In this article we shall consider methods of reducing exposure to noise from traffic, as distinct from the methods for reducing noise at the source through improved vehicle design.
These methods, which are applied in different ways from place to place, are essentially of three types:
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1. Noise reduction by urban planning and road design (including the use of sound screens or barriers).
2. Noise reduction by architectural means, such as sound proofing and arrangement of rooms.
3. Noise reduction by means of traffic control and police measures.
Urban Planning and Road Design:
We have seen that a large number of parameters influence the noise levels caused by traffic. Substantial reductions in noise can be achieved by changing the design of roads and their surroundings, that is, by appropriate urban planning.
From the standpoint of suppressing the noise transmitted towards dwellings, tunnels are the most radical solution—yet a solution that creates many difficulties. In traditional cities and towns, considerable topographical and technical problems can arise because of the state of the subsoil, the space taken up by sewers, underground railways, etc.,—factors that make excavation difficult and sometimes even impossible.
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Furthermore, when an underground road can be built, it is generally entirely new rather than a substitute for an existing surface road to be closed as soon as the underground road is opened. In addition, building a road underground is by far the most costly solution.
According to a British estimate, building a road in a bored tunnel is 13 times more expensive than building it at ground level. Noise and pollution from ventilation systems are also among the various problems which arise with tunnels (as they do with underground parking lots). The outlets from these systems must be provided with suitable deflectors and in some way must remove (filter) the exhaust gases, the noxious fumes and pollutants.
Tunnels must also be treated acoustically, so that sound levels inside do no exceed the levels which can be tolerated by motorists. Further, the effect on surface buildings of vibrations due to traffic can be serious. In brief, solving a given problem should not go so far as to create a new type of annoyance.
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Noise from a raised road affects a greater area than noise from a road at the level of the adjacent ground. Thus building a raised road is clearly the least desirable solution from one standpoint of noise transmission, unless the road is bordered by a solid parapet.
On the other hand, an effective way of preventing the propagation of noise is, as we have seen, to place a road in a cut. In general, vertical walls give better noise protection than sloping banks, but there is a danger of reflection towards adjacent buildings.
A road in a cut with sloping banks is likely to protect only the lower floors from noise, but when the adjacent ground is grassed over, reflection effects are avoided. According to British estimates, if the cost of constructing an urban roadway at ground level is equal to 100, building it in a cut with sloping banks will vary between 130 and 200 and with vertical walls between 400 and 600.
Roads in cuts can prove especially suitable for expressways and main roads in suburban areas, at some distance from dwellings, provided these are not too high. The vertical walls or sloping sides of the cut thus act as barriers and noise reduction at any point will depend on the distance and height of this point with respect to the barrier.
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However, from an acoustic standpoint, to build roads in cuts is virtually valueless in congested districts—as in old towns—with narrow streets and high buildings. It is only in new or renovated districts, where it is possible to alter the width of streets and the height of adjacent buildings, that construction in cuts—particularly in the case of expressways—can prove really worthwhile.
In order to reduce to a minimum noise due to the road surface, all roads that are still stone-paved should be covered with asphalt: this can result in noise reductions possibly exceeding 5 dBA. The economics of surfacing is, however, difficult to assess in view of the substantial variations (such as geography) that affect the cost of surfacing.
Yet an asphalt paving needs to be replaced or repaired much more often (about every 3-5 years) than a stone or concrete surface. Thus, the expenditures for road maintenance (and the resulting disturbance to traffic) should not be overlooked in attempting to assess the advantages and costs of different types of road surface.
The side walls or banks of roads built in cuts act as acoustic barriers. But it is also possible to erect barriers along roads built on grade, at ground level.
The effectiveness of trees and shrubs as acoustic screens is hard to predict. Foliage density varies growth patterns are affected by the environment, car exhausts, salt spray, disease etc. Deciduous trees cost less and more adaptable to city conditions than evergreens, but are of limited use during winter. In favour of natural barriers, it must be remembered that the interaction of wind and trees produces a pleasant, soothing sound.
The effectiveness of an artificial barrier depends on its height and type of construction and also on the distance and the height of the observer with respect to the barrier. A barrier is better at reducing noise peaks than background noise (since part of the sound energy diffracts around it). Thus, the closer the observer is to the barrier, the more effective will be the barrier.
The maximum noise reductions that can be expected in practice from an acoustic barrier are limited to between 10 and 15 dBA. Figure 9.4 shows the noise reduction obtained at different heights and distances from a barrier 3 metre high. A specific mass of 15 kg/m2 is sufficient for a barrier to be effective. However, the requirements a barrier has to meet because of its position in the open air—safety, wind resistance and durability—are such that this specific weight will in fact need to be exceeded.
The main acoustic requirement for a barrier is that it should be airtight. Special attention must be given to any joints. Furthermore, when dwellings are so situated that they can receive reflected sound from the barrier, it may prove necessary to cover it with some absorbent material. The process is very costly, however and gives rise to problems of maintenance and weather resistance.
