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4

FACTORS EFFECTING THE ACOUSTIC PROPERTIES OF DATE PALM

FIBER

LAMYAA ABD ALRAHMAN JAWAD

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Doctor of Philosophy (Mechanical Engineering)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

JANUARY 2015

iii

To my family, my beloved Mom and Uncle Ahmad,

My lovely sisters Damyaa and Alyaa

iv

ACKNOWLEDGEMENT

First of all, I would like to express my deepest and most sincere thanks and

gratitude to Allah, the Most Gracious and the Most Merciful, for giving me the

strength, perseverance and endurance to complete this research work. It is through

His Special Grace and Mercy that the thesis could be completed.

I would like to thank my supervisors Dr. Raja Ishak, and, and Prof. Dr.

Roslan.Abdul Rahman. Their guidance and mentorship have been like a lighthouse

during whole time at UTM. I have learned much from their experience and

knowledge about the noise and acoustics. Their suggestions and guidance‘s helped

me to maintain on the correct path., also whose throughout the endeavor, never left

me by myself. I owe my deepest respect to them.

My deepest gratitude goes to my family for their unflagging love and support

throughout my life. This dissertation is simply impossible without them. Also special

thanks to my Mom who gave me opportunity to study abroad and let me experience

all the things I would have never done before. Last but not the least, my Uncle

Ahmad, and my sisters (Damyaa and Alyaa) always encourage me and give lots of

supports in life, for their endless understanding and support. All the happiness

moments shared with my family has kept me going forward.

I want to express my sincere respect and grateful to engineer, technician and

staff in Universiti Technologi Malaysia (UTM), Malaysian Palm Oil Board MPOB,

Universiti Tun Hussein Onn Malaysia (UTHM), and Universiti Kebangsaan

Malaysia (UKM), for their assistance of this work, especially to Mr. Md Kasron, Mr.

Helmifairuz., and Mr. Fadli. I would also like to thank all other people who have

provided help of various kinds during my doctoral study, including my colleagues.

Over this time I have found myself indebted to many colleagues and friends for

sharing of their knowledge, guidance, patience and good humour.

Finally, I hope this work is a humble reflection of the values and

opportunities you have provided. If we could only bloom so bright.

v

ABSTRACT

This research focuses on factors that affect the acoustic performance of a

porous material, namely date palm fiber (DPF). Natural DPF is chosen as a potential

replacement to synthetic fibers which raise a lot of health concerns. DPF is

considered as abundant material which can be obtained cheaply and easily,

particularly in the Middle East region. Analytical approaches, namely Delany-

Bazley, Biot-Allard and Johnson-Allard were used to estimate the acoustical

performance of DPF with previously tested natural fiber. All of the analytical values

were then validated with the experimental results. Two types of tests, namely

acoustic absorption coefficient (AAC) and sound transmission loss (STL) were

undertaken to investigate the acoustical performance of the DPF. A series of tests

were undertaken to evaluate the parameters that influence the acoustical performance

of the samples. These include tests on the sample of DPF panel treated with latex;

sample treated with Urea Formaldehyde as well as samples backed with porous

material such as woven cotton cloth and polyester. The findings showed that the

AAC of DPF improved with the increase in thickness and density. It was also found

that there were improvements on the AAC for sample panels backed with porous

materials, particularly at low frequency ranging from 90 Hz to 1.6 kHz. In addition,

the STL of DPF panels treated with Urea Formaldehyde produced higher acoustic

insulation (in the range of 30 dB) due to the increase in panel stiffness. In short, the

results showed that acoustical performance of the DPF is slightly less than synthetic

fibers, but better than other previously tested natural fibers. This is mainly due to the

smaller diameter of DPF as compared to other natural fiber counterparts. The good

acoustical performance of DPF suggests that it has a very good potential to replace

the currently used synthetic fibers for acoustical treatment application.

vi

ABSTRAK

Kajian ini memfokus kepada faktor yang memberi kesan kepada prestasi

akustik satu bahan poros tulen baru iaitu fiber pelepah tamar (DPF). DPF semula

jadi dipilih sebagai satu bahan berpotensi untuk menggantikan fiber sintetik yang

menyebabkan banyak masalah kesihatan. DPF dianggap sebagai bahan buangan yang

boleh didapati dengan murah dan mudah, terutamanya di rantau Timur Tengah.

