**3.1 Acoustic textiles**

The definition of sound is a pressure change in air, water, or similar elastic medium, which can be perceived by the ear as a stimulus of hearing. Loudness and tone are two aspects of sound. Loudness is defined as the sound pressure expressed in decibel (dB), and tone is defined as the sound frequency expressed in Hertz (Hz). Human ear is not sensitive to all the sound frequencies, and depending on its frequency and intensity, the sound may or may not be audible to the human ear. The range of 20–20,000 Hz is the audible sound frequency for humans [4].

As the material or item is vibrated, the sound is created. These vibrations spread in solid, liquid, or gas medium in a waveform from the emitter to the receiver. Therefore, a sound wave is the transfer of energy radiated by a source material or an object into the medium as it travels. A sound wave is considered by its frequency, wavelength, and amplitude. The wave properties can change according to the interaction of sound waves with the receiver's surface and the properties of the receiver's object. The sound wave can be absorbed, transmitted, reflected, refracted, and diffracted from the surface [5].

The classic solution to the problem of noise pollution is to remove the noise from the source; however, it is not always possible to remove it in this way. Thus, noise isolation methods are usually used for the reduction of noise emission. For this purpose, two types of methods are used, defined as sound absorption and sound insulation, as shown in **Figure 1**. In sound absorption, air particles rub inside the insulating material, and with the conversion of the sound kinetic energy into heat energy, the sound power is reduced. Therefore, sound absorption refers to the absorption level of the sound from the sound source in the environment of the

**89**

noise pollution.

*Investigation of Sound Absorption Characteristics of Textile Materials Produced from Recycled…*

source. Since frictional and momentum losses and heat fluctuation occur as the sound goes through the sound absorbing material, these are the essential reasons of

Sound insulation deals with the transmission of sound between the adjacent rooms. Sound transmission loss indicates the sound insulation ability of a material and is denoted as the difference between the transmitted sound power and the sound power level of the incident wave [24]. An applying barrier to prevent transmission is the main principle. Materials that have high density such as thick glass,

Sound absorptive materials are known as the materials reducing the acoustic energy of a sound wave as the wave passes through it. They are usually used to soften the acoustic environment of a closed volume by reducing the amplitude of the reflected waves [25]. There are three types of absorbers: porous absorbers, membrane absorbers, and resonance absorbers. Inside the porous absorbers, sound transmission occurs in such a way that viscous and thermal effects cause acoustic energy to be dissipated. They are usually fuzzy, fibrous materials such as textiles, mineral wool, curtains, clothing, carpets, and certain types of foam plastic. As the sound energy penetrates into the material on hitting the surface, the soundabsorbing effect is obtained. Generally, since the required thickness is large with porous textile absorbers, obtaining adequate absorption at low frequencies is difficult. Membrane absorber is basically a flat box, 100–200 mm deep, installed in the wall with a thin sheet of plywood or similar on the front and with a light mineral wool filling the box cavity. Resonance absorber takes the form of perforated plasterboard, perforated metal corrugated sheets, and metal boxes. The working principle of the resonant absorbers is dissipating acoustic energy with structure vibration. Resonance absorbers are usually efficient in a thin tunable low-frequency band, whereas the porous absorbers are good for mid-to-high frequency range [26, 27]. Even though all materials absorb a part of incident sound, acoustic material term is used, especially for materials providing high values of absorption for the specific application fields. The acoustic materials are used in various applications such as in building construction; transportation; industries for specific purposes; such as concert rooms, schools, theaters, recording studios, lecture halls, and so forth. In recent days, acoustic textiles as acoustic materials are critical for the reduction of

The sound intensity, sound pressure levels, and sound classification systems can be used to assess the performance of the acoustic structure. Sound absorption defines the energy amount absorbed by the material and stated as sound absorption coefficient (α) ranging between 0 and 1, where 0 means no absorption and 1 means the highest or total absorption. The higher coefficient expresses lower reverberation

*DOI: http://dx.doi.org/10.5772/intechopen.92792*

the acoustic energy loss [2, 10].

*Sound isolation and absorption [23].*

**Figure 1.**

solid, metal, and brick are used for this aim [5].

