Research of Chosen Acoustics Descriptors of Developed Materials from Old Automobile Recycled Materials †

Legislative regulations and standards have been approved for noise control, aimed at controlling noise minimization. This problem is also under the public interest, because noise is increasing in many counties. EU directive 70/157/eec determines and controls limits of environmental noise and is aimed at creating less noisy and more pleasant outdoor and indoor environments for European residents within “sustainable development in Europe”. This study focused on the utilization of new, so-called acoustic more convenient materials, based on and produced from old materials from automobiles, e.g. foam, textile, rubber, and tires. The chosen acoustic parameters—sound absorption coefficient and sound transmission loss—of these materials were tested, and the acoustic properties of materials were subsequently improved compared to tested values and potential applications for them were found.


Introduction
The negative effects of environmental noise are various and can be caused by many different sources of noise.According to EU directive 70/157/eec [1] and the World Health Organization (WHO) [2], there are approximately 80 million people in the European Union exposed to high noise levels.It is unacceptable in the long term.There are 170 million people living in so-called "gray areas" that are characterized by the permanent production of annoying noise [3][4][5].
The significant source of noise is transport (road, rail and air) while the dominant source is road traffic [6,7].The problem of road traffic noise is often tackled through the construction of noise barriers [7][8][9] from various absorbing and reflective materials.There are several research teams dealing with the research of acoustic materials [9][10][11].
This paper presents the results of scientific and research activities focusing on the development and study of acoustically suitable materials based on recycled components from vehicles after the expiry of their life cycle [12][13][14].The results obtained by measuring selected acoustic parameters (the sound absorption coefficient (α) and the reduction index R) are presented [15,16].

Measurement of Selected Acoustic Parameters of the Acoustic Materials under Development
The following acoustic parameters were chosen from several possible acoustic descriptors: • the reduction index R; and • the sound absorption coefficient (α).
The sound absorption coefficient (α) and the reduction index R can be measured by two methods: • the standing wave ratio method; and • the transfer function method.
The transfer function measurement method was utilized.This method can be used for measuring the sound absorption coefficient, the reflection factor, the normal impedance and the normal admittance [17][18][19][20].
Figure 1 shows an impedance tube based on the transfer function method.

Measurement of Selected Acoustic Parameters of the Acoustic Materials under Development
The following acoustic parameters were chosen from several possible acoustic descriptors:  the reduction index R; and  the sound absorption coefficient (α).
The sound absorption coefficient (α) and the reduction index R can be measured by two methods:


the standing wave ratio method; and  the transfer function method.
The transfer function measurement method was utilized.This method can be used for measuring the sound absorption coefficient, the reflection factor, the normal impedance and the normal admittance [17][18][19][20].
Figure 1 shows an impedance tube based on the transfer function method.The proposed measuring methodology includes the utilization of a Kundt tube with two positions of the positioning microphones and the system of numerical frequency analysis for the determination of normal incidence sound absorption coefficient of sound absorbers.It can be applied for the determination of the acoustic surface impedance or acoustic surface admittance for the sound absorbing materials.The impedance ratios of sound absorbing materials are proportional to their physical properties, such as airflow resistance, elasticity, porosity and density.
In compliance with STN EN ISO 10534-1 standard, the proposed test method utilizes an impedance tube with a sound source connected to one end and a test specimen mounted in the tube on the other end.This test method is an alternative measurement method, which is, in general, much faster than the method included in STN EN ISO 10534-1 [21][22][23][24].
The measurement of the absolute and transmission characteristics of the developed acoustic materials are used by researchers for the acoustic design of devices such as automobiles, household appliances, etc.

Measurement of Chosen Acoustic Parameters of Developed Acoustically Suitable Materials
The proposed measurement methodology was verified by experimental tests with newly developed materials (Ekomolitan and recycled rubber).The experimental tests included the measurements of acoustic parameters, namely the sound absorption coefficient (α) and the reduction index R for the tested materials.
The experimental research tests were designed and verified at the Department of Process and Environmental Engineering, Faculty of Mechanical Engineering, Technical University of Kosice, Kosice, Slovakia.

