Improvement of the Acoustic Attenuation of Plaster Composites by the Addition of Short-Fibre Reinforcement

Noise attenuation is a key contemporary issue associated with the protection of human health. In this study, the possibilities of affecting acoustic properties of plaster composites by the addition of short-fibre reinforcement are described.-e improvement of attenuation abilities was first verified using a simple numerical model with a pure plaster followed by using a reinforced plaster. -emodel results revealed amutual correlation between the fibre ratio and dissipated acoustic energy. Hence, typical plasters used in the building industry (e.g., plaster, lime cement, and cement) are used as the base materials of the tested composites. -e reinforcing dispersion in the form of short fibres (basalt and glass) with a defined length was selected after evidence from previously reported studies and after the comparison of some other fibres with respect to the trade-off between the rendered mechanical properties and cost. Transfer functions of the tested samples were measured using an impedance tube with two microphones, followed by the calculation of the total acoustic absorption. On the other hand, cement and plaster materials exhibited a low damping ability, and the absorption could be considerably increased by the addition of fibres, especially in the area around 1 kHz. In contrast, the UM plaster exhibited good damping properties even without the dispersion, and the addition of improper fibres such as glass ones possibly worsened the properties. -e acoustic attenuation of the plaster composites can be improved by the appropriate combination of the base material and fibre dispersion. However, it is not possible to generalise this improvement for all possible combinations.


Introduction
Currently, the enormous technological development could be observed in a lot of everyday life sectors such as transport and other industries.In addition, this phenomenon exerts some negative effects, one of them being the increasing noise levels.
e increase in industrialisation and transport technologies has led to a rapidly increasing number of people being affected by residing in an extremely noisy environment.Several detailed physiological and hygienic studies have revealed that noise is one of the most widespread harmful factors not only in workplaces, but also in everyday life [1].Long-term, frequent exposure to noise affect not only the mental state (e.g., tiredness or concentration ability), but also the overall health of the human beings, as well as cause some irreversible harm.Hence, it is crucial to protect human beings by decreasing noise levels [1,2].Issues associated with noise in such environments are constantly increasing; therefore, common people, employers, and employees require their homes and working places to be protected [3].Requirements related to this issue are reflected in the directives and regulations, and the European Union also recognises environmental noise as one of the most important environmental issues.Because of this fact, laws regulating noise levels are valid in several countries [4].Noise reduction should be conceptually solved with regard to the interactive approach, especially with respect to the noise source and its surroundings.Despite the fact that passive measures, such as wall cladding panels with porous materials and resonators, address only the consequences but not the main causes of noise, these factors are often the only possible and viable solution.e ability of building materials to absorb sound waves typically depends on their sti ness, density, and porosity [5][6][7].e function of the absorption coe cient exists in order to quantify the ability of materials to absorb sound waves at a speci c frequency.When sound reaches a speci c part, it dissipates, passes through, or reects depending on the physical properties and microstructure of materials [8].erefore, parts such as walls and roofs of buildings, which constitute the main parts of a residential or industrial object require materials with a high absorption coe cient such as wool glass, foam, mineral wools, or their composites [9].Currently, various products such as chopped bres, grids, steel truss, rovings, and veils can improve the mechanical and acoustic properties of the main building materials.Pedreño-Rojas et al. have added some recycled wood to the gypsum matrix [10].eir experiments revealed that by increasing the percentage of the added wood waste, the mass of the new material decreases while simultaneously improving its thermal and sound absorption properties.eir group has revealed that the best results (with respect to the acoustic and mechanical properties) are achieved by the addition of up to 10%-20% of wood shavings or sawdust composites.
Generally, the mechanical and physical properties of typically used construction materials ( red and silicate bricks, bricks, concrete, aerated concrete, steel-reinforced concrete, glass wool, and polystyrene) have been extensively investigated.However, the materials used in this sector are often produced from nonrenewable natural resources, and the production of these materials adversely a ects the environment [11,12].Hence, in recent years, increased e orts to use materials, including some typical natural materials such as ax, hemp, wood, straw, reeds, and others that can reduce the negative impact on the environment have been observed.Typically, ax and hemp bres are used to insulate buildings.Wood and wood sawdust can be used on walls and as a ller for building materials, respectively.Another wellknown conventional method is to use reed bundles as roo ng and straw in combination with clay for wall lling and reinforcement [13].Randazzo et al. [14] have tested some commercially available clay plasters typically used in Europe for exploring the manner in which mineralogical and textural properties can a ect their physical parameters; that is moisture absorption, thermal conductivity, and noise attenuation.Results obtained from the tests revealed that the addition of natural bres slightly increases the open porosity; hence, because of its relatively higher pore volume, the obtained nal sound absorption is possibly higher.
Porosity considerably a ected the material's ability to absorb sound waves.
e principle is mainly caused by di erent resistivities of individual materials (such as solid and air) and phenomena occurring at their mutual interfaces.Generally, several routes of energy dissipation simultaneously occur, that is, re ection at the boundaries, de ection caused by di erent speeds of sound propagation in individual materials, and friction of air on the walls in micropores.Pore size can be categorised at di erent scales (i.e., micropores, interparticle pores, and mesopores).Cannabis and lime-based composites exhibit one of the highest porosities (70%-80%) [15].Gle et al. [16] have extensively examined the acoustic bene ts observed with the use of cannabis composites. is study revealed that the level of sound absorption in hemp-reinforced concrete can be controlled and signicantly increased using suitable components with speci ed manufacturing processes.
As has been previously mentioned, brous materials are typically added to building materials as a reinforcing element to modify their mechanical properties, such as tensile strength, compressive strength, or impact strength.Previous studies [17] have reported signi cant improvement in the main mechanical properties of plaster composites by the addition of an appropriate volume ratio of bres (1%-2% of reinforcement).Glass and basalt bres were used as the reinforcing elements.Similar issues have been reported by Blankenhorn and Soroushian [18,19] with focus on cellulosecement composites.Neithalath et al. [20] have investigated the e ects of natural bre reinforcement to the morphological changes in acoustic and mechanical properties.e results obtained from their study clearly revealed the increasing dependence of the acoustic absorption coe cient on the bre volume.In other independent studies reported by Kalinova and Mohrova [21,22], the use of nano ber structures as sound-absorbing elements and nanomembranes as resonators has been described.
Experimental studies have reported improvement in the acoustic, thermal, and mechanical properties of plasters by the addition of short-bre cellulose [20,23] or polypropylene [24] as reinforcement.However, according to [24], the improvement of the aforementioned properties by ber count is questionable.
is study focuses on the sound attenuation characteristics in plaster composites.ree typical plasters (viz., gypsum, lime cement, and cement, resp.) were reinforced with glass and basalt bres, and impedance tube measurements were carried out.