*Investigation of Sound Absorption Characteristics of Textile Materials Produced from Recycled… DOI: http://dx.doi.org/10.5772/intechopen.92792*

**Figure 1.** *Sound isolation and absorption [23].*

*Waste in Textile and Leather Sectors*

**3.1 Acoustic textiles**

diffracted from the surface [5].

secondary recycling technology class [13, 20].

**3. Sound absorption characteristics of textile materials**

The definition of sound is a pressure change in air, water, or similar elastic medium, which can be perceived by the ear as a stimulus of hearing. Loudness and tone are two aspects of sound. Loudness is defined as the sound pressure expressed in decibel (dB), and tone is defined as the sound frequency expressed in Hertz (Hz). Human ear is not sensitive to all the sound frequencies, and depending on its frequency and intensity, the sound may or may not be audible to the human ear. The

As the material or item is vibrated, the sound is created. These vibrations spread

range of 20–20,000 Hz is the audible sound frequency for humans [4].

in solid, liquid, or gas medium in a waveform from the emitter to the receiver. Therefore, a sound wave is the transfer of energy radiated by a source material or an object into the medium as it travels. A sound wave is considered by its frequency, wavelength, and amplitude. The wave properties can change according to the interaction of sound waves with the receiver's surface and the properties of the receiver's object. The sound wave can be absorbed, transmitted, reflected, refracted, and

The classic solution to the problem of noise pollution is to remove the noise from the source; however, it is not always possible to remove it in this way. Thus, noise isolation methods are usually used for the reduction of noise emission. For this purpose, two types of methods are used, defined as sound absorption and sound insulation, as shown in **Figure 1**. In sound absorption, air particles rub inside the insulating material, and with the conversion of the sound kinetic energy into heat energy, the sound power is reduced. Therefore, sound absorption refers to the absorption level of the sound from the sound source in the environment of the

waste is included in a different application area than its original form and transformed into a new product having lower levels of physical, mechanical, and chemical properties. In tertiary recycling, pyrolysis occurs by turning gas into simple chemicals or fuels by gasification and hydrolysis. The fourth recycling process is realized by utilizing the heat released by incineration of solid wastes. The recycling method of the fibers differs on the type of fibers, for instance, synthetic fibers are chemically recycled, whereas all others are mechanically recycled [19]. Recycling of synthetic fibers such as polyester and polyamide is included in primary recycling technologies and is evaluated within the scope of the "closed loop recycling" class. Today, the most applied recycling is "open loop recycling," which belongs to the

Since recycled materials demonstrate good sound absorbing characteristics, these materials are becoming an interesting alternative to typical materials for functional applications. Acoustic barriers and acoustic ceilings, passenger vehicle noise absorbers, and wall claddings are some of the implementation of noise control functions of the nonwoven fabrics. Nonwoven fabrics made from recycled fibers are having more advantageous in terms of environmental pleasantness compared to traditionally utilized polyurethane foams produced by environmental harmful manufacturing methods and cannot be recycled [21]. Lower product cost, good processing, and environmental protection are some of the benefits of using recycled polyester nonwovens compared to conventional sound absorbers. Former studies about the noise absorption of nonwovens have indicated that the noise absorption coefficients of these materials in the high frequency range (*f* > 2000 Hz) are equivalent to that of the traditional sound absorbers such as rockwool and glass fiber [22].

**88**

source. Since frictional and momentum losses and heat fluctuation occur as the sound goes through the sound absorbing material, these are the essential reasons of the acoustic energy loss [2, 10].

Sound insulation deals with the transmission of sound between the adjacent rooms. Sound transmission loss indicates the sound insulation ability of a material and is denoted as the difference between the transmitted sound power and the sound power level of the incident wave [24]. An applying barrier to prevent transmission is the main principle. Materials that have high density such as thick glass, solid, metal, and brick are used for this aim [5].

