The Potential of Napier Grass Leaf Fibres as an Acoustic Absorber

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

The Potential of Napier Grass Leaf Fibres as an

Acoustic Absorber

Zaiton Haron , Khairulzan Yahya , Nurathirah Mohd Fasli , Nadirah Darus , Suhaida Galib ,

Herni Halim , Tuan Nor Farazila Tuan Mat

Department of Structures and Materials, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Johore Bahru, MALAYSIA

School of Civil Engineering, University Sains Malaysia, Penang, Malaysia

Received: 14/09/2019

Accepted: 17/04/2020

Published: 20/05/2020

Abstract

Acoustic absorbers are introduced to treat poor acoustic enviroment in rooms. However, many available acoustic absorbers in the market

are composed of hazardous materials. Therefore, there are demands for the use of sustainable materials in the production of acoustic absorbers.

This research investigated the sound absorption potential of grass leaf fibres, i.e. Napier grass, as material for acoustic absorbers. Various

bound Napier grass fibre samples, with and without binder under normal press with different thickness, were prepared and tested by using an

impedance tube test for sound absorption coefficient (SAC) determination. Samples with binder under hot press with thickness of 10mm

were also prepared. The results revealed that 5mm and 20mm fibres, respectively, when bonded with urea formaldhyde (UF) under normal

press and with thickness of 30mm, produced a relatively high SAC for frequencies at 500 Hz, 1000 Hz and 2500 Hz, and thus resulted in a

high average NRC value of 0.59. This exceeded the value for synthetic fibre-glass and was similar to rockwool. Moreover, the sound

absorption performance of 20 mm fibre size hot pressed samples were better than hot pressed 5mm fibre size at 500 Hz frequency until 1000

Hz, as well as the bulk and fibre bonded with UF samples for all frequencies. This study concluded that non-toxic Napier grass fibres can be

used in the production of sustainable acoustic absorbers.

Keywords: Napier grass, Sound absorber, Natural fibre, Sound absorption coefficient

Introduction¹

Napier's fibres cellulose content is relatively high and this

indicates that the fibres are strong and of good quality and may be

suitable for sound absorber. According to Daud et al. [19], the

percentage of chemical composition content of Napier fibres is

said to be 12.3% of cellulose, 80.4% of holocellulose, 68.2% of

hemicellulose, 52.0% of NaOH and 10.7% of lignin. This grass

Acoustic absorbers alter the quality of acoustics inside a

room. Absorber materials consist of synthetic fibres like glass

wool and rock-wool, which are normaly used due to its good

acoustic performance. However, the production of synthetic

absorbers have ‘negative’ impacts to the environment due to

-1

produces a higher dry matter yield of 70 tonnes ha yr [20].

When noise incident strikes an absorber the sound energy can be

dissipated in the form of heat, and thus results in the reduction of

sound energy. The absorbed sound energy can be described by

sound absorption coefficient (SAC) between 0 and 1 at some

frequencies. A value of 0 indicates that the surface of absober has

poor absorption while that approaching 1 shows good absorption.

According to Mamtaz et al. [21], natural fibres were found to have

good (SAC) at mid and high frequencies, provided that the

thickness is sufficient to provide the viscous friction through air

vibration. Xiang et al. [8] found that kapok fibre with thickness

significant release of CO into the atmosphere. Not only that, but

also the disposition of this synthetic absorber is harmful to the

environment. Therefore, alternative green materials that can

produce comparable sound absorbers are needed and of current

interest. Researches had so far explored the capability of natural

fibres as an acoustic absorber, such as tea-leaf ﬁbres [1, 2], paddy

ﬁbres [4, 5], jute ﬁbres [6], ijuk (Arenga pinnata) [7], kapok ﬁbres

[8], kenaf ﬁbres [9], sisal-kenaf composite [10], pineapple threads

[11] and sugarcane baggasse [12, 13]. However, none of the

studies was conducted on grass leaf fibres. One of the grass types

that is fast-growing, easily found and abundant in Malaysia is

Napier or elephant grass. Napier grass is also easily found in the

tropical and subtropical regions across the world [14] (Figure 1).

Napier resembles sugarcane in height and in method of

propagation [15]. But in terms of value added, sugarcane fibres

are not only useful for some new product inventions [16-18] but

are also good at absorbing sound [12].