Protective barriers have been tested in a number of countries. The barrier consists of plastic panels mounted on a metal frame, which are reported to give a 10 dBA attenuation.
Often choices are possible. Dwellings may be built near a main road and protected from traffic noise by a screen. Or, they may be built farther away from the road. The selection between the two solutions must be guided by an assessment of the respective costs of a barrier and of the amount of land necessary if no barrier is used.
In building a barrier, a number of factors need to be considered in addition to cost and acoustic effectiveness:
1. Wind resistance.
2. Degree of inflammability.
3. Degree of sound reflection or non-reflection under different weather conditions.
4. Ground area required.
5. Aesthetic aspects (opacity or transparence).
In general, like roads in cuts, protective barriers are not effective in traditional streets of congested districts, since the buildings are too near the road and the intersections too numerous. On the other hand, the use of barriers along certain sections of highways can be an optimum, short or medium-range solution to reduce peak traffic noise until the noise from engines and exhausts can be substantially reduced; once engines and exhaust are quieter, it can also be a long-range solution to reduce tyre noise.
Separation of noisy areas from residential areas (zoning), whenever feasible, can be an adequate noise control solution in certain cases. Geographical separation according to function can prove to be relatively simple in building new factories away from residential areas or new residential areas away from industrial zones. But it is a far more difficult process when it comes to the location of roads, since their raison is to link together as many points as possible rather than to isolate them.
In spite of this difficulty, some amount of road planning is possible. Fast roads can skirt congested residential districts, while the residential character of certain areas can be preserved by adopting strict zoning directives, such as limits to the height of buildings or restrictions in building areas.
In new districts, the construction of screening buildings (air-conditioned offices, garages, stores, factories) between roads and dwellings can be promoted. This has been done, where buildings exclusively used for car parking act as screens between residential areas and main roads. The effectiveness of screen buildings of course depends on their height and their arrangement with respect to the dwellings they are intended to protect.
The placing of non-residential buildings to screen buildings from arterial roads is not without problems, however: if land in the area is expensive, the screening buildings may prove impossible or difficult to build economically. Furthermore, the screening buildings themselves can become sources of noise (as in the case of factories) or can be unattractive in appearance.
The best solutions often consist of adopting a set of complementary measures. For instance, if a road is built in a vertically walled cut, it could be bordered by low buildings. High buildings could be erected farther away from the road, thus benefiting from the cumulative protection provided by the cut and by the screen of low buildings.
Urban planning schemes involve considerations that range from road design, to land-use planning and to the location and architecture of buildings. Thus their configuration and cost are heavily influenced by local conditions. If an area between the road and dwellings must be kept clear, the costs may well turn out to be high, especially if the road is in an urban area.
Soundproofing and Arrangement of Living Space:
Methods of attenuating the noise between the road and the front of the dwelling—through appropriate urban planning and road design—need to be coupled with measures for reducing the noise at the dwelling— through soundproofing and appropriate arrangements of the living spaces.
In a building, in general, the insulation provided by the external walls is appreciably greater than that attainable with windows. Thus, in almost every case, the determining factors in soundproofing a dwelling is soundproofing of the windows and, if necessary, air-conditioning the entire dwelling.
The acoustic insulation of a window depends on the spectrum of external noise.
If windows must be intensively soundproofed, care must be taken to provide mechanical ventilation. In practice, soundproofing possibilities must match minimum comfortable summer temperature requirements (corresponding to a renewal of the air at least five times per hour).
Soundproofing costs—and the cost of providing air-conditioning where necessary depend on the amount of insulation required, the size of the openings, the method adopted and on different geographical and economic variables such as whether the dwellings to be soundproofed are detached houses or flats, whether the windows are of standard size or not, etc. Costs also vary substantially according to whether existing dwellings must be modified or special soundproofing measures are adopted at the design stage. If modifications are made to existing dwellings, the cost will obviously be much higher.
The cost of soundproofing new dwellings to afford protection not only from noise outside but inside the building varies in the United States between 2 per cent and 10 per cent of the total cost of the dwelling. In the United Kingdom, it has been calculated that to protect all houses in the country exposed to a level of traffic noise considered to be unsatisfactory (greater than 65 dBA at the front). In France, an ‘acoustic comfort’ standard has been instituted that permits the grant of an additional 2-3 per cent to low-cost housing financed by the State if their acoustic qualities are deemed to be adequate.
In the United Kingdom, a recent investigation included a calculation of unit costs as determined by the desired acoustic attenuation.
In any event, acoustic insulation of dwellings (and where appropriate, hospitals, schools and all premises requiring a certain amount of silence) is not a universal remedy. It offers no protection against noise perceived outside the buildings. Furthermore, in order to be really effective, it requires an air- conditioning system and permanently closed windows.
However, soundproofing becomes essential to provide protection against the most serious cases of noise, primarily those along expressways as well as near airports and other very noisy places. Acoustic insulation, although having the disadvantage of only giving protection against noise at the point of reception, has the advantage of giving protection against all types of external noise.