Guidelines On Design of Noise Barriers

Contents

2. Design Considerations

The primary function of noise barriers is to shield receivers from excessive noise generated by road traffic. While the onus of mitigating road traffic noise lies with the road projects, noise barriers are considered the most reasonable noise mitigation measures available.

Many factors need to be considered in the detailed design of noise barriers. First of all, barriers must be acoustically adequate. They must reduce the noise as identified in the EIA and NIA studies. A proper design of noise barriers would need due considerations from both acoustic and non-acoustic aspects. Acoustical design considerations include barrier material, barrier locations, dimensions and shapes. However, they are not the only requirements leading to proper design of noise barriers.

A second set of design considerations, collectively labeled as non-acoustical design considerations, is equally important. As is often the case, the solution of one problem (in this case noise), may cause other problems such as unsafe conditions, visual blight, maintenance difficulties, lack of maintenance access due to improper barrier design and air pollution in the case of full enclosures or deck over. With proper attention to maintainability, structural integrity, safety, aesthetics, and other non-acoustical factors, these potential negative effects of noise barriers can be reduced, avoided, or even reversed.

2.1 Acoustical Design Considerations

The material, location, dimensions, and shapes of noise barriers can affect the acoustical performance.

Figure 2.1.1 is a simplified sketch showing what happens to road traffic noise when a noise barrier is placed between the source (vehicle) and receiver. The original straight line path from the source to the receiver is now interrupted by the noise barrier. Depending on the noise barrier material and surface treatment, a portion of the original noise energy is reflected or scattered back towards the source. Other portions are absorbed by the material of the noise barrier, transmitted through the noise barrier, or diffracted at the top edge of the noise barrier.

The transmitted noise, however, continues on to the receiver with a "loss" of acoustical energy (acoustical energy redirected and some converted into heat). The common logarithm of energy ratios of the noise in front of the barrier and behind the barrier, expressed in decibels (dB), is called the Transmission Loss (TL). The TL of a barrier depends on the barrier material (mainly its weight), and the frequency spectrum of the noise source.

Figure 2.1.1 Alteration of Noise Paths by a Noise Barrier

The transmitted noise is not the only noise from the source reaching the receiver. The straight line noise path from the source to the top of the barrier, originally destined in the direction of A without the barrier, now is diffracted downward towards the receiver (Figure 2.1.2). This process also results in a "loss"of acoustical energy.

Figure 2.1.2 Barrier Diffraction

The receiver is thus exposed to the transmitted and diffracted noise. Whereas the transmitted noise only depends on barrier material properties, the diffracted noise depends on the location, shape, and dimensions of the barriers.

Where there are noise sensitive receivers on the opposite side of the road, absorptive type noise barriers, either alone or in combination with reflective type, could be used to avoid causing reflection of noise to these receivers. The same may also be required for barriers along the medium barrier in the case of a dual carriageway. In case where this is required, the lower portion of at least 2 to 3 meters should be of absorptive materials.

Sometimes enclosures may be required. If the enclosure is extended to cover the footway(s) as well, attention should be paid to the reverberation noise inside the enclosure. To reduce the noise disturbance on the pedestrians, it is recommended to limit the reverberation time inside the enclosure. Though there is no specific noise level standard applicable here, the general guideline to address reverberation noise is to specify the reverberation time at 500 Hertz to no more than 2 seconds.

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2.2 Transmission Loss of Various Barrier Materials

All materials permit sound energy to pass through, although in varying degrees depending on the material and the frequency of sound. The attenuation of sound passing through a material is referred to as Transmission Loss (TL).

For a barrier to be fully effective the amount of sound energy passing through it must be significantly less than that passing over the top (or around the edge). When noise levels of two sources L_A and L_B are added, a difference between them larger than 10 dB adds less than 0.5 dB to the higher level.

