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1. INTRODUCTION
The development and growth of human activities have led to rapid increase in traffic density on roads and highways. As a result, the intensity of traffic noise has gone up to unacceptable levels. The consciousness on noise pollution has become significant in developed nations like USA, Japan, China, UK and Australia. Considering the importance of noise abatement, highway noise barriers have been erected at sensitive locations in these countries.

In Indian context, the highways, busy roads and railway tracks close to densely populated area need attention. The reduced noise level will benefit the nearby residential, commercial or public places in terms of better living environment, reduced irritation and impairment of hearing capacity. As per the norms laid down by Federal Highway Administration and Transport authorities of US, the noise level in such localities should not exceed 65dB. Generally, the intensity of noise varies between 70 to 90dB on busy roads and highways. A reduction of noise level by 5 to 25dB through appropriate selection of acoustic material and design of barriers will mitigate the problem to a great extent.

The noise pollution generated by traffic is a significant and growing problem for transportation agencies. Generally, traffic noise is addressed in one of the three ways: controlling the noise at its source, directing the path of sound, or using regulatory and receiver controls or policies. Transportation agencies typically use the second alternative (directing the path of sound), because it falls within their jurisdictions.
1.1 WHAT ARE NOISE BARRIERS
Noise barriers are solid obstructions built between the highway and homes along highway. They do not completely block all noise they only reduce the overall noise levels. Effective noise barriers typically reduce noise levels by 5 to 10 decibels, cutting the loudness of traffic noise by as much as one half.

Highway traffic noise barriers:
¢ Can reduce the loudness of traffic noise by as much as half.
¢ Do not completely block all traffic noise.
¢ Can be effective, regardless of the material used.
¢ Are most effective within 61 meters of a highway.
¢ Must be designed to be visually appealing, preserve aesthetic values and scenic vistas.
¢ Do not increase noise levels perceptibly on the opposite side of a highway.
¢ Substantially reduce noise levels for people living next to highways.
1.2 WHEN ARE NOISE BARRIERS REQUIRED
Noise barriers are not required at locations where an absolute threshold is met. The federal highway administration noise regulations apply only to projects where a state transport department has requested federal funding for participation in the improvements. The state transport department must determine if there will be traffic noise impacts, when a project is proposed for (1) the construction of a highway on a new location or (2) the reconstruction of an existing highway to either significantly change the vertical or horizontal alignment or increase the number of through- traffic lanes. If the state transportation department identifies potential impacts, it must implement abatement measures, possibly including the construction of noise barriers, where reasonably and feasible.
2. MECHANISM OF NOISE PROPOGATION
Noise is defined as unwanted sound. It propagates through different media either sound or liquid or gas. In case of air, it propagates in the form of longitudinal waves having compression and rarefaction. The speed of sound is 340 m/s in air. Without any barrier, noise takes direct path to the receiver. The presence of a barrier reflects, diffracts and transmits different parts of sound energy. A reduced level of sound energy still propagates in the direct path due to transmission. Sharp edges of a barrier diffract or split the sound waves in different directions and reduce its intensity. Sometimes, multiple edges are provided on barriers to further diffract sound energy. It can be seen that even in the shadow zone; a receiver gets diffracted and transmitted noise.


Fig: 2.1 working of a noise barrier
2.1. HOW DOES A NOISE BARRIER WORK
Noise barrier reduce the sound which enters a community from a busy highway by absorbing the sound, transmitting it, reflecting it back across the highway, or forcing it to take a longer path over and around the barrier. A noise barrier must be tall enough and long enough to block the view of a highway from the area that is to be protected. A noise barrier can achieve a 5dB noise reduction, when it is tall enough to break the line of sight from the highway to the receiver. After it breaks the line-of-sight, it can achieve approx 1.5dB of additional noise level reduction for each meter of barrier height. To effectively reduce the noise coming around its ends, a barrier should be at
least eight times as long as the distance from the receiver to the barrier. Openings in the noise barriers for driveway connections or intersecting streets destroy their effectiveness. Noise barriers are normally more effective in reducing noise for areas that are within approx 61 meters (200 meters) of a highway.

