Development of Composite Acoustic Panels of Waste Tyre Textile Fibres and Paper Sludge

Abstract

Since society is moving towards sustainable development, interest in secondary use of waste has recently become significant. This paper investigates a process to develop an acoustic material, using two types of waste. Composite acoustic panels were developed using waste tyre textile fibres (WTTF) and paper sludge (PS), and polyvinyl acetate (PVA) were used as a binder. Non-acoustic (bulk density, airflow resistivity) and acoustic (sound absorption coefficient, sound transmission loss) parameters were studied. Composite acoustic panels with different proportions of WTTF/PS/PVA (sixteen samples) were subjected to testing for the sound absorption coefficient according to ISO 10534-2 and sound transmission loss according to ASTM E2611. The density of all samples varied between 155.2 and 709.9 kg/m³, the thickness between 14.4 and 20.5 mm, and the airflow resistivity between 29.5 and 101.5 kPa∙s/m². The results reveal that the proportion of various waste materials in mixtures can improve the acoustic performance of panels. The combination that gives the highest α_avg. with a value of 0.50 was experimentally found to be 70% WTTF mixed with 15% PVA and 15% H₂O. The average sound absorption coefficient with a value of 0.46 was also found to be 25% WTTF mixed with 25% PS and 25% PVA and 25% H₂O. In sound transmission loss, the most effective was 50% PS and the 50% PVA composite, the TL_eq was 28.3 dB, while the composites together with 30% WTTF, 20% PS and 25% PVA, and 25% H₂O showed 18.9 dB loss. The results obtained using WTTF and/or PS wastes are attractive and show great and promising development potential.

1. Introduction

The rapid and extensive growth of populations in cities causes severe environmental problems [1,2]. Noise pollution is considered to be one of the main problems that has a negative impact on both human well-being and the environment itself. It is well known that long-term noise exposure causes premature deaths and contributes to new cases of heart diseases, as well as reduces healthy life years of the population [3,4]. More than 113 million inhabitants of Europe alone suffer from the long-term noise of L_DEN > 55 dB caused by traffic; 22 million people are exposed to railway noise; 4 million people to aircraft noise; and 1 million inhabitants exposed to industrial noise [5]. It was estimated that more than 1.6 million healthy life years are lost due to the extensive noise in living surroundings [6].

Sound-absorbing materials are often used to control environmental noise. Such materials are used in buildings, placed on walls or ceilings [7,8]; noise reduction in the environment is mainly based on noise barriers, which also have sound absorbing materials in their structure [9], as well as new generation quieter road surfaces [10]. However, these materials are often produced from non-renewable material sources and their production requires high amounts of energy, and excavation sites often damage the landscape and cause serious environmental concerns [11,12]. Commonly found materials in the construction field examples could be: mineral wools (rock and glass wools), polyurethane foams, sound absorbing panels, etc. [13,14]. Those materials often characterised high porosity, which is one of the main parameters that influence the effectiveness of sound absorption. According to any acoustic models and pore-elastic theory, higher open porosity values lead to a higher sound absorption coefficient, especially in higher frequencies. Porosity has a direct influence on the airflow resistivity that describes the sound absorption of the material [15,16].

Nowadays, the European Union and the rest of the world are moving towards sustainable and green economies development, and the use of recycled materials has become inseparable. The current EU target until 2030 is to reduce carbon emissions by 55%, which is also directly related to waste generation [17]. The use of recycled materials in the field of acoustics became significant in recent years, since scientists are looking for new, sustainable and green materials which will have sound absorption values similar to or higher than those of widely used materials produced from non-renewable sources. In general, the reuse of waste material shows its importance in the bioeconomy, the reduction of dependence on raw materials, and the reduction of greenhouse gas emissions [18]. Much attention is paid to the acoustic materials that are produced from recycled tyre rubber, plastic waste, various fabric waste, etc. [19,20,21,22,23,24,25]. The trend in various natural fibres in recent years became evident as well. Scientific studies also pay attention to natural materials such as hemp, jute, flax fibres, etc. Such materials show promising results for sound absorption applications [26,27,28].

The tyre recycling process is complicated. During the recycling process, three different parts should be separated: rubber, metals, and textile fibres [29]. Rubber and metals have their niche in reuse; however, waste tyre textile fibre (WTTF) has not yet been widely used. Recently, studies on the use of WTTF have been conducted in the construction field for its use in concrete reinforcement, as well as to improve hot asphalt mix and strengthen different soils [30,31,32,33]. However, there is a lack of research done in the field of acoustics, especially in applications for sound absorption. The authors Maderuelo-Sanz et al. [34] and Thai et al. [35] studied the possibility of using WTTF for sound absorption applications. Studies showed that it has potential, as its sound absorption coefficient (NRC value) was 0.56. Ružickij and Grubliauskas [36] made the first attempts to investigate the sound absorption coefficient of WTTF, and investigating the dependence of rubber remains on the sound absorption capability. They found out that an increase in rubber remains decreases sound absorption coefficient.

Paper sludge (PS) is formed during the paper production and recycling processes. PS is produced during the wastewater treatment process. Paper sludge waste is considered to be a big problem because often this waste ends up in the landfills or incineration plants. The use of paper sludge has been studied to reduce environmental pollution. The applications studied are mainly focused on building and insulating materials, and bioethanol and biogas production [37,38,39]. Applications for sound insulation and sound absorption have been studied in several studies. The sound absorption coefficient at 1000 Hz was found to vary between 0.40 and 0.70, which are promising results for sound absorption applications. Ref. [40] in their research studied different binders used for panel production. The maximum sound absorption coefficient was obtained at lower frequencies (500 Hz) and the result was 0.98 when the PS composite acoustic panels were backed with an air cavity.

