*3.2.1 Measurement of acoustic absorption characteristics by impedance tube method*

One of the methods mostly utilized to determine the sound absorption coefficients of textile surfaces is the ISO 10534-2 double microphone impedance tube method. The sound pressure difference with the help of a microphone placed in the impedance tube is measured. The sound is created by the signal generator in the apparatus to define the sound absorption coefficient and transmitted through the impedance tube. The performance of the material is examined by the software.

The measurement principle of the impedance tube method depends on measuring the reflected sound wave and calculating the sound absorption coefficient from the surface impedance value and transfer function. Obtaining the surface impedance and sound absorption coefficient values in one measurement for each frequency generates the advantage of this method [36].

During the measurement, sound is generated by the sound source, and the receiving decibels are evaluated by the decibel meter with and without the sample, and subsequently, the sound insulation by the fabric samples is calculated [37]. The tube is produced by rigid, transparent, or opaque materials to limit the sound within

**93**

range of 50 Hz to 6.4 kHz.

2 π f/c (m<sup>−</sup><sup>1</sup>

**Figure 2.**

*r* = |*r*| *e<sup>j</sup><sup>r</sup>*

*α* = 1 − |*r*|

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

the tube along one direction toward the direction of transmission. Consequently, it simplifies the three-dimensional wave equations to one-dimensional wave equation.

In the impedance tube arrangement schematically shown in **Figure 2**, first, the test sample is attached at one end of the impedance tube, and the signal characterizing the voice is generated by the software of test system. Plane waves are created in the tube by sound source emitting random or pseudo-random cycle, and pressure is measured at two places close to the sample. Then, the signal passing across the amplifier is transformed into a planar progressive sound wave in the tube through the speaker. The transfer function between two microphones is a ratio of the pressure values measured separately from the two microphones. The transfer function is related to the value of the reflection factor and the value of the sound absorption coefficient of that frequency (*R*) is obtained from this factor [36]. The frequency scale is based on the diameter of the tube and the gap between the two microphones. With the impedance tube method, the sound absorption coefficient values of the materials can be measured in the frequency

The sample size is small compared to reverberation method [38].

The normal incidence reflection factor is computed by Eq. (1).

*p*2/*p*1 = *S*12/*S*21; and *H*R and *H*I are the real and imaginary part of *H*12.

 = \_ *H*<sup>12</sup> − *H*<sup>1</sup> *HR* − *H*<sup>12</sup>

where *r* is a reflection factor of normal incidence; *x*1 is the distance between the sample and the further microphone location; *j* is the square root of minus one; *k* is

is the transfer function from microphone one to two, defined by the complex ratio

The sound absorption coefficient (*α*) is calculated by Eq. (2) [39, 40]:

*3.2.2 Measurement of acoustic absorption characteristics by alpha cabinet method*

ISO 354 and ASTM C423-17 standards explain the measurement method of sound absorption by the reverberation room. In reverberation room, reverberant sound field closely approaches a diffuse sound field. This approach occurs, when the source is on, that is the stable condition and also after the sound source

); *Фr* is the phase angle of the normal incidence reflection factor; *H*<sup>12</sup>

2

*e*<sup>2</sup>*jk*0*x*<sup>1</sup> (1)

. (2)

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

*Impedance tube sound absorption measurement method [41].*

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

**Figure 2.** *Impedance tube sound absorption measurement method [41].*

*Waste in Textile and Leather Sectors*

result, lower sound insulation [30].

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

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

sound absorption coefficients by the reverberation room method.

The performance of the material is examined by the software.

frequency generates the advantage of this method [36].

The impedance tube method and alpha cabinet methods, which are used in

One of the methods mostly utilized to determine the sound absorption coefficients

of textile surfaces is the ISO 10534-2 double microphone impedance tube method. The sound pressure difference with the help of a microphone placed in the impedance tube is measured. The sound is created by the signal generator in the apparatus to define the sound absorption coefficient and transmitted through the impedance tube.

The measurement principle of the impedance tube method depends on measuring the reflected sound wave and calculating the sound absorption coefficient from the surface impedance value and transfer function. Obtaining the surface impedance and sound absorption coefficient values in one measurement for each

During the measurement, sound is generated by the sound source, and the receiving decibels are evaluated by the decibel meter with and without the sample, and subsequently, the sound insulation by the fabric samples is calculated [37]. The tube is produced by rigid, transparent, or opaque materials to limit the sound within

experimental part, will be explained in detail in the following paragraphs.

*3.2.1 Measurement of acoustic absorption characteristics by impedance tube* 

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

**92**

*method*

the tube along one direction toward the direction of transmission. Consequently, it simplifies the three-dimensional wave equations to one-dimensional wave equation. The sample size is small compared to reverberation method [38].