Fig: 2.2 Modes of noise propagation and shadow zone

3. ACOUSTIC MATERIALS
The materials used for noise prevention can be classified into three types, namely, absorbing materials, barrier materials and damping materials. The sound absorbing materials like glass wool or open cell polyurethane foam are usually porous and their noise attenuation property is defined by the specific absorption co-efficient. The sound barrier materials are usually heavy and do not have pores in them. A higher mass of barrier material leads to higher loss of transmission loss of noise. Typically, metal, stone, glass etc are good barrier materials. Damping material such as felt and thermocol dampen structural vibration to prevent resonance. Since, higher noise transmission loss is a major objective of noise barriers, a suitable barrier material need to be selected. The transmission loss characteristic of different barrier material is provided in the table 3.1 below.

It can be see that for a particular type of material, transmission loss increases with surface density (kg/m2) and thickness. Concrete and brick walls have the advantage of lower capital cost and minimal maintenance. Steel scores over concrete in terms of lower mass, reduced wall thickness for similar performance, quick erection and ease of dismantling. Weather resistant steel or galvanized steel is ideally suited for outdoor usage. Polycarbonate, though transparent and effective, requires frequent replacement due to shorter life.

Material Thickness
(mm) Surface density (kg/m2) Transmission loss dB
Polycarbonate 8-12 10-14 30-33
Acrylic 15 18 32
Dense concrete 100 244 40
Light concrete 100 161 36
Lightweight concrete 200 151 34
Brick 150 288 40
Steel 1.27 9.8 25
Steel 0.64 4.9 18
Aluminum sheet 1.59 4.4 23
Aluminum sheet 3.18 8.8 25
Wood 25 18 21
Plywood 13 8.3 20
Plywood 25 16.1 23
Absorptive panel backed by metal sheet
50-125
20-30
30-47

Table 3.1 Sound transmission loss of various materials

4. HIGHWAY NOISE BARRIER AND APPLICATIONS

On highways, noise is created by sources like automobile engine, horn, road friction etc. receivers are human beings like drivers, computers and nearby residents. There are three approaches to reduce the noise propagated to these receivers. The first one is to reduce the level of noise emitted by sources through technological improvement. The second is by having an optimal distance between the source and the receivers, particularly the residents. The last one is by having a noise barrier to isolate the receivers from the noise source.
Highway noise barrier is a wall for abatement of highway noise. The noise is higher in case of multiple vehicles, higher speed and presence of higher commercial vehicles. If one vehicle generates a noise level of 70 dB, ten vehicles of similar kind will generate 80 dB. In other words, noise level from different sources gets added in logarithmic scale.
Apart from highway noise abatement, noise barriers can be used for the following application:
¢ Railway tracks in metro cities.
¢ Railway switchyards, bridges and tunnels.
¢ Airport terminals and boundaries.
¢ Enclosures for noisy equipments.
However for each of the above applications, the specific requirements and design considerations will be different

5. DESIGN CONSIDERATIONS FOR HIGHWAYS
The acoustic considerations are based on the existing and desired noise levels in the protected area. The material and barrier thickness are selected accordingly. The barrier can be on one or both sides of the highway depending on which locations need to be protection. The barrier height depends on the desired shadow zone. Relatively taller barriers are necessary to protect multistoried buildings. Inorder to prevent noise reflections to highways, thereby causing irritation to the drivers and passengers, the bottom portion (around 1.8m above ground) of barrier is made of absorptive panels. Reflective panels are mounted at higher elevations. Transparent panels are used sometimes to permit ingress of light. The top edges of the barriers are designed to diffract the noise. In general, the total height of barrier varies between 2.8 to 3.2 m, which is considered to maximize the performance to cost ratio. The wind load and structural rigidity are important considerations in designing the panels. The cost of fabrication and erection is a prime consideration. Selections of cheaper and weather resistant materials and modalities of funding are essential for incorporation of noise barriers. Life cycle and periodic maintenance cost are also taken into account during material selection. Besides cost and acoustics, aesthetic consideration is also important. Inorder to reduce monotony, various wall patterns, colors and height alterations are used.
Sometimes, earth berm or its combination with concrete barrier suit local topography.
Finally, the following factors also merit consideration:
¢ The need of way maintenance.
¢ Water and electricity provision.
¢ Drainage.
¢ Access of pedestrians and smaller vehicles.
¢ Vehicle impact.