Waste tyre textile fibre and paper sludge are fibrous and granular materials, respectively. For a panel to be formed, binders should be used. For binding WTTF fibres different binders are used, such as polyurethane, epoxy, phenol resins and polylactic acid. However, in the research it was found that the ratio of fibres and the binder is very important, as the optimum must be found to obtain the best sound absorption coefficient [41,42,43]. For the binding of PS granules, different natural materials can be used, such as clay, slake lime, cement, corn starch, etc. In both cases, the best ratio of material-to-binder must be found to obtain good porosity [44].

As mentioned above, there is little knowledge of WTTF and PS as acoustic sound absorbers. Furthermore, there is a lack of information on the use of both in composite materials, and thus in particular, together, this study is useful for future development. This study contributes to the reduction of WTTF and PS waste while taking into account the newly developing principles of bioeconomy and circular economy. With this study, we ultimately aim to reduce the amount of waste that is stored in special areas or incinerated.

The considered composite acoustic panels were made of waste tyre textile fibres and paper sludge with polyvinyl acetate binder. Sixteen different panels with different proportions of waste and binder are considered for sound absorption and insulation applications. The aim of this study is to propose a new sound absorbing panel, which is based on the recycled waste, which consists of waste tyre textile fibres and paper sludge.

The article is organised as follows: in Section 2 the materials and methods used in the study are presented, in Section 3 the main results are presented, and Section 4 presents conclusions and prospects.

2. Materials and Methods

In this section, methods of panel preparation based on WTTF, PS and PVA will be described, as well as non-acoustic (bulk density, airflow resistivity) and acoustic (sound absorption coefficient, sound transmission loss) parameters determination methods.

2.1. Method of Sample Preparation and Physical Properties Estimation

The waste tyre textile fibre and paper sludge were provided by the recycling companies, one that recycles tyres and the other that recycles and produces paper products (Figure 1). Both WTTF and PS were dried in advance in a drier. The drying process in both cases took 72 h at a temperature of 60 °C, to ensure that all moisture was eliminated. PS particles 2.5–5.0 mm in diameter (sorted using sieves) were used for panel production. The bulk density of the PS used was 355.3 ± 8.7 kg/m³. The bulk density of WTTF was 40.1 ± 2.3 kg/m³. According to Maderuelo-Sanz et al. [34], the diameter of the fibre ranges from 20–30 μm. It consists of 10% remains of rubber impurities.

To prepare the composite acoustic panels based on the incorporation of WTTF and PS, it was mixed with PVA and water binder. PVA is known for its good binding properties and being a non-toxic, biodegradable binder, which can be considered an eco-friendly option for the production of composite acoustic panels [45]. The role of water in the experimental part was to dilute PVA, since PVA has a high viscosity.

The dry part was placed into the mixing container and then the PVA binder solution was added. The mixing was carried out using a thin spatula and the mixing time was 3 min. After the mixing of the materials was complete, the mixture was placed in the 3D printed containers, which were made of polylactic acid (PLA) (Figure 2). The top and bottom of the container were perforated with square holes of 2 × 2 mm. The holes ensured an even drying process over the sample area.

When the containers were filled with the study material, they were pressed (pressure applied 490 Pa) in order to hold the correct shape. This container shape was used for the preparation of the composite acoustic panels, since its area is large enough to cut 3 samples for the measurements and to eliminate possible shrinkage of the samples.

The composite acoustic panels were dried for 7 days at room temperature. For the sound absorption coefficient measurement test, in total, 16 samples were prepared (Figure 3). The description of the samples is provided in

Table 1. Mix proportion of the materials in the samples (percentage by weight (wt%)).

Sample	WTTF, wt%	PS, wt%	PVA, wt%	H2O, wt%
S1	0	50	50	0
S2	0	60	40	0
S3	0	70	30	0
S4	0	80	20	0
S5	50	0	25	25
S6	60	0	20	20
S7	70	0	15	15
S8	80	0	10	10
S9	25	25	25	25
S10	20	30	25	25
S11	15	35	25	25
S12	10	40	25	25
S13	25	25	25	25
S14	30	20	25	25
S15	35	15	25	25
S16	40	10	25	25

.

Using a circular cutter, 3 samples were cut from the composite acoustic panels. The diameter of the samples for the airflow resistivity, sound absorption coefficient, and sound transmission loss measurements was 30 mm.

The bulk density of the samples was determined by weighting each cut sample and by measuring the thickness of the samples (diameter in all cases was 30 mm). Afterward, by performing simple mathematical calculations, the bulk density was determined.

2.2. Method of Determination of Airflow Resistivity

The method of determination of airflow resistivity is based on the ISO 9053-1 standard [46]. The direct measurement method is used in the study. The method is based on the measurement of the pressure difference before and after the sample. The measurement set consists of the tube, air compressor, and manometer (Figure 4). The inner diameter of the tube was 37 mm. The length is designed so that the air flow in the tube is close to the laminar flow (24.5 Re). The sample was placed in the holder and the holder with the sample was placed inside the tube on a support structure. The sample holder has more than 50% of the area of open cavities. The diameter of the cavities is greater than 3 mm. During measurement, the tube was sealed with grease to prevent air leakage.