Materials and Methods
Typical plaster and cement plaster were completed with limecement plaster that exhibits enhanced thermoinsulating properties ( ermo UM); these plaster materials are typically suitable for the central European climate, which can be possibly prepared even under laboratory conditions, without the requirement of any specific machinery.Furthermore, plasters such as gravel, perlite, mineral, or stucco plaster are also available in the market.Flass, carbon, basalt fibres, and even some nanofibrous dispersions were used as reinforcing elements for composites.In previously reported papers by the authors of this study [17], all possible combinations of typical plasters and fibre reinforcements have been investigated with respect to their mechanical properties (Figure 1).e results revealed that when using a suitable mass ratio (1%-2% of fibre reinforcement), the mechanical properties (flexural or tensile strength, compressive strength, and impact strength) of the plaster composites possibly improved significantly [17].Based on those results, three plasters and two fibre reinforcement materials, the mechanical properties of which can be improved, or at least not worsened, were selected for their acoustic attenuation.Although the carbon fibres should be considered on the basis of the obtained mechanical properties, the fibres were not included for additional tests because the cost of the dispersion was greater than that of the base plaster material.Table 1 summarises the fundamental mechanical properties of the base materials.Similar to that carried out during the initial verification of the model, the pure plaster without any fibre dispersion was investigated.
e second set of samples contained 7.7 mm-long basalt fibres with a diameter of 13 μm.e final sets were created from alkalised glass fibre with a length of 12 mm and a diameter of 14 μm.e fibre length was selected on the basis of the determined critical length of the individual types of fibre dispersion [25].