5.1 Barrier material selection
Noise barriers are generally constructed of one of the following: concrete, earth, wood, brick or masonry units, metal, vegetation, mineral aggregates, plastic, glass, or composites of these materials. These materials can produce barriers in a variety of shapes and sizes, and each material has specific characteristics that make it suitable for a particular noise situation.
Free-standing thin wall is the most commonly used noise barrier, and number of factors use to select barrier materials, includes site geometry and compatibility, durability and integrity, cost, acoustic properties, and community preference. Another criterion in barrier selection was low cost or low bid price; lowest-cost criteria virtually eliminated public participation and aesthetic considerations from the decision-making process.

Metal barriers (particularly steel) fared well in the durability portion than that of concrete which is a commonly used noise barrier material. The low cost and availability made wood the material a good choice but there are problems with repairing and replacing wooden barriers. Wood's acoustic properties are poorer than those of more solid materials (a transmission loss of between 18 and 23 dB), and it has a tendency to shrink, warp, deteriorate, and discolor.

Brick or masonry units, on the other hand, offer an excellent transmission loss of 33 dB, but they cost more to construct and need to be replaced when damaged. Information about plastic and glass barriers was limited, since only few states are using these materials. The literature review revealed that lexan, glass, and fiberglass are the three main plastic or glass materials used in noise barriers. In particular, fiberglass is weather-resistant and durable, and it offers effective sound absorption and design flexibility.

5.2 Aesthetics
The visual quality of noise barriers is a critical factor, since they become a major line element [in the highway corridor], second only to the roadway itself. Design considerations such as color, texture, scale, line, proportion, and form must be carefully evaluated. Both transportation agencies and the public are becoming increasingly aware of the significance of aesthetics in relation to noise barriers.

5.3 Are residents view considered
A major consideration in the design of a noise barrier is its visual impact on the surrounding area. A tall barrier near a one-story, single family, detached residential area can have a negative visual effect. One solution to addressing the size relationship in visual quality is to provide staggered horizontal elements to a noise barrier to reduce the visual impact by planting landscaping in the foreground. Native plantings are preferable. The visual character of noise barriers in relationship to their environmental setting should be carefully considered. In general, it is desirable to locate a noise barrier approximately four times its height from residences and to provide landscaping near the barrier to avoid visual dominance.
Noise barriers should reflect the character of their surroundings as much as possible. It is always desirable to preserve aesthetic views and scenic vistas, to the extent possible.
5.4 Are motorists view considered
The psychological effect of noise barriers on the passing motorist should be a part of barrier design and construction. Noise barriers in dense, urban settings should be designed differently than barriers in more open suburban or rural areas, and they should be designed to avoid monotony for the motorist. At normal roadway speeds, motorists tend to notice noise barriers overall form, color, and surface texture. A primary objective of noise barrier design should be to avoid a tunnel effect for the motorist. This can be accomplished by varying the forms, materials, and surface treatments.
Graffiti on noise barriers can be a potential problem. One solution is to use materials that can be readily washed or repainted. Landscaping and plantings near barriers can also be used to discourage graffiti, as well as to add visual quality.
5.5 Does construction of a noise barrier increase noise levels on the opposite side of the highway
Residents adjacent to a highway sometimes feel that their noise levels have increased substantially, because of the construction of a noise barrier on the opposite side of the highway. However, field studies have shown that this is not true. If all the noise striking a noise barrier were reflected back to the other side of a highway, the increase would be theoretically limited to 3 dB. In practice, not all of the acoustical energy is reflected back to the other side. Some of the energy goes over the barrier, some is reflected to points other than the homes on the opposite side, some is scattered by ground coverings (for example, grass and shrubs), and some is blocked by the vehicles on the highway. Additionally, some of the reflected energy is lost due to the longer path that it must travel. Measurements made to quantify this reflective increase have never shown an increase of greater than 1-2 dB an increase that is not perceptible to the average human ear.
5.6 Does construction of noise barriers on both sides of a highway increase noise levels
Multiple reflections of noise between two parallel plane surfaces, such as noise barriers or retaining walls on both sides of a highway, can theoretically reduce the effectiveness of individual barriers. However, studies of this issue have found no problems associated with this type of reflective noise. Any measured increases in noise levels have been less than can be perceived by normal human hearing, that is, less than 3 dB. Studies have suggested that to avoid a reduction in the performance of parallel reflective noise barriers, the width-to-height ratio of the roadway section to the barriers should be at least 10:1.
The width is the distance between the barriers, and the height is the average height of the barriers above the roadway. This means that two parallel barriers 3 meters (10 feet) tall should be at least 30 meters (100 feet) apart to avoid any reduction in effectiveness. These studies have also shown that any reduction in performance can be eliminated through the use of sound absorptive noise barriers.