For example: L_A = 70 dB L_B = 60 dB

L_A+B = 10 x log₁₀ [log₁₀^-1(70/10) + log₁₀^-1(60/10)] = 70.4 dB

Thus, if the portion of sound transmitted through the barrier is 10 dB lower than that which goes over the barrier, the overall sound received is essentially determined by the energy travelling over the barrier.

For acoustical purposes, any material may be used for a barrier between a noise source and a noise receiver as long as it has a TL of at least 10 dB(A) greater than the desired noise reduction (i.e. Insertion Loss (IL) determined in the EIA or NIA studies). This ensures that the only noise path to be considered in the acoustical design of a noise barrier is the diffracted noise path, i.e. the path over (or around) the barrier.

For example, if a noise barrier is designed to reduce the noise level at a receiver by 8 dB(A), the TL of the barrier must be at least 18 dB(A). The transmitted noise may then be ignored, because the diffracted noise is at least 10 dB(A) greater and hence the noise propagation path must be over the barrier.

Table 2.2.1 gives approximate TL values for some common materials, tested for typical A-weighted traffic noise frequency spectra. They may be used as a rough guide in acoustical design of noise barriers. For accurate values, consult material test reports prepared by accredited laboratories.

Table 2.2.1

In terms of noise reduction, the maximum value that can be achieved theoretically is 20 dB(A) for thin screens (walls) and 23 dB(A) for berms. A material that has a TL of 33 dB(A) or greater would therefore always be adequate for a noise barrier in any situation.

Small adjustments in surface density to reach a preferred material gauge or a preferred construction thickness do not greatly affect the TL.

Similar to the practice in other countries, a material surface density of 10 kg/m² is typically sufficient but this should be reviewed on a case-by-case basis to meet the requirements of the project.

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2.3 Reduction in Noise Barrier Performance due to Holes, Slits or Gaps

Sound "leaks" , due to holes, slits, cracks or gaps through or beneath a noise barrier, can seriously reduce the barrier performance, and should be avoided. Any gaps represent segments of the barrier with zero Transmission Loss; that is, the gap can be considered to transmit 100% of the energy incident on it. Therefore, extra efforts should be spent at design and construction stages to avoid holes, slits or gaps, either with the adjoining panels, along the bottom edge or gaps for road traffic signs, lighting poles, fire hydrants, construction joints or expansion joints. See Figure 2.3.1 for examples.

While site specific situation warrants provision of gaps like necessary opening of maintenance doors in a very long barrier or provision of access into special areas, special attention should be paid to provide overlapping of barriers etc. (also see section 4.5). In such cases, the sound transmission loss of the barrier is reduced by the amounts shown in Table 2.3.1 for various percentages of the barrier area comprising leaks.

If the noise barrier TL were reduced by at most 3 dB, the overall barrier performance will be reduced by at most 1 dB(A). It may be seen from Table 2.3.1 that the percent area occupied by leaks ranges from at most 1.5% of the total area for situations where the minimum TL requirement is 10 dB, to nearly zero for situations where the required barrier TL exceeds 20 dB. Thus, the significance of leaks increases dramatically where a high amount of noise barrier attenuation is needed.

For noise barriers made of concrete, or other "planks" , the planks must be tongue-and-grooved, carefully lapped, or extremely well butted, to ensure a good air seal at joints. "Alternating boards" , planks mounted on alternate sides of horizontal supports, should not be used.

Table 2.3.1 Reduction in Transmission Loss due to Leaks

% area occupied by leaks	Transmission Loss without leaks at 500 Hz
	10dB*	15dB*	20dB*	25dB*
	reduction in transmission loss, dB
50	10+	15+	20+	25+
25	10	15	20	25
13	8	12	17	22
6	5	10	14	19
3	4	7	11	16
1.5	2	5	9	13
0.78	1	3	6	10
0.39	1	2	4	8
0.20	0	1	3	5
0.10	0	1	1	4
0.05	0	0	1	2
* Required transmission loss for the proposed barriers