Pendekatan analitikal, iaitu Delany-Bazley, Biot-Allard dan Johnson-Allard telah

digunakan untuk menganggarkan prestasi akustik daripada DPF berbanding serat

semula jadi yang telah diuji sebelum ini. Kesemua nilai-nilai analisis kemudiannya

disahkan dengan keputusan eksperimen. Dua jenis ujian, iaitu pekali penyerapan

akustik (AAC) dan kehilangan transmisi bunyi (STL) telah dijalankan untuk

menyiasat prestasi akustik daripada DPF. Beberapa siri ujian telah dijalankan untuk

menilai parameter yang mempengaruhi sampel prestasi akustik. Ini termasuklah

ujian ke atas sampel panel DPF yang dirawat dengan latex; sampel yang dirawat

dengan Urea Formaldehyde dan juga sampel yang dilapik dengan bahan poros seperti

kain kapas tenunan dan poliester. Dapatan kajian menunjukkan bahawa AAC

daripada DPF bertambah baik dengan peningkatan ketebalan dan kepadatan. Selain

daripada itu terdapat juga peningkatan pada AAC untuk sampel panel yang dilapik

dengan bahan poros, terutamanya pada frekuensi rendah dalam lingkungan 90 Hz

hingga 1.6 kHz. Di samping itu, STL bagi panel DPF dirawat dengan Urea

Formaldehyde menghasilkan penebat akustik yang baik (dalam lingkungan 30 dB)

hasil daripada peningkatan kekerasan panel. Secara ringkas, keputusan menunjukkan

bahawa prestasi akustik daripada DPF adalah sedikit kurang berbanding serat

sintetik, tetapi lebih baik berbanding gentian asli lain yang telah diuji sebelum ini.

Ini adalah disebabkan oleh diameter DPF yang lebih kecil berbanding gentian semula

jadi yang lain. Prestasi akustik DPF yang baik menunjukkan bahawa ia mempunyai

potensi yang sangat baik untuk menggantikan gentian sintetik yang masih banyak

digunakan bagi aplikasi rawatan akustik.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xiii

