Research of Insertion Loss of Multilayered Construction with Devulcanized Waste Rubber

– In this study, the insertion loss of devulcanized waste rubber baffles were evaluated. Acoustic baffles are suitable to reduce noise from the devices or machines by interfering with their emitting sound waves. Knowledge of the acoustic properties of the material used is of significant importance in ensuring the effectiveness of the acoustic properties of the baffle. Basic properties include airborne sound insulation, which is usually determined during laboratory tests. Baffles consists of sound absorbing and sound insulating materials. In this study, plasterboards were used as sound insulating material and devulcanized waste rubber as sound absorbing material. During the devulcanization process, porous granules are obtained, which can be used as an acoustic material. In this study, two types of rubber granules were devulcanized by grinding method and one other type was chemically devulcanized. Three types of rubber granules were mixed together in increasing 25 % proportion steps and glued with patented polyurethane glue. A total of 15 different composition devulcanized waste rubber granule boards were made. Rubber boards were attached together with the plasterboards. Insertion loss of the different composite baffles was measured in semi-anechoic chamber in a purposefully designed stand in 1/3rd octave bands. The results showed that the insertion loss of the baffles was mostly dependent on the rubber granule board density. When the density of the rubber board increased, the insertion loss also increased. The 5–6 dB insertion loss difference was measured between the most and least dense rubber granule board baffles


INTRODUCTION
Globally, due to the growing number of cars, approximately 1.5 billion new tyres are produced each year, making tyre waste one of the biggest environmental problems [1] It is estimated that about 1 billion tires end of life each year, and more than 50 % of this amount is landfilled.By 2030, the number of used tires can reach 1.2 billion, including storage -5 billion [2], [3].Used tires can be used in a variety of ways, disposed of at special collection sites, burnt as fuel for heat generation, pyrolyzed [4], used to create new materials etc.
In recent years, the industry has taken the challenge to apply the principles of the circular economy to find environmentally friendly materials, as well as to use waste to develop new products.One way to reuse tires would be to embed them in concrete and replace some natural materials such as sand.This method is economically effective, because natural materials used in the production of concrete are quite expensive and replacing them with rubber will save a significant amount of natural resources [5]- [7].Tire waste can also be recycled by separating the rubber from the carcass.Shredded tyre waste is used in engineering due to its size, shape, high elasticity, good vibration and noise reduction.The properties of rubber components are directly dependent on their microstructure, which is formed by elastomeric chains (rubber, polymers, resins) and filler, which forms a continuous and homogeneous polymeric composite [8].The most important problem in recycling of rubbers is their crosslinks created during vulcanization.One of well-known recycling method is devulcanization.During the process, the crosslinks between C-C, C-S, S-S are broken in order not to damage the main C-C link.Mechanical, thermal, chemical microwave irradiation devulcanization methods have been used for devulcanization of rubbers [9].During the devulcanization process, porous granules are obtained, which can be used as an acoustic material.Materials with high sound insulation and absorption properties can be made from recycled rubber waste.Rubber particle size has an effect on sound absorption behaviour [10].Due to its inherent good damping properties, rubber materials after devulcanization are available in different acoustic applications.
In order to determine how well the sound is transmitted through a medium, the insertion loss -the loss in the transmitted signal from the insertion of an object or device in the acoustic path -is one of the most valuable acoustic parameters for performance evaluation [11].
Insertion loss of the acoustical enclosures is determined from the coupled motion of the sound field and vibration of wall panels [12].In early experiments Lyon computed noise reduction of a small enclosure by assuming that the critical frequency of the wall lies above the first acoustic resonance of the enclosure [13].Later, insertion loss of the small enclosures whose sizes were less than 0.5 m was studied by [14].They solved the coupled motion of the air cavity and the wall panel using an acoustic velocity potential and the finite element method.Al-Bassyiouni investigated sound transmission through a flexible panel into an enclosure, in which a spherical wave was generated outside the enclosure, and the largest dimension of the enclosure was 60.96 cm [15].They proposed an active noise control scheme based on a structural-acoustics model, in which they used piezoelectric patches as actuators.Later studies were focused on insertion loss of homogenous and multi-layer structures.Kosala studied single homogeneous walls and the results of his research proved that insertion loss research corresponds to the research methodologies of single structures [16].Studies of multilayer structures were described in [17].During the research, it was found that the medium between two homogeneous plates determines the occurrence of mass air mass resonance in structures.

