4 Physical Techniques to Reduce Noise Impacts — Audible Landscape — Federal Approach — Noise

The Audible Landscape

4 Physical Techniques to Reduce Noise Impacts

This section describes some of the physical methods which architects, developers and builders can employ to reduce noise impacts. There are four major actions which can be taken to improve noise compatibility for any type of land use or activity. These are site planning, architectural design, construction methods, and barrier construction.

Acoustical site design uses the arrangement of buildings on a tract of land to minimize noise impacts by capitalizing on the site’s natural shape and contours. Open space, nonresidential land uses, and barrier buildings can be arranged to shield residential areas or other noise sensitive activities from noise, and residences can be oriented away from noise.

Acoustical architectural design incorporates noise reducing concepts in the details of individual buildings. The areas of architectural concern include building height, room arrangement, window placement, and balcony and courtyard design.

Acoustical construction involves the use of building materials and techniques to reduce noise transmission through walls, windows, doors, ceilings, and floors. This area includes many of the new and traditional soundproofing concepts

Noise barriers can be erected between noise sources and noise-sensitive areas. Barrier types include berms made of sloping mounds of earth, walls and fences constructed of a variety of materials, thick plantings of trees and shrubs, and combinations of these materials.

These physical techniques vary widely in their noise reduction characteristics, their costs, and especially, in their applicability to specific locations and conditions. This section is not designed to provide complete criteria for selecting a solution to particular noise problems and is not intended as a substitute for acoustical design. Rather, its purpose is to illustrate the wide range of possible alternatives which could be considered in the architectural and engineering planning process. Knowledgeable municipal officials can provide valuable assistance to designers, developers, and builders who may not be familiar with sound attenuation techniques that are most applicable locally.

4.1 Acoustical Site Planning

The arrangement of buildings on a site can be used to minimize noise impacts. If incompatible land uses already exist, or if a noise sensitive activity is planned, acoustical site planning often provides a successful technique for noise impact reduction.

Many site planning techniques can be employed to shield a residential development from noise. These can include:

  1. increasing the distance between the noise source and the receiver;
  2. placing nonresidential land uses such as parking lots, maintenance facilities, and utility areas between the source and the receiver;
  3. locating barrier-type buildings parallel to the noise source or the highway; and
  4. orienting the residences away from the noise.

The implementation of many of the above site planning techniques can be combined through the use of cluster and planned unit development techniques.

Distance Noise can be effectively reduced by increasing the distance between a residential building and a highway. Distance itself reduces sound: doubling the distance from a noise source can reduce its intensity. Distance itself reduces sound: doubling the distance from a noise source can reduce its intensity by as much as 6 dBA. In the case of high rise buildings, distance may be the only means, besides acoustical design and construction, of reducing noise impacts. This is because it is nearly impossible to provide physical shielding for the higher stories from adjacent noise. (See Figure 4.1.)

4.1 Noise barriers can shield only the lowest floors of a building.

Noise Compatible Land Uses as Buffers Noise protection can be achieved by locating noise-compatible land uses between the highway and residential units. Whenever possible, compatible uses should be nearest the noise source. Figure 4.2 which follows shows a proposed parking garage along two sides of a development in Boston. Both the Fitzgerald Expressway and the entrance to the Callahan Tunnel which are shown on the site plan are major and noisy traffic routes.

4.2 Parking Garage to shield residential area.

In addition to protecting the residential development from the noise and dirt of highway traffic, the parking garage provides needed facilities for the residents

Buildings as Noise Shields Additional noise protection can be achieved by arranging the site plan to use buildings as noise barriers. A long building, or a row of buildings parallel to a highway can shield other more distant structures or open areas from noise. One study shows that a two-story building can reduce noise levels on the side of the building away from the noise source by about 13dBA. 1

If the use of the barrier building is sensitive to highway noise, the building can be soundproofed. This technique was used in a housing project under construction in England where a 3,900 foot long, 18 foot wide and 45-70 foot high wall (depending on the terrain) serves as both residence and a sound shield. 2

The wall/building will contain 387 apartments in which the kitchens and bathrooms are placed towards the noise, and the bedrooms and living rooms face away from the highway. The wall facing the highway will be soundproofed and windows, when they exist, are sealed. Substantial noise reductions are expected.