A detailed description of the equipment used for sound absorption measurements is presented in

Table 2. The list of equipment used for airflow resistivity determination.

Name of the Device	Measurement Limits	Accuracy	Advanced Uncertainty
Dimanometer Retrotec DM32	−2488–+2488 Pa	1st class	±5.0 Pa
Air Velocity Metre testo 400	0.01–14.0 m/s		–
Digital Caliper	0–150 mm		±0.013 mm

.

The pressure difference is determined by the differential pressure manometer, which provides the pressure difference. Using an air flow metre, the velocity of the air flow is determined. According to Equation (1), the airflow resistance of the material is calculated [46]:

$$R_{air}=\frac{\Delta p}{q_v}$$

(1)

where

Δp—air pressure difference, Pa;

q_v—air flow that passes through the sample, m³/s.

Then, the specific airflow resistance R_S is calculated, which can be described as the product of the air resistance of the sample and the cross-sectional area of the sample (Equation (2)) [46]:

$$R_S=R·A$$

(2)

where

R—airflow resistance, Pa∙s/m³;

A—cross-sectional area of the sample, m².

In the last step, by Equation (3), the static airflow resistivity σ is calculated [46]:

$$\sigma =\frac{R_S}{d}$$

(3)

where

R_S—specific airflow resistance, Pa∙s/m;

d—thickness of the sample, m.

2.3. Method of Determination of Normal Incidence Sound Absorption Coefficient

The determination method of sound absorption coefficient (α) is based on the ISO 10534-2 standard [47]. The tube used for the measurements was 30 mm in diameter, and all samples were rigidly backed to eliminate transmission. The impedance tube principle scheme is presented in Figure 5. The distance between microphone No. 3 and the sample was x_3S = 60 mm, the high-frequency measurement microphones No. 2 and No. 3, x₂₃ = 20 mm, and the distance between the low-frequency measurement microphones was x₁₂ = 100 mm. The measurement range was 160 to 5000 Hz, and the results are presented in narrowband. The number of averages was set to 50.

The tube consists of two parts, one which has a sound source, together with microphones, and the other where the sample is placed. The part of the device where the sample is placed is screwed off, the sample is placed in the tube, the rigid backing is pushed to the sample and screwed on. Then, both parts of the tube are connected. Measurements are performed using software. In the first step, the background sound pressure level is measured; in the second step, sound absorption/sound transmission loss is measured.

A detailed description of the equipment used for sound absorption measurements is presented in

Table 3. The list of equipment used for sound absorption coefficient determination.

Name of the Device	Measurement Limits	Accuracy	Advanced Uncertainty
Sound Pressure Level Metre Sinus Apollo Light	Dynamic range 110 dB	1st class	–
Microphone No. 1 Microtech Gefel M370	Dynamic range 28 dBA–132 dBA		from ±0.12 dB to ±0.27 dB
Microphone No. 2 Microtech Gefel M370
Microphone No. 3 Microtech Gefel M370
Sound Level Calibrator Larson Davis CAL200	At 1000 Hz, 94 dB and 114 dB		±0.13 dB ±0.02 Hz
Digital Caliper	0–150 mm		±0.013 mm

.

The transfer function method was used for the determination of the sound absorption coefficient described in the ISO 10534-2 standard [47]. The technique is based on the transfer function between two microphones: x₁₂—low frequency range microphone set and x₂₃—high frequency range microphone set.

The transfer function H₁₂ and H₂₃ is obtained as the pressure ratio between the pressures registered by microphones No. 1 and No. 2, and microphones No. 2 and No. 3 at any frequencies. Calculation presented in Equation (4) [47]:

$${\mathrm{H}}_{12}=\frac{{\mathrm{p}}_{2\left(\mathrm{f}\right)}}{{\mathrm{p}}_1\left(\mathrm{f}\right)},{ \mathrm{H}}_{23}=\frac{{\mathrm{p}}_{3\left(\mathrm{f}\right)}}{{\mathrm{p}}_2\left(\mathrm{f}\right)}$$

(4)

$${\mathrm{H}}_{\mathrm{I} \left(160-1000 \mathrm{Hz}\right)}=\frac{{\mathrm{p}}_{2\mathrm{I}}}{{\mathrm{p}}_{1\mathrm{I}}}={\mathrm{e}}^{-{\mathrm{jk}}_0\left({\mathrm{x}}_{12}+{\mathrm{x}}_{23}\right)},{ \mathrm{H}}_{\mathrm{I} \left(1000-5000 \mathrm{Hz}\right)}=\frac{{\mathrm{p}}_{3\mathrm{I}}}{{\mathrm{p}}_{2\mathrm{I}}}={\mathrm{e}}^{-{\mathrm{jk}}_0\left({\mathrm{x}}_{23}\right)}$$

(5)

$${\mathrm{H}}_{\mathrm{R} \left(160-1000 \mathrm{Hz}\right)}=\frac{{\mathrm{p}}_{2\mathrm{R}}}{{\mathrm{p}}_{1\mathrm{R}}}={\mathrm{e}}^{-{\mathrm{jk}}_0\left({\mathrm{x}}_{12}+{\mathrm{x}}_{23}\right)},{ \mathrm{H}}_{\mathrm{R} \left(1000-5000 \mathrm{Hz}\right)}=\frac{{\mathrm{p}}_{3\mathrm{R}}}{{\mathrm{p}}_{2\mathrm{R}}}={\mathrm{e}}^{-{\mathrm{jk}}_0\left({\mathrm{x}}_{23}\right)}$$

