PE materials increase with the increase of PE film thickness because the greater vibration of PE film caused by increase in thickness create the more scattering when sound waves pass through nonwoven fabrics and transmit on the surface of PE film. FromFigure 7(b), it can be seen that the maximum sound absorption coefficient shifts to the lower frequency with the increase of PE film thickness for PE/kapok materials. The same phenomena also happen for PE/kapok/PE three-layer materials and kapok/PE/kapok/PE and PE/kapok/PE/kapok four-layer materials inFigures 8 and9. The specific surface impedance curves inFigure 10 reveal that the sound energy loss in multi-layer materials increases and reflection on the materials surface decreases with the increase in PE film thickness at the lower frequency, but those are opposite at the medium frequency. That may be because that the greater cavity resonant effect caused by increase in PE film thickness improves sound absorption at the low frequency, but reduces sound absorption at the medium frequency. That means that the sound absorption at the low frequency of the sample can be further improved by increasing the PE film thickness.

Figure 7. Effects of PE film thickness on sound absorption properties at the low frequency of two-layer materials.

Figure 8. Effects of PE film thickness on sound absorption properties at the low frequency of three-layer materials.

Figure 9. Effects of PE film thickness on sound absorption properties at the low frequency of four-layer materials.

Figure 10. The real part and imaginary part of the specific surface impedance of two-layer (a-1), (a-2), three-layer (b-1), (b-2), and four-layer (c-1), (c-2) materials with different PE film thickness.

Effects of the nonwoven fabrics gradient thickness on sound absorption properties at the low frequency

In this paper, the nonwoven fabrics gradient thickness means a gradual increase or decrease in nonwoven fabrics thickness in the direction of sound wave incidence. As we know, the gradient change of nonwoven fabrics thickness has directionality. As an example, sample KPKP3 has opposite gradient change direction to sample KPKP5. The comparison of sound absorption coefficients and specific surface impedance of KPKP and PKPK multi-layer materials with the gradient thickness structures of kapok-based fiber nonwoven fabrics is presented inFigures 11 and12. FromFigure 11(a) and (b), it can be seen that the gradient thickness of nonwoven fabrics has the important influence on sound absorption properties of multi-layer materials.Figure 11(a) shows that sound absorption coefficients of KPKP3 gradient structure material were higher than those of KPKP2 regular materials at the frequency range of 100–2500 Hz.Figure 11(b) also shows that sound absorption coefficients of PKPK3 gradient structure material were higher than those of PKPK2 regular materials at the frequency range of 100–2500 Hz.Figure 12(a-1) to (b-2) shows that the real part of specific surface impedance of sample KPKP3 is nearer to one and the imaginary part is closer to zero than those of sample KPKP2 at the frequency range of 100–2500 Hz. These indicate that the less reflection on the multi-layer surface and the more sound energy loss in the materials of sample KPKP3 create the better sound absorption of sample KPKP3 than that of sample KPKP2.

Figure 11. The effect of the gradient thickness of nonwoven fabrics on sound absorption coefficients.

Figure 12. The real part and imaginary part of the specific surface impedance of KPKP (a-1), (a-2), and PKPK samples (b-1), (b-2) with the gradient thickness of nonwoven fabrics.

In addition, comparing the sound absorption coefficients of sample KPKP3 with sample KPKP5 inFigure 11(a), it clearly appears that the direction of the gradient structure also has the significant effect to sound absorption of multi-layer materials. The sound absorption coefficients of sample KPKP5 are higher than those of sample KPKP3 below 1468 Hz, while lower above 1468 Hz.Figure 11(b) also shows that compared to sample PKPK3, the max sound absorption coefficients of sample PKPK5 shift to the lower frequency. The repeated reflection of sound waves in the multi-layer materials between the two media interfaces with different acoustical impedance causes the improvement of sound absorption of the gradient structure materials [26,27].

Conclusion

In this paper, two-layer, three-layer, and four-layer materials composed of kapok fibers nonwoven fabrics reinforced by PE films were prepared as a system to further improve the sound absorption coefficients at the low frequency. The results showed that sound absorption at the low frequency can be further improved by the combination of nonwoven fabrics with kapok and PE film. In addition, the layering sequence, the PE film thickness, and the gradient thickness of kapok fiber nonwoven fabrics have the significant effects to the sound absorption of multi-layer materials.

The PE film position change has the significant effect to sound absorption of multi-layer materials. Nonwoven fabrics with kapok backed by PE film have the better sound absorption than nonwoven fabrics with kapok backed by rigid wall at the frequency range of 100–2500 Hz because of the elastic vibration of PE film. When kapok fiber nonwoven fabric is backed by the rigid wall, multi-layer materials reinforced by PE film have the better sound absorption at the low frequency. The increase of PE film thickness has contributed to improvement of sound absorption at the low frequency of multi-layer materials, but decline of that at the medium frequency for PK, PKP, PKPK, and KPKP materials. The gradient thickness structure and gradient change direction of kapok fiber nonwoven fabrics have the significant effects to sound absorption of multi-layer PE film-reinforced kapok fiber nonwoven fabrics materials. The study in this paper may be used to further research the sound-absorbing mechanism of multi-layer structure with porous materials.

Acknowledgements

Thanks for Caitang Zhang's warm-hearted help in sample testing.

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is supported by Scientific Research foundation of Wuhan Textile University (grant number 165015) and Technical innovation special project of Hubei province (2019AAA005).

ORCID iDs

Xueting Liuhttps://orcid.org/0000-0003-4708-470X

Xiaoning Tanghttps://orcid.org/0000-0002-8133-0882

References

1. Xiang HF, Wang D, Liu HC, et al. Investigation on sound absorption properties of kapok fibers.Chin J Polym Sci 2013; 31: 521–529.

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Abstract

Introduction

Experimental

Sample preparation

Measurement of material parameters

The normal incidence sound absorption coefficients

The specific surface impedance

Thickness

Results and discussion

Effects of the layering sequence on sound absorption at the low frequency

Effects of the thickness of PE film on sound absorption at the low frequency

Effects of the nonwoven fabrics gradient thickness on sound absorption properties at the low frequency

Conclusion

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iDs

References

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