Although of a different nature, as for pollutant emission control, trade-offs between conflicting objectives relating to motor vehicles, noise control requires an analysis of the problems and related factors, prior to defining control levels, involving different technological approaches.
Determination is required on the part of authorities to define priorities and to tackle environmental problems in relation to noise. In this respect, decision makers should bear in mind the following considerations:
(i) Particularly in metropolitan areas, noise directly affects senses, and consequently, the quality of the urban environment, more than any other nuisance stemming from technical progress.
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Urban noise is an erratic combination of “Peak noise” and background noise of a lower intensity, ranging from about 50 to 70 dBA for which motor vehicles are regarded as one of the major contributors.
This is in spite of the significant progress achieved in the last decades in silencing exhausts and reducing mechanical noise. Noise disturbs sleep and may be a cause of stress.
(ii) Four-w heeled vehicles (passenger cars, public transports and commercial vehicles) and two-wheeled vehicles (motor cycles and mopeds) present different characteristics, as far as their acoustic energy emission and radiation are concerned. Experience has shown that, especially in urban areas, commercial vehicles and motor cycles are responsible for the majority of “Peak noise” emission recorded in traffic.
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(iii) Concerning the four-w heeled vehicles, both acoustic energy generation and radiation should be considered. For these vehicles, the noise control levels enforced in Europe, Japan and the United States of America, generally, result in a minimum exhaust pipe noise, achieved by means of advanced silencers, using gas expansion and noise frequency interference techniques.
It may be difficult technically to further reduce this source of noise. Conversely, exploration of ways and means should continue to reduce the following acoustic energy emissions and radiations which are unrelated to exhaust:
(i) Fan aerodynamic noise, particularly from air-cooled engines,
(ii) Mechanical noise from the engine cylinder block and crank case, gear box transmission and alternators,
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(iii) Rolling noise from tyres in contact with the road.
The substitution of larger engines, with lower RPM for engines of smaller cubic capacity and higher RPM coupled with higher transmission ratios, should also be considered in association with related parameters which may be included in vehicle taxation criteria.
Many Industrialized countries are considering the possibilities of progressive reduction in noise emission levels of motor vehicles of 5 to 10 dBA according to the vehicle type. Attention is drawn to the fact, however, that different noise measurement methods are used in different parts of the world.
(iv) Diesel equipped vehicles are noisy by the nature of the engine combustion, under very high compression. Public transport and heavy commercial vehicles require special consideration, and authorities should pay particular attention to their proper maintenance and periodic inspection, including their silencing devices.
Authorities should also encourage research and development on advanced diesel engine technology, and appropriate engine and transmission encapsulation which when applied at the vehicle design stage, may reduce their acoustic energy emission and radiation.
Incentives should also be given to implement “quiet truck and bus” demonstration programmers where the cost-effectiveness of the solutions demonstrated would be the major aspect to be considered. Such programmers could be complemented by appropriate durability testis to provide data on degradation of noise emission as a function of use, age and service. Concerning buses, the interior noise issue should also be considered.
(v) Noise control for motor cycles appears to be one of the most complicated issues amongst those relating to environmental protection and motor vehicle use. Owing to the weight limit, high engine power/ cubic capacity ratio, resulting in high engine RPM for maxi torque; have to be used on most motor cycles.
Moreover, since, for reasons of cooling and handling in cross-winds, the engine and gear box cannot easily be encapsulated or protected by sound proofing cover, the problem of their acoustic energy radiation remains difficult to solve.
However, more than other current road transportation noise problems; the motor cycle noise issue may be seen more as a problem of use rather than one of an originally noisy machine. In this connection, in every country, many new motor cycle owners erroneously believe that they can get extra engine power by tampering with the silencer equipment.
As a result, more replacement silencers, which are less effective than the original, are manufactured and sold, and consequently, more “Peak noise” from motor cycles, when overtaking, is observed in urban areas. Both public education and Government regulation are needed to, progressively, correct this situation.
Authorities who have not done so are encouraged to adopt both road-user education programmers and noise regulations such as those already enforced or in preparation in Europe, Japan and the United States of America.
These regulations should include noise standards and a labeling requirement which for example, would certify to vehicle owners and the police that a motor vehicle, and especially a motor cycle, equipped with a specific replacement silencer, complies with noise level prescribed.
Amongst the technological approaches which may be considered to decrease motor vehicle noise emission, some may conflict with other environmental objectives, such as safety and fuel economy.
For instance, reduction of rolling noise, under wet and dry conditions through smoother road surface, may give rise to dangerous deterioration of the tyre grip on roads under wet conditions, whilst stone or wood-paved streets give rise to both poor and noisy rolling operation.
In the same way, some tyre designs, developed to provide good adherence, lower rolling resistance and improved fuel economy, may be noisier than conventional tyres.
It has also to be noted that, for a given vehicle, larger engines with lower maxi RPM, favorable to the reduction of exhaust and mechanical acoustic energy emission, may penalize fuel economy when associated with weight increase or inappropriate transmission ratios.
These examples highlight the complexity of the noise reduction issue, and necessity for the vehicle owner to respect the manufacturers’ specifications (i.e., type and pressure of tyres, axle ratio) and for authorities to have an integrated approach towards environmental and fuel economy requirements.
Within an overall strategy aimed at improving the noise situation in urban areas, abatement of acoustic energy emissions from motor vehicles should not be developed in isolation, but be associated with appropriate measures related to improved traffic conditions and road surface, land use planning and sitting of residential areas, as well as to the progressive use of sound absorbing materials in buildings and in sound barriers along major highways in urban areas.
In this respect, for example, the possibility of two people holding a conversation at normal levels, between 2 and 3 metres apart in a dwelling, along any urban road, should be, amongst others, a reasonable and practicable objective. This implies that peak noise should be eliminated and background noise level during the daytime should be less than 60 dBA (as measured at the facade of the building).
As for pollutant emission control, there is a need for an internationally acceptance test procedure for noise measurement, and efforts should be to evolve an acceptable test procure for noise measurement and efforts should be addressed to reaching agreement on the definition of a common noise testing procedure which, in any case, would better reflect traffic noise than the procedures currently in use.
Proposals have been made by the United States of America Environment Protection Agency and the Committee of the Common Market Constructors (CCMC), whilst the Group of Experts on the construction of Vehicles of the UN/ECE has recently adopted a slightly modified and more appropriate version of the ISO R 362 method.
However, these solutions differ significantly, mainly with regard to vehicle approach speed and acceleration and position of microphones in the measurement area. No agreement has yet been concluded as regards the adoption of a common and appropriate noise testing procedure.
At the same time, new trends are being observed in the evolution of the car market, where there is an increased registration of smaller and lower powered cars in the