Figure 2.3.1 Examples of Gaps

DON'T	DO
Gap at lamp post	Recess formed at lamp post
Gap at gantry sign	Barrier continues at gantry sign
Gap at bottom edge	Gap at bottom edge filled with concrete and sealant

Therefore, to avoid reduction in acoustic performance of noise barriers, recess should be formed along the barrier to accommodate the street furniture as far as possible. See Figure 2.3.2. However, if this is not possible for whatever reason, an integrated design of the noise barrier may be required to accommodate the street furniture. In case where space (headroom and side clearance), sight line and maintenance are permissible, traffic signs may be integrated with the noise barrier.

Figure 2.3.2 Recess for Emergency Telephone

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2.4 Barrier Shapes

Calculation of Road Traffic Noise (CRTN), the methodology used in predicting road traffic noise in Hong Kong, assumes that a barrier has insignificant thickness, but diffraction over the top edge of a barrier is affected by its cross section. It may be appropriate to use an equivalent effective height for barriers which are very wide such as buildings. This can be estimated from the geometry as shown in Figure 2.4.1. Barriers with cross sections having corners and curved shapes are not as effective at reducing noise as those with sharp edges. Wedge shapes with internal angles greater than 90 ° and rounded shapes are least effective. It may therefore be advantageous to use an acoustic screen on the top of a mound, to increase its effectiveness.

The effectiveness of a thin barrier of given height may be increased by bringing the diffracting edge nearer to the source of noise - thus increasing the path difference. Where a tall barrier is placed near to the carriageway, tilting the upper section towards the source can provide additional benefit. Increasing the number of diffracting edges can also improve attenuation considerably.

In most cases it will be relatively expensive to provide more than one barrier, however, by attaching short side panels to a barrier so that there are several edges at the same level may increase the number of diffracting edges (see Figure 2.4.1). Full scale trials with a triple edged barrier have shown benefits of as much as 3 dB(A) in certain circumstances. Such modifications may increase the wind loading on the barrier slightly, but probably by less than would occur if the barrier was made taller to achieve the same acoustic benefit.

Figure 2.4.1 Thick Barrier and Multiple Edged Barrier

Barriers do not necessarily have to be of constant height - it may be cost effective to increase the height in the vicinity of isolated noise sensitive receivers and to reduce it between them. Some computer programs can optimise the profile of a barrier to screen such properties efficiently. Varying the height of the barrier may also help to alleviate the monotonous appearance of long lengths of barrier and may lessen the visual impact of the barrier as well.

While the position (relative to the carriageway) and height of the barrier have been fixed in the EIA study, the outlook is still open to the designer to refine. It is very often that only limited space is allowed at planning stage for noise barrier erection which not only imposes restriction on the choice of noise barrier design but also makes it impossible to soften the structure with landscape planting. If necessary, the designer should explore the feasibility of increasing the space required for the erection of noise barrier.

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2.5 Choice of Material

In general, roadside noise barriers could be divided into the following categories:-

• Reflective type - transparent and non-transparent
• Absorptive type - sound absorbent materials and possible finishes of absorptive panels
• Earth landscaped mound and retaining structures
• Mixed type - a combination of the above types

One of the key features in all structures is the material ultimately chosen. Despite the above categorization, the materials could largely be categorized as reflective and absorptive. The determination whether reflective or absorptive or the combination of both are already done in the EIA or NIA studies. In general the following could be used :

• Steel (painted, galvanized, stainless)
• Aluminium
• Polycarbonate or acrylic sheets
• Concrete, brick or glass fibre reinforced concrete (GRC)
• Proprietary-made acoustic panels
• Landscaped earth berm

An acoustic panel is typically made up of a perforated cover sheet enclosing noise absorptive material (mineral wool or fiberglass inside and wrapped up with polyester film). An absorptive GRC noise barrier relies on noise absorptive material inside the GRC surface grill for noise absorption.