6. STEEL COMPOSITE NOISE BARRIER DEVELOPMENT
6.1 WHAT IS A COMPOSITE MATERIAL
A composite material should satisfy the following conditions:

¢ It is manufactured (i.e. naturally occurring composites such as wood, are excluded).
¢ It consists of two or more physically and/or chemically distinct, suitably arranged or disturbed phase with an interface separating them.
¢ It has characteristics that are not depicted by any of the components in isolation.
A prototype steel composite noise barrier was designed and erected at RDCIS (Research and Development Centre for Iron & Steel), SAIL (Steel Authority of India Limited) (shown in FIG: 6.1). It consisted of 12 panels, of which bottom 8 were of absorptive type and top 4 were of reflective. The absorptive panels had rectangular openings on front side to permit ingress of noise to the glass fiber inserts. The back plate of the noise barrier had trapezoidal ribs, by cold forming, capable of withstanding a wind load of 1500 N/m2. The panel front and back plates were made of galvanized sheets, which possess reasonably good atmospheric corrosion properties. The glass fiber mats were integrated and covered with roving to protect the atmospheric inserts. The core of the panel is filled with glass fiber, wrapped with nylon net, which provides excellent sound absorbing capacity. All the panels were erected between grouted steel joists.


FIG: 6.1 PROTOTYPE NOISE BARRIER
The schematic diagram of absorptive panel is shown in the fig: 6.1. The joist and panel interface was lined with shims. The steel joist / column foundation was made of RCC. A reflective panel of 85*1000*2500 mm size costs around Rs 3500 where as an absorptive panel made of glass fiber costs Rs 10500. The support structure cost is additional. To reduce the cost, it is necessary to select a cheaper sound absorbing material, with similar performance of glass fiber.

7. TESTS AND RESULTS

The glass fiber used as absorbing material was tested for absorption co-efficient as per IS: 3348.
The absorption co-efficient is given by: a =1- R2
Where, a = absorption co-efficient,
R= Reflection component
a = 0.55 means, 55% of sound energy has entered the material and 45% has been reflected out. The measurement was done as per standing radio technique.

Transmission loss for the steel-glass fiber composite sample was done as per SAE J 1400. It is the difference in sound level measured with and without barrier. It is given by,
TL = L1- L2
Where,
TL = Transmission loss
L1 = sound level without barrier
L2 = sound level with barrier
Both the tests indicate a superior performance of the barrier material at higher frequencies. Transmission loss was also determined through field test of prototype as per ASTM E 1014 and E 1779 guidelines. A random noise level was kept 5 m in front of the barrier at a height of 1 m above the ground. The noise level in front of and at selected distances behind the barrier was measured with a noise level meter. Measurement was done in an A-weighted scale, closely resembling human hearing, as well as in selected frequencies. The noise reduction at varying source levels and the transmission loss at select frequencies are illustrated in figures shown below.




Fig: 7.1 Lab test results of noise barrier material


Fig: 7.2 Field test results of barrier at 90 dB noise level



Fig: 7.3 Field test results of barrier at 80 and108dB noise level
The results indicate that the barrier is equally effective when the source noise level varies between 80 to 108 dB. A drop of around 25 dB was observed within 10 m behind the barrier. The effectiveness was relatively less in the case of lower frequencies like 250 and 500 Hz. Fiber glass steel combination was found to be highly effective in the frequency range of 1000 Hz and above.