(6)

After determination of transfer functions, the sound reflection coefficient can be calculated from the Equations (4)–(6) [47]:

$${\mathrm{R}}_{\left(160-1000 \mathrm{Hz}\right)}=\frac{{\mathrm{H}}_{12}-{\mathrm{H}}_{\mathrm{I} \left(160-1000 \mathrm{Hz}\right)}}{{\mathrm{H}}_{\mathrm{R} \left(160-1000 \mathrm{Hz}\right)}-{\mathrm{H}}_{12}}{\mathrm{e}}^{2{\mathrm{jk}}_0\left({\mathrm{X}}_{12}+{\mathrm{X}}_{23}+{\mathrm{X}}_{3\mathrm{s}}\right)},$$

$${\mathrm{R}}_{\left(1000-5000 \mathrm{Hz}\right)}=\frac{{\mathrm{H}}_{23}-{\mathrm{H}}_{\mathrm{I} \left(1-5 \mathrm{kHz}\right)}}{{\mathrm{H}}_{\mathrm{R} \left(1-5 \mathrm{kHz}\right)}-{\mathrm{H}}_{13}}{\mathrm{e}}^{2{\mathrm{jk}}_0\left({\mathrm{X}}_{23}+{\mathrm{X}}_{3\mathrm{s}}\right)}$$

(7)

where

p₁, p₂, p₃—pressure recorded by the microphones, Pa;

H_I—incident wave transfer function;

H_R—reflected wave transfer function;

x₁₂—distance between microphone No. 1 and No. 2, mm;

x₂₃—distance between microphone No. 2 and No. 3, mm;

x_3S—distance between microphone No. 3 and the sample, mm.

R—sound reflection coefficient;

j—complex number;

k₀—wave number;

After the sound reflection coefficient is determined, the sound absorption coefficient is calculated according to Equation (8) [47]:

$$\mathsf{\alpha }=1-{\left|\mathrm{R}\right|}^2$$

(8)

where

R—sound reflection coefficient.

The sound absorption coefficient ranges from 0 to 1, and is dimensionless.

To briefly summarise the results, an average sound absorption coefficient (α_avg.) was calculated according to the Equation (9):

$${\mathsf{\alpha }}_{\mathrm{avg}.}=\frac{\sum {\mathsf{\alpha }}_{\mathrm{n}}}{\mathrm{n}}$$

(9)

where

α_avg.—average sound absorption coefficient;

α_n—sound absorption coefficient in the 1/3 octave frequency band;

n—total number of 1/3 octave frequency bands (n = 16).

2.4. Method of Determination of Sound Transmission Loss

The determination method of sound transmission loss (TL) is based on the ASTM E2611 standard [48]. The tube used for the measurements was 30 mm in diameter; the anechoic termination was used at the end of the tube. The transmission loss had 1 load and 4 microphones combination. The tube is presented in Figure 6.

The distances between the microphones and the sample for low frequency (160–1000 Hz) determination were: x₁₂ = 120 mm, x_2S = 60 mm, x_3S = 40 mm, x₃₄ = 120 mm, L_SH = 100 mm. The distances for high frequency (1000–5000 Hz) sound transmission loss determination were: x₁₂ = 20 mm, x_2S = 60 mm, x_3S = 40 mm, x₃₄ = 20 mm, L_SH = 100 mm. The number of averages was set to 50.

A detailed description of the equipment used for sound absorption measurements is presented in

Table 4. The list of equipment used for sound transmission loss determination.

Name of the Device	Measurement Limits	Accuracy	Advanced Uncertainty
Sound Pressure Level Metre Sinus Apollo Light	Dynamic range 110 dB	1st class	–
Microphone No. 1 Microtech Gefel M370	Dynamic range 28 dBA–132 dBA		from ±0.12 dB to ±0.27 dB
Microphone No. 2 Microtech Gefel M370
Microphone No. 3 Microtech Gefel M370
Microphone No. 4 Microtech Gefel M370			from ±0.13 dB to ±0.28 dB
Sound Level Calibrator Larson Davis CAL200	At 1000 Hz, 94 dB and 114 dB		±0.13 dB ±0.02 Hz
Digital Caliper	0–150 mm		±0.013 mm

.

The transfer matrix can be expressed as [48]:

$$\mathrm{T}=\left[\begin{array}{cc}{\mathrm{T}}_{11}& {\mathrm{T}}_{12}\\ {\mathrm{T}}_{21}& {\mathrm{T}}_{22}\end{array}\right]$$

(10)

The transmission coefficient for anechoically-backed samples can be determined according to Equation (11) [48]:

$$\mathsf{\tau }=\frac{2{\mathrm{e}}^{\mathrm{jkd}}}{{\mathrm{T}}_{11}+\left({\mathrm{T}}_{12}+\mathsf{\rho }\mathrm{c}\right)+{\mathsf{\rho }\mathrm{cT}}_{21}+{\mathrm{T}}_{22}}$$

(11)

where

τ—sound transmission coefficient;

k—wave number in the air, m⁻¹;

d—thickness of the sample, m;

ρ—density of air, kg/m³;

c—speed of sound in air, m/s.

After all, the normal incidence sound transmission loss is expressed as [48]:

$${\mathrm{TL}}_{\mathrm{n}}=20{\log }_{10}\left|\frac{1}{\mathsf{\tau }}\right|$$

(12)

where

TL_n—sound transmission loss in the octave band n, dB;

τ—sound transmission coefficient.

