PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Advanced investigation using the EApu method on theeffect of quantitation and particle size of charcoal in clay bricks
on sound absorption coefficient
Siwat Lawanwadeekul1; Toru Otsuru2; Reiji Tomiku2; and Hiroyasu Nishiguchi2
1
2

Graduate School of Engineering, Oita University, 700 Dannoharu, Oita, 870–1192 Japan

Faculty of Science and Technology, Oita University, 700 Dannoharu, Oita, 870–1192 Japan

ABSTRACTDue to rapid urbanization, noise pollution becomes the environmental problems in Asia and theirharmful effects on people's health are undeniable. Reducing noise by absorption material is a goodalternative. Bricks are one of the most common construction supplies in Asia because of the cost.
Therefore, this study aims to apply for the authors' EApu method (sound absorption characteristics
measurement method using ensemble averaging technique and a pressure -velocity sensor) to clay
bricks to clarify the relationship between the sound absorption coefficient and the amount and size ofan additive. The measurement was conducted in a reverberation room following the procedure
presented in our previous papers. Measured bricks have four types of porous that were de signed by
selecting from different sizes of charcoal that was added into the specimens in the amounts of 0 30%wt. Next, samples were fired at 1100 ̊C and tested for sound absorption coefficient. Lastly, themicrostructure analysis was conducted. The results indicated that the larger size and higher percentage
of charcoal added in fired clay bricks presented the excellent absorption coefficient. Such a detailed
investigation can be performed by the EApu method, and the EApu method helps material designing
process become more accessible to control sound absorption.
Keywords: Quantitation, Particle size, Clay bricks, Sound absorption coefficient, and EApu method.

1. INTRODUCTION
Noise pollution is one of the most severe pollutions found in the world. Reducing noise by using
absorption materials are a good option and have many research guarantee as well as an economic
viewpoint on the manufactures in an around South East Asian countries that leads us to startinvestigating the sound absorption characteristics of clay bricks.
Porous sound absorption bricks are composed of channels, cracks, or cavities, which allow thesound waves to enter the materials. Sound energy is dissipated by thermal loss caused by the frictionof air molecules within the pore walls, and viscous loss brought the viscously of airflow within thematerials. These energy consumption principles endow porous materials with broad frequency bandfor sound absorption (1, 2).
Two techniques commonly used to perform as measurements sound absorption coefficients ar e areverberation room method (ISO 354:2006) (3) and an Impedance tube method (10534-2:1998) (4). Thesemethods are not suitable for materials under development. The reverberation room method determinedthe dimensions of samples should be the minimum and maximum sample size as 10m 2 – 12m2
respectively, while impedance tube determines are tightly fixed to one end of the tube during themeasurement. It found that the size and shape of samples are very problematic to form clay brick . Thus,
the EApu method was adapted to this measurement.
1
2

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v18f2003@oita-u.ac.jp
otsuru@oita-u.ac.jp, tomiku-reiji@oita-u.ac.jp, and nishiguc@oita-u.ac.jp

Firstly, Takahashi et al . (5 ) proposed “EA method ” as to measure materials ’ sound absorption
characteristics, surface normal impedance and absorption coefficient, in - situ using a two - microphone
utilizing environmental ambient noise . Next, Otsuru et al . (6 ) resumed the method using a pressure velocity sensor, pu - sensor, and showed that the ensemble averaging technique serves to obtain stablesound absorption characteristics eliminating the material ’ s edge- effect . Then, our recent paper (7)presented an improved calibration method of pu -sensor using acoustical tube taking account of relative
humidity change . From the calibration by using the EA method and using a pu -sensor, it was found the
EApu method have excellent reproducibility .
Herein, EApu method is applied onto clay bricks and clarify the relationship of this material, soundabsorption coefficient, and physical properties. Meanwhile, an adequate averaging number for thematerial is discussed because individual differences between bricks as well as positions are inevitable
for standard economic clay bricks.