LIST OF FIGURES xv

LIST OF ABBREVIATIONS xxiii

LIST OF APPENDICES xxvi

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Background 2

1.3 Porous Materials 4

1.4 Problem Statement 5

1.5 Objectives of the Study 6

1.6 Scope of the Research 6

1.7 Significance of the Study 8

1.8 Contribution of the Research 8

1.9 Organization of the Thesis 9

viii

2 LITERATURE REVIEW 12

2.1 Introduction 12

2.2 Background 13

2.3 Synthetic Absorption Materials 13

2.4 Natural Fiber Absorption Materials 24

2.5 Theoretical Applications 34

2.5.1 The Main Parameters of Porous

Medium 34

2.5.1.1 Flow resistivity 34

2.5.1.2 Porosity 36

2.5.1.3 Tortuosity 37

2.5.1.4 Viscous and Thermal

Characteristic Lengths 37

2.5.2 The Basic Formulation of the

Impedance Function 39

2.5.2.1 Acoustic Impedance 40

2.5.2.2 Characteristic Impedance 41

2.5.2.3 Propagation Constant 42

2.5.3 Measuring Absorption Performance 43

2.5.3.1 Absorption Coefficient 43

2.5.3.2 Empirical Technique 44

2.5.4 Theoretical Predictions for Acoustic

Properties

46

2.5.5 Theories Using Perforated Plate 49

2.7 Research Gaps 51

2.8 Summary 52

3 RESEARCH METHODOLOGY 54

3.1 Introduction 54

3.2 Background 55

3.3 Research Methodology 57

3.4 Microscopic Properties of Fibers 62

3.5 Measuring Flow Resistivity 67

ix

3.6 Experimental Set Up 70

3.6.1 Acoustic Absorption Coefficient Testing 70

3.6.2 Sound Transmission Loss Testing 73

3.7 Summary 76

4 ANALYTICAL ESTIMATION 78

4.1 Introduction 78

4.2 Analyzing Acoustical Properties of Date Palm

Fiber 79

4.2.1 Delany-Bazley Approach 79

4.2.2 Biot-Allard Approach 81

4.2.3 Calculation of Absorption Coefficient 86

4.3 Analysis of Compression Effectiveness on

Acoustical Properties 87

4.3.1 Johnson-Allard Approach 88

4.3.2 Simulation Procedure for the

Compression Ratio (n) 89

4.3.3 Calculation of Absorption

Coefficient 92

4.4 Effect of Perforated Plates on Acoustic

Properties via Various Perforation Ratios and

Aperture Diameters 92

4.4.1 Allard Approach 94

4.4.2 Beranek and Ver Approach 97

4.4.3 Atalla and Sgard Approach 97

4.4.4 Allard Transfer Function Approach 99

4.4.5 Estimation of the Acoustic

Absorption Coefficient 102

4.5 Results and Discussion 103

4.5.1 Estimation of AAC by Delany-

Bazley and Biot- Allard Approaches 103

4.5.2 Simulation of Impacts from the

Compression Ratio to Improve 110

x

Acoustical Performance

4.5.3 Absorption Behaviour of Perforated

Plate 114

4.5.3.1 Comparison of Absorption

Behaviour between all

Analytical Approaches 116

4.6 Summary 121

5 EXPERIMENTAL TESTING 123

5.1 Introduction 123

5.2 Experimental Testing 124

5.2.1 Acoustic Absorption Coefficient Test 124

5.2.2 Sound Transmission Loss Test 125

5.3 Results and Discussion 127

5.3.1 Effect of Latex Application 127

5.3.2 Effects of Porous Material Backing 130

5.3.2.1 Experimental Effects of a

Woven Cotton Cloth

Backing 130

5.3.2.2 Experimental Effect of

Polyester Backing 131

5.3.3 Influence of Thickness on Sound

Transmission Properties 133

5.3.3.1 Panels Treated with Latex 133

5.3.3.2 Panels Treated with Urea

Formaldehyde 135

5.3.4 The Effect of Density on DPF Panels

Treated with Urea Formaldehyde 136

5.3.5 Sound Transmission Class 141

5.6 Summary 147

6 COMPARASION BETWEEN ANALYTICAL

AND EXPERIMENTAL MEASUREMENT 149

xi

6.1 Introduction 149

6.2 Result and Discussion 150

6.2.1 Influence of Thickness in AAC 150

6.2.2 Effect of Bulk Density 153

6.2.3 The Effect of Bulk Density on a

Compressed Panel 155

6.2.4 Influence of Perforated Plates on

Absorption Behaviour Using: 160

6.2.4.1 Beranek and Ver Approach 160

6.2.4.2 Allard Approach 162

6.2.4.3 Atalla and Sgard Approach 163

6.2.4.4 Allard Transfer Function

Approach 165

6.2.5 Effect of Fiber Diameter 168

6.2.6 Physical Effects 177

6.3 Comparative between Synthetic Material and

Natural Fiber 179

6.4 Summary 181

7 CONCLUSION AND RECOMMENDATIONS 184

7.1 Conclusion 184

7.2 Recommendation for Future Work 187

REFERENCES 189

Appendices A-B 200-214

xii

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Properties of each type of natural fiber latex

panels 61

3.2 Properties of DPF Formaldehyde panels 62

3.3 Physical properties of date palm fiber DPF 66

3.4 Physical properties of oil palm fiber OPF 66

3.5 Physical properties of coconut coir fiber CCF 67

3.6 Flow resistivity: theoretical and measurement for

four samples of date palm fiber DPF 68


two samples of oil palm fiber OPF 69


two samples of coconut coir fiber CCF 69

4.1 Illustrates values for fixed regression 80

4.2 Illustrates the perforation ratio; aperture diameter;

distance between apertures of the perforated plates 93

4.3 The values for physical parameters of DPF

samples 108

4.4 Analytical result on the maximum value of AAC

for different sample thicknesses 109

4.5 Variation of physical parameters for compression

ratios for panel thickness of 30 mm: = 130

kg/m3, = 4011.31 kg/ m

3, =

123 , = 242 m 113

xiii

4.6 Variation of physical parameters for compression

ratios from panel thickness of 50 mm: = 150

kg.m-3

, = 4011.31 kg. m

-3, =

123 m, = 241 m 114

4.7 Physical properties of DPF 115

4.8 The value of flow resistivity depends on

perforation ratio; for each perforated plates 116

5.1 Maximum acoustic absorption coefficients at

different sample thicknesses (experimental results)

128

5.2 Preferred frequency band 140

6.1 Illustrates values for physical parameters and

compression time of heat compression during

manufacturing (5 min) 156

6.2 Illustrates values for physical parameters and

compression time during manufacturing of heat

compression (7 min) 158

6.3 Results of CCF and DPF absorption tests 173

6.4 Physical properties of DPF and CCF fibers 173

6.5 Maximum acoustic absorption coefficient (AAC)

values for each thickness and frequency range 174

6.6 Physical properties of OPF and DPF fibers 174

6.7 Acoustic absorption coefficient of synthetic

material in octave bands 180

6.8 Acoustic absorption coefficient of three natural

fibers in octave bands 180

xiv

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 The modified Ingard and Dear impedance tube

system 16

2.2 Configurations of the problem 18

2.3 Schematic of double-layer micro-perforated

membrane (MPM) of oblique incidence 19

2.4 Typical structure of a perforated panel 20

2.5 Generalization of an acoustic transmission analysis

diagram for a composite absorber 21

2.6 Electron microscopy of melamine foam. The solid

phase or skeleton of the foam appears in white 22

2.7 Validation of proposed model by comparing model

prediction to external test results on three different

constructions (1, 2 and 3) for each type of Shoddy

absorber studied: (a) mechanically bonded; (b)