Sound absorptive materials are known as the materials reducing the acoustic energy of a sound wave as the wave passes through it. They are usually used to soften the acoustic environment of a closed volume by reducing the amplitude of the reflected waves [25]. There are three types of absorbers: porous absorbers, membrane absorbers, and resonance absorbers. Inside the porous absorbers, sound transmission occurs in such a way that viscous and thermal effects cause acoustic energy to be dissipated. They are usually fuzzy, fibrous materials such as textiles, mineral wool, curtains, clothing, carpets, and certain types of foam plastic. As the sound energy penetrates into the material on hitting the surface, the soundabsorbing effect is obtained. Generally, since the required thickness is large with porous textile absorbers, obtaining adequate absorption at low frequencies is difficult. Membrane absorber is basically a flat box, 100–200 mm deep, installed in the wall with a thin sheet of plywood or similar on the front and with a light mineral wool filling the box cavity. Resonance absorber takes the form of perforated plasterboard, perforated metal corrugated sheets, and metal boxes. The working principle of the resonant absorbers is dissipating acoustic energy with structure vibration. Resonance absorbers are usually efficient in a thin tunable low-frequency band, whereas the porous absorbers are good for mid-to-high frequency range [26, 27].

Even though all materials absorb a part of incident sound, acoustic material term is used, especially for materials providing high values of absorption for the specific application fields. The acoustic materials are used in various applications such as in building construction; transportation; industries for specific purposes; such as concert rooms, schools, theaters, recording studios, lecture halls, and so forth. In recent days, acoustic textiles as acoustic materials are critical for the reduction of noise pollution.

The sound intensity, sound pressure levels, and sound classification systems can be used to assess the performance of the acoustic structure. Sound absorption defines the energy amount absorbed by the material and stated as sound absorption coefficient (α) ranging between 0 and 1, where 0 means no absorption and 1 means the highest or total absorption. The higher coefficient expresses lower reverberation time. The reverberation time is the time lag, in seconds, measured for the sound to decay by 60 dB in a space after a sound source has been stopped. The noise reduction coefficient is the average absorption coefficients of an acoustic material at a typical frequency set of 250, 512, 1024, and 2048 Hz defined according to the tube type and acoustic measuring instrument used for the tests [5, 24].

Nonwoven materials are ideal materials for sound insulation and sound absorption applications in order to decrease sound pollution in the environment due to their fibrous structure and high total surface area [28]. Areal density (mass), porosity, volumetric density, tortuosity, particle size distribution, and thickness constitute significant physical properties of nonwoven fabrics for acoustic applications. Acoustic ceilings, noise reducing quilts, and noise proof barriers are some of the applications of nonwoven fabrics serving as noise absorption elements. A wide variety of studies on the acoustic properties of nonwoven products are available, which some of them are given in the following paragraphs.

Lee and Joo investigated the usage of the recycled polyester nonwovens as a sound absorber instead of conventional materials such as glass wool and rockwool by using a two-microphone impedance tube. Nonwoven having more fine fiber was found to be better at contacting the sound wave due to more resistant characteristic. The nonwoven absorber having an unoriented web in the middle layer had a higher noise absorption coefficient (NAC) than the ones having entirely oriented web structure, but the difference was insignificant. The panel resonance effect had contributed to increase the noise absorption coefficient. In the case of coating structure, the panel promoted the NAC in low- and middle-frequency regions, but reverse effect was obtained in the high frequency region by the coincidence effect [22].

Na et al. measured the sound absorption features of the fabric-nonwoven system produced by adding a microfiber fabric to a layer of 15-, 30-, and 45-mm nonwoven by reverberation chamber method. The results revealed that the fabrics made from microfibers were quite advantageous compared to the conventional fabrics of the similar thickness or weight in terms of sound absorption characteristics [29].

Tascan and Vaughn studied the effects of total fiber surface area and fabric density on needle punched nonwovens. It was reported that the needle punched nonwoven fabrics produced from polyester fibers having octalobal and trilobal cross-sectional shape had better sound insulation results than the nonwoven fabrics produced from round fibers. Moreover, nonwovens with finer fibers in various cross-sectional shapes had better sound absorption and insulation than the ones made from coarser fibers. Fabric density and total fiber surface area in needle punched nonwoven fabrics were found to be in the tendency of improving fabric sound insulation [28].

Sengupta searched the effect of fabric type, density, the number of layers, source intensity, the distance of fabric from the receiver, the distance of the fabric from the sound source, and fiber type on the sound reduction of various needle-punched nonwoven fabrics. A sound insulation box was used for measurement. It was found that higher area density was one of the reasons of higher sound reduction. A negative correlation between the area density and bulk density of needle punched nonwoven and sound reduction was determined. Moreover, maximum sound reduction among jute, polypropylene, polyester, and other jute-polypropylene blended (3:1 and 1:3) nonwovens was obtained by jute-polypropylene (1:1) blend [30, 31].