Basic eory for the Study of Acoustic Quantities.
e mechanical oscillation of liquid, solid, or gas particles spread as waves called sound, which is always connected to energy transfer.Pressure oscillations in the surroundings, typically gaseous environment, can produce acoustic field or just generally the sound.ese pressure changes are caused by the oscillation of the cover surfaces of individual parts or entire systems.When considering noise in a structure, subjective noise perception must be considered, particularly in an urban environment [26].Sound field is described by changes in steady-state variables, such as pressure, speed, temperature, and density, which describes the state of the environment in which sound spreads.
e actual sound velocity c, which depends on the modulus of elasticity E and the density of the material ρ, is determined by the spreading velocity of the longitudinal waves in an unbounded space according to (1).Energy dissipation occurs because of the absorption of sound by the material; therefore, the velocity potential φ, where the velocity is equal to the negative velocity gradient (2), could be introduced.e speed potential permits the definition of sound pressure p according to (3).
e specific mechanical impedance Z (4) can be used to quantify the sound transfer between the two materials as the ratio of the passing and impacting energy.To understand the basics of sound propagation and attenuation, it is possible to use the basic assumptions that can be well characterised by a simplified 1D structure as mentioned in references [26,27].Considering a 1D structure in which some planar or longitudinal wave spreads, this phenomenon is associated with structural deformation.en, the ratio between the passing energy and the incident energy can be expressed as the ratio between the through and incident wave intensities, according to (5).Based on this assumption, transfer attenuation ( 6) can be defined using the transfer coefficient (5).An example involves the steel-aluminium combination when it is possible to obtain a transmission attenuation coefficient of only 1.6 dB.On the other hand, for steel PVC, the attenuation could reach up to 14 dB [26].By simplifying the problem according to (5) and (6) where some factors of the acoustic system were neglected, it is possible to obtain the corresponding mathematical expression that can be used to optimise the intensity of the passing waves.
where Z i and Z j represent i and j-materials, respectively.e final attenuation is attributed to the combination of different damping properties of individual materials.erefore, for modelling and describing the attenuation, it is crucial to introduce the complex dynamic modulus  E (7), where the inner damping coefficient η of materials can be Advances in Materials Science and Engineering de ned by relation (8).Table 2 summarises the internal damping coe cients of several selected materials.Extremely dense materials that re ect 100% sound exhibit a sound absorption coe cient equal to 0 [28].
e sound absorption of plasters at a frequency of 1000 Hz is 0.02-0.05[29].e absorption coe cient of the brous layers is a ected by the thickness and density of these layers.Moretti et al. [30] have tested basalt layers of di erent thicknesses and densities.From the results obtained (Figure 2), brous layers exhibit an extremely good absorption coe cient, especially at high frequencies.Figure 2 shows sound absorption properties for some chosen bres with a layer thickness T of 9, 18, and 27 mm and densities D of 175 and 200 kg•m −3 , which is taken from the study by Moretti et al. [30].In particular, at low frequencies, sound absorption depends on the bre thickness.
Wang and Torng have also carried out a similar study [31] in which the absorption characteristics of porous materials reinforced with glass and basalt bres were investigated.
e 50 mm-thick glass bre layer with a bulk density of 100 kg•m −3 exhibits an absorption coe cient of 0.82.e basalt layer with the same thickness and bulk density exhibits an absorption coe cient of 0.9.eir results revealed that both bres exhibit similar sound absorption.

Plaster.
Plasters are mainly categorised according to the binder used.e binder ensures adhesion to the masonry as well as plaster strength and durability (the ability to withstand environmental impacts).e plaster is formed after hardening of the mortar mixture.
e rst of the plasters selected herein was lime cement with enhanced thermal insulation properties, that is, ermo UM.Another signi cant feature of this material is low bulk density compared to typically used plasters: is plaster is 4 times lighter.Because of its low bulk density and low modulus of elasticity, the plaster eliminates volume or shape changes in the base wall material, thereby not creating any cracks.
ermo UM plaster is characterised by a high thermal conductivity coe cient and the plaster's chemical composition eliminates microbial, algal, and fungal growth on its surface [32].
Another plaster, referred to as gypsum plaster, is a typically cheaper material.It exhibits good thermal insulation and acoustic attenuation.Owing to its thermal properties, the plaster is thought to be a hygroscopic material, which permits the regulation of the room microclimate [33,34].Vapour can permeate into the plaster and does not close the water in the construction while simultaneously permitting the natural drying of residual water from the blocks.Gypsum plaster is suitable for antibacterial environment because its smooth surface prevents dust from settling on it.At high temperatures, water vapour is released by gypsum, which is typically used to increase re resistance [35].
e last plaster, which is referred to as cement plaster, is designed for highly loaded areas; it exhibits increased strength, water resistance, and adhesion to the base substrate, as well as frost and weather resistance.