2. MATERIALS AND METHODS
2.1 Raw Materials, Forming and Sintering Process
The local clay used as raw material was obtained from Ko Kha district in Lampang province,
northern of Thailand. Then, the local clay was grounded for 8 hours in a ball mill and passed a sievewith 230 mesh in order to determine the extent of the pore-forming effected by using charcoal. The
charcoal was a dry sieve and separated into four different sizes; large (L): 2 mm, medium (M): 1.5mm, small (S): 0.5 mm, and (F) fineness: less than 0.05 mm. Next, the prepared charcoal powder wasadded into the local clay brick with seven different percentages: 0, 5, 10, 15, 20, 25 and 30 percent of
the total weight (%wt). After that, the specimens were formed by pressing. Each group of green
specimens was fired at 1100 degree Celsius with two hours soaking time in the electric kiln furnace.
The specimens were natural cool down to room temperature in the furnace. Finally, the specimens
were tested physical properties, sound absorption coefficient, and microstructure analysis to find acorrelation between properties.
2.2

Test and Specimens Description

Table 1 – Description testing method and specimens dimension which explain in results and discussion
Result and discussion
3.1
3.2

Test
EApu Method
EApu and Tube Methods

Specimens Dimensions
100×100×10 mm
ø10×5 mm

2.3 Microstructure Analysis, Physical Properties Testing and Multivariate Linear Model
The Microstructure Analysis was done by using optical microscopy. The Physical properties test
method was carried out under the standard of Thai Industrial Standard (TISI 77-2545) to determine thewater absorption (8) while the method based on ASTM C373-88 was used to determine the bulk density,and apparent porosity (9).
The physical properties data were analyzed to find the relationship between physical properties
and absorption coefficient from the EApu method in term of noise reduction coefficient (NRC).
Statistics, commonly known as simple linear regression (SLR) and multiple linear regression (MLR)
was calculated for this research.
2.4 EApu Measurement Setup
The EApu method configuration has followed in our previous papers (7,11,12): a pu-sensor(Microflown; PR-900782) was placed 1 cm above the material’s surface and is plugged into a 2ch-FFT
instrument (B&K; 3160-A-042). All measurements were conducted in the reverberation room having a
volume of 168m3 at Oita University. The incoherent filtered white noises of 100–3,150 Hz emitted
from four loudspeakers (Fostex; FE-103E) mounted in a wooden box and a sub-woofer (JVC; SXDW77). All loudspeakers and a subwoofer were distributed on the floor close to the walls of thereverberation room.

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In a measurement following the EApu method, the 2ch-FFT instrument calculates a material’s
ensemble averaged surface normal impedance Z EA as the transfer function between particle velocity
v and sound pressure p. Then, the corresponding sound absorption coefficient α EA was calculated byusing the following equation:௓‫ן‬ா஺ ൌ ͳ െ ቚ ಶಲିఘ௖ ଶ௓ಶಲ ାఘ௖

(1)ቚ

Where, ρc is air density, and speed of sound, respectively.

Figure 1 – the sound absorption coefficient method measurement setup.

3. RESULTS AND DISCUSSION
3.1

The Microstructure Analysis and Physical Properties
Figure 2 showed the surface of the clay bricks that charcoal size contents increased with thesame magnification; the position on these optical microscopy images was the spot of pu - sensor
measurement. The fired samples presented the different porous sizes on the surface after being fired
at 1100 degree Celsius. The specimens with large charcoal size also showed great visible pores about
1. 5- 0. 8 mm, and when the size decreased, the pores were being smaller to 0 . 7- 0. 3 mm and 0. 3-0. 08
mm respectively. Fineness size and none charcoal added could not appreciate the porous because
sintering behavior made the clay particle closer.

L30 1100

M30 1100

F30 1100

S30 1100

None 1100

Figure 2 – microstructure analysis of clay brick

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The data shown in Table 2 demonstrated the physical properties of clay, both none added and added
charcoal 30% in the different sizes. The bulk density of specimens was decreased after increasing theamounts of charcoal from 0%wt to 30%wt, and it was decreased as charcoal size rose. The results
show that bulk density in the ranges of 0.87–1.84 g/cm3 . The bulk density is not only related toapparent porosity but also related to the water absorption because of the porous inside of bricks.
Table 2 – Physical properties of clay brick both none added and added charcoal 30 %Charcoal Size
None
F
S
M
L