Küçük and Korkmaz tested eight different nonwoven composites produced from various blends. Sound absorption properties, weight per unit area, thickness, and air permeability parameters of the samples were measured. An increase in sound absorption properties of the material was determined along with the increase in thickness and the decrease in air permeability. About 70% cotton and 30%

**91**

*Investigation of Sound Absorption Characteristics of Textile Materials Produced from Recycled…*

Recently, recycled nonwovens have become important sound absorption materials. Some of the studies related to sound absorption characteristic of the recycled

Seddeq et al. searched the acoustic properties of the nonwovens produced from recycled natural fibers blended with synthetic fibers and studied PET/cotton-wool (70/30), PP/cotton-wool (80/20), cotton/PET (50/50) blends and 100% jute fibers

nonwoven samples had high sound absorption coefficients, whereas at low frequencies (100–400 Hz), low sound absorption coefficients were obtained, and at mid frequencies (500–1600 Hz), better sound absorption coefficients were found. The increased thickness of nonwovens improved the sound absorption coefficients of

Carvalho et al. studied qualitative analysis of the acoustic insulation behavior of various thermo-bonded nonwoven fabrics. Nonwoven fabrics were produced from mineral wool and recycled fibers, containing a mixture of polypropylene, cotton, acrylic, and polyester. Some samples were laminated with aluminum foil. Thermobonded nonwovens with high thickness value and laminated with aluminum foil displayed better sound reduction performance than the other single-layered nonwovens made from recycled fibers. They showed better performance than the

Manning and Panneton studied the acoustic properties of post-consumer and industrial recycled fiber absorbers. Three different shoddy samples, needlepunched mat, thermally bonded mat, and resin-bonded mat, were compared in the study. A lower noise absorption coefficient value of 0.20 was obtained at low frequency (0–1000 Hz) for all samples. The results showed that the noise absorption coefficient values of the samples in frequency range of 0–4000 Hz were similar

Rey et al. designed novel green sound absorbing materials as a part of noise barriers. They used recycled textile materials and nontoxic binder fibers to manufacture the eco-materials. Acoustic characterization of noise barriers was measured in a small-scale reverberation room designed for the testing of small samples. New materials used in noise barrier prototypes performed very well in accordance with the international standards. The performance of them was comparable with those of commercially available noise barriers made of typical sound

Patnaik et al. studied the sound insulation properties of needle-punched samples produced from waste wool and recycled polyester fibers (r-PET) for building industrial applications. Waste wool fibers were mixed with r-PET fibers in 50/50 proportion to prepare needle-punched mats, and their acoustic properties were evaluated with other performance properties. Nonwoven mats were produced from waste wool fibers that are coring wool (CW) and Dorper wool (DW); r-PET fibers; and blended r-PET fibers in 50/50 proportion with these fibers (DWP, CWP). Good sound absorption properties in the overall frequency range (50–5700 Hz) were obtained for all nonwoven samples. The sound absorption was lower at low frequencies (50–1000 Hz) and increased from medium (1000–2000 Hz) to high frequency range (200–5700 Hz) for all the samples. The lowest noise absorption coefficient value was 0.61 for r-PET, and the highest was 0.75 for DWP for all frequency ranges.

and 2.53–5.64 mm. At high frequencies (2000–6300 Hz),

polyester nonwoven supplied the best sound absorption coefficient in the midto-high frequency ranges. It was revealed that the increase in the amount of fiber per unit area caused an increase in sound absorption of the material. The results indicated that acrylic and polypropylene addition into a cotton and polyester fiber mixture increased the sound absorption properties of the composite in the low- and

*DOI: http://dx.doi.org/10.5772/intechopen.92792*

nonwoven surfaces are given in the following paragraphs.

mid-frequency ranges [3].

in the range of 422–561 g/m2

to each other [33].

absorbing materials [34].

the materials at all frequency ranges [10].

nonwovens made from mineral wool as well [32].

## *Investigation of Sound Absorption Characteristics of Textile Materials Produced from Recycled… DOI: http://dx.doi.org/10.5772/intechopen.92792*

polyester nonwoven supplied the best sound absorption coefficient in the midto-high frequency ranges. It was revealed that the increase in the amount of fiber per unit area caused an increase in sound absorption of the material. The results indicated that acrylic and polypropylene addition into a cotton and polyester fiber mixture increased the sound absorption properties of the composite in the low- and mid-frequency ranges [3].