0mm produced an average noise reduction coefficient higher

than that of 20mm, especially at low frequencies. In addition,

density of materials also changed the sound absorption

performance. Koizumi et al. [22] found that increased sample

density could increase acoustic performance in the range of

medium to high frequency regions.

Corresponding author: Zaiton Haron, Department of Structures and Materials, School of Civil Engineering, Faculty of Engineering,

Universiti Teknologi Malaysia, Johore Bahru, MALAYSIA .Email: zaitonharon@utm.my.

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

(a)

Napier grass/elephant

(b) Napier grass distribution arround the world [12]

Figure 1: Napier grass

Density is related to porosity because at high frequency, the

higher the density the higher the porosity and sound absorption.

Yahya and Desmond [23] found that when bamboo fibre densities

increased, the number of bamboo fibre per unit area of the sample

will also increase. This would allow increased friction between

surfaces, causing the high acoustic energy to be dissipated, and

thus increased the SAC of sample.

In these mixtures, samples of approximately the same density

were developed by using 5mm and 20 mm sized fibres that were

treated with 5% of NAOH to remove lignin. Meanwhile, for the

third mixture hot pressed method was carried out with samples of

higher density. In the first mixture, absorber samples which

contained 5mm NAF fibres without binder and have bulk density

of 160 kg/m with thickness of 10mm, 20mm and 30 mm were

Resin may change sound absorption performance of natural

fibres as it filled the spaces within and between fibres

obtained, and denoted as Bulk (5mm)_160 kg/m . This density

was selected after several trials for its workable criteria. Next,

[

21](Mamtaz et al. ), and thus increased flow resistivity and sound

absorption higher than those unbonded. Furthermore, Antonio

24] suggested that a fibrous layer which underwent compression

with the same density of 160 kg/m , absorber samples that

contained 20mm NAF combined with 5mm NAF fibres in the

ratio of 70:30 were prepared, also with thickness of 10mm, 20mm

and 30 mm. These absorber samples were denoted as Bulk

[

produced high tortuosity behaviour due to decrease in

characteristic lengths and porosity. Tortuosity for porous media is

(20mm)_160 kg/m . The second mixture samples were bonded

the ratio of the characteristic length (L

) to the sample thickness

with UF to achieve slightly higher density samples with

thickness of 10mm, 20mm and 30 mm for 5mm and 20mm fibre

size, respectively. The series for 5mm NAF bonded with 40% UF

(L) [25]. Antonio [24] mentioned that passageways for sound

travels became small and caused the sound waves to use these

available small passageways for their propagation, and thus

reducing the SAC at certain frequencies.

by weight was to achieve a density of 186 kg/m (5mm+UF-186

kg/m ). Meanwhile, the series for 20mm NAF combined with

Therefore, this paper , demonstrates the acoustic performance

of Napier grass leaf fibre (NAF) as a sound absorber. Sound

absorber specimens made from two different sizes of fibre,

bonded and unbonded with resin were produced. The effects of

fibre sizes, density and thickness of absorber specimens on

absorption coefficient and noise reduction were investigated.

Microstructures and porosity were used to explain the behaviour

of acoustical performance. The benefit of study is

thedetermination of NAF potential as an alternative material for

sound absorber.

5mm NAF and bonded with UF in mix ratio of 60:40, was for

density of 212 kg/m (20mm+UF-212 kg/m ). The mix

ratio,which was also obtained after several trials, were made to

obtain a good intact between fibres and UF. After the fibres were

mixed, the mixture of materials was manually pressed in a 98mm

diameter cylindrical casing with specified thickness (Figure 4),

and then it underwent a drying process.

Bulk 5mm

Bulk 20mm

5mm+UF

Materials and Methods

Three types of mixtures were developed to obtain the NAF

20mm+UF

absorber specimens with specific density. The first and second

mixtures were to obtain samples without and with binder,which

were fabricated with hand lay up dan normal press method.

Figure 4: Specimens with and without binder

Third samples were specimen with UF under the hot press method

for high density samples (927 kg/m and 1059 kg/m ) of 10mm

thickness (hot pressed 5 mm +UF and hot pressed 20mm+UF).

The mixtures were developed by mix ratio of 50:50 (fibre:UF).