Theoretical Background
Early models for the analysis of sound insulation of multi-layered structures were concentrated on the dispersion of airborne noise through panels.Structural connections between the walls were not calculated, and the panels were modelled as infinite.The first models such as Beranek and Work applied only to ordinary sound waves.In 1950 London introduced a model of sound propagation that included incident waves and also analysed the stiffness of the panels, and these models included additional physical factors such as resonance [18].
Mulholland and colleagues extended Baranek and Work's model to include incident shear waves [19] and then extended his model to include reflections in the cavity between the two plates.Researcher Heckl went back to Beranek and Work's original model to include soundabsorbing materials based on the stiffness of the materials per unit area inside the cavity [20] and modelled the sound insulation of double walls consisting of two thin plates with an elastic material between them.Scientist Fahy Baranek and Work introduced an equivalent fluid

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____________________________________________________________________________ 2024 / 28 model that allowed the analysis of hollow spaces between two plates [21], [22].Au and Byrne modelled the effect of porous layers and air spaces influence on sound insulation [23].Kang compared the results of previous models with experimental data and found that there was a dependence on the incidence of the sound wave on the surface of the material, the higher it was, the greater the sound insulation.Scientist Donato proposed to correct the final size of the walls using the Beranek and Work and London model wave number method [24].
In previous models, the joints between two panels were not included in the calculations.Rigid connections between baffle plates, called acoustic bridges, were introduced by the scientist Sharp, later by Cremer and Heckl [25].Lin and Garrelik found an analytical solution for the sound insulation of double walls of infinite length.The connecting plates and rigid frames were modelled as springs.This method was later applied by Brunskog in his model, taking into account the interaction between rigid frames [26].Other authors have also presented extended models that include flexible joints.They were modelled as a series of springs [27]- [31].This modelling of flexible connections was very approximate, so empirical formulas were used for the spring stiffness [32].
Nowadays, many scientists are also working on the theoretical prediction of the sound insulation index of single and double structures based on experimental studies as accurately as possible.Theoretical models are mostly focused on predicting the sound insulation index.Much work in this field was done by scientists Sharp, Davy, Craik and others [29], [30], [33].There are studies that try to apply the models of single structures by extending them to double structures [34].By replacing the usual models proposed by Sharp and Davy by supplementing them with empirical formulas, double-layer constructions can be predicted.The principal graph of sound insulation in the entire frequency band of single construction is presented in Fig. 1.The graph clearly shows that there were three fundamental frequencies, resonant Fp, critical Fc, and angular deflection frequency Fs.The sound insulation of single-layer structures improves with increasing frequency, and their sound insulation is largely dependent on the mass of the structure itself.Theoretical studies of single-layer constructions were analysed by comparing them with the results of experimental studies and numerical modelling.The application of these laws was also analysed by Kosala, who compared the results of PVC and other single-layer materials obtained during experimental and numerical modelling studies with Davy and Sharp models.From the results obtained in Fig. 2, it can be seen that the sound insulation indicators were very close.In his conclusions, the scientist stated that these methods were accurate enough and reliable to theoretically calculate the sound insulation of single-layer materials.
A typical sound insulation for a double construction is given in Fig. 3.The theoretical calculation of multilayer structures is much more complicated.The two panels are usually separated by a wooden, metal or other frame.It is necessary to take into account the vibrations transmitted by the frame, which lead to a decrease in sound insulation at certain frequencies.The reduction of sound transmission in a double structure is based on a theory that first evaluates a structure whose panels are not structurally connected, and then calculates the impact of various connections on sound insulation.
In this article small-area structures ware investigated based on the practice of insertion loss method.Constructions were made of recycled rubber granules panels with different granulometric composition and plasterboard and to compare the results with simulated results.