Fibre Types and Proportion.
In this study, short glass and basalt bres were used as dispersion elements.Such bres are typically used in the building industry particularly because of the cost-e ectiveness as well as acoustic and thermal insulation properties.
Basalt is typically formed by decompression melting in the Earth's upper mantle.is rock is made up of di erent species such as pyroxene, magnetite, olivine, and plagioclase [36].e basic technological condition for selecting optimal kind of basalt for producing bres is the so-called acidity factor M k (9).Militky [37] has reported that the acidity factor ranges from 1.1 to 3. e most appropriate technological condition for producing bres is at M k 1.65.
e selection of basalt rocks for producing fibres is affected not only by the chemical and mineralogical composition, but also by the rock texture [38].
e resulting quality of fibres is affected by the level of technological equipment and the entire technological production process.Moretti et al. [30] have developed a mineral fibre acoustic absorption panel for a highly efficient power system in buildings.e results of their application revealed extremely good acoustic and thermal properties for these panels.e additional advantage is chemical stability, low thickness, and considerable mechanical strength.
Compared to glass, basalt fibres are more stable in a strongly alkaline environment, but in a strongly acidic environment, the fibres exhibit lower stability [39].e glass fibre base comprises tetrahedrons (SiO 4 ) 4− and contains different oxides.e individual types of glass fibres can be distributed according to their oxide content.For producing fibres, not only glass stones, but also different processed glass metaproducts such as glass beads, rods, frits, or waste glass shards were used.e base glass composition significantly affects the properties of the resulting fibres [40].
One of the most frequently declared fibre parameters is their linear mass density t (10): where m is the mass (g), l is the length (km), s is equivalent to the fibre diameter (μm), and ρ is the fibre density (kg/m 3 ).e d value can also be calculated according to (11) for noncircular cross sections.en, d is called the equivalent fibre diameter.
2.4.Porosity.With respect to the sound attenuation and thermal insulation purposes, the porosity of the resulting part of the fibre-filled materials is crucial compared to the basic physical properties.e porosity ψ (12) represents the proportion of the fibre structure that is filled with air.As has been already mentioned, the sound absorption mechanism depends on the amount and mutual bonding of the pores inside the material.Good sound-absorbing materials contain interconnected pores, which enable further sound propagation and permit air flow under acoustic oscillation.Hence, the first principle of acoustic energy dissipation caused by friction is used.To express pore volume, an analogy based on the determination of porosity in thick nonwoven formations has been used.is idea could be valid on the basis of the assumption [41] that the fibre surface is approximately equal to the pore surface. is method is probably not the most suitable one, and direct measurement based on some screening methods can be used to possibly set statistically more precise results, but only to estimate porosity, sufficiently precise results should be obtained: where V c is the total volume of the body, V p is the pore volume, and V is the fibre volume.
As the same volume of air may be found in a few large pores or in a number of small pores, the median pore size ξ is determined: where A p is the pore area.
V p (or simply the air volume) in fibre formation can be calculated as follows: An important prerequisite according to reference [41] is that where the fibre ends, the air around it begins.us, the fibre surface is also the pore surface.Based on this assumption, the pore surface A p can be obtained from where A is the fibre surface, L is the total length of all fibres in the fibrous structure, and p is the cross-sectional circumference of the fibre.e shape of the interfacial pores is complicated; hence, the interfibrous pores are considered to be shaped as capillaries.e pores can be imagined as air fibres, and then their surface can be calculated as follows: where L p is the length of the pore capillaries in the fibrous form, d p is the equivalent pore diameter, and q p is the pore shape factor, which is defined as Because the geometric characteristics ξ (13) depend on the pore shape, it is possible to use the simple assumption that the shape of the pores is still the same.erefore, for the pore shape factor, it is possible to set q p � constant and fibre shape factor q � constant and hence Subsequently, pore volume (19) can then be expressed as follows: Figure 3 shows the ratio of pores in the studied samples determined by the aforementioned method.
Advances in Materials Science and Engineering 2.5.Sample Preparation and Testing.Typically, 6% of the bre reinforcement is used in concrete materials.However, blending the plaster and bres with such a high content can cause some problems in their bonding.
e bre reinforcement content in this experiment was 1% (Figure 3 and Table 3) because the optimal improvement of mechanical properties as has been reported in a previous study in which the glass and basalt laments were used as the reinforcing elements was observed at weight ratios of 1%, 2%, and 3% [17].e calculation of the mixing ratio ( 20) is based on the percentage of the components in the composite and their densities: where w i (%) is the mass fraction of the ith component, V i (%) is the volume fraction of the ith component, and ρ i (kg•m −3 ) is the density of the ith component.Experimental samples (Figure 4) were created in circular releasable moulds with a diameter 99.8 mm (required for an impedance tube inner diameter of 100 mm at the tolerance eld K7) and a thickness of 10 mm. Figure 5 shows the microscopic pictures of all combinations of plaster and bre.e legend of Table 4 provides an overview of the photograph and material combination.