8. CONCLUSION
Most residents near a barrier seem to feel that highway noise barriers effectively reduce traffic noise and that the benefits of barriers far outweigh the disadvantages of barriers. While noise barriers do not eliminate all highway traffic noise, they do reduce it substantially and improve the quality of life for people who live adjacent to busy highways.
The prototype noise barrier developed by R&D Centre for Iron and Steel (RDCIS), SAIL (Steel Authority of India Limited) Ranchi found to be effective by achieving a transmission loss of 25 dB in A-wtd scale. Also fiber glass“steel combination was found to be highly effective in the frequency range of 1000 Hz and frequency of highway noise usually ranges between 400 to 1600 Hz. Wide ranges of properties that can be possible through the proper designing and fabrication of composite materials, development of cost effective and more advanced noise barriers can be developed. A further optimization of cost and development of suitable cheaper material need to be carried out. Matching of noise pattern in terms of frequency with the absorption characteristic and transmission property of barrier material will ensure development of cost effective superior noise barriers in India.

9. REFERENCES
1. Noise and Vibration Control Engineering
Principles and Applications - LEO L BERANEK
ISTVAN L VER
2. Noise Pollution and its Control - V.P.KUDESIA
T.N. TIWARI
3. Noise Pollution and Control “ S.P.SINGAL
4. Fundamentals of Machine Vibration and Industrial noise control “
Dr. T.N SATHYANESAN
5. Composite Materials “ K.K.CHAWLA


ABSTRACT
The growing consciousness of noise pollution has led to installation of highway noise barriers in several countries. To mitigate the level of noise pollution within the city premises due to the increasing traffic intensity in the roads and highways noise barrier are necessary. Considering the futuristic importance of such barrier in India, a prototype noise barrier was designed and erected at RDCIS (Research and Development Centre for Iron & Steel), SAIL (Steel Authority of India Limited), Ranchi. Such noise barriers are gaining popularity in USA, Europe, Japan and Australia for traffic noise abatement in dense urban localities.
There are wide ranges of properties which are possible through composite materials. A prototype composite barrier made of steel which enables lower thickness of barrier walls, suitable for places having space limitation and quick erection has been designed, fabricated and tested here. The testing of transmission loss was measured at site as per ASTM specification. In this seminar the design, acoustic performance and cost of prototype has been discussed. Some of the acoustic and aesthetic requirements of highway noise barriers have also been discussed.
CONTENTS
1. INTRODUCTION 1
1.1. WHAT ARE NOISE BARRIERS 1
1.2. WHEN ARE NOISE BARRIERS REQUIRED 2
2. MECHANISM OF NOISE PROPOGATION 3
2.1. HOW DOES A NOISE BARRIER WORK 3

3. ACOUSTIC MATERIALS 5
4. HIGHWAY NOISE BARRIER AND APPLICATION 7
5. DESIGN CONSIDERATIONS FOR HIGHWAYS 8
5.1. BARRIER MATERIAL SELECTION 9
5.2. AESTHETICS 10
5.3. ARE RESIDENTS VIEW CONSIDERED 10
5.4. ARE MOTORISTS VIEW CONSIDERED 10
5.5. DOES CONSTRUCTION OF A NOISE BARRIER
INCREASES NOISE LEVEL ON THE OPPOSITE SIDE OF
THE HIGHWAY 11
5.6. DOES CONSTRUCTION OF NOISE BARRIERS ON BOTH SIDES OF A HIGHWAY INCREASE NOISE LEVELS 11

6. STEEL COMPOSITE NOISE BARRIER DEVELOPMENT 13
6.1. WHAT IS A COMPOSITE MATERIAL 13
7. TESTS AND RESULTS 15
8. CONCLUSION 18
9. REFERENCES 19
LIST OF FIGURES
FIG 2.1 WORKING OF A NOISE BARRIER 3
FIG 2.2 MODES OF NOISE PROPOGATION AND SHADOW
ZONE 4
FIG 6.1 PROTOTYPE NOISE BARRIER 14

FIG 6.2 SHEMATIC DIAGRAM OF ABSORPTIVE PANEL 14

FIG 7.1 LAB TEST RESULTS OF NOISE BARRIER MATERIAL 16
FIG 7.2 FIELD TEST RESULTS OF BARRIER AT 90dB NOISE
LEVEL 16
FIG 7.3 FIELD TEST RESULTS OF BARRIER AT 80 TO 108 dB
NOISE LEVEL 17