To briefly summarise the results, equivalent sound transmission loss was calculated according to the Equation (13):

$${\mathrm{TL}}_{\mathrm{eq}}=10·\log \left({\displaystyle \sum }_{\mathrm{i}=1}^{\mathrm{n}}{10}^{\frac{{\mathrm{L}}_{\mathrm{i}}}{10}}\right)$$

(13)

where

TL_eq—total equivalent sound pressure level, dB;

L_i—sound pressure level in the 1/3 octave frequency band, dB.

3. Results and Discussion

In this section, the variation of two different aspects will be discussed, namely, non-acoustic (bulk density, airflow resistivity) and acoustic (sound absorption coefficient, sound transmission loss) parameters.

3.1. Characterisation of Non-Acoustic Parameters of the Samples

The non-acoustic properties of all composites are listed in

Table 5. Non-acoustic parameters of the measured samples.

Sample	Thickness, mm	Density, kg/m3	Airflow Resistivity, kPa∙s/m2
S1	18.1 ± 0.7	709.9 ± 29.8	101.5 ± 18.7
S2	14.4 ± 0.3	673.0 ± 14.7	82.3 ± 4.1
S3	18.2 ± 0.9	626.0 ± 24.8	37.3 ± 6.8
S4	20.5 ± 1.1	471.2 ± 22.5	13.6 ± 4.3
S5	16.5 ± 0.4	199.7 ± 8.6	50.7 ± 1.1
S6	19.4 ± 0.5	180.2 ± 9.1	72.4 ± 20.6
S7	19.7 ± 0.4	163.2 ± 8.5	83.9 ± 3.6
S8	19.0 ± 0.8	155.2 ± 3.4	144.3 ± 3.4
S9	17.1 ± 0.3	321.8 ± 16.8	59.4 ± 12.7
S10	18.1 ± 0.5	338.6 ± 10.1	63.0 ± 8.5
S11	18.3 ± 0.5	359.5 ± 18.5	56.4 ± 3.9
S12	16.7 ± 0.3	389.2 ± 12.1	29.5 ± 4.9
S13	18.1 ± 0.6	315.6 ± 15.5	76.9 ± 2.5
S14	16.2 ± 0.5	290.7 ± 17.4	98.2 ± 2.5
S15	18.0 ± 0.1	260.4 ± 13.4	76.2 ± 3.8
S16	18.4 ± 0.4	224.0 ± 6.8	41.1 ± 3.9

. Depending on the type of sample, it can be seen that the airflow resistivity has different dependences on the density. It is known that the acoustic parameters are influenced by the non-acoustic parameters.

In samples S1–S4 where 50–80 wt% PS was used and 20–50 wt% PVA was used, increasing density influences the increase in airflow resistivity values. The bulk density of these samples varies from 471 kg/m³ to 709 kg/m³, airflow resistivity values vary from 13.6 kPa∙s/m² to 101.5 kPa∙s/m². This phenomenon can be justified as the density of samples increases, and the air entering the sample more difficult, thus increasing the airflow resistivity.

In the set of samples S5–S8 (where 50–80 wt% WTTF, 10–25 wt% PVA, and 10–25 wt% H₂O were used), airflow resistivity was inversely proportional to density. This may be connected to the amount of WTTF fibre in the samples increases and the amount of binder decreases, as this affects the amount of voids formed and their size in the composite acoustic panels. This phenomenon can lead to an increase in airflow resistivity [49]. The airflow resistivity of these samples varies from 50.7 kPa∙s/m² to 144.3 kPa∙s/m².

The bulk density of the samples S9–S16 (10–40 wt% WTTF, 10–40 wt% PS, 25 wt% PVA and H₂O) varies from 224 kg/m³ to 389 kg/m³ and the airflow resistivity values range from 29.5 kPa∙s/m² to 98.2 kPa∙s/m². In samples S9–S12 (when PS was the dominant component), airflow resistivity stayed within the error margin and then decreased. The increase in the content of PS grains in the composites affects the airflow resistivity. When the PS content is relatively high (35–40 wt%), the airflow resistivity decreases due to a much larger particle size of PS compared to the WTTF fibres. Increasing the number of PS particles reduces the number of voids in the matrix and contributes to the increase in the density of the composite. In samples S13–S16 (when WTTF was the dominant component), airflow resistivity increased, compared to samples S9–S12. The airflow resistivity values of the samples S13–S16 varied from 41.1 kPa∙s/m² to 98.2 kPa∙s/m².

Although increasing density should increase airflow resistivity, taking into account that composite acoustic panels are produced from two different materials, the obtained porosity of the samples may influence airflow resistivity, and the phenomenon may not occur as it should.

The uncertainties obtained in these samples can be explained as an unequal distribution of raw materials in the sample, although efforts were made to do so. In some cases, some samples had a higher percentage of PS and in some cases lower, which led to the fact of high uncertainties.

3.2. Characterisation of Acoustic Parameters of the Samples

The acoustic properties of materials depend on many factors, including the proportion of the mixture proportion, technological process, type and structure of raw materials, compatibility, cohesion, etc. The acoustic performance of composite materials containing porous particles is also related to porosity, which plays a very important role in the propagation of sound in the material.