The Hardware and Software Equipment Used
The architecture of hardware equipment used for measuring the sound absorption coefficient (α) (for the frequency bands 100-800 Hz and 400-2500 Hz) is shown in Figure 2. The proposed measuring methodology includes the utilization of a Kundt tube with two positions of the positioning microphones and the system of numerical frequency analysis for the determination of normal incidence sound absorption coefficient of sound absorbers.It can be applied for the determination of the acoustic surface impedance or acoustic surface admittance for the sound absorbing materials.The impedance ratios of sound absorbing materials are proportional to their physical properties, such as airflow resistance, elasticity, porosity and density.
In compliance with STN EN ISO 10534-1 standard, the proposed test method utilizes an impedance tube with a sound source connected to one end and a test specimen mounted in the tube on the other end.This test method is an alternative measurement method, which is, in general, much faster than the method included in STN EN ISO 10534-1 [21][22][23][24].
The measurement of the absolute and transmission characteristics of the developed acoustic materials are used by researchers for the acoustic design of devices such as automobiles, household appliances, etc.

Measurement of Chosen Acoustic Parameters of Developed Acoustically Suitable Materials
The proposed measurement methodology was verified by experimental tests with newly developed materials (Ekomolitan and recycled rubber).The experimental tests included the measurements of acoustic parameters, namely the sound absorption coefficient (α) and the reduction index R for the tested materials.
The experimental research tests were designed and verified at the Department of Process and Environmental Engineering, Faculty of Mechanical Engineering, Technical University of Kosice, Kosice, Slovakia.

The Hardware and Software Equipment Used
The architecture of hardware equipment used for measuring the sound absorption coefficient (α) (for the frequency bands 100-800 Hz and 400-2500 Hz) is shown in Figure 2.
The measured system includes the tube with the inner diameter of 60 mm (marked as SW060-L) and the holder of the tested sample with the inner diameter of 60 mm (marked as SW060-S).
Recycling 2017, 7, x 3 of 12 The measured system includes the tube with the inner diameter of 60 mm (marked as SW060-L) and the holder of the tested sample with the inner diameter of 60 mm (marked as SW060-S).The system for measuring of the reduction index R (for the frequency bands 100-800 Hz and 400-2500 Hz) is shown in Figure 3. Similarly, the measured system consists of the tube with the inner diameter of 60 mm (marked as SW060-L) and of the tube extension piece with the inner diameter of 60 mm (marked as SW060-E).

The Experimental Materials Used in Testing
For experimental measurements, materials from selected components of vehicles after the expiry of their life cycle were used.Materials tested were recycled by operations such as crushing, cutting, grinding, heating, stamping, etc.
Specifically, the following materials were used.The system for measuring of the reduction index R (for the frequency bands 100-800 Hz and 400-2500 Hz) is shown in Figure 3. Similarly, the measured system consists of the tube with the inner diameter of 60 mm (marked as SW060-L) and of the tube extension piece with the inner diameter of 60 mm (marked as SW060-E).
Recycling 2017, 7, x 3 of 12 The measured system includes the tube with the inner diameter of 60 mm (marked as SW060-L) and the holder of the tested sample with the inner diameter of 60 mm (marked as SW060-S).The system for measuring of the reduction index R (for the frequency bands 100-800 Hz and 400-2500 Hz) is shown in Figure 3. Similarly, the measured system consists of the tube with the inner diameter of 60 mm (marked as SW060-L) and of the tube extension piece with the inner diameter of 60 mm (marked as SW060-E).

The Experimental Materials Used in Testing
For experimental measurements, materials from selected components of vehicles after the expiry of their life cycle were used.Materials tested were recycled by operations such as crushing, cutting, grinding, heating, stamping, etc.
Specifically, the following materials were used.

The Experimental Materials Used in Testing
For experimental measurements, materials from selected components of vehicles after the expiry of their life cycle were used.Materials tested were recycled by operations such as crushing, cutting, grinding, heating, stamping, etc.
Specifically, the following materials were used.
• Ekomolitan (Figures 4 and 5): The material was manufactured by the recycling of a pure polyurethane foam (PUR foam) from seats taken from old vehicles by adhesive-free hot pressing of the recycled PUR products technology: Figure 4 shows the specimens of the PUR foam ground fraction.Figure 5 shows the result after pressing, i.e., Ekomolitan foam.
Figure 4 shows the specimens of the PUR foam ground fraction.Figure 5 shows the result after pressing, i.e., Ekomolitan foam.