The mixture then placed in a 190.5mm x 190.5 mm x 20mm box

mould made of woods (Figure 5(a)). This wooden mould was

designed for producing a sample that run the compression process

in a hot press machine to form the required sample size. The

mould was then removed from its position and transferred to a hot

press machine and heated up to 130 °C for 2 min (Figure 5). After

(

a) Napier grass leaves

(b) Shredded Napier

grass leaves fiber

formaldehyde

(UF)

fiber with 5mm size

(size 20mm)

Figure 3: Materials used

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

that, the 'hot press' machine was released and the composite was

cooled at room temperature. This resulted in density of 927 kg/m

and 1059 kg/m for 5 mm and 20 mm NAF bonded with UF,

respectively.

mm HP

(a) Impedance Test according

to ISO 10534-2

(b) Specimen holder

m N

Figure 6: Sound absorption test

50:50

Compacted

Hot pressed

20 mm HP

Result and Discussions

3.1 Microstructures of materials

Figures 7(a) depicts SEM micrographs for the magnification

of 300x which shows fibrillar structure of NAF while Figure 7(b)

for magnification 1000x evidences in a rough surface topography

of open cell as well as the porosity existing in fibres. An energy-

dispersive X-ray spectroscopy analysis (EDX) analysis shows

that the open NAF cells contain Carbon (55.8%), Oxygen

Figure 5: Specimen preparation of hot press NAF: (a) UF binder (b)

mixture in wooden mould (c) hot press machine (d) specimens cut after

hot press

The acoustic performance of each absorber sample was

measured by using an impedance tube in accordance with ASTM

E1050-98 [26] (Figure 6). Absorber samples were placed at a

sample holder located the end of the impedance tube. The surface

of the sample is normally encountered towards incoming sound

waves, while the other end of the impedance tube is the location

of the sound source. The sound source consists of white noise

that is generated from the computer. Two microphones are

performed with a loud generator of loud speakers. SAC for one

third octave band were meaured and stored in the computer.

Based on the SAC values, noise reduction coefficient (NRC),

a scalar quantity to represent the amount of sound energy

absorbed when it pounds a surface, were determined. Similar with

SAC, NRC can define the nature of a surface based on the value,

in which approaching 0 indicates the surface has good reflection,

whereas approaching 1 depicts good absorption. NRC is

determined by calculating the arithmetic mean of the absorption

coefficient at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz. The acoustic

performance of rockwool and fibreglass absorber available in the

current market were also tested for comparison. A scanning

electronic microscope (SEM) was used for specimens’

microstructure observation.

(43.8%), Natrium (0.3%) and Calcium (0.1%) (Table 2).

(

a) Structure of fibres under

(b) Structure of fibres under

magnification 1000x

magnification 300x

Figure 7: SEM micrographs for Fibres

Figure 8(a) and Figure 8 (b) shows the UF filled in and

between fibres (UF) and evidence that some macro pores (p)

existed in the hot press sample surfaces. Based on these SEM

micrographs, it showed that both fibre sizes were bonded well

with UF but there were less pores for the 20mm sized fibre

samples as compared to 5mm sized fibre samples. The bonding

between fibres and UF matrix for 5mm sized fibre samples were

examined by EDX showed that chemical elements of UF matrix

Table 1: Mix proportion of NAF absorber

Type of

Sample

5mm

20mm

NAF,

NAF

UF,

Thickness,

Density,

ρ(kg/m3)

NAF %

(among others carbon, oxygen, silicon, alluminium, and kalium)

were present in the fibres bonded with UF, which indicated the

penetration of UF in the open cells (Table 2).

Bulk – NAF

100

10,20,30

10, 20,30

10, 20, 30

160

186

212

927

Bulk-NAF

0mm

NAF 5mm +

NAF

0mm+ UF

Hot

100

press_NAF

Hot

1059

press_NAF

0mm

(

a) Hot pressed 5 mm fibres

(b) Hot pressed 20mm fibres

size bonded with UF

Figure 8: SEM micrographs for hot pressed samples

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

Table 2: Chemical composition detected by using EDX test

waves energy to decrease due to a longer time taken by sound

waves to travel through the sample. This can be seen through the

SEM analysis in Figure 8, whereby 20mm sized fibre samples

demontrated a denser surface. Therefore, sound energy loss

through friction between sound waves and air molecules; hence

improved sound absorption, especially at 1000 Hz. It was also

observed that the bonded samples with higher density, either

compacted manually or hot pressed, had higher NRC despite their

different SAC curves.