Recently, recycled nonwovens have become important sound absorption materials. Some of the studies related to sound absorption characteristic of the recycled nonwoven surfaces are given in the following paragraphs.

Seddeq et al. searched the acoustic properties of the nonwovens produced from recycled natural fibers blended with synthetic fibers and studied PET/cotton-wool (70/30), PP/cotton-wool (80/20), cotton/PET (50/50) blends and 100% jute fibers in the range of 422–561 g/m2 and 2.53–5.64 mm. At high frequencies (2000–6300 Hz), nonwoven samples had high sound absorption coefficients, whereas at low frequencies (100–400 Hz), low sound absorption coefficients were obtained, and at mid frequencies (500–1600 Hz), better sound absorption coefficients were found. The increased thickness of nonwovens improved the sound absorption coefficients of the materials at all frequency ranges [10].

Carvalho et al. studied qualitative analysis of the acoustic insulation behavior of various thermo-bonded nonwoven fabrics. Nonwoven fabrics were produced from mineral wool and recycled fibers, containing a mixture of polypropylene, cotton, acrylic, and polyester. Some samples were laminated with aluminum foil. Thermobonded nonwovens with high thickness value and laminated with aluminum foil displayed better sound reduction performance than the other single-layered nonwovens made from recycled fibers. They showed better performance than the nonwovens made from mineral wool as well [32].

Manning and Panneton studied the acoustic properties of post-consumer and industrial recycled fiber absorbers. Three different shoddy samples, needlepunched mat, thermally bonded mat, and resin-bonded mat, were compared in the study. A lower noise absorption coefficient value of 0.20 was obtained at low frequency (0–1000 Hz) for all samples. The results showed that the noise absorption coefficient values of the samples in frequency range of 0–4000 Hz were similar to each other [33].

Rey et al. designed novel green sound absorbing materials as a part of noise barriers. They used recycled textile materials and nontoxic binder fibers to manufacture the eco-materials. Acoustic characterization of noise barriers was measured in a small-scale reverberation room designed for the testing of small samples. New materials used in noise barrier prototypes performed very well in accordance with the international standards. The performance of them was comparable with those of commercially available noise barriers made of typical sound absorbing materials [34].

Patnaik et al. studied the sound insulation properties of needle-punched samples produced from waste wool and recycled polyester fibers (r-PET) for building industrial applications. Waste wool fibers were mixed with r-PET fibers in 50/50 proportion to prepare needle-punched mats, and their acoustic properties were evaluated with other performance properties. Nonwoven mats were produced from waste wool fibers that are coring wool (CW) and Dorper wool (DW); r-PET fibers; and blended r-PET fibers in 50/50 proportion with these fibers (DWP, CWP). Good sound absorption properties in the overall frequency range (50–5700 Hz) were obtained for all nonwoven samples. The sound absorption was lower at low frequencies (50–1000 Hz) and increased from medium (1000–2000 Hz) to high frequency range (200–5700 Hz) for all the samples. The lowest noise absorption coefficient value was 0.61 for r-PET, and the highest was 0.75 for DWP for all frequency ranges.

*Waste in Textile and Leather Sectors*

time. The reverberation time is the time lag, in seconds, measured for the sound to decay by 60 dB in a space after a sound source has been stopped. The noise reduction coefficient is the average absorption coefficients of an acoustic material at a typical frequency set of 250, 512, 1024, and 2048 Hz defined according to the tube

Nonwoven materials are ideal materials for sound insulation and sound absorption applications in order to decrease sound pollution in the environment due to their fibrous structure and high total surface area [28]. Areal density (mass), porosity, volumetric density, tortuosity, particle size distribution, and thickness constitute significant physical properties of nonwoven fabrics for acoustic applications. Acoustic ceilings, noise reducing quilts, and noise proof barriers are some of the applications of nonwoven fabrics serving as noise absorption elements. A wide variety of studies on the acoustic properties of nonwoven products are available, which some of them are given in the following paragraphs. Lee and Joo investigated the usage of the recycled polyester nonwovens as a sound absorber instead of conventional materials such as glass wool and rockwool by using a two-microphone impedance tube. Nonwoven having more fine fiber was found to be better at contacting the sound wave due to more resistant characteristic. The nonwoven absorber having an unoriented web in the middle layer had a higher noise absorption coefficient (NAC) than the ones having entirely oriented web structure, but the difference was insignificant. The panel resonance effect had contributed to increase the noise absorption coefficient. In the case of coating structure, the panel promoted the NAC in low- and middle-frequency regions, but reverse effect was obtained in the high frequency region by the coincidence