Each of these materials will have its special advantages and disadvantages and it is dependent upon the nature and requirement of a specific project to determine the suitability. As a general rule, the following should be noted :

• Except for absorptive GRC composites, acoustic panels and earth berms, all other materials to various degree reflect sound (i.e. reflective) to premises on the opposite side of the receiver to be protected;
• Metallic and transparent material can produce "glare" effects at certain incident angles;
• The appropriate surface treatment of polycarbonate must be chosen to avoid weathering, ultra-violet attack and consequent loss of transparency; and
• Non-transparent materials such as steel, aluminium and concrete normally require greater efforts in surface treatment to soften the visual impact.

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2.6 Barrier Materials

The following sections give a brief introduction to various materials that could be used for the construction of noise barriers. The design of and the materials used in noise barriers shall be selected to ensure that factors such as aging/corrosion resistance, stone impact resistance, colour resistance and fire resistance etc. can satisfy the requirements specified in noise barrier standard ZTV-Lsw 88.

2.6.1 Concrete

Concrete is used in various ways in the construction of noise barriers. Precast planks slotted into H shaped uprights provide a rapid means of construction and can be easily repaired. One form of proprietary concrete noise barrier is constructed from linked precast panels set at varying angles so as to obviate the need for separate post supports. Concrete noise barriers benefit from low-maintenance, but prefabricated noise barriers are relatively expensive. Special designed surface features can be beneficially employed to reflect sound at a desired angle, away from noise sensitive receivers. On a highway contract involving other concrete structures it may be economical to use in-situ concrete to construct noise barriers. Concrete noise barriers are usually sufficiently robust to withstand vehicle impact damage, but an untensioned corrugated beam barrier may be needed to prevent excessive damage to vehicles if the surface finish is heavily textured. Alternatively, concrete profile barriers could be used to form the lower portion of a noise barrier. For structural design of concrete profile barriers, please refer to Highways Department's Standard Drawings No. H2101A and H2102 to H2107.

2.6.2 Alternative Materials

A variety of materials can be used in barriers including glass, acrylic and other synthetic materials, hollow sheet metal box sections and porous concrete. Landscaped barrier systems, including living barriers of willow or similar woody plants may also be aesthetically attractive.

2.6.3 Metal

Metal noise barriers can be painted or coated in a wide range of colours. Steel is commonly used for supports. Sheet metal can be formed into lightweight hollow sections, which may contain fibreboard or mineral wool absorbent materials. A number of profiled barrier systems, comprising horizontal panels spanning between galvanized steel posts, are commercially available. The metal sheeting on one side may be perforated to allow noise to interact with absorbent material within, and the corrugated profile provides structural rigidity. Aluminium is often used in proprietary systems because of its high strength to weight ratio; large panels may be easily erected with fewer supports (up to 5-meter spans).

2.6.4 Transparent Materials

Transparent materials allow light to properties or areas which would otherwise be placed in the shadow of the barriers. At the top of a noise barrier, transparency (i.e. by using transparent panels) will reduce the visual impact of tall noise barriers and tinted material may enhance the appearance. "Windows" (i.e. incorporation of transparent panels at eye level of the noise barrier) may allow road users to orientate themselves by providing views of the surrounding area. But designers should be aware of the oblique and narrow angle of view from the driving position and of the obscuring effect of supporting structures. Potential problems with birds flying into transparent barriers may be reduced by either using tinted material or by superimposing a pattern of thin opaque stripes.

Transparent materials are noise reflecting and their use might therefore be restricted where reverberation would cause problems. Transparent panels may need to be protected from impact by errant vehicles. Consideration should also be given to the use of laminates, toughened glass, embedded mesh or other systems in order to control the spread of fragments in the event of damage.

Some transparent panels can become semi-opaque relatively quickly, either through superficial or material deterioration. It may be appropriate to make some allowance for this in specifying requirements. Grit can abrade surfaces - plastics are more vulnerable to this than glass. Maintenance requirements and expected life need to be considered when the use of transparent materials is proposed. There are products on the market that claim to be self-cleansing. The adoption of these materials should be considered to reduce the need of maintenance.