Experimental Measurement and Modelling.
Basically, the improvement of the acoustic absorption was rst veri ed by the addition of bres using a simple numerical model simulating the passage of sound waves through the plaster as well as bre-reinforced plaster.Even if the rst simpli ed model of inner porosity and all necessary mechanical properties were not precise, the simulation revealed that the principle should work in theory.is is a model of acoustic absorption using a porous acoustic open cell sample.In porous materials, sound propagates in a network of small interconnected pores.Because of small pore dimensions, losses occur particularly because of thermal conduction and viscous friction.
is model aims to characterise the absorption properties, more speci cally, the speci c surface impedance and absorption coe cient in terms of sound incidence angle and frequency.e model uses a 2D geometry of such a system.e models were created in COMSOL.Base models of the samples (plaster ermo UM, gypsum, and cement) were created using the Pressure Acoustics user interface with a rigid Biot equivalent uid model [42,43].e surrounding uid is air, and the incident pressure p inp is expressed as shown in the following equation: where θ and k 0 are the incidence angle and wavenumber in the free eld (air domain), respectively.e pressure p solved in this model is the total eld, and the scattered eld p scat is expressed as p scat p − p inp .Notably, this expression for the scattered eld is only valid in the air domain as the incident eld is not known apriori for porous materials.Two parameters that characterise the absorption properties of the porous absorber are the speci c surface impedance Z and the absorption coe cient α (Table 1).e absorption coe cient, which represents the ratio of the absorbed and incident energy, is de ned as follows: where R is the pressure re ection coe cient that expresses the ratio of the scattered to the incident pressure.Subsequently, geometric dimensions (sample diameter D 100 mm and sample thickness 10 mm) and material properties of the sample were determined and maintained constant (porosity, speed of sound, dynamic viscosity, thermal conductivity, and density).e modelled frequencies range from 100 Hz to 1.2 kHz at a minimal wavelength of 0.0344 m, and the absolute pressure is 1 bar.Figure 6 shows the layout of the modelled domain.An approach similar to the boundary conditions has been used according to the study reported by Cox and Antonio [44].For investigating the acoustic absorption, it is crucial to prepare samples with a precise cylindrical pro le such that they can accurately t into the clamping part of the impedance tube.As mentioned previously, three plasters (i.e., gypsum, cement and lime, resp.) and two bres (i.e., glass and basalt, resp.) have been tested.Based on the basic characteristics of the individual materials, the best results for acoustic absorption can be assumed to be achieved using basalt-bre-reinforced  Advances in Materials Science and Engineering lime cement according to the study by Wang and Torng [31].e reason for this assumption was that hollow silicate beads were present inside the mixture of lime-cement plaster, which basically serve as pores that render high thermal and acoustic properties.Figure 7 schematically shows the experimental apparatus, where the test was carried on a Brüel & Kjaer impedance tube 4206, with two microphones and ampli ers.
is device permits the measurement of α for perpendicularly impacting waves.
e impedance tube comprises a loudspeaker and two measuring microphones that are connected to the evaluation unit.Lengths of X 1 0.15 m, X 2 0.1 m, and Xv 10 mm re ect the actual thickness of tested sample.
As the measurement signal, pink noise was used.e socalled pink noise is a signal with a frequency spectrum such that the power spectral density (PSD, energy, or power per frequency interval) is proportionally inverse to the signal frequency [45].In the pink noise, each octave (halving/doubling in frequency) carries an equal amount of noise energy.Noise records with various statistical characteristics, for example, uniform and Gaussian distributions, can be generated.However, generally, this noise exhibits a "white" spectrum, implying that the noise retains its statistical characteristic at all frequencies.With respect to power at a constant bandwidth, pink noise falls o at 3 dB per octave.e pink or typically socalled 1/f noise is one of typical ambient noises in nature and transport or even could describe a owing river and beating heart [46].Gilden et al. have reported an unusual study where they described connection with human cognition [47].at is, when a person attempts to remember, for example, a temporal interval, some errors are inevitably associated with the estimate.e sequences of errors in spatial and temporal replications were found to uctuate as 1/f noises.As reported by McClain [45], researchers often create 1/f 2 noise as a mistake instead of their required results.e generation of 1/f noise involves amplitude scaling in the frequency domain by 1/√f and not just by 1/f itself.Generally, the magnitude of acoustic variables depends on the frequency f.In actual cases in which frequency spectra would be measured with a step df 1 Hz, their processing would be extremely complicated.In these cases, frequency bands with the percentually constant width are introduced.is condition is satis ed by octave or thirdoctave frequency bands.
Typically, ten octaves are characterised by f 2 /f 1 2, where indexes 2 and 1 represent the higher and lower frequencies, respectively.Each octave is marked by the middle frequency f m : e third-octave band is created by the octaves to their logarithmical thirds.erefore, 30 bands are obtained.
e frequencies f 1 and f 4 border the one octave band and f 2 and f 3 indicate the frequencies of the inner thirds of the octave.en, it is possible to claim that the log of inner frequencies ratio is equal to Based on (25), the ratio between any adjacent frequencies is constant.
Finally, the dependence between two consecutive lying middle frequencies of the third-octave bands is In our case, the third-octave bands were used to describe the acoustic absorption level.
During measurements, the actual distance between the microphones and sample must be considered relative to the speed of sound in the ambient medium, which was done on 1 mm en, H is equal to the multiplication of complex numbers K and distance x i .Based on those results and from the data measured on the two microphones at sampling ratios of 48 kHz and 4097 complex samples, the final value of the absorption coefficient can be obtained.During operation, it is crucial to find the real and imaginary parts of reflection, transfer function H 12 , phase shift φ 12 , and their exponential shape e jφ12 and set the absorption coefficient r i .Finally, the absorption coefficient η is computed as (29) or in the same manner only using a different formula (8):