In Figure 7 narrow-band sound absorption results are presented from samples S1–S4 and S5–S8. Samples S1–S4 are based only on the PS and the binder. As the percentage of PS increased in the sample, the sound absorption coefficient increased throughout the spectrum. According to the Figure 7a, the lowest sound absorption in the whole spectrum is visible when the density was highest (709.9 kg/m³) and, respectively, the airflow resistivity was also highest (101.5 kPa∙s/m²). This could be justified because the sample was too rigid for a sound wave to enter the material, as those samples compared to others had a higher percentage of the binder, which supposedly could also clog the pores [50]. In samples S2–S4, the sound absorption coefficient increased in the high frequency range. At approximately 1600 Hz frequency samples with a higher percentage of PS surpassed sample S1 and peaked at 2300 Hz. Specifically, as the percentage of the PS in the sample increased, the density decreased (673.0 → 626.0 → 471.2 kg/m³) and airflow resistivity decreased (82.3 → 37.3 → 13.6 kPa∙s/m²), respectively. The decrease in airflow resistivity shows that the material is more porous and air could pass through the material.

Single-panel sound absorption tends to increase with frequency until it reaches the first peak value. The sound absorption values in this frequency region are defined mainly by the thickness and density of the sample. Furthermore, knowing that low-frequency sounds are more difficult to absorb, it can be seen that PS samples are not very effective for sound absorption at low frequencies. Considering the results presented, it can be seen that the samples are more effective in the range between 1250 and 2500 Hz. Although it is known that, as the density increases sound absorption in the lower frequencies should also increase, that phenomenon is found within a margin of error.

In Figure 7b the results of the samples S5–S8, which are based on WTTF and the binder, are presented. According to the results, a similar trend is observed. As the percentage of WTTF in the sample increased, sound absorption shifts to the lower frequencies. As previously analysed, the sample, which contains the lowest percentage of WTTF, shows the lowest results in the whole spectrum. It is important to note that as the density decreases, the sound absorption increases at lower frequencies. This can be justified by the increase in airflow resistivity (50.7 → 72.4 → 83.9 → 144.3 kPa∙s/m²) with decreasing density (199.7 → 180.2 → 163.2 → 155.2 kg/m³). This phenomenon can be explained by an increase in fibre content in the sample, a reduction in pore size, and the consequence of this is an increase in airflow resistivity [49]. The peak absorption of the sample was in the range of 2500 and 3000 Hz. The peak in such high frequencies can be explained as an insufficient thickness since it was approximately 20 mm. To obtain a peak at lower frequencies, the thickness of the sample should be at least ¼ wavelength of the desired frequency [51].

For example, when both composites are compared, it can be clearly seen that WTTF composites have greater potential for use for sound absorption in lower frequencies. Samples S1–S4 at 500 Hz (from the acoustic point of view of the room, those frequencies are among the most important) showed results between 0.09 and 0.14, while samples S5–S8 showed 0.14 to 0.20. When the mid-frequency range, 1000 Hz frequency, is compared, the difference becomes significant. PS composite sample S4 shows the highest result of 0.28, whereas WTTF composite sample S7 shows 0.53. Both samples obtained their peak sound absorption at a similar frequency range due to the similar thickness of the samples.

In Figure 8 narrowband sound absorption results of samples S9–S12 and S13–S16 are presented. In the first part of the study, separate compounds were tested. In the second part, both materials were mixed in different proportions and tested. In Figure 8a, the results are presented, where PS was the dominant material and WTTF was additional. Based on the results, it can be clearly seen that samples S9–S11 show very similar results, as well as peaking in the same frequency range. Based on the airflow resistivity results obtained, it can be seen that the variation was very close (varying between 56.4 and 63.0 kPa∙s/m²). It could be meant that there is no big difference in whether in the sample 25, 30, or 35% of PS. However, S10 had a higher airflow resistivity result and its maximum sound absorption was slightly higher than the others. The results of sample S12 were the lowest, when the percentage of PS was the highest. Its’ peak sound absorption was at approximately 3200 Hz, while samples S9–S11 peak in the range of 2500–2700 Hz.

The airflow resistivity of the sample S12 was also the lowest 29.5 kPa∙s/m², although the density was the highest. The reason for such low results may be the fact that the use of a high percentage of PS created large pores that air could pass through; however, to prove this, an additional test should be performed on SEM.

In Figure 8b sound absorption results are presented, when samples were prepared with WTTF being the dominant material in the composite. According to the results presented, sample S13 showed the highest results in the low-frequency range (160–500 Hz) and mid-frequency range (501–2000 Hz). The S14–S16 sound absorption coefficient at low and mid-frequencies was similar with much exception. However, in the high-frequency range samples started to differ. The sound absorption peak obtained by the samples was also different: the peak of S13 was at 2300 Hz, S14—2600 Hz, S15—2600 Hz, and S16—2800 Hz. Although a trend in the samples was visible (increasing percentage of WTTF in the sample decreased density; airflow resistivity increased and afterwards decreased), but those trends are not visible in the measurement results. An increase in airflow resistivity explains the fact that the porosity of the sample decreases; however, it is not reflected in the measurement results.

When comparing samples that were prepared using different dominant materials in the composite, it can be seen that when WTTF was the dominant material, the results were slightly higher compared to the composites when PS was dominant. Again, comparing low frequencies at 500 Hz, it can be seen that the results vary from 0.10 to 0.14 in samples S9–S12, while in samples S13–S16 the results varied from 0.15 to 0.16. According to that, we can see that the results do not differ much. For example, comparing the results as in the previous figure at 1000 Hz, the sound absorption coefficient of S9–S12 varied between 0.18 and 0.34, while the results of the samples S13–S16 varied between 0.36 and 0.48. Here, the results are more different, meaning that WTTF being dominant has more advantages. The samples prepared by both methods had peak absorption in the same frequency range and their results were between 0.95 and 0.99.