Na et al. measured the sound absorption features of the fabric-nonwoven system produced by adding a microfiber fabric to a layer of 15-, 30-, and 45-mm nonwoven by reverberation chamber method. The results revealed that the fabrics made from microfibers were quite advantageous compared to the conventional fabrics of the similar thickness or weight in terms of sound absorption characteristics [29]. Tascan and Vaughn studied the effects of total fiber surface area and fabric density on needle punched nonwovens. It was reported that the needle punched nonwoven fabrics produced from polyester fibers having octalobal and trilobal cross-sectional shape had better sound insulation results than the nonwoven fabrics produced from round fibers. Moreover, nonwovens with finer fibers in various cross-sectional shapes had better sound absorption and insulation than the ones made from coarser fibers. Fabric density and total fiber surface area in needle punched nonwoven fabrics were found to be in the tendency of improving fabric

Sengupta searched the effect of fabric type, density, the number of layers, source intensity, the distance of fabric from the receiver, the distance of the fabric from the sound source, and fiber type on the sound reduction of various needle-punched nonwoven fabrics. A sound insulation box was used for measurement. It was found that higher area density was one of the reasons of higher sound reduction. A negative correlation between the area density and bulk density of needle punched nonwoven and sound reduction was determined. Moreover, maximum sound reduction among jute, polypropylene, polyester, and other jute-polypropylene blended (3:1 and 1:3)

Küçük and Korkmaz tested eight different nonwoven composites produced from various blends. Sound absorption properties, weight per unit area, thickness, and air permeability parameters of the samples were measured. An increase in sound absorption properties of the material was determined along with the increase in thickness and the decrease in air permeability. About 70% cotton and 30%

nonwovens was obtained by jute-polypropylene (1:1) blend [30, 31].

type and acoustic measuring instrument used for the tests [5, 24].

**90**

effect [22].

sound insulation [28].

DWP presented higher *α* value than CWP due to the presence of longer fiber length. It was stated that sound absorption depends on thickness of the material among other factors. They also determined that the r-PET/wool mats (DWP, CWP) could absorb more than 70% of the incident noise in the overall frequency range [35].

Kalebek investigated acoustic behavior of needle-punched nonwoven fabrics produced from recycled PES fibers for the automotive industry. The physical properties such as density, thickness, weight per unit area, air permeability, tensile strength, and elongation were measured and compared to each other. The fabric mass per unit area and the thickness of the needle-punched nonwoven fabrics were found to be positively effective on the sound insulation. Additionally, it was observed that higher air permeability caused higher sound transmission and, as a result, lower sound insulation [30].

### **3.2 Measurement methods of acoustic absorption properties of materials**

There are various standards related to test procedures for determination of acoustic features of textile materials. In terms of standards, sound absorption properties of nonwovens can be defined in different parameters such as sound absorption coefficient, transmission coefficient, reflection coefficient, sound transmission loss, airflow resistivity, and sound power ratio. Some of the commonly used standards are as follows: ISO 354:07 Acoustics—measurement of sound absorption in a reverberation room; ISO 11957:2009 Acoustics—determination of sound insulation performance of cabins (laboratory and in situ measurements); ISO 10534-1:96 Acoustics—determination of sound absorption coefficient and impedance in impedance tubes—Part 1: method using standing wave ratio; ISO 10534-2:98 Acoustics—determination of sound absorption coefficient and impedance in an impedance tube—Part 2: transfer-function method; ASTM E2611-19 Standard test method for measurement of normal incidence sound transmission of acoustic material based on the transfer matrix method; ASTM E1050-19 Standard test method for impedance and absorption of acoustic materials using a tube, two microphones, and digital frequency analysis system; and ASTM C423-17 Standard test method for sound absorption and sound absorption coefficients by the reverberation room method.

The impedance tube method and alpha cabinet methods, which are used in experimental part, will be explained in detail in the following paragraphs.