Results and Discussion
e fundamental material properties of samples significantly affect sound absorption.In particular, one of the most important parameters is porosity.According to the FEM simulation, the best absorption should be observed for ermo UM (Figure 8).Figures 8-10 show the model results obtained from the sound absorption shown at a frequency of 1000 Hz.However, the model can be used at any frequency within the defined range.Based on the model results obtained in Figures 8-10, it is also possible to determine the effective thickness in which samples should be used.As can be observed for the cement sample (Figure 9), only ∼2/3 of the thickness effectively shield, thereby decreasing absorption.Hence, the structure of the material does not use the entire sample thickness with a constant acoustic attenuation efficiency.With the possible improvement in the effective absorption thickness of the samples and internal material attenuation, the fibres were added into FEM.In the model simulation of the gypsum sample, the effect of glass and basalt fibres was examined.Clearly, fibres significantly affect the total absorption (Figure 10).Basalt fibres, not glass fibres, are extremely suitable for increasing the sound absorption at a high thickness.However, this statement cannot be generalised because it depends on the combination of the plaster and fibres used.With other plasters, better results can be achieved with glass fibres and worse results were observed with the basalt fibres (Figure 11).Probably, compared simply to the fundamental physical properties, the extent to which the added fibre reinforcement can increase the inner porosity of the material used is key.Hence, in this case, it is not possible to strictly rely only on model simulation, but it is necessary to conduct experiments with actual reinforced materials.
At the start of measurements, the repeatability of the main process and sample behaviour were verified.For the same samples, repeatable results were obtained, without any significant deviation from several sets of samples.However, the testing process was extremely sensitive to the actual sample shape (Figure 9).In case of a gap between the specimen and inner tube wall, measurement results were overestimated on account of the undiluted waves as well as the occurrence of vibrations from the imperfectly touching parts.Figure 12 shows the comparison of two ideal samples (dashed) and one nonideally circular sample number 3 (straight grey line).A comparison of the FEM model revealed similar results obtained from the measurements; however, Figure 13 shows a more uniform course.Figures 14-16 show the dependence of the acoustic absorption of each tested plaster material on the type of fibres used.e best acoustic absorption was achieved by the basalt-fibre-reinforced cement plaster.For better clarity, the results are also shown in the third-octave band in Figure 17.
In addition, the cement plaster with no fibre reinforcement exhibited the worst acoustic absorption.On the other hand, a lime-cement plaster with thermal insulating properties exhibited good acoustic absorption, even without fibre reinforcement, which was related to the nature of the inner mixture.
Dry plaster mixture contained hollow silicate beads, which positively affected the acoustic properties of the mixture and led to increased attenuation (Figure 14).However, by the addition of a fibrous dispersion into this mixture, the acoustic properties were degraded.On the other hand, the most significant increase in acoustic absorption occurred by the addition of fibres into the cement plaster, which was predicted after image analysis because in the basic material, the inner sample structure was clearly homogeneous with minimum pores.erefore, it is not possible to state that the addition of a fibrous dispersion into plaster materials will generally improve their acoustic damping or favour a particular reinforcement that would be universal for all plasters.Randazzo et al. [14] have tested clay plasters, and their results revealed that the addition of natural fibres leads to increased open pores and subsequently improved acoustic absorption.With these results, it is possible to agree, but not generalise.Similar results were also reported by Neithalath et al. [20], who examined the effect of cellulose fibres on the acoustic absorption of cement composites.He found that the acoustic absorption coefficient increases with the volume of fibres in the composite.
is viewpoint can be slightly misleading because with a high fibre content, the mechanical properties are degraded, and the effectiveness of the increased attenuation is not proportional.Bentchikou et al. [23], who examined the mechanical properties of the resultant cement composite by the addition of cellulosic fibres to cement, were aware of this issue.eir results, similar to those discussed herein, revealed that the addition of a high amount of fibres generally leads to the decrease in the