Bulk density
1.84 g/cm3
1.44 g/cm3
1.08 g/cm3
1.09 g/cm3
0.89 g/cm3

Apparent porosity
10.24 %
27.99 %
35.99 %
38.37 %
42.13 %

Water absorption
6.37 %
23.46 %
26.27 %
35.33 %
35.40 %

Pore size
0.3-0.08 mm
0.7-0.3 mm
1.5-0.8 mm

3.2 Effect of Quantitation and Particle Size of Charcoal on Sound Absorption
Coefficient
The sound absorption coefficient was measured by the EApu method at three positions, and eachmeasurement was repeated twice. Hereafter, mean values and standard deviations of the six times
measurement were focused on the comparison. The mean values of the sound absorption coefficientcompared among different sizes, and the amount showed in Figure 3 ( a) . The horizontal axis is afrequency, and the vertical axis is a sound absorption coefficient — the large size and high percentage
of charcoal added and fired at 1100 degree Celsius demonstrate a good result, with approximate 0. 30
while the second most numerous is the medium size the same as amount and temperature, approximate
0.15. As a result, the sound absorption coefficient corresponds to the sign of both size and amount. As
we know well, the sound absorbability can be characterized by the available sound absorption
coefficient. The main influencing factors are physical properties (apparent porosity, water absorption,and bulk density) ( 13) . When sound waves impinge on porous material, they propagate into theinterstices in which a part of the sound energy that was dissipated by frictional and viscous losses
within the porous.

Figure 3 – (a) the mean values of sound absorption coefficient, and (b) the standard deviation
The standard deviation values of the clay bricks in the reverberation room are depicted in figure 3(b). In this study, the standard deviations of specimens are tiny, from 0.01 to 0.03. i.e., all the uncertainty
values stay less than 0. 04 ( 14) . It means that the specimens were not distinct in measuring theabsorption materials by using the EApu method.

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A comparison of the relationship between the sound absorption coefficient presented in term ofnoise reduction coefficient and the physical properties data collected from the samples with measured
the bulk density and apparent porosity demonstrated in figure 4. A correlation of data between noise
reduction coefficients and bulk density is evident, however when it compared with water absorption
and apparent porosity, it is more spacious.

Figure 4 – the noise reduction coefficient versus physical properties.
Not surprisingly, the simple linear regression cannot be explained the complex relation because
the physical properties have been more affected together and probably linked in a more sophisticated
way to the microstructure. Table 3, the multiple linear regression was used to compare the relationshipbetween the noise reduction coefficient and physical properties, bulk density, apparent porosity, andwater absorption. The result showed a good correlation and R Square, 0.92 and 0.85, respectively.
Table 3 – multiple linear regression of noise reduction coefficient and physical properties
Regression Statistics
0.921027
Correlation
0.848291
R Square
0.783273
Adjusted R Square
0.039886
Standard Error
3.3 Comparison EApu and Tube Methods
In figure 5, the data showed sound absorption coefficients of the clay brick added charcoal 30 %wt,
and they were fried at 1100 degree Celsius. The test pieces were circle sheets, diameter 100 mm, andthickness 5 mm. Different measurement methods tested them. In EApu method, the maximum accuracy
was 3150 Hz, and the tube method was 1600 Hz. The behavior of sound absorption coefficients at
1600 Hz showed that the maximum of the tube method was double comparing with EApu method .
The difference in values between the EApu and those of the tube methods are also attributed to thefact that the first method utilizes the reverberant field where p lane sound waves come from all
directions, and the tube techniques utilize a sound source at normal incidence. However, both methods
show similar tendencies.
It is worth mentioning that erroneous estimates of the absorption coefficients are acquired with t hemeasurement method. For some sample types like clay brick, the sample size is limited . In figure 5(a,b) as a result, the estimated average absorption and standard deviation having fair agreements basedon the maximum dispersion being below 0.04. At this stage, it can be concluded that the reproducibilityof the proposed method is satisfactory, and the method gives appropriate absorption coefficients
despite the geometrical differences of the reverberation rooms.

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Figure 5 – (a) the average sound absorption coefficients of clay brick with different method,and (b) the standard deviation of measurement.