METHODOLOGY
Tests of insertion loss were conducted to measure the acoustic parameters of materials using smaller sized panels.The dimensions of the measured samples were 300×300 mm.Insertion loss tests were performed in an anechoic room of the VGTU noise reduction chamber.The walls of the chamber ware covered with acoustic foam, which prevents extraneous sound from entering the room while creating an anechoic environment Fig. 4. Insertion loss was calculated as the difference between the sound pressure level without enclosure and the sound pressure level with the enclosure.Insertion loss was calculated using the Eq. ( 1): without 1 2 with 20 log where Pwithout Sound pressure level without enclosure, dB; Pwith Sound pressure level with enclosure, dB.
For the research, a special stand was created, consisting of two loudspeakers mounted in a box, which emit white sound.The tested panels were mounted on the stand and the sound level was measured Fig. 5.For the tests, the sound pressure level shall be measured without any test sample being equipped and then sound pressure level measured with the test sample.The results were calculated according to Eq. ( 1).
The following conditions shall be met during experimental measurements: − Distance between source and microphone 1 m; − Measurement time 30 s; − White sound was used for research.

Tested materials
Rubber granule panels were produced from three different rubber granule fractions, which was obtained by devulcanization process.The size of different fractions was represented in Table 1.Two rubber fractions were obtained by grinding method, and the third fraction was obtained by chemical devulcanization process.After chemical processing of the rubber granules, rubber granules with higher porosity and partially fibrous structure were obtained, which, due to their higher porosity, have better sound absorption.The difference between mechanically devulcanized rubber granules and our innovative chemically devulcanized granules is shown in Fig. 7.During chemical devulcanization of rubber, turning it into devulcanized rubber flour, mechanical shear causes stress in the S-S bridges between rubber chains, the devulcanizing agent compounds promote the delocalisation of the S-S bridges, so the adhesion of the rubber particles is inhibited, resulting in rubber granules with higher surface porosity.

TABLE 1. GRANULES SIZE
A total of 15 different rubber granule panels were produced, the composition of which is shown in the The rubber granule boards were made by mixing different fractions as represented in Table 2 with polyurethane waste resin and hardener.Each composition of rubber granule was mixed in a mixing tank to ensure an even distribution of granules.After primary mixing a polyurethane waste resin was poured into the mixed sample, and the sample was mixed.Then, the hardener was added, and everything was mixed again.The amount and proportions of hardener and polyurethane resin in each sample were the same but were not disclosed for confidentiality reasons.For the measurements, multilayer constructions were made, which consisted of two plasterboards (GKB and GKFI) and rubber board between them.Three different measurements of each sample were made.

EXPERIMENTAL RESULTS
In the anechoic chamber, on a specially designed stand (Fig. 8), the insertion loss of 15 different constructions was measured.Measurement results represented insertion loss values in 1/3 octave band of different sound insulating constructions, which were made two plasterboards and rubber composite between them.GKP (density 680 kg/m 3 ) and GKFI (density 1000 kg/m 3 ) plasterboards were used in this research.
The first measured structures were made by connecting the rubber granule board on both sides with two GKB plasterboard boards.The thickness of the structure was 39 mm.Research has shown that the sound level decreased at low frequencies by 20-25 dB, and the resonant frequency was set at 200-250 Hz.From 250 Hz to half the critical frequency, which was equal to 1250 Hz, the sound reduction increased according to the mass law and reached 35-42 dB, and at 2500 Hz the critical frequency was reached, where the sound reduction values decreased to 20-30 dB.From the critical frequency in the high frequency band, the sound insulation values increased, reaching the peaks at 6300 Hz.The highest insertion loss values reached 40-55 dB.The research results showed that structures with higher density panels were better at damping sound.Panels with a higher than average density 4, 7, 8, 9, 13 had better sound insulation.Considering the dynamic stiffness of the panels, there were no clear trends, but panels with slightly lower dynamic stiffness isolated the sound better.