thermally bonded; (c) resin bonded. experimental

result ( ), proposed model ( ) and Delany–

Bazley model ( ) 23

2.8 Experimental data compared to numerical

simulations of the absorption coefficient for

coconut coir fiber having layer thickness 20 mm,

35 mm and 50 mm with and without air gap. (i) 20

mm coir fiber layer thickness, (ii) 35 mm coir fiber

xv

layer thickness, and (iii) 50 mm coir fiber layer

thickness 26

2.9 Effect of perforation size (opening area) and air

gap thickness on the acoustic properties of coir

fiber sound absorption panels 27

2.10 The effect of the perforation ratio. Is (1) 0%

(unperforated), (2) 0.1%, (3) 0.2%, (4) 0.4% and

(5) 0.8% 28

2.11 Simulations of the absorption coefficient of 50 mm

(original thickness) coir fiber for varying

compression rate 30

2.12 The three main types of porous absorbing material 31

2.13 A Typical Green Wall 32

2.14 The dependence of reduction in sound level on the

thickness of the specimens of chopped reeds within

frequency bands 33

2.15 The bi-microphone impedance tube system 45

3.1 Components of the date palm and raw Fibers 56

3.2 Fibers covering petiole and rachis 56

3.3 Research procedure flowchart 58

3.4 Flow charts of Preparation Samples 59

3.5 (a) DPF: same thickness, three densities; (b) DPF:

different thickness and densities; (c) OPF: different

thickness and densities; (d) CCF: different

thickness and densities 60

3.6 Six samples of different thickness and density 62

3.7 DPF microstructure of four randomly fiber

diameters 64

3.8 OPF microstructure of four randomly fiber

diameters 67

3.9 CCF microstructure of four randomly fiber

diameters 68

xvi

3.10 Flow resistivity device (a) Vacuum pump

connected with PC (b) Sample holder

68

3.11 Schematic diagrams of microphone positions 70

3.12 Impedance tube instrument and set-up for acoustic

characteristic measurement 72

3.13 Schematic diagram of impedance tube 72

3.14 Photograph of impedance tube of STL test 73

3.15 Schematic diagram of impedance tube for the

measurement of STL 74

3.16 Schematic diagram identifying microphone

positions 75

3.17 Illustration of incidence and reflected waves inside

the tube 76

4.1 Block diagram of process flow in developing the

theoretical absorption plot for DPF 87

4.2 The thickness of porous fibers after compression 90

4.3 Sketch diagram for each perforated plate illustrates

the distance between apertures (D), diameter of

aperture (d), and perforation ratio 93

4.4 (a) The impinging normal incidence of acoustic on

porous layer; (b) porous layers media 1, 2 99

4.5 Illustrated diagrams of required parameters for

calculations as per both analytical approaches: (a)