Fibres

bonded with

Fibres

without UF

Composition

UF matrix

56.5

41.0

1.0

0.6

0.1

0.4

0.2

0.1

Carbon (C)

Oxygen (O)

Silicon (Si)

Aluminum (Al)

Kalium (K)

68.5

30.9

0.1

0.5

71.4

23.0

2.6

2.3

0.2

0.1

Ferum (Fe)

Magnesium (Mg)

Calsium (Ca)

Natrium (Na)

Phosphorus (P)

0.9

.2 Effect of fibres size and density on sound absorption

Generally, sound incidents were absorbed by the rough

surface of topogphy and porosity of fibres (Figure 7). In detail,

the effect of fibre sizes and density on the SAC of NAF absorber

samples with the same thickness of 10mm can be seen in Figure

0.5

0.2

0.1

500

1000

1500

2000

. Bulk sample made of 5 mm sized NAF had a high SAC value

Frequency, Hz

as compared to those bulk sample made of 20 mm sized NAF

fibres. The finding was similar with a study reported by Mamtaz

et al. [21] which found that more subtle fibres absorbed more

sound than thick and large-sized fibres. However, the trend was

not similar with samples bonded with UF due to slight disparities

in density. The binder increased the density of samples and caused

uncertain effect on SAC. NAF with sizes of 5mm and 20mm

bonded with UF produced a similar SAC with those bulk of 5mm.

This implied that the coarser fibre of 20mm bonded with UF

sample showed a consistent increase in sound absorption

performance. The SAC for the 20mm NAF bonded with UF

sample increased at 630 Hz up to 2000 Hz higher than the bulk

sample.

Bulk(5mm)_160 kg/m3

Bulk(20mm)_160 kg/m3

mm + UF_186kg/m3

0mm+ UF_212kg/m3

Hot press_5mm_927 kg/m3

Hot press_20mm_1059 kg/m3

Figure 9: Effect of size of fibres and density on sound absorption of 10

mm thickness of samples

00 Hz

1000 Hz

2000 Hz

NRC

The highest SAC was produced by the 20mm fibre size hot

pressed sample with value of 0.74 at a frequency of 1000 Hz.

Moreover, the sound absorption performance of 20 mm fibre size

hot pressed samples was better than the hot pressed 5mm fibre

size samples at 500 Hz frequency to 1000 Hz and the bulk and

fibre bonded with UF samples for all frequency. This difference

was actually related to the density value of the absorber samples,

whereby the density value for 20mm fibre size hot pressed

absorber samples was the highest, followed by fibre size of 5mm

hot pressed samples, 20mm fibre size bonded with UF, 5mm fibre

size bonded with UF and all bulk samples.

It could also be seen that the hot press samples had a

bell-

shaped pattern of SAC. This implied that the SAC curves for hot

pressed demonstrated the tortuosity effect (with peak value) and

higher SAC for frequency that ranged below 1600 Hz. The denser

sample caused the peak of SAC curve to shift to the 1000Hz. It

can be seen that the hot pressed sample with coarser fibres yielded

a more tortuous path. As the fibres were compressed, the sound

that travelled through the ﬁbres decreased and air flow resistivity

increased. The sound waves found their way through the small

passageways available for their propagation, and thus the viscous

losses also decreased [24], reducing the SAC at frequency greater

than 1600 Hz. Further, Figure 10 shows the effect of density on

sound absorption at 500 Hz, 1000 Hz and 2000 Hz by absorber

samples of 10 mm thickness. Generally, with the same sample

thickness, hot pressed samples with 20 mm fibres showed higher

SAC for 500 Hz, 1000 Hz and 2000 Hz, because hot press the

characteristic length increase giving a more tortuous path. Air

flow resistivity increased and this caused the incident sound

Figure 10: Effect of absorbers samples density on sound absorption at

00 Hz, 1000 Hz, 2000 Hz and NRC of 10 mm thickness of samples

.3 Effect of thickness on sound absorption

Figure 11 indicates the SAC of NAF 20mm sized fibre

absorber samples, which increases by thickness, with and without

binder. Generally, as the sample thickness increases, the SAC for

all frequency ranges are also increased. Based on the achieved

results, the variation in thickness altered the SAC curve and peak

shape. The sample of 30 mm thickness, with binder and without

binder, and manually compacted had a bell-shaped SAC curve,

denoting the more tortuous path and reached a peak SAC

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

approximation of more than 0.99 at 1250 Hz. The SAC for 20mm

and 10mm thicknesses samples reached 0.99 and 0.52,

respectively, at frequency 2000Hz. For low frequency at 500 Hz

the bonded samples with UF reached a higher SAC of 0.60 as

compared to those unbonded samples.