Recycled rubber (Figures 6 and 7): It is the product of the process of recycling used tires.To produce sound-absorbing material, the rubber sawdust, rubber cords and rubber grinding layer from tire retreading is suitable.The mono-structural material is most often produced from a rubber fraction.The technical material selected: The author of this paper filed patent application No. 1/288237 [8] dealing with the production of this sound-absorbing material.Figure 4 shows the specimens of the PUR foam ground fraction.Figure 5 shows the result after pressing, i.e., Ekomolitan foam.


Recycled rubber (Figures 6 and 7): It is the product of the process of recycling used tires.To produce sound-absorbing material, the rubber sawdust, rubber cords and rubber grinding layer from tire retreading is suitable.The mono-structural material is most often produced from a rubber fraction.The technical material selected: The author of this paper filed patent application No. 1/288237 [8] dealing with the production of this sound-absorbing material.

•
Recycled rubber (Figures 6 and 7): It is the product of the process of recycling used tires.
To produce sound-absorbing material, the rubber sawdust, rubber cords and rubber grinding layer from tire retreading is suitable.The mono-structural material is most often produced from a rubber fraction.The technical material selected: Ground fraction: 4-6 mm; Pressing temperature: 900-1200 The author of this paper filed patent application No. 1/288237 [8] dealing with the production of this sound-absorbing material.


Nobasil (Figure 8): Material was manufactured from mineral fibers (bead wool) bonded by a modified artificial resin with the addition of hydrophobic and antifungal substances.The material is sewn on one side by glass thread with stitches fastened with glue onto a lightweight glass fiber mat.This material has wide application in civil engineering owing to its good thermal-insulation and sound-absorbing properties.The material has been used in comparisons with newly developed material properties.Nobasil is often used as one of the materials in noise barrier sandwich construction.

Preparation of Test Samples
Three materials, including their combinations, were used for the preparation of the test samples (Ekomolitan, recycled rubber and Nobasil).The specific examples are shown in Figure 9.
The test samples had diameters of 30 mm and 60 mm.The samples had thicknesses of 20 mm, 30 mm, 40 mm, 50 mm and 60 mm.


Nobasil (Figure 8): Material was manufactured from mineral fibers (bead wool) bonded by a modified artificial resin with the addition of hydrophobic and antifungal substances.The material is sewn on one side by glass thread with stitches fastened with glue onto a lightweight glass fiber mat.This material has wide application in civil engineering owing to its good thermal-insulation and sound-absorbing properties.The material has been used in comparisons with newly developed material properties.Nobasil is often used as one of the materials in noise barrier sandwich construction.

Preparation of Test Samples
Three materials, including their combinations, were used for the preparation of the test samples (Ekomolitan, recycled rubber and Nobasil).The specific examples are shown in Figure 9.
The test samples had diameters of 30 mm and 60 mm.The samples had thicknesses of 20 mm, 30 mm, 40 mm, 50 mm and 60 mm.

•
Nobasil (Figure 8): Material was manufactured from mineral fibers (bead wool) bonded by a modified artificial resin with the addition of hydrophobic and antifungal substances.The material is sewn on one side by glass thread with stitches fastened with glue onto a lightweight glass fiber mat.
Recycling 2017, 7, x 5 of 12   This material has wide application in civil engineering owing to its good thermal-insulation and sound-absorbing properties.The material has been used in comparisons with newly developed material properties.Nobasil is often used as one of the materials in noise barrier sandwich construction.

Preparation of Test Samples
Three materials, including their combinations, were used for the preparation of the test samples (Ekomolitan, recycled rubber and Nobasil).The specific examples are shown in Figure 9.
The test samples had diameters of 30 mm and 60 mm.The samples had thicknesses of 20 mm, 30 mm, 40 mm, 50 mm and 60 mm.This material has wide application in civil engineering owing to its good thermal-insulation and sound-absorbing properties.The material has been used in comparisons with newly developed material properties.Nobasil is often used as one of the materials in noise barrier sandwich construction.

Preparation of Test Samples
Three materials, including their combinations, were used for the preparation of the test samples (Ekomolitan, recycled rubber and Nobasil).The specific examples are shown in Figure 9.
The test samples had diameters of 30 mm and 60 mm.The samples had thicknesses of 20 mm, 30 mm, 40 mm, 50 mm and 60 mm.