In the impedance tube arrangement schematically shown in **Figure 2**, first, the test sample is attached at one end of the impedance tube, and the signal characterizing the voice is generated by the software of test system. Plane waves are created in the tube by sound source emitting random or pseudo-random cycle, and pressure is measured at two places close to the sample. Then, the signal passing across the amplifier is transformed into a planar progressive sound wave in the tube through the speaker. The transfer function between two microphones is a ratio of the pressure values measured separately from the two microphones. The transfer function is related to the value of the reflection factor and the value of the sound absorption coefficient of that frequency (*R*) is obtained from this factor [36]. The frequency scale is based on the diameter of the tube and the gap between the two microphones. With the impedance tube method, the sound absorption coefficient values of the materials can be measured in the frequency range of 50 Hz to 6.4 kHz.

The normal incidence reflection factor is computed by Eq. (1).

age of 50 Hz to 6.4 kHz.

The normal incidence reflection factor is computed by Eq. (1).

$$r = \left| r \right| e^{j\omega} = \frac{H\_{12} - H\_1}{H\_R - H\_{12}} e^{j\omega\_{0X}}\tag{1}$$

where *r* is a reflection factor of normal incidence; *x*1 is the distance between the sample and the further microphone location; *j* is the square root of minus one; *k* is 2 π f/c (m<sup>−</sup><sup>1</sup> ); *Фr* is the phase angle of the normal incidence reflection factor; *H*<sup>12</sup> is the transfer function from microphone one to two, defined by the complex ratio *p*2/*p*1 = *S*12/*S*21; and *H*R and *H*I are the real and imaginary part of *H*12.

The sound absorption coefficient (*α*) is calculated by Eq. (2) [39, 40]:

$$
\alpha = \mathbb{1} - \left| \mathbf{r} \right|^2. \tag{2}
$$

#### *3.2.2 Measurement of acoustic absorption characteristics by alpha cabinet method*

ISO 354 and ASTM C423-17 standards explain the measurement method of sound absorption by the reverberation room. In reverberation room, reverberant sound field closely approaches a diffuse sound field. This approach occurs, when the source is on, that is the stable condition and also after the sound source is stopped, which is defined as decomposition condition. The room is isolated adequately to keep outside noises and structural vibrations from impeding during the measurements (ASTM C423-17).

Reverberation room is naturally large empty room having long reverberation times. The volume of the room is higher than 200 m3 , and it has nonparallel wall and ceiling sides. An exactly truly reverberant room is designed such that energy from a noise source is diffused throughout the room to keep the sound pressure level the same everywhere. Absorption coefficients of a material can be computed with and without the material by measuring reverberation times in a reverberation room [42].

There are different types of cabinets used to measure acoustic sound absorption characteristics on the market. Some of the cabin examples are as follows: Rieter Alpha Cabin Instructions: Technical Note 591; Toyota Engineering Standard: test method for acoustic materials, TSL0600G; Renault Test Method: fibrous and cellular material sound absorption in diffuser field, D49-1977-B; and the design of small reverberation chambers for transmission loss measurements [36].

The name of alpha cabinet comes from the name of the sound absorption coefficient "alpha," and their structure is like a miniature reverberation room (**Figure 3**). During the measurements, the relative humidity in the alpha cabinet is 55%, and the temperature is set at 25°C. Before testing in the alpha cabinet, it is necessary to compare the noise levels in the cabinet with the background noise levels from outside (motor noise and electrical noise), while the speakers are in operation. The difference between these levels should be at least 45 dB. Alpha cabinets are quite similar to the reverberation rooms (**Figure 4**).

Sabine's reverberation formula is used for determining the material absorption coefficients with the help of reverberation rooms and is still commonly accepted as a very practical evaluation method for the reverberation time in rooms. Sabin is a unit of sound absorption in square meter, indicating the area of open window. The main principle of the method is that sound energy moving in the direction of an open window in a room will not be reflected at all; however, it entirely disappears in the open air outside. The same effect will be obtained if the open window would be replaced with 100% absorbing material in the same sizes [45].

The Sabine formula is applied for the sound absorption coefficient measurements, and the coefficient is analyzed by taking the difference between the resonance times of the sampled and nonsampled measurements in the cabinet [36]. *<sup>α</sup>* = \_

$$\text{ни попазапреси пасамисисисиси и исе саошиси } \lfloor \gg \rfloor.$$

$$\alpha = \frac{0.163 \times V}{S} \left( \frac{1}{TR} - \frac{1}{TR\_0} \right) \times C \tag{3}$$

where *V* is the cabinet volume, *S* is the sample area, *TR* is the sound receiving time with the test sample inside the cabinet, *TR*0 is the sound receiving time without the test sample inside the cabinet, and *C* is the cabinet correction coefficient.