Orientation The orientation of buildings or activities on a site affects the impact of noise, and the building or activity area may be oriented in such a way as to reduce this impact.

Noise impacts can be severe for rooms facing the roadway since they are closest to the noise source. The noise impact may also be great for rooms perpendicular to the roadway because a) the noise pattern can be more annoying in perpendicular rooms and b) windows on perpendicular walls do not reduce noise as effectively as those on parallel walls because of the angle of the sound. Road noise can be more annoying in perpendicular rooms because it is more extreme when it suddenly comes in and out of earshot as the traffic passes around the side of the building, rather than rising and falling in a continuous sound, as it would if the room were parallel to passing vehicles.

Whether the noise impact is greater on the perpendicular or the parallel wall will depend on the specific individual conditions. Once the most severely impacted wall or walls are determined, noise impacts may be minimized by reducing or eliminating windows from these walls.

Buildings can also be oriented on a site in such a way as to exploit the site’s natural features. With reference to noise, natural topography can be exploited and buildings placed in low noise pockets if they exist. If no natural noise pockets exist, it is possible to create them by excavating pockets for buildings and piling up earth mounds between them and the noise. Such a structure would obstruct the sound paths and reduce the noise impacts on the residences.

Cluster and Planned Unit Development A cluster subdivision is one in which the densities prescribed by the zoning ordinance are adhered to but instead of applying to each individual parcel, they are aggregated over the entire site, and the land is developed as a single entity. A planned unit development, or P.U.D. is similar but changes in land use are included, such as apartments and commercial facilities in what would otherwise be a single-family district. Examples of grid, cluster and P.U.D. subdivisions follow in Figures 4.4, 4.5, and 4.6.

Figure 4.3 provides another example of locating noise-compatible uses near a highway (West Street) in Springfield, Massachusetts. From the plan, one can see that parking spaces, ends of buildings, and a baseball diamond are near the highway.

4.3 Parking spaces, end of buildings, and a baseball diamond are placed near the highway. A berm is constructed and trees are planted to shield residences from traffic noise.

4.4 Conventional Grid Subdivision

4.5 Cluster Subdivision

4.6 Placement of noise compatible land uses near highway in Planned Unit Development

From Figure 4.4 it can be seen how the conventional grid subdivision affords no noise protection from the adjacent highway. The first row of houses bears the full impact of the noise. In contrast, the cluster and P.U.D. techniques enable commercial uses and open space respectively to serve as noise buffers. Examples of this are shown in Figures 4.6 and 4.7.

4.7 In cluster development, open space can be placed near the highway to reduce noise impacts on residences

A word of caution is necessary: in a cluster development, the required open space can be located near the highway to minimize noise to the residences. However, many recreation uses are noise sensitive, and when one takes advantage of the flexibility of cluster development to minimize noise, care must be taken not to use all of the available open space in buffer strips, thus depriving the development of a significant open space area. Where high noise levels exist, a combination of buffer strips and other techniques (such as berms and acoustical sound proofing) can be employed.

The flexibility of the cluster and planned unit development techniques allows many of the above site planning techniques to be realized and effective noise reduction achieved.

1 Hans Bernard Reichow, Town Planning and Noise Abatement, Architect’s Journal, 137-7 (February 13, 1963) pp. 357-360.

2 Live-in Wall, 3,900 Feet Long, is Also a Sound Shield, Engineering Record, (September 6,1973).

4.2 Acoustical Architectural Design

Noise can be controlled in a building with proper architectural design. By giving attention to acoustical considerations in the planning of room arrangement, placement of windows, building height, balconies, and courtyards, the architect may achieve significant noise impact reduction, without the need for costly acoustical construction.

Room Arrangement Noise impacts can be substantially reduced by separating more noise sensitive rooms from less noise sensitive rooms; and placing the former in the part of the building which is furthest away from the noise source. The less sensitive rooms should then be placed closest to the noise source where they can act as noise buffers for the more sensitive rooms.