Delany-Bazley, (b) Biot- Allard 104

4.6 Illustrates the Biot-Allard and Delany approaches

as well as experimental results for a DPF panel of

20 mm thickness 105



30 mm thickness 106



40 mm thickness

106

xvii

4.9 Illustrates the Biot- Allard and Delany approaches


50 mm thickness

107

4.10 Simulation of a compression ratio for 30 mm

thickness 111

4.11 Simulation of a compression ratio for 50 mm

thickness 112

4.12 Comparison of all approaches: perforation ratio

= 22%, D = 4 mm, and d = 2 mm (a) 30 mm, and

(b) 50 mm panel 118


= 10%, D = 8 mm, and d = 3 mm (a) 30 mm, and

(b) 50 mm panel 119


= 10%, D = 11 mm, and d = 4 mm (a) 30 mm,

and (b) 50 mm panel 120

5.1 The arrangement of a DPF panel backed by a

porous material and rigid wall 124

5.2 Photos of PDF Sample treated with Latex 125

5.3 Photos of DPF sample treated with Urea

Formaldehyde 126

5.4 Experimental results: acoustic absorption

coefficient treated without latex 129

5.5 Experimental results: acoustic absorption

coefficient treated with Latex 129

5.6 Photograph of samples: (a) polyester, (b) woven

cotton cloth 130

5.7 Comparison of the acoustic absorption coefficient

of date palm fiber (30 mm) with woven cotton

cloth and polyester 132

5.8 Comparison of the acoustic absorption coefficient

of date palm fiber (50 mm) with woven cotton

cloth and polyester

132

xviii

5.9 Comparison of STL for three DPF panels treated

with Latex 134

5.10 Comparison of STL for three PDF panels treated

with Urea Formaldehyde Panels 136

5.11 Comparison of PDF panels treated with Urea

Formaldehyde of three densities with the same

thickness (10 mm) 137

5.12 Comparison of PDF panels treated with Urea

Formaldehyde panels of two densities with same

thickness (20 mm) 138

5.13 STC & STL for 20 mm panel treated with latex 142

5.14 STC& STL for 30 mm panel treated with latex 143

5.15 STC & STL for 50 mm panel treated with latex 143

5.16 STC & STL for 50 mm panel treated with Urea

Formaldehyde 144


Formaldehyde 144


Formaldehyde 145


Formaldehyde 145


Formaldehyde 147


Formaldehyde 147

6.1 AAC comparison between experimental and

theoretical using Delany – Bazley approaches for a

DPF panel of 20 mm thickness 150



DPF panel of 30 mm thickness

151

xix



DPF panel of 40 mm thickness

151



DPF panel of 50 mm thickness 152

6.5 The effect of bulk density of DPF panel on AAC 154

6.6 Comparison between Johnson-Allard theoretical

values and experimental results of 10 kg.m-3

and l1

kg.m-3

panel densities 156

6.7 Johnson-Allard vs. experimental results for 4.76

kg.m-3

, 7.15 kg.m-3

, and 9.20 kg.m-3

panel densities 159

6.8 Comparison of experimental results with the

theoretical approach of Beranek & Ver at 30 mm 161


theoretical approach of Beranek & Ver at 50 mm 161

6.10 Comparison of experimental results with the Allard

approach (30 mm) 162

6.11 Comparison of experimental results with the Allard

approach (50 mm) 163

6.12 Comparison of experimental results with Atalla and

Sgard (30 mm) 164

6.13 Comparison of experimental results with Atalla

and Sgard (50 mm) 165


Transfer Function approach (30 mm) 166


Transfer Function approach (50 mm) 167

6.16 Analytical and experimental comparison of CCF

(20 mm, 40 mm) (b) 50 mm panel 169

6.17 Analytical and experimental comparison of DPF

(20 mm, 40 mm) 170

xx

6.18 Analytical and experimental comparison of OPF

(30 mm, 50 mm) 170

6.19 Analytical and experimental comparison of DPF

(30 mm, 50 mm) 171

6.20 Comparison of analytical and experimental results

of two fibers (a) 20 mm thick panels: CCF vs. DPF

(b) 40 mm thick panels: CCF vs. DPF 175

6.21 Comparison of analytical and experimental results

of two fibers. (a) 30 mm thick panels: OPF vs. DPF

(b) 50 mm thick panels: OPF vs. DPF 176

6.22 Comparison between fiber glass boards, date palm

fiber and oil palm fiber at same thickness panels 50

mm in octave band 181

xxi

LIST OF ABBREVIATIONS

A - Dimensionless Parameter in Delany-Bazley Approach

A - Complex Steady State Pressure or Force

B - Complex Steady State Velocity or Volume Velocity

C - Speed of Sound

C0 - Sound Speed in the Air

D - Distance between Apertures

d - Hole Diameter

dmix - Diameter of Fibers Mixed with Latex

f - Frequency

G - Viscous Correction Factor

k - Wave Number in Free Air

Kb - Bulk Modulus of the Frame

K (ω) - Bulk Modulus of Rigid Framed Porous Material

Km,n - Wave Number in a Cylinder

l - Total Length of Fiber per Unit Volume of Material

lmix - Total Length per Unit Volume of Fibers Mixed with Latex

L - Thickness of Perforated Plate

Ii - Incident Sound Intensity

Ir - Incident Sound Intensity

n - Compression Ratio

N - Shear Modulus of Frame

P - Pressure (Pa)

P, Q, and R Biot‘s Experiments Elasticity Coefficients

P0 - Atmospheric Pressure

r - Prandtl Number of air

xxii

r - Fiber Radius

R - Part-Resistance from Impedance (real)

RS - Surface Resistance

- Real Dynamic Tortuosity

S - Surface Area of the Sample

S = R2 Cross Section Area of the Perforated Plate

Sp - Surface Area of the Pores

T - Time Period

T - Transfer matrix

T1 and T2 Transfer Matrices of Layer 1and 2

T1 2 - Interface Matrix Relating Stresses and Velocities for the two Adjacent

Porous Layers

- Poisson‘s coefficient

V - Volume of Air

V (A) - Acoustic Field Vector at Point A

V (B) - Acoustic Field Vectors at Point B

X - Imaginary Part of Acoustic Impedance

Z - Acoustic Impedance

Za - Characteristic Impedance of Air

ZB - Normal Surface Impedance of the Backing Layer

Zf - Characteristics Impedance of Fibrous Material

Zj - Characteristics Impedance of jth Layer

Z0 - Characteristics Impedance

- Complicated impedance

| | - Amount of complicated impedance

aiZ - Characteristic Impedance Corresponding to Transmission in Air

fiZ

- Characteristic Impedance Corresponding to Transmission in Frame

- Acoustic Absorption Coefficient

- Tortuosity

- Phase Change Constant (Coefficient)