Conclusion
Modern science does not emphasise only on the improvement of technologies, materials and their properties, but also focuses on the environment as well as the protection and comfort of human health.Hence, the e ect of natural bre reinforcement on the acoustic absorption of plaster composites is investigated herein.After the initial study, typical plasters were selected for experimental and numerical studies to determine the possibilities observed by the e ects of the addition of bre reinforcement on their acoustics properties.Lime cement, cement, and gypsum plasters were selected not only for their physical properties but also for their typical use in the construction industry under geographical conditions.Plaster materials are disadvantageous because of their low tensile strength, which decreases the plaster life and causes visible defects and cracks.After previous studies reported by Samková et al. [17] on the amount of materials and bres, glass and basalt bres were added to the plasters as 1% by weight ratio of the brous reinforcement to improve the mechanical properties.e acoustic attenuation of plaster composites was not considerably a ected by the mechanical properties of the bres, but it was a ected by nal structure of the plaster mixture, especially the amount, size, and pore structure.Both bres are typically used in the industry in di erent applications and forms, which is attributed to their good mechanical and physical properties, cost-e ectiveness, and medical benign nature.Another important conclusion was the dependence Surface: sound pressure level (dB) Advances in Materials Science and Engineering of the e ective attenuation on the material thickness in di erent plasters.at is, it is not true to generalise that the wider the plaster layer, the higher the acoustic attenuation will be proportionately (i.e., twice the layer thickness does not exhibit two times better absorption).Nevertheless, when the e ective thickness is known, it is possible to combine the suitable bre dispersion so as to achieve even better results with a signi cantly thin material layer, which could Advances in Materials Science and Engineering considerably lead to material and cost savings.As part of our future work, we intend to improve the existing numerical models, especially material models for reinforced plasters, and to extend their applications to electromagnetic shielding.

Figure 2 :
Figure 2: Results from the absorption coe cient of basalt layers from the study by Moretti et al. [30].

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Porosity of materials with the speci ed bre volume ratio.

Figure 5 :
Figure 5: Microstructure of all tested samples and scale used in micrometres.

Figure 5
Science and Engineeringcompressive and tensile strength.Hence, 1% of bre reinforcement is examined to improve the acoustic properties and prevent the degradation of mechanical properties in the tested materials.

Figure 9 :
Figure 9: FEM model with the cement sample.

Table 1 :
Mechanical properties of the base materials.

Table 3 :
Volume fraction of the individual components.

Table 4 :
e created combinations of matrix (plaster), dispersion (fibres), and number of their picture below.