The Results of Experimental Measurements
The sound absorption coefficient (α) is a dimensionless number ranging from 0 to 1.The closer the measured value is to 1 (or if it is equal to 1), the better (higher) is the sound absorption of the measured material (absorber).
The reduction index R was also measured in the experiment.This is the value given in "dB" based on the ratio of the wave of incident sound to the front side of the acoustically absorbing material to the sound waves transmitted from the rear side.The reduction index R represents the damping properties of the material, i.e., the is higher value, the more muffled is the sound.
Many measurements of the sound absorption coefficient (α) and reduction index R were made for homogeneous material (Ekomolitan, recycled rubber, or Nobasil) and for two-and three-layered sandwich structures with various thickness.
The values obtained for the sound absorption coefficients of the materials with a thickness of 60 mm are given in Table 1, where red numbers mean the worst values of the parameters obtained from the materials measured and green numbers mean the best values of the parameters obtained from materials measured.
The reduction index R values for materials with thickness of 60 mm are shown in Table 2, where red numbers mean the worst values of the parameters obtained from materials measured and green numbers mean the best values of the parameters obtained from the materials measured.
The values for materials with thickness of 20 mm, 30 mm, 40 mm, and 50 mm [9] were obtained similarly.
The results of measurements of the sound absorption (α) of the homogeneous material (Ekomolitan, recycled rubber, and Nobasil) with thickness of 60 mm are shown in Figure 10.
The results of the reduction index R measurements of 60-mm-thick homogeneous materials are shown in Figure 11.

The Results of Experimental Measurements
The sound absorption coefficient (α) is a dimensionless number ranging from 0 to 1.The closer the measured value is to 1 (or if it is equal to 1), the better (higher) is the sound absorption of the measured material (absorber).
The reduction index R was also measured in the experiment.This is the value given in "dB" based on the ratio of the wave of incident sound to the front side of the acoustically absorbing material to the sound waves transmitted from the rear side.The reduction index R represents the damping properties of the material, i.e., the is higher value, the more muffled is the sound.
Many measurements of the sound absorption coefficient (α) and reduction index R were made for homogeneous material (Ekomolitan, recycled rubber, or Nobasil) and for two-and three-layered sandwich structures with various thickness.
The values obtained for the sound absorption coefficients of the materials with a thickness of 60 mm are given in Table 1, where red numbers mean the worst values of the parameters obtained from the materials measured and green numbers mean the best values of the parameters obtained from materials measured.
The reduction index R values for materials with thickness of 60 mm are shown in Table 2, where red numbers mean the worst values of the parameters obtained from materials measured and green numbers mean the best values of the parameters obtained from the materials measured.
The values for materials with thickness of 20 mm, 30 mm, 40 mm, and 50 mm [9] were obtained similarly.
The results of measurements of the sound absorption (α) of the homogeneous material (Ekomolitan, recycled rubber, and Nobasil) with thickness of 60 mm are shown in Figure 10.
The results of the reduction index R measurements of 60-mm-thick homogeneous materials are shown in Figure 11.Measurements of sound absorption coefficients (a) and reduction index R of sandwich absorbents consisting of different combinations of two (with thickness of 20 mm, 40 mm and 60 mm) and three (with thickness of 30 mm and 60 mm) materials (Ekomolitan, recycled rubber, Nobasil) were also obtained.The sandwich structures are applicable for noise barrier construction (shields).The results of measurements in the frequency band ranging from 500 Hz to 1500 Hz are presented in Figures 12  and 13.
Recycling 2017, 7, x 9 of 12 Measurements of sound absorption coefficients (a) and reduction index R of sandwich absorbents consisting of different combinations of two (with thickness of 20 mm, 40 mm and 60 mm) and three (with thickness of 30 mm and 60 mm) materials (Ekomolitan, recycled rubber, Nobasil) were also obtained.The sandwich structures are applicable for noise barrier construction (shields).The results of measurements in the frequency band ranging from 500 Hz to 1500 Hz are presented in Figures 12 and 13     Measurements of sound absorption coefficients (a) and reduction index R of sandwich absorbents consisting of different combinations of two (with thickness of 20 mm, 40 mm and 60 mm) and three (with thickness of 30 mm and 60 mm) materials (Ekomolitan, recycled rubber, Nobasil) were also obtained.The sandwich structures are applicable for noise barrier construction (shields).The results of measurements in the frequency band ranging from 500 Hz to 1500 Hz are presented in Figures 12 and 13