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____________________________________________________________________________ 2024 / 28 128 Fig. 9. Insertion loss results of rubber granule panels (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) with two GKB plasterboards on both sides.A sound insulation structure consisting of a rubber granule board joined on both sides with two GKFI plasterboard boards was also constructed and tested.The thickness of the structure was 39 mm.Gypsum boards of higher density (>1000 kg/m 3 ) were used for the construction.Research results showed that the sound insulation of such constructions, using different rubber granule panels, depended on the density of the material, but the differences were not large.Meanwhile, there was no clear trend in dynamic stiffness, but structures with 6, 7, 11 rubber panels with lower-than-average dynamic stiffness slightly better reduced sound than structures with higher dynamic stiffness rubber granule panels.
In conclusion of all the studied constructions, consisting of two additional plates on both sides, it could be stated that the properties of the materials to dampen vibrations between the two plates depend on the deformation characteristics of the material and the acoustic impedance.Research had established that resonant frequencies occurred at 500 Hz in all structures, which was influenced by the mass of the material.The critical frequencies occurred at 2000-2500 Hz.Since the thicknesses of the structures were similar, the critical frequency also occurred at the same frequency.When comparing constructions with different plasterboard panels, it was found that constructions with higher density plasterboard panels (GKFI) isolated the sound 3-5 dB better.The resonant frequencies in both cases were at 200 Hz, whereas the critical frequency with denser panels shifts to higher frequencies at 2500 Hz.

DISCUSSION
During the experimental studies, the aim was to investigate the results of the insertion loss of constructions with different devulcanized rubber panels.After research, it was found that structures with higher density rubber granule panels insulate sound better.It was also found that when a higher density plasterboard was used, the sound reduction values also increased by 3-5 dB.Such trends were fully correlated with theoretical models.According to the mass law, the sound insulation depended on the mass of the structure.However, as stated in the theory, it was additionally important to evaluate the dynamic properties of materials.In theory, single-layer and multilayer constructions were distinguished.Single-layer constructions can consist of several different layers, but the main feature of such constructions is that they are rigidly connected to each other.Meanwhile, multilayer structures consist of two different layers connected by a certain connection.Therefore, the sound insulation of single-layer structures depends on the mass law, and the sound insulation values theoretically increase by 6 dB per octave and only at the frequency when the thickness of the structure coincide with the sound wavelength, a critical frequency occurs when the sound energy begins to vibrate the structure, which causes energy losses.Meanwhile, a mass-air-mass resonance occurs at low frequencies in a multilayer structure, and it depends on the gap between the two plates and the density of the medium between them.From this frequency, the structure behaves like a single-layer structure according to the mass law, until a critical frequency occurs at high frequencies, which depends on the plates of the structure.When evaluating the structures described in this article, it is also important to identify the type of the structure.In this case, a multi-layered structure was created, which should act as a single-layer structure, but due to the low dynamic stiffness of the rubber granule plates, part of the energy was lost according to the theory of double structures.When the average values of the two different measured structures were compared with the simulation results, it was found that a mass-airmass resonance was found at low frequencies, which depended on the thickness of the rubber plate.According to theory, the resonant frequency moves towards lower frequencies by increasing the gap between the two plates.Since the thickness of the rubber granule plates were relatively uniform at 12±2 mm, in both cases the resonant frequency was at 200-250 Hz, which coincides with the results of the simulation of the theory.However, in medium and high frequencies, the sound insulation values of the structure were close to the result of the simulation of single layer structures, especially in the medium frequency band, which depends on the mass of the structure, while compared with the theory of multi-layer structures, it was clearly seen that the simulated values in medium frequencies were significantly higher.The resonant frequency of the structures was slightly different from the simulation results.
Therefore, summarizing the results, it can be stated that the created multi-layered structure with two plasterboards and a rubber granule panel between them acted as a single-layer structure, the sound insulation of which depended mainly on the entire mass of the structure.However, the massair-mass resonance measured at low frequencies allowed us to say, that the rubber granule plate also acted as a junction connecting two plates.Therefore, such constructions can be used in various constructions designed to reduce the noise emitted by devices, forming barriers, enclosures, etc.By increasing the weight of the structures, greater sound insulation can be obtained, while by changing the thickness of the rubber granules, the resonant frequency of the structure can be controlled, considering the sound characteristics of the noise source.The research results showed that the sound insulation of these constructions, which uses different rubber granule panels, depends on the density of the material.The higher the density, the better insertion loss results.Meanwhile, there was no clear trend in dynamic stiffness, but structures with 6, 7, 11 rubber panels with lower-than-average dynamic stiffness slightly better reduced sound than structures with higher dynamic stiffness rubber granule panels.3. A multi-layered structure with two plasterboard panels and a rubber granule panel between them acted as a single layer structure.Sound insulation depended on the entire mass of the structure, but the mass-air-mass resonance measured at low frequencies allowed us to say that the rubber granule panel works as a junction connecting two panels.When the weight of the structures, greater sound insulation can be obtained, while changing the thickness of the rubber granules, the resonant frequency of the structure can be controlled, considering the sound characteristics of the noise source.