4. CONCLUSIONSThe objectives of this study which we tried to apply the ensemble -averaging technique using withclay bricks and concentrated on the amount and size of additive in clay bricks that affected on soundabsorption coefficient. It can be concluded that
1. The ensemble-averaging technique can be used for measurement sound absorption
coefficient in this clay bricks, and the standard deviation presents the satisfactory values.
2. There is a direct effect of the amount and size of additive on the sound absorption because
the physical properties changed. The large size and high percentage of charcoal added todemonstrate the excellent result.
3. The methods of measurement affect data. According to this research, we found that the
tube method was higher sound absorption coefficient than EApu methods. However, there
are a similar tendency and low standard deviation of both data.

Future work
1.
2.

Produce the clay brick with different sizes and measured by comparing with three methods,
tube, reverberation room, and EApu methods.
Clarify the influence of the sample size, physical properties, and microstructure with effecton sound absorption coefficient by using EApu method.

ACKNOWLEDGEMENTS
Authors are thankful to the Ministry of Science and Technology, Office of the Civil Service
Commission, and Lampang Rajabhat University for supporting implementation of this study . We
would like to thank all staff in the Graduate School of Engineering laboratory at Oita University whohelped us conduct this research.

REFERENCES
1.
2.
3.
4.

U. Berardi, G. Iannace, Acoustic characterization of natural fibers for sound absorption applications,
Build. Environ. 94. 2015. p. 840–852.
X. Xinzhao, L. Guoming, L. Dongyan, S. Guoxin, Y. Rui, Electrically conductive graphene- coatedpolyurethane foam and its epoxy composites, Compos. Commun. 7.2018. p. 1–6.
International Organization for Standardization, Acoustics- Measurement of sound absorption in areverberation room, ISO354, 2006.
International Organization for Standardization, Acoustics– Determination of sound absorption coefficientand impedance in impedance tubes – Part 2: Transferfunction method, ISO10534-2, 1998.

1371

5.

Y. Takahashi, T. Otsuru, R. Tomiku, In situ measurements of surface impedance and absorption
coefficients of porous materials using two microphones and ambient noise. Applied Acoustics, 66(7),2005, 845-865.
6. Otsuru, T., Tomiku, R., Din, N. B. C., Okamoto, N., & Murakami, M. Ensemble averaged surface normal
impedance of material using an in-situ technique: Preliminary study using boundary element method. The
Journal of the Acoustical Society of America, 125(6). 2009. pp 3784-3791.
7. Noriaki Sakamoto, Toru Otsuru, Reiji Tomiku ,and Saki Yamauchi Reproducibility of sound absorption
and surface impedance of materials measured in a reverberation room using ensemble averaging
technique with a pressure-velocity sensor and improved calibration Applied Acoustics 142. 2018. p. 87–
94.
8. Thai industrial Standard. Building brick, TISI 77-2545, Thai Industrial Standards Institute Ministry ofIndustry. 2002.
9. ASTM C373-88. Standard test method for water absorption, bulk density, apparent density and apparent
specific gravity of fired white wares products. West Conshohocken, PA: ASTM International. 2002.
10. Otsuru T, Tomiku R, Din NBC, Okamoto N, Murakami M. Ensemble averaged surface normal impedanceof material using an in-situ technique: preliminary study using boundary element method, J Acoust Soc
Am. 2009. p. 125:3784–91.
11. Din NBC, Otsuru T, Tomiku R, Okamoto N, Asniawaty K. Measurement method with a pressure-velocity
sensor for measuring surface normal impedance of materials using ensemble averaging:comparison withother methods and its geometrical configuration, Acoust Sci Tech. 2012. p. 33:86–95.
12. Din NBC, Otsuru T, Tomiku R, Okamoto N, Asniawaty K. Reproducibility and applicability of ensemble
averaged surface normal impedance of materials using an in-situ technique, Acoust Aust. 2013. p.41:207–
212.
13. P. S. Liu, G. F. Chen, Chapter Ten - Characterization Methods: Physical Properties. Porous
Materials ;Processing and Applications. 2014. p.122-125
14. M. Vorla¨nder, ‘‘Computer simulations in room acoustics: Concepts and uncertainties,’’ J. Acoust. Soc.
Am., Vol.133. 2013. p. 1203–1213.

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