Conclusions
The frequency spectrum maximum of traffic noise ranges from 500 Hz to 1500 Hz, with the loudest noise produced by frequency of 1000 Hz.Based on the measured sound absorption coefficient (α) values of the materials tested (Ekomolitan, recycled rubber and Nobasil) of different thicknesses (20 mm, 30 mm, 40 mm, 50 mm and 60 mm) at frequencies ranging from 500 Hz to 1500 Hz, and especially at frequency of 1000 Hz, Nobasil appears to be the most suitable material at a thickness of ≥20 mm, followed by Ekomolitan and finally recycled rubber.The Ekomolitan appears to be the most suitable materials at thickness of ≥ 20 mm followed by Nobasil, and finally recycled rubber.
It is obvious from the measured values of the sandwich absorbers' sound absorption coefficient (α) that the order of sandwich layers (of materials used) is important.The recommended order of sandwich (absorber) layers from the noise source at the measured materials is as follows: (a) Two-layered sandwiches:

Conclusions
The frequency spectrum maximum of traffic noise ranges from 500 Hz to 1500 Hz, with the loudest noise produced by frequency of 1000 Hz.Based on the measured sound absorption coefficient (α) values of the materials tested (Ekomolitan, recycled rubber and Nobasil) of different thicknesses (20 mm, 30 mm, 40 mm, 50 mm and 60 mm) at frequencies ranging from 500 Hz to 1500 Hz, and especially at frequency of 1000 Hz, Nobasil appears to be the most suitable material at a thickness of ≥20 mm, followed by Ekomolitan and finally recycled rubber.The Ekomolitan appears to be the most suitable materials at thickness of ≥ 20 mm followed by Nobasil, and finally recycled rubber.
It is obvious from the measured values of the sandwich absorbers' sound absorption coefficient (α) that the order of sandwich layers (of materials used) is important.The recommended order of sandwich (absorber) layers from the noise source at the measured materials is as follows: (a) Two-layered sandwiches:

Conclusions
The frequency spectrum maximum of traffic noise ranges from 500 Hz to 1500 Hz, with the loudest noise produced by frequency of 1000 Hz.Based on the measured sound absorption coefficient (α) values of the materials tested (Ekomolitan, recycled rubber and Nobasil) of different thicknesses (20 mm, 30 mm, 40 mm, 50 mm and 60 mm) at frequencies ranging from 500 Hz to 1500 Hz, and especially at frequency of 1000 Hz, Nobasil appears to be the most suitable material at a thickness of ≥20 mm, followed by Ekomolitan and finally recycled rubber.The Ekomolitan appears to be the most suitable materials at thickness of ≥ 20 mm followed by Nobasil, and finally recycled rubber.
It is obvious from the measured values of the sandwich absorbers' sound absorption coefficient (α) that the order of sandwich layers (of materials used) is important.The recommended order of sandwich (absorber) layers from the noise source at the measured materials is as follows: (a) Two-layered sandwiches:


Ekomolitan (Figures 4 and 5): The material was manufactured by the recycling of a pure polyurethane foam (PUR foam) from seats taken from old vehicles by adhesive-free hot pressing of the recycled PUR products technology: o Ground fraction: 2.00-10 mm; o Pressing at 200 °C (Steaming by superheated steam; o Pressing pressure: 5.7 kPa; o Hold time under the pressure at given temperature: 12 min; and o Pressed piece density: 1.2 g•cm −3


Ekomolitan (Figures 4 and 5): The material was manufactured by the recycling of a pure polyurethane foam (PUR foam) from seats taken from old vehicles by adhesive-free hot pressing of the recycled PUR products technology: o Ground fraction: 2.00-10 mm; o Pressing at 200 °C (Steaming by superheated steam; o Pressing pressure: 5.7 kPa; o Hold time under the pressure at given temperature: 12 min; and o Pressed piece density: 1.2 g•cm −3

Figure 7 .
Figure 7. Recycled rubber after pressing.Nobasil(Figure8): Material was manufactured from mineral fibers (bead wool) bonded by a modified artificial resin with the addition of hydrophobic and antifungal substances.The material is sewn on one side by glass thread with stitches fastened with glue onto a lightweight glass fiber mat.

Table 1 .
Sound absorption coefficient values for materials with thickness of 60 mm.

Table 2 .
Reduction index R values for materials with thickness of 60 mm.