Vandalism may also be a material factor. Laminated safety glass has the advantage that tends not to accumulate static electricity, which would attract dirt. Polycarbonate may become opalescent over time as it can absorb water, especially at exposed edges.

2.6.5 Plastics

Apart from their use in transparent panels, plastics have also been used in absorbent panels and for supporting planted systems. Plastics may be coloured as required, but colour may bleach in strong sunlight. Susceptibility to bleaching can be tested in a weatherrometer. Plastics are prone to damage from fire and vandalism and some, e.g. polyethylene, become brittle after prolonged exposure to sunlight.

2.6.6 Recycled Materials

An increasing number of products are available which claim to be "environmentally friendly" by incorporating various recycled materials in their manufacture. Examples are: recycled plastics in supporting structures, waste materials from industrial processes in absorbers, sections of old tyres as planters, domestic waste transformed into compost. There may be limitations in the suitability of recycled products. The use of mixed scrap and surplus may affect choice of colour; eliminating contamination and reprocessing reclaimed materials will add to costs. It is important to establish whether the recycled product is comparable with new material and to ensure it will not tend to degrade more quickly.

2.6.7 Sound Absorbent Materials

Tests in a reverberation chamber (BS 3638 or similar) will produce a frequency response curve. It is desirable for absorption coefficients to be better than 0.8 at frequencies which are significant in the traffic noise spectrum. In general, the peak traffic noise frequencies lie between 500 - 1500 Hz. In some cases, tests may indicate absorption coefficients larger than 1. Although theoretically impossible, this can occur with highly absorbent materials where the shape of the product differs markedly from the ideal of a flat sheet. Some products are strongly tuned to prevent reverberation of low frequencies (100 - 300Hz). These are unlikely to prove useful in connection with high speed roads, but may be appropriate in urban centres where heavy vehicles will be stationary at junctions and accelerating in low gear.

Acoustic requirements should be specified for the whole noise barrier structure (including panels and supporting structure) and allowance should be made for a proportion of reflective supporting elements. An overall performance rating may be quoted for products, obtained by combining sound absorption coefficients in a similar manner to that described above for insulation performance.

Sound absorbent material may be fixed to a backing structure such as a framework of timber or steel, or the surface of a solid wall. Sound absorbent panels are often based on noise absorbent products developed for use in industrial environments and may be available in a range of colours. The aesthetic aspects including shape, colour and surface texture should be considered.

The case for using absorbent barriers in specific situations must be argued on the basis of their cost effectiveness, but where a high quality finish is already required, the additional cost of similar absorbent panels may not be excessive. The geometry of sound reflections may also permit the use of the absorbent material to be limited to that part of the surface where it will be most effective. Materials placed close to the carriageway can quickly become dirty and clogged with pollutants.

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2.7 Earth Berms and Retaining Structures

If a road construction contract would otherwise have surplus material, landscaped berms can be provided at negligible cost; at the same time the inevitable impact on the surrounding area of hauling the surplus material off site can be avoided. The design of berms should be compatible with the local landscape character and topography. The surplus material may only be suitable for gentle slopes and large quantities may be needed to achieve a significant amount of screening. Long roadside slopes are visually attractive but acoustically inefficient and increase landtake, which is always a constraint in Hong Kong. On the protected side, gentle slopes may serve other design objectives such as returning landscaped areas to agriculture.

Where insufficient land is available to construct earth berms high enough with natural slopes, geotextile reinforcement may be used to steepen slopes, but at the risk of being visually incompatible. Alternatively, retaining methods such as reinforced and anchored earth construction, gabions, concrete or timber cribs, and other proprietary support systems may be used to support the traffic face with advantage.