Fig. 1 .
Fig. 1.Principle graph of sound insulation of a single-layer construction over the entire frequency band [35].

Fig. 10 .
Fig. 10.Construction of rubber granules and two GKFI plasterboards.The research results showed that at low frequencies the sound insulation values of the structure reached 20-25 dB, but at 200 Hz a mass air mass resonant was founded and the sound reduction values decreased to 15-22 dB.From the resonant frequency, the sound reduction values increased in the mid-high frequency band up to 1250 Hz, where the results reached 38-43dB.At 2500 Hz, a critical frequency occurred and the results decreased again to 28-35 dB.From the critical frequency, sound insulation increased, and the best values were reached at a high frequency of 6300 Hz.The reduction in sound pressure level reached 44-53 dB.Research results showed that the sound insulation of such constructions, using different rubber granule panels, depended on the density of the material, but the differences were not large.Meanwhile, there was no clear trend in dynamic stiffness, but structures with 6, 7, 11 rubber panels with lower-than-average dynamic stiffness slightly better reduced sound than structures with higher dynamic stiffness rubber granule panels.In conclusion of all the studied constructions, consisting of two additional plates on both sides, it could be stated that the properties of the materials to dampen vibrations between the two plates depend on the deformation characteristics of the material and the acoustic impedance.Research had established that resonant frequencies occurred at 500 Hz in all structures, which was influenced by the mass of the material.The critical frequencies occurred at 2000-2500 Hz.Since the thicknesses of the structures were similar, the critical frequency also occurred at the same frequency.When comparing constructions with different plasterboard panels, it was found that constructions with higher density plasterboard panels (GKFI) isolated the sound 3-5 dB better.The resonant frequencies in both cases were at 200 Hz, whereas the critical frequency with denser panels shifts to higher frequencies at 2500 Hz.

Fig. 12 .
Fig. 12.Comparison between measured sound reduction values ant simulated according single and double structure theory.

1 .
Research has established that when studying constructions with GKB panels, the sound level reduction in low frequencies reached 20-25 dB, and the resonant frequency was set at 200-250 Hz.At high frequencies, values up to half of the critical frequency reached 35-42 dB, and at 2500 Hz, the critical frequency was reached, where the sound reduction decreased to 20-30 dB.The peaks of the sound insulation values were at 6300 Hz.The highest values reached 40-55 dB.Boards with a density higher than average had better sound insulation.2. Research has established that when studying constructions with GKFI plasterboards, the sound insulation values of the structure reached 20-25 dB at low frequencies.When the resonant frequency reached 200 Hz, the sound reduction values increased.In the medium and high frequencies, up to 1250 Hz the results reached 38-43dB, and at 2500 Hz a critical frequency occurred and insertion loss values decreased to 28-35 dB.The best values were achieved at a high frequency of 6300 Hz.The reduction in sound pressure level reached 44-53 dB.

Table 2 .TABLE 2 .
COMPOSITION OF DIFFERENT RUBBER GRANULES BOARDS