To briefly summarise the results, an average sound absorption coefficient of all samples was calculated and is presented in

Table 6. Average sound absorption coefficient results of samples S1–S16.

Sample	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	S13	S14	S15	S16
αavg.	0.27	0.30	0.34	0.38	0.44	0.48	0.50	0.49	0.44	0.43	0.42	0.35	0.46	0.44	0.45	0.45

.

According to the table presented, comparing the materials used separately (PS with the binder and WTTF with the binder alone, without mixing together), an average sound absorption coefficient was highest in sample S7 with a result of 0.50. That can be seen as well in Figure 7b that the sample S7 had higher sound absorption results at low- and midrange frequencies. When comparing composite samples with different proportions of mix, we can see that α_avg. do not differ drastically; however, when WTTF was the dominant component, the absorption coefficient was higher. The highest α_avg. was obtained by the sample S13—0.46. Taking into account that when WTTF used with the binder only showed higher results, the use of both materials can be used to solve the waste problem; however, samples S5–S8 were more effective at the low- and mid-range frequencies.

In the second part of this study, the sound transmission loss (TL) of these samples was measured. In the Figure 9 the results of sound transmission loss of the samples S1–S4 and S5–S9 are presented.

As it was expected in Figure 9a, sound transmission loss increased with increasing density. That is justified by the fact that such panels are mainly dependable on the density (overall mass) of the material; however, there is also the influence of the airflow resistivity. As the airflow resistivity of the composite material increases, the sound transmission through the sample decreases, meaning that we have high sound insulation.

The highest result of TL was obtained by the sample S1, which had the highest density (709.9 kg/m³). The TL values in the low frequencies (500 Hz) reached 12.0 dB, while the peak TL was at 5000 Hz with a result of 23.8 dB. For example, compared to sample S4, which showed the lowest results, the difference in low frequencies was 5 times; however, the peak TL of this sample was at approx. 3200 Hz with the results of 6.1 dB.

In Figure 9b, we see that density does not influence the results, while the airflow resistivity shows the main trends of the TL change. We can see that S8 shows the highest TL results, which can be justified that it has the highest airflow resistivity, presuming that sound cannot pass through the material due to low porosity. This happened because for these samples preparation was used with 80% WTTF and only 20% of the binder. As the fibre content in the sample increased, the porosity was reduced due to the small size of the fibre. However, we can see that S6 surpassed S7 and, though it should not happen, this may be explained by the high uncertainty of the airflow resistivity results, meaning that such a sample may have higher airflow resistivity compared to S7.

For example, by comparing the two composite materials, we can see that S1–S4 are more effective in the transmission of sound through the material. The highest S8 highest result was 10.6 dB, while the TL of the sample S1 was 23.8 dB, more than twice the difference. Furthermore, at low frequencies, at 500 Hz, the TL of the sample S1 was 12.0 dB; however, the highest results showed that the TL of the sample S8 was only 6.3 dB.

In Figure 10 the results of sound transmission loss of the samples S9–S12 and S13–S16 are presented. As with previously overviewed results, it can be seen that those samples are mainly influenced by the airflow resistivity, not the density. According to the results presented in the Figure 10a, sample S11 shows the highest results of sound transmission loss throughout the spectrum; however, its airflow resistivity is not the highest (56.4 kPa∙s/m², the highest airflow resistivity result was of sample S10—63.0 kPa∙s/m²). The airflow resistivity of sample S10 has quite a large uncertainty, which may be the reason why we cannot see its result to be dominant. The lowest results were obtained by sample S12, which accordingly had the lowest airflow resistivity, yet had the highest density. Its maximum was 6.5 dB at 5000 Hz. However, when analysing the results, it can be highlighted that, though S9 and S11 had similar airflow resistivities, 59.4 and 56.4 kPa∙s/m², respectively, there may be an influence of the density on the TL results.

Lastly, in Figure 10b, the results of the samples S13–S16 are presented. Here, it can definitely be seen that airflow resistivity mainly influences TL results. The airflow resistivity of sample S14 was 98.2 kPa∙s/m², compared to, for instance, S13, which was 76.9 kPa∙s/m² or S15—76.2 kPa∙s/m². The highest TL results were observed at 5000 Hz—9.7 dB.

When comparing the two prepared samples in different ways, it can be seen that the samples S13–S16 that had the dominant material WTTF had higher results at high frequencies. S11 TL results maximum was 8.6 dB, while S14 showed 9.7 dB. In the low frequencies—500 Hz, S11 showed 5.0 dB TL, while S14 showed 5.6 dB, although the results do not differ much. If we compare, for example, density, we can see that samples S13–S16 are more effective in TL with lower density than samples S9–S12. As previously discussed, this phenomenon may be influenced by the fact that the amount of fibres increases in the sample, decreases the porosity of the sample, and increases sound transmission loss. As mentioned, an interesting phenomenon can be observed that in all measurements there is no visible resonance and coincidence frequencies of structures that caused a drop in TL.

To summarise the results, the equivalent sound transmission loss of all samples was calculated and is presented in

Table 7. Equivalent sound transmission loss results of samples S1–S16.