- Pi parameter

- Density

xxiii

- Inertial Coupling Term

- Air Density

- Bulk Density of the Porous Media

- Density of Fiber

- Flow Resistivity

- Correction Length

11 ,

12 , Inertial Coupling Parameters between the Frame and Fluid

and 22

- Air Viscosity

- Porosity

- Perforation Ratio

- Viscous Characteristic Length

- Thermal Characteristic Length

- Ratio of Specific Heat at Constant Pressure to Specific Heat at

Constant Volume of air

j - Complex Wave Propagation Constant of jth Layer

f -

Surface Acoustic Impedance of Material

j - Surface Acoustic Impedance of the jth Layer

- Angular Frequency

21 , and

22 Squared Wave Numbers of Two Compressional Waves

i (i=1, 2) Ratio of Velocities for Two Compressional Waves

xxiv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Preparation of the Raw Fibers 200

B Publications 214

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

A comfortable environment free from unwanted noise is the dream of every

person. Unwanted noise can be detrimental to those exposed: negative effects may

include hearing loss, sleep loss, and increased stress. Additionally, acoustic radiation

may be damaging to sensitive mechanical and electrical systems because excess

vibration and fatigue may be induced. For these reasons, noise reduction is of great

interest to engineers. Approaches to noise control may be classified as active or

passive. Passive noise control widely employs acoustical treatments with porous

materials, known to be effective sound absorbers. The sound absorption coefficient

provides a quantitative measure of the acoustic energy absorption for rigidly backed

porous materials.

Therefore, sound absorbing materials are used to control the sound levels in

these sources. It is a well-known subject on which much research has been done

already, although much material and absorbing structure design remains empirical.

The compromises between the thickness of the materials, their cost, their practicality

and their performances are often important. In most cases, rooms, acoustical walls

2

and others have different absorption qualities which depend on the range of

frequencies that are produced in their surroundings. A common constraint in design

settings is the space (thickness) available for the sound absorbing materials. The

sound absorbers must generally be placed adjacent to more rigid walls and surfaces.

Good natural material absorption can be the new to obtain.

Furthermore, utilization of agricultural waste materials enhances economics

by booming retrieve industry in one hand and reducing the need for non-degradable

synthetic matters on the other hand. Studying the acoustical behavior of date palm

fiber and analytical method for predicting its characteristics will abate to optimize

the performance of absorber panel in various practical usages. Only then can the

development of relevant performance be achieved under conditions that are accepted

by the market. Additionally, products form date palm fiber will have the benefit of

low raw materials and manufacturing cost, at the same time providing a suitable end

use for fibers.

1.2 Background

The traditional way for diminishing the undesirable or harmful noise in the

noisy places is by using porous materials to block the sound transmission path [1, 2].

These porous materials that reduce the acoustic energy of sound wave passes through

it by the phenomenon of absorption are called acoustic absorption materials.

Absorption materials consist of various synthetic materials like glass wool minerals

and agriculture-based foams and fibers [3-5]. These materials have been widely

accepted as acoustic absorption materials for acoustical applications. The ability of

these materials absorb at nearly all frequencies, but the performance increasing when

increasing the thickness at low frequency [6].

3

In recent years the interesting investigation has been developed for a wide

range of absorber for porous materials where the researchers seek for a new materials

and ranking for preparing in order to strengthen noise attenuation techniques.

Therefore, the products of sound absorption are mostly synthetic materials because

of domination in the market. For this reason many researches has been working in

developing a new materials which have a high potential in the acoustical applications

[7-11]. The natural materials have interesting advantages such as renewable source

and easily available. There is a limited study on alternative materials for acoustical

absorber, but during recent years it has increased because of the side effect related to

the potential health risks of the synthetic materials which are commonly used in

industrial applications such as asbestos, rock wool, glass wool. In USA/ U.E these

materials have been banned for nonessential uses especially in risky range due the

risk of injury to the workers leading to many diseases especially for lungs and eyes

[12]. It is also to give an opportunity for the natural materials to be developed as

sound absorbers to be used in applications traditionally occupied by synthetic

materials.

Furthermore, nowadays agricultural waste materials has been widely used to

replace synthetic fiber for noise absorption purposes [13, 14]. This natural fiber is

suitable as a substitute to synthetic fibers and wood-based materials for acoustic

absorption purposes. The advantage of this fiber is cheaper, renewable, and non-

abrasive which does not give rise to health and safety issues during processing and

handling of the materials [15, 16]. The sound absorption attribute of previously

natural fiber such as tea leaf fiber, rice straw fiber, coconut coir fiber, oil palm fiber

and reed [17, 18] were investigated and the results showed high potential to be used

as sound absorption panel. They can be very useful for various usages in many

structural and non-structural applications.

In summary, the literature indicates that natural fibers have many advantages

such as cheap, non-hazardous, environmentally friendly, and renewable. Moreover,

the exploitation of waste materials (natural wealth) boosts the economy by booming

the industry through re-cycling technology and diminishes the environmental

4

pollution due waste deposition. Moreover, the synthetic materials are replaced to the

natural fibers because less healthy and safety issue during processing.