Whether or not a room is noise sensitive depends on its use. Bedrooms, livingrooms, and dining rooms are usually noise sensitive, while kitchens, bathrooms, and playrooms are less so. Figure 4.8 shows a layout designed to reduce the impact of highway noise. This technique was used extensively in England in a 100 acre residential development adjacent to a planned expressway. 1 Kitchens and bathrooms were placed on the expressway side of the building, and bedrooms and living rooms were placed on the shielded side. In addition, the wall facing the expressway is sound insulated.

4.8 Use of acoustical architectural design to reduce noise impacts on more noise sensitive living spaces

1 Live-ln Wall is Also Sound Shield, Engineering News-Record, September 6,1973.

Solid Walls Noise can be reduced by eliminating windows and other openings from the walls of a building close to noise sources. The solid wall can then have the effect of a sound barrier for the rest of the building. As previously discussed in Figure 4.1, walls directly adjacent, and those perpendicular to the noise source can be the most severely impacted. When a solid wall is impractical, illegal, or highly undesirable; the same effect can be achieved by reducing window size and sealing windows airtight. This technique is used in the housing project described above. 1

One Story Houses In cases where either the house or the highway is slightly recessed or a barrier has been placed in the sound path, the noise impact may be further reduced if the house has only one story 2 (See Figure 4.9). If the single story design is inefficient, the split level design may be effective. In any case the path of the sound waves should be assessed before the building design is drawn.

4.9 Noise impacts can be reduced by use of single story houses.

Balconies If balconies are desired they should be given acoustical consideration. The standard jutting balcony, facing the road, may reflect traffic noise directly into the interior of the building in the manner illustrated in Figure 4.10. In addition to reflecting noise into the building, the balcony may be rendered unusable due to the high noise levels. This problem is particularly applicable to high rise apartment buildings where balconies are common. If balconies are desired, the architect may avoid unpleasant noise impacts by placing them on the shielded side of the buildings.

4.10 The standard jutting balcony facing the road may reflect traffic noise directly into the interior of the building.

Courtyards Proper architectural design may also provide for noise reduction in an area outside of the building. The court garden and patio houses can provide outdoor acoustical privacy. (See Figure 4.11). Schools, rest homes, hotels, and multi-family apartment dwellings can also have exterior spaces with reduced noise by means of court yards.

4.11 Use of courtyard house to obtain quite outdoor environment

4.3 Acoustical Construction

Noise can be intercepted as it passes through the walls, floors, windows, ceilings, and doors of a building. Examples of noise reducing materials and construction techniques are described in the pages that follow.

To compare the insulation performance of alternative constructions, the sound transmission class (STC) is used as a measure of a material’s ability to reduce sound. Sound Transmission Class is equal to the number of decibels a sound is reduced as it passes through a material. Thus, a high STC rating indicates a good insulating material. It takes into account the influence of different frequencies on sound transmission, but essentially it is the difference between the sound levels on the side of the partition where the noise originates and the side where it is received. For example, if the external noise level is 85 dB and the desired internal level is 45 dB, a partition of 40 STC is required. The Sound Transmission Class rating is the official rating endorsed by the American Society of Testing and Measurement. It can be used as a guide in determining what type of construction is needed to reduce noise.

Walls provide building occupants with the most protection from exterior noise. Different wall materials and designs vary greatly in their sound insulating properties. Figure 4.12 provides a visual summary of some ways in which the acoustical properties can be improved:

4.12 Factors which influence sound attenuation of walls

    • Increase the mass and stiffness of the wall.

    In general, the denser the wall material, the more it will reduce noise. Thus, concrete walls are better insulators than wood walls of equal thickness. Increasing the thickness of a wall is another way to increase mass and improve sound insulation. Doubling the thickness of a partition can result in as much as a 6 dB reduction in sound. 3 However, the costs of construction tend to limit the feasibility of large increases in wall mass. The relative stiffness of the wall material can influence its sound attenuation value. Care must be taken to avoid wall constructions that can vibrate at audible frequencies and transmit exterior sounds.