Sabine also derived a definition for the time, *T*, of the residual sound to decay below the audible intensity, starting from 1,000,000 times higher initial amount given in Eq. (4).

$$T = \mathbf{0.161 V/A} \tag{4}$$

**95**

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

value of NRC indicates the perfect reflection. The NRC value 1 means that the perfect absorption happens. Reverberation room techniques can also be used to

When studies carried out in the literature are examined, it can be concluded that impedance tube measurements and alpha cabinet measurements do not give comparable results, as also found in this study. Therefore, correlation analysis between

Today, while studies on new sound insulating materials continue intensively, the usage of recycled products in this area is also increasing. Recycling technology provides environmental, social, and economic benefits. With environmental awareness and increased sensitivity to sustainability, recycled materials provide significant

In this study, nonwoven materials produced from conventional polyester (PES), mechanically recycled polyester (rm-PES), recycled polyester (r-PET), polypropylene (PP), and recycled polypropylene (r-PP) were compared with each other in terms of their sound absorption characteristics [47]. In **Figure 5**, photographs of the

Supplying the raw materials and producing the nonwoven fabrics were carried out in a Turkish company working especially on recycled products. The processes

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

measure this parameter [5].

*Twin mini reverberation cabinets [43].*

*Alpha cabinet measurement arrangement [44].*

**Figure 4.**

**Figure 3.**

the results is not necessary.

**4. Experimental part**

samples are given.

advantages for sound insulation.

where *V* is the room volume in m3 and *A* is the total absorption area in m<sup>2</sup> [45, 46].

The noise reduction coefficient (NRC) is calculated by deriving the arithmetic mean of the absorption coefficients in the 250, 500, 1000, and 2000 Hz 1/3-octave frequency bands. This number is rounded to the nearest multiple of 0.05. The zero

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

**Figure 3.** *Twin mini reverberation cabinets [43].*

*Waste in Textile and Leather Sectors*

room [42].

the measurements (ASTM C423-17).

times. The volume of the room is higher than 200 m3

similar to the reverberation rooms (**Figure 4**).

*<sup>α</sup>* = \_

where *V* is the room volume in m3

reverberation chambers for transmission loss measurements [36].

replaced with 100% absorbing material in the same sizes [45].

is stopped, which is defined as decomposition condition. The room is isolated adequately to keep outside noises and structural vibrations from impeding during

Reverberation room is naturally large empty room having long reverberation

There are different types of cabinets used to measure acoustic sound absorption characteristics on the market. Some of the cabin examples are as follows: Rieter Alpha Cabin Instructions: Technical Note 591; Toyota Engineering Standard: test method for acoustic materials, TSL0600G; Renault Test Method: fibrous and cellular material sound absorption in diffuser field, D49-1977-B; and the design of small

The name of alpha cabinet comes from the name of the sound absorption coefficient "alpha," and their structure is like a miniature reverberation room (**Figure 3**). During the measurements, the relative humidity in the alpha cabinet is 55%, and the temperature is set at 25°C. Before testing in the alpha cabinet, it is necessary to compare the noise levels in the cabinet with the background noise levels from outside (motor noise and electrical noise), while the speakers are in operation. The difference between these levels should be at least 45 dB. Alpha cabinets are quite

Sabine's reverberation formula is used for determining the material absorption coefficients with the help of reverberation rooms and is still commonly accepted as a very practical evaluation method for the reverberation time in rooms. Sabin is a unit of sound absorption in square meter, indicating the area of open window. The main principle of the method is that sound energy moving in the direction of an open window in a room will not be reflected at all; however, it entirely disappears in the open air outside. The same effect will be obtained if the open window would be

The Sabine formula is applied for the sound absorption coefficient measurements, and the coefficient is analyzed by taking the difference between the resonance times of the sampled and nonsampled measurements in the cabinet [36].

> \_1 *TR* <sup>−</sup>

where *V* is the cabinet volume, *S* is the sample area, *TR* is the sound receiving time with the test sample inside the cabinet, *TR*0 is the sound receiving time without the test sample inside the cabinet, and *C* is the cabinet correction coefficient. Sabine also derived a definition for the time, *T*, of the residual sound to decay below the audible intensity, starting from 1,000,000 times higher initial amount

*T* = 0.161 *V*/*A* (4)

The noise reduction coefficient (NRC) is calculated by deriving the arithmetic mean of the absorption coefficients in the 250, 500, 1000, and 2000 Hz 1/3-octave frequency bands. This number is rounded to the nearest multiple of 0.05. The zero

\_1 *TR*<sup>0</sup> 

and *A* is the total absorption area in m<sup>2</sup>

) × *C* (3)

0.163 × *V S* (

and ceiling sides. An exactly truly reverberant room is designed such that energy from a noise source is diffused throughout the room to keep the sound pressure level the same everywhere. Absorption coefficients of a material can be computed with and without the material by measuring reverberation times in a reverberation

, and it has nonparallel wall

**94**

given in Eq. (4).

[45, 46].

**Figure 4.** *Alpha cabinet measurement arrangement [44].*

value of NRC indicates the perfect reflection. The NRC value 1 means that the perfect absorption happens. Reverberation room techniques can also be used to measure this parameter [5].

When studies carried out in the literature are examined, it can be concluded that impedance tube measurements and alpha cabinet measurements do not give comparable results, as also found in this study. Therefore, correlation analysis between the results is not necessary.