Sample	S1	S2	S3	S4	S5	S6	S7	S8	S9	S10	S11	S12	S13	S14	S15	S16
TLeq	28.3	20.5	17.7	15.9	17.3	18.9	18.5	19.5	17.4	17.8	18.2	16.5	18.5	18.9	18.0	17.0

.

According to the table presented, comparing the single materials used with the binders, the equivalent sound transmission loss was the highest of sample S1, with the result of 28.3 dB. For example, when comparing the TL_eq of samples S5–S8, the values were very similar. Additionally, comparing them with PS composites, it can be seen that the results are in some cases higher than those of samples S1–S4. When comparing mixed composite samples, it can be seen that the results between S9–S12 and S13–S16 are also similar, but when WTTF was the dominant material, the results were higher.

Summarising all the results, we can see that the WTTF and PS composite acoustic panels α_avg. varied between 0.27 and 0.50. The authors [52] performed sound absorption measurements in different granular media, and their findings show that the mean values of sound absorption of granular materials ranged from 0.31 to 0.33 (at thickness of 20 mm). Such results indicate that the addition of WTTF to the composite increases the sound absorption performance of the granular PS.

The WTTF composites (as only material) showed the highest α_avg. results, with values of 0.35 to 0.50. When comparing these results with other researchers [19], at double thickness they obtained results greater than 0.90 at 1000 Hz, while in our case it was 0.52 (thickness 19.7 ± 0.4 mm). It is well-known that the sound absorption peak tends to move to lower frequency range when increasing thickness. It is likely that our studied composites would show similar peaks at the same frequency if thicker samples were measured. The gained peak sound absorption in this study was 0.99 at the 2616 Hz frequency.

The WTTF and PS composites show promising results in TL. Transmission loss values varied between 15.9 and 28.3 dB. Compared with similar study results of waste tyre recycled rubber material, the transmission loss value was around 30 dB [53]. One of the main parameters that influence sound transmission loss is density and thickness of the material. Knowing that the composite target density of the study mentioned was 1000 kg/m³ (in this study case, it ranged from 163.2 ± 8.5 to 709.9 ± 29.8), it can be assumed that some of the proposed composite acoustic panels could be good for use for sound insulation purposes.

4. Conclusions and Prospects

The use of waste tyre textile fibres and paper sludge is an interesting way to extend the life of these wastes in environmental, materials, and civil engineering applications, providing panels with good acoustic parameters. In this first attempt of non-acoustic (bulk density, airflow resistivity) and acoustic (sound absorption coefficient, sound transmission loss) characterisation, the composite acoustic panels of waste tyre textile fibres and paper sludge have shown potentially good properties. The sixteen different composite panels were produced and tested. Different mixing ratios were used for the panel production. The results show that we were able to produce a sturdy acoustic panel made of recycled WTTF and PS raw material. The results obtained establish that the amount of waste and binders present in the composite material is the important factor in explaining the variation in the acoustic properties of the composite panels.

The α_avg. of the samples S1–S4 (where 50–80 wt% PS and 20–50 wt% PVA were used) varied between 0.27 and 0.38; with a greater amount of presence of PS in the composite, the sound absorption increased. Samples S5–S8 (where 50–80 wt% WTTF, 10–25 wt% PVA and 10–25 wt% H₂O were used) showed α_avg. between 0.44 and 0.50. When the fibre content in the composite increased, the sound absorption at low- and mid-frequencies increased. The composite mix with the dominant PS material (S9–S12, where 10–25 wt% WTTF, 25–40 wt% PS, 25 wt% PVA and 25 wt% H₂O was used), showed that an average sound absorption coefficient varied between 0.35 and 0.44, while when WTTF was the dominant component (S13–S16, where 25–40 wt% WTTF, 10–25 wt% PS, 25 wt% PVA and 25 wt% H₂O was used), the coefficient varied between 0.44 and 0.46. The higher amount of more porous WTTF raw materials in the composite decreased the density and improved the sound absorption capacity of the panel.

The equivalent sound transmission loss results showed that the highest loss is obtained in the dense sample S1 (50 wt% PS, 50 wt% PVA) with a result of 28.3 dB. As the density decreased, sound transmission loss decreased as well. Such a phenomenon for single-panel structures can be explained by the mass law. In the samples S9–S12 and S13–S16, sound transmission loss was similar, as the values ranged from 16.5 to 18.9. Since WTTF is a fibrous material and PS is granular, their combination cannot improve sound transmission loss due to increased porosity, which leads to the passage of sound through the material. It was noticed that as the airflow resistivity increased in the sample, sound transmission loss would increase as well.

The results obtained in the experimental study of the samples show only macro-parameters of the materials (sound absorption, transmission loss, airflow resistivity and density). This is one of the main limitations of the study. In continuation of this study, micro parameters (which do influence macro parameters) need (pore size, tortuosity, porosity) to be determined. Furthermore, non-acoustic mechanical parameters such as compressive and bending strength were not considered in this study.

Despite the limitations, the possible applications of composite acoustic panels can be seen in sound-absorbing structures for reverberation time reduction or as a substitute to mineral wools in wall insulation in the future.

Future work will focus on life cycle assessment (LCA), finding the impact on the environment of such composite acoustic panels, and conducting cost-benefit analysis to find whether it is feasible to produce composite acoustic panels of WTTF and PS. Furthermore, the mentioned limitations will be taken into account in future studies to gain substantial knowledge of the composite materials studied (micro and mechanical parameters).