1.3 Porous Materials

Porous materials are extensively used as sound absorption materials in noise

control engineering. There are different kinds of porous materials which are

generally categorized as fibrous medium and porous foam. Many researchers have

studied the absorption properties of different materials to reduce the influence of

noise via sound absorption [19-22]. Most researchers attempted to improve the

absorption factor. This research demonstrates that the thickness of a substances‘

porosity and the depth of the air gap have significant relationships to acoustic

attenuation, hereinafter referred to as the ‗acoustic absorption factor‘ (AAF).

The researchers [23-25] improved the acoustic absorption properties of the

materials via multi-layer different forms of flow porosity has been good factors in

determining absorbent factor. Gardner [26] studied polyurethane foam to predict

acoustic characteristics and suggested a neural network model for polyurethane by

gauging multiple parameters such as frequency, the flow of air resistance, and

acoustic density from which a model of acoustic characteristics such as an absorption

coefficient and surface resistance could be predicted. This model is very accurate

and readily expands for the study of different types of materials with different sets of

parameters. Hence, the model used for testing foam is also used to derive numerical

and other analytical results for other materials.

The problem of noise becomes even more critical the predominant

frequencies (low and high) lie within the crucial frequency range of the human ear

which is most sensitive to 4 kHz [27]. There is great need for noise reduction by

5

using absorption materials (synthetic materials) such as glass wool, rock wool etc. of

various densities. However, glass wool was banned in the USA/E.U. for non-

essential uses, especially in this risk range [12]. Furthermore, there is reduction in

the transmission of frequencies in nonessential spaces with a double wall. Such

hardened building materials have higher absorption coefficients for incidental sound.

1.4 Problem Statement

From the review of reported investigations, it is clear that a synthetic material

is harmful to workers. Utilization of agricultural waste materials enhances

economics by booming retrieve industry in one hand and reducing the need for non-

degradable synthetic materials on the other hand. Two distinct kinds of problem

arise of waste natural materials. The first is illustrated the deposition of waste

materials (natural fiber) which are renewable annually, due to inevitable

imperfections in the accumulation of waste and due to the periodic character of the

removal wastes by burning. Therefore, the pollutants are increasing significantly in

the environmental directly through the air. Environmental pollution resulting from

waste materials deposition and removal of them by burning is one of the biggest

problems faced by the human race and a lot of efforts are being put into solving this

problem. The second problem is the reduction of acoustic energy by using synthetic

materials, namely, asbestos, rock wool, glass wool which have been banned due to

potential of injury and risk especially for lungs and eyes.

Furthermore, the use of synthetic materials has always been prohibitive.

Therefore, appropriate solutions may offer simpler means for noise reduction. By

having absorption natural material with suitable characteristic, engineers can have

safer option in tackling acoustical related issue.

6

1.5 Objectives of the Study

This research focuses on the acoustical properties and development of a new

natural material, specifically, date palm fiber (DPF) as acoustic panels. Hence, the

three major objectives of this research are as follow:

To study the capability of date palm fiber (DPF) as a replacement to the

synthetic materials for acoustic application.

To identify major contributor that affect the acoustic properties of date

palm fiber (DPF).

To determine the physical properties that affecting on compression of

date palm fiber (DPF).

To improve the acoustical properties of date palm fiber (DPF).

1.6 Scope of the Research

The scope of this research comprises the following areas:

Scanning microscopy of the network structure of three types of fiber

diameter:

Date palm fiber

Oil palm fiber

Coconut coir fiber

Develop samples of DPF with different densities and thickness as well

as different material combinations, to include:

Four thicknesses and densities

Three densities of the same thickness

Develop samples of OPF and CCF with different densities and

7

thickness as well as material combinations, to include:

Two various thicknesses and densities

Measuring flow resistivity of three types of fiber panels.

An analytical approach utilized to estimate AAC for three types of

investigation:

Acoustical properties:

i) Delany-Bazley

ii) Biot -Allard

Compression effects:

i) Johnson-Allard

Effects of perforated plate:

i) Allard

ii) Beranek and Ver

iii) Atalla and Sgard

iv) Allard Transfer Function

An experimental test utilize to measure AAC for two types of

investigation:

The effect of latex application

The effect of porous materials

An experimental test utilize to measure sound transmission loss (STL)

on two types of samples at various densities and thicknesses:

i) Latex panel

ii) Urea Formaldehyde panels

iii) Identify the sound transmission class (STC)

Comparative study between experimental and analytical.

Compare results of three types of fibers according to fiber diameter.

Compare results of DPF with synthetic material fiber glass board.

8

1.7 Significance of the Study

Natural fiber materials are increasingly being used for different purposes in

many specialized applications. Date palm fiber is one of the most important

agricultural products harvested in Iraq. The purpose of this study is to make a

contribution towards solving the problem of deposition waste material in one hand,

reduce pollution and hazards health on the other hand by using natural fibers that are

perhaps more suitable as an alternative to synthetic materials such as glass wool

(fiberglass) and asbestos which are now widely been used in industrial applications.