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2.8 Non-acoustical Considerations

While standard guidelines on ventilation, lighting, visual impact and drainage requirement specifically for noise enclosure are not available, the designers are recommended to principally specify that agreements on requirements from Electrical & Mechanical Services Department, Lighting Division of Highways Department, Landscape Unit of Highways Department, Structures Division of Highways Department and Drainage Services Department should be sought.

Some other important issues including fire fighting and emergency access requirements for road users as well as nearby residents should have already been considered during EIA or NIA. Nevertheless, designers should not overlook these during detailed design stage.

2.8.1 Safety/Vehicle Impact

Whenever there is a likelihood of a noise barrier being struck by an errant vehicle, for example, when it is closer than 4.5 m from the carriageway, it should be protected from the impact of errant vehicles by a vehicle restraint system. Untensioned corrugated beam barrier or concrete profile barrier should be installed between it and the hard shoulder/kerb or carriageway on roads where speeds of 70 km/h or more are permitted.

The criteria for the provision of untensioned corrugated beam barrier and clearance required behind it for the various categories of untensioned corrugated beam barrier are given in the Highways Department Standard Drawings No. HH2128 and HH2129. The requirements for the horizontal clearance from a carriageway to an obstruction including any railing barrier are given in TPDM Volume 2 Table 3.5.2.1 (copy attached at Annex A for reference).

Alternatively, where space is limited, say less than 1.5m, untensioned corrugated beam barrier or concrete profile barrier can be integrated with the noise barrier.

Where noise barriers are required to be installed on bridge structures, these should only be combined with a parapet if the assembly has been designed to accept the consequences of vehicle impact. Materials and finishes for attached noise barriers need to allow for the considerable distortions of metal parapets under impact. A freestanding noise barrier vulnerable to vehicular impact should be located behind vehicle parapet with adequate clearance for it to deflect upon impact.

The risk associated with noise barriers on flyovers falling onto vehicle/pedestrian paths upon impact by vehicles should also be considered in the design. Additional catching devices such as laminated panels or panels with embedded filaments or mesh should be provided as appropriate. Further requirements are set out in the "Particular Specification for Noise Barrier" published by Highways Department, February 2001.

2.8.2 Fire Resistance and Emergency Access

Noise barriers, particular transparent type, are subjected to fire hazards. In the case of noise barriers whose elements, whilst having the fire retardant properties as required, are nevertheless combustible, either the posts must be non-combustible and function as a fire barrier, or a length of at least 4 metre made of non-combustible elements shall be inserted in every 100 metres of noise barrier. Emergency access/exit points are also required for roadside barriers and barriers erected along the central reserve to assist evacuation. Attention should be paid to the design of these doors so as to avoid sound leakage and these doors should be kept closed under normal circumstances. (Also see section 4.5.) Advice from relevant authority should be sought on the frequency of these emergency access/exit points. If necessary, a risk assessment should be conducted to evaluate the anticipated risk associated with the noise enclosure.

2.8.3 Lighting Considerations

Lighting inside noise enclosures should be uniform and should avoid glare and flicker effects whilst the switch-on time of artificial lighting during daytime should be minimized. The Lighting Division of the Highways Department, the "Final Report of the Noise Enclosure Lighting - Engineering Study" and/or the "Public Lighting Design Manual" should be consulted / referred to as appropriate for the design of lighting conditions inside noise enclosures.

2.8.4 Maintenance Considerations

Chapter 4 would give a full and detail account of maintenance considerations.

2.8.5 Installation

The contract drawings should show methods of fixing noise barriers to structures, which ensure that gaps below the bottom edge of the barrier are avoided.

The drawings should also show the position and height of the noise barrier and where applicable, the position of gates, also the fittings required and the proposals for treatment at gaps to maintain the acoustic attenuation. The length and position of noise barriers behind any gap should ensure that there is adequate deviation of the noise path from the carriageway to any sensitive receiver being protected by the noise barrier. It should be noted that additional width may be required on embankments to install panels behind the barrier line where gaps are required.

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