0.6

500

1000

1500

2000

0.5

Frequency, Hz

Bulk 5mm-10mm

Bulk 5mm-20mm

Bulk 5mm-30mm

5mm+UF-10mm

500

1000

1500

2000

Frequency, Hz

5mm+UF-20mm

Figure. 12: SAC of absorber samples with fibre size of 5mm for

thickness of 10mm, 20mm, and 30mm with and without binder.

Bulk 20mm-10mm

Bulk 20mm-30mm

Bulk 20mm-20mm

20mm+UF-10mm

20mm+UF-30mm

20mm+UF-20mm

Figure 11: Sound absorption of absorber with NAF fibre size of 20mm

for thickness of 10mm, 20mm, and 30mm with and without binder.

Similarly, absorber samples of 5 mm sized fibres with UF and

bulk at 30 mm thicknesses resulted in more tortuous paths and

reached similar values of SAC (Figure 12). Moreover, both have

higher SAC values as compared to the other thickness. The

thicker samples demonstrated a higher SAC at lower frequency.

Furthermore, the NRC increased as the thickness increased, for

both samples with bulk density and bonded with UF (Figure 11).

The trend that NRC showed was that the thicker the sample, the

higher the NRC; this was similar to Xiang et al. [8] finding which

used kapok fibre. This showed that sample thickness caused the

incident sound waves energy to decrease due to the longer time

taken by sound waves to travel through the sample. UF filled the

spaces within and between fibres and in the fibre samples. It

increased flow resistivity and increased sound absorption higher

than those unbonded. This was in agreement with Mamtaz et al.

Figure 13: NRC of absorber samples with fiber size of 5 and 20mm for

thickness of 10mm, 20mm, and 30mm with and without binder.

[

21] who concluded that resin may achieve effective sound

absorption performance. Also, NRC is average absorption, and

thus it showed that finer fibre samples, both bonded and

unbonded, demonstrated higher noise reduction capability,

whereas the coarser fibre samples showed a higher peak SAC

curve. This finding was similar with a study reported by Mamtaz

et al. [21], in which more subtle fibres absorbed more sound than

the thick and large-sized fibres.

0.4

.4 Comparison with current synthetic fibres

Figure 14 and Figure 15 compare the acoustic performance of

500

1000

1500

2000

absorber samples with the samples of rockwool and fibreglass

absorber available in the current market of about the same range

of thickness. It can be seen that except for bulk 5mm sample, all

Napier fibres were able to achieve a high SAC value of greater

than 0.8 as compared to the SAC value of synthetic fibres at a

frequency range of 1000 Hz and above.

Frequency, Hz

Bulk 5mm-30mm

5mm-UF-30mm

Bulk 20mm-30mm

Fiber galss-25 mm

Rockwool 35 mm

0mm-UF-30mm

Figure 14: Comparison of sound absorption of NAF absorber samples

with rockwool and fibreglass.

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 797-803

ii.

NAF with UF binder has higher acoustic performance as

compared to the bulk sample

250 Hz

500 Hz

1000 Hz

2000 Hz

NRC

iii.

Under the hot press condition, the 20mm fibres size has

higher acoustic performance than those 5 mm fibre size

hot press. SAC curves are more tortuous as compared to

those bulk or bound with UF

iv.

Generally, the thicker the sample, the higher the acoustic

performance

The best sound absorption performance by considering

the low and high frequencies is given by 5mm fibre size

bound with UF. It is better than the 25 mm thick

fibreglass and 35 mm thick rockwool samples.

It can be inferred that NAF poses great significant potential

to be utilised as raw material for acoustic absorber production.

Therefore, this may overcome the environmental problem caused

by the production of synthetic acoustic absorbers.

Acknowledgements.

This work is supported by the Research University Grant

Scheme (GUP: Q.J130000.2522.19H76) financed by the Ministry

of Higher Education, Malaysia.

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