  • Use cavity partitions

    A cavity wall is composed of two or more layers separated by an airspace. The airspace makes a more effective sound insulator than a single wall of equal weight, leading to cost savings.

  • Increase the width of the airspace.

    A three inch airspace provides significant noise reduction, but increasing the spacing to six inches can reduce noise levels by an additional 5 dBA. Extremely wide air spaces are difficult to design.

  • Increase the spacing between studs.

    In a single stud wall, 24 inch stud spacing gives a 2-5 dB increase in STC over the common 16 inch spacing. 4

  • Use staggered studs.

    Sound transmission can be reduced by attaching each stud to only one panel and alternating between the two panels.

  • Use resilient materials to hold the studs and panels together.

    Nails severely reduce the wall’s ability to reduce noise. Resilient layers such as fiber board and glass fiber board, resilient clips, and semi-resilient attachments are relatively inexpensive, simple to insert, and can raise the STC rating from 2-5 dB.1

  • Use dissimilar leaves.

    If the leaves are made of different materials and/or thickness, the sound reduction qualities of the wall are improved. 2

  • Add acoustical blankets.

    Also known as isolation blankets, these can increase sound attenuation when placed in the airspace. Made from sound absorbing materials such as mineral or rock wool, fiberglass, hair felt or wood fibers, these can attenuate noise as much as 10 dB. 3 They are mainly effective in relatively lightweight construction.

  • Seal cracks and edges.

    If the sound insulation of a high performance wall is ever to be realized, the wall must be well sealed at the perimeter. Small holes and cracks can be devastating to the insulation of a wall. A one-inch square hole or a 1/16 inch crack 16 inches long will reduce a 50 STC wall to 40. 4

    Figure 4.13 shows a sample of wall types ranging from the lowest to the highest sound insulation values. The cost of these walls in dollars per square foot is given for comparison of cost effectiveness. 5

    4.13 Wall Types with STC Rating and Approximate Cost.

    1 Live-in Wall.

    2 This technique is used extensively in Cerritos, California.

    3 R.K. Cook and P. Chrzanowski, Transmission of Noise Through Walls and Floors, Cyril Harris, ed. Handbook of Noise Control, McGraw-Hill Book Company, Inc. (New York, 1957).

    4 T. Doelle, Environmental Acoustics, (New York, McGraw-Hill Book Company, 1972), pp. 232-233.

    Windows Sound enters a building through its acoustically weakest points, and windows are one of the weakest parts of a wall. An open or weak window will severely negate the effect of a very strong wall. Whenever windows are going to be a part of the building design, they should be given acoustical consideration. Figure 4.14 illustrates the effects of windows on the sound transmission of walls. For example, if a wall with an STC rating of 45 contains a window with an STC rating of 26 covering only 20% of its area, the overall STC of the composite partition will be 33, a reduction of 12 dB.

    4.14 Graph for calculating STC of composite barriers.

    The following is a discussion of techniques that can be used to reduce noise in a building by means of its windows. These techniques range from a blocking of the principal paths of noise entry to a blocking of the most indirect paths.

    • Close windows The first step in reducing unwanted sound is to close and seal the windows. The greatest amount of sound insulation can be achieved if windows are permanently sealed. However, openable acoustical windows have been developed which are fairly effective in reducing sound.6 Whether or not the sealing is permanent, keeping windows closed necessitates the installation of an air-conditioning system. The air conditioning system may in addition provide some masking of noise. (Masking is discussed below). If windows must be openable, special seals are available which allow windows to be opened. 7
    • Reduce window size The smaller the windows, the greater the transmission loss of the total partition of which the window is a part. Reducing the window size is a technique that is used because (a) it precludes the cost of expensive acoustical windows, and (b) it saves money by cutting down the use of glass. The problems with this technique are (a) it is not every effective in reducing noise; e.g. reducing the proportion of window to wall size from 50% to 20% reduces noise by only 3 decibels; and (b) many building codes require a minimum window to wall size ratio.
    • Increase glass thickness If ordinary windows are insufficient in reducing noise impacts in spite of sealing techniques, then thicker glass can be in stalled. In addition, this glass can be laminated with a tough transparent plastic which is both noise and shatter resistant. Glass reduces noise by the mass principle; that is, the thicker the glass, the more noise resistant it will be. A 1/2-inch thick glass has a maximum STC rating of 35 dB compared to a 25 dB rating for ordinary 3/16 inch glass.