Secondly, the acoustical performance of natural fiber such as date palm fiber is

important in order to be used efficiently in wide range of applications such as

transportation sector, building etc. Market demands increases for porous absorbers

with the aspirations like lower frequency absorption ability, wide band frequency

absorption capability, and thin structure for limited space absorber, specific acoustic

absorption spectrum and low cost materials. Another advantage is to be of benefit to

the natural wealth of acoustics absorption applications, especially in barrier

application because natural fibers are cheaper, non-hazardous and environmentally

friendly. To help the greening of technology, wide use is made of manufactured

substances derived from agricultural products as materials for industrial fiber and

wood based products for the acoustical application. The application of natural fiber

may also assist in the preservation of health. Studies conducted on date palm fiber

have demonstrated its potential for use in the manufacture of acoustical related

application.

1.8 Contribution of the Research

A brief outline of the main contributions of this research is given in this

subsection as follows:

9

Agricultural waste material is used instead of synthetic materials for

acoustical applications to diminish health hazards.

Date palm fiber has a high potential of acoustical performance to encourage

the acoustics engineers for using in various applications such as in buildings,

for automotive interior noise reduction, in wall linings and room interior

surfacing, transportation sector efficiently and effectively.

This research is focusing on the use of waste materials (natural fiber) for the

purpose of environmental protection against pollution.

Another contribution of this research is compression ratio simulator approach

that can be applied to all thicknesses panels to improve the desired

performance as the main goal in the limited space structure.

1.9 Organization of the Thesis

This thesis is divided into seven chapters. A brief outline of the contents of

the thesis is as follows:

Chapter 1 presents an introduction to the research problem. It involves the

background and significance of the research as well as the problem statement and

contributions. The logistical flow and structure of the thesis are also outlined in this

chapter.

Chapter 2 is devoted to a literature study on absorption materials for acoustic

applications. Firstly, prior works on synthetic materials and natural materials on

absorption materials to improve the absorption factor are addressed. In addition, the

theoretical background of the study is presented to assist the reader‘s comprehension

of this thesis. Experimental testing is then reviewed and discussed wherein three

10

approaches are described and reviewed prior to the presentation of their application

in subsequent chapters. Finally, natural absorption materials that have previously

been applied for absorption purposes are reviewed.

Chapter 3 focuses on the three types of natural materials are utilized. The

micro-structure of fibers is studied in order to measure physical parameters such as

diameter, length and mass to estimate bulk density, flow resistance and so on. Flow

resistance is measured to achieve validation via experimental equations.

Consequently, average values for physical parameters are then applied in the

equations for utilization in all analytical approaches. The experimental setup of the

two types of test is clearly introduced.

Chapter 4 presents analytical approaches that were implemented to evaluate

the absorption coefficient of date palm fiber. In the first section two approaches

modifications of physical parameters using latex are introduced to the equation. In

the second section the proposed validated method was implemented within the

compression ratio simulation analysis by Johnson-Allard approach. Proposed optimal

compression ratio and appropriate thicknesses for improved AAC were based on the

simulation which revealed that the compressed DPF considerably improved

absorption. Afterwards, presents the effectiveness of perforated plate methodology

based on perforation ratio and aperture diameters. This section concentrates on

utilizing three types that enhance performance absorbers. The proposed analytical

approaches were used as objective functions to improve acoustical properties.

Chapter 5 presents the experimental evaluation of rigid walls for application

two type of testing with latex and without. The performances of the two porous

materials were selected for enhanced absorption was experimentally investigated. In

this chapter also focuses on sound transmission loss (STL) and experimental

evaluations. Latex panels with Urea Formaldehyde panels were tested in the

impedance tube device. A Urea Formaldehyde panel was employed in the

transmission loss studies of rigid panels. The greatest thickness and effectiveness of

11

high density panels were found for optimal insertion losses. Afterwards, sound

transmission class (STC) was identified for two types of panels according to STL

measure. Finally, the performance of the Urea Formaldehyde panel is suggested for

barrier use due to its high potential for transmission.

Chapter 6 is devoted to a comparative between approaches and an

experimental result is discussed. Firstly presents the application of the Johnson-

Allard approach for the effect of bulk density on the acoustical properties of same

thickness panels at various heat compression times during manufacturing. This

procedure was utilized for different times of heat compression during manufacturing

and the analysis was employed as a valid method to estimate the bulk density

required to improve the AAC. Secondly, a suitable perforated plate was selected by

the analytical study and the performance of the proposed perforated plates for

enhanced absorption was experimentally investigated. In the last section, the

effectiveness of each fiber is described and the superiority of DPF compared to OPF,

CCF according to fiber diameter and then comparative with synthetic materials

discussed.

Chapter 7 sums up the research project and directions for future research

works are outlined.

189

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