    However, glass thickness are only practical up to a certain point, when STC increases become too insignificant to justify the cost. For example, a 1/2 inch thick glass can have an STC of 35; increasing the thickness to 3/4 inch only raises the STC to 37. However, a double glass acoustical window consisting of two 3/16 inch thick panes separated by an airspace will have an STC of 51 and can cost less than either solid window

    In addition to thickness, proper sealing is crucial to the success of the window. To prevent sound leaks, single windows can be mounted in resilient material such as rubber, cork, or felt.

    Install Double-Glazed Windows Double-glazed windows are paired panes separated by an airspace or hung in a special frame. Generally, the performance of the double-glazed window may be increased with:

    1. increased airspace width
    2. increased glass thickness
    3. proper use of sealing
    4. slightly dissimilar thickness of the panes
    5. slightly non-parallel panes

    In general the airspace between the panes should not be less than 2-4 inches if an STC above 40 is desired. If this is not possible, a heavy single-glazed window can be used. The use of slightly non-parallel panes is a technique employed when extremely high sound insulation is required, such as in control rooms of television studios.

    The thickness of double-glazed panes may vary from 1 /8 to 1 /4 inch or more per pane. Although thickness is important, the factors which most determine the noise resistance of the window is the use of sealant and the width of the airspace.

    As in the case of all windows, proper sealing is extremely important. To achieve an STC above 43, double-glazed windows should be sealed permanently. If the windows must be openable, there are available special frames and sealers for openable windows which allow a maximum STC of 43.1

    Permanently sealed double-glazed windows often require an air pressure control system to maintain a constant air pressure and minimal moisture in the airspace. Without this system, the panes may deflect, and, in extremely severe cases, pop out of the frames.

    To further insure isolation of noise between double-glazed panes, the panes could be of different thicknesses, different weights, and slightly non-parallel to each other. This prevents acoustical coupling and resonance of sound waves.

    Doors Acoustically,doors are even weaker than windows, and more difficult to treat. Any door will reduce the insulation value of the surrounding wall. The common, hollow core door has an STC rating of 17 dB. Taking up about 20% of the wall, this door will reduce a 48 STC wall to 24 STC. To strengthen a door against noise, the hollow core door can be replaced by a heavier solid core door that is well sealed 2 and is relatively inexpensive. A solid core door with vinyl seal around the edges and carpeting on the floor will reduce the same 48 STC wall to only 33dB 3. An increased sound insulation value can be achieved if gasketed stops or drop bar threshold closers are installed at the bottom edge of the door. (See Figure 4.15) The alternative solution to doors is to eliminate them whenever possible from the severely impacted walls and place them in more shielded walls.

    4.15 Increased sound insulation can be achieved with gasketed door stops or drop bar threshold closers.

    Ceilings Acoustical treatment of ceilings is not usually necessary unless the noise is extremely severe or the noise source is passing over the building. The ordinary plaster ceiling should provide adequate sound insulation except in extremely severe cases. An acoustically weak ceiling which is likely to require treatment is the beamed ceiling.4 Beamed ceilings may be modified by the addition of a layer of fiberglass or some other noise resistant material. Suspended ceilings are the most effective noise reducers but they are also the most expensive.

    Floors In the case of highway noise, floors would only require acoustical treatment if the highway were passing under the building. In this case, flooring would have to provide protection against structural vibrations as well as airborne sound.

    Two ways to insulate a floor from noise are to install a solid concrete slab at least 6 inches thick or install a floating floor. In general, the floating floor gives the greatest amount of sound and vibration insulation; however, it is extremely expensive. Basically, a floating floor consists of a wood or concrete slab placed over the structural slab, but separated by a resilient material. The resilient material isolates the surface slab from the structural slab and the surrounding walls.

    Interior Design Overall interior noise levels can be reduced by the extensive use of thick, heavy carpeting, drapes, wall hangings, and acoustical ceiling tiles. These materials absorb sound. They cannot prevent noise from coming through the walls, but they can reduce overall sound levels by reducing sound reverberations.

    Masking Another way of coping with noise is to drown it out with background noise. This technique is known as masking. It can be very effective in reducing noise fluctuations which are often the most annoying aspects of noise. Masking can be produced by air conditioning and heating systems, soft music, or electronic devices.

    4.4 Barriers

    A noise barrier is an obstacle placed between a noise source and a receiver which interrupts the path of the noise. They can be made out of many different substances:

    1. sloping mounds of earth, called berms
    2. walls and fences made of various materials including concrete, wood, metal,plastic, and stucco
    3. regions of dense plantings of shrubs and trees
    4. combinations of the above techniques

    The choice of a particular alternative depends upon considerations of space, cost, safety and aesthetics, as well as the desired level of sound reduction. The effectiveness of the barrier is dependent on the mass and height of the barrier, and its distance from the noise source and the receiver. To be effective a barrier must block the line of sight between the highest point of a noise source, such as a truck’s exhaust stack, and the highest part of the receiver. This is illustrated in Figure 4.16.

    4.16 To be effective, a barrier must block the line of sight between the highest point of a noise source and the highest part of a receiver.

    To be most effective, a barrier must be long and continuous to prevent sounds from passing around the ends. It must also be solid, with few, if any, holes, cracks or openings. It must also be strong and flexible enough to withstand wind pressure.

    Safety is another important consideration in barrier construction. These may include such requirements as slope, the distance from the roadway, the use of a guard rail, and discontinuation of barriers at intersections. Aesthetic design is also important. A barrier constructed without regard for aesthetic considerations could easily be an eyesore. A well designed berm or fence can aesthetically improve an area from viewpoints of both the motorist and the users of nearby land.

    Earth Berms An earth berm, a long mound of earth running parallel to the highway, is one of the most frequently used barriers. Figure 4.17 shows a cross-section of a berm. Berms can range from five to fifty feet in height. The higher the berm, the more land is required for its construction. Because of the amount of land required, a berm is not always the most practical solution to highway noise. Different techniques must be applied in urban as distinct from rural settings. A berm can provide noise attenuation of up to 15 dBA if it is several feet higher than the line of sight between the noise source and the receiver. This is comparable to the noise reduction of various walls and fences which are used as barriers. However, earth berms possess an added advantage: instead of reflecting noise from one side of the highway to another, as walls do,1 and thus increasing the noise heard on the opposite side, they deflect sound upwards. Figure 4.18 illustrates this phenomenon. The cost of building a berm varies with the area of the country and the nature of the project. In California, the statewide average for building a berm is about $1 per cubic yard when the earth is at the site.2 In planning a berm, one must include seeding and planting in figuring cost. Also to be included are land costs and maintenance in relation to erosion, drainage, snowplowing, mowing, and perhaps future seeding. It costs approximately $1,000 per acre per year to maintain a berm which is accessible to maintenance equipment.3

    4.17 Cross section of a berm

    4.18 Wall barriers may reflect sound from one side of the highway to the other.

    Walls and Fences as Barriers In addition to the more usual function of keeping people, animals and vehicles from entering the highway right of way at undesired locations, a properly designed fence or wall can also provide visual and acoustical separation between highway noise sources and adjacent land areas. This method can reduce noise as much as 15 dBA.4 The vertical construction and minimal width of walls and fences makes installation possible when space is severely limited. This is especially important when land costs are high, and where buildings are already adjacent to the highway. The advantages and disadvantages of wall and fence barriers are summarized in Figure 4.19. The number of design variations for fence and wall barriers is virtually unlimited. Acoustically, any solid continuous structure will suffice, provided that it is high enough, and provided that the barrier is of adequate mass and density. The cost of a fence or wall type barrier can vary considerably according to the type of construction, the material used, local availability of materials and skills, and the barrier’s dimensions. Not all types of barriers are suited for all climates, and local conditions may cause significant differences in the maintenance cost of the various barrier types. The cost questions must be evaluated on a local basis. Some of the frequently used materials for fence and wall construction are masonry, precast concrete, and wood. Masonry noise barriers can be made of concrete blocks, brick or stone. A concrete block barrier might range in cost from $10a linear foot for a 6-ft. high wall, to $75 a linear foot for a 12-ft. high wall. This latter figure includes a safety railing. In general, a concrete block wall would cost $50 to $60 a linear foot.1 To alleviate the monotony of a long run of wall, pilasters can be used: a 20 ft. high concrete wall with pilasters might cost $300 per linear foot.2 Brick and stone are extremely expensive and should only be used for special aesthetic considerations.3 Precast concrete panels offer opportunities for cost reduction. A 13′ 4 high wall in Fairfield, California constructed of precast concrete panels cost only $29.50 per linear foot Wood noise barriers are another possibility. They tend to be less expensive than other methods but are not as durable. An estimated cost for a 6′ high 5/8 plywood fence is $5.00 per linear foot. 4

    Plantings Plants absorb and scatter sound waves. However, the effectiveness of trees, shrubs, and other plantings as noise reducers is the subject of some debate. Some conclusions can, however, be drawn:

    • Plantings in a buffer strip, high, dense,and thick enough to be visually opaque, will provide more attenuation than that provided by the mere distance which the buffer strip represents. A reduction of 3-5 dBA per 100 feet can be expected. Shrubs or other ground cover are necessary in this respect to provide the required density near the ground.
    • The principal effect of plantings is psychological. By removing the noise source from view, plantings can reduce human annoyance to noise. The fact that people cannot see the highway can reduce their awareness of it, even though the noise remains.
    • Time must be allowed for trees and shrubs to attain their desired height.
    • Because they lose their leaves, deciduous trees do not provide year-round noise protection.

    In general, plantings by themselves do not provide much sound attenuation. It is more effective, therefore, to use plantings in conjunction with other noise reduction techniques and for aesthetic enhancement. The cost of plantings varies with the species selected, the section of the country, the climate, and the width of the buffer strip. For deciduous trees and evergreens, costs range from $10 to $50 a linear foot. The width of such a strip would be approximately 40 feet for deciduous trees and 20 feet for evergreens. Planting shrubs between the trees so as to form a dense ground cover would double the price.

    Combinations of Various Barrier Designs Often, the most economical, acoustically acceptable, and aesthetically pleasing barrier is some combination of the barrier types previously discussed. For example, the Milwaukee County Expressway and Transportation Commission feels that barriers constructed of precast concrete on top of an earth berm provide maximum benefit for the cost.5 They estimate that such a combination costs $51 per linear foot. In addition to cost advantages, an earth berm with a barrier wall on top of it possesses several other advantages over both a wall or a berm alone: 1) it is more visually pleasing than a wall of equivalent height; 2) the berm portion of this combination is less dangerous for a motorist leaving the roadway; 3) the non-vertical construction of the berm does not reflect noise back to the opposite side of the highway the way a wall does; 4) the combination requires less land than would be required for a berm of equivalent height and slope; and 5) the wall provides a fencing function not provided by a berm. Another combination to be considered is that of plantings in combination with a barrier. Not only do plantings and ground cover provide some additional noise attenuation, but they also increase visual appeal.

    1 Reflection of noise from one side of the highway to another can increase sound levels by 3 dBA. Scholes, Salvidge, and Sargent, Barriers and Traffic Noise Peaks,Applied Acoustics, 5:3 (July 1972) p. 217.

    2 This estimate was provided by the California Highway Department.

    3 Ibid.

    4 California Division of Highways, Highway Noise Control, A Value Engineering Study, (October 1972).

    4.5 Conclusion

    Figure 4.19 provides a summary of the physical techniques which can be used by designers, builders, and developers to reduce highway noise impacts. Some conclusions follow which may be useful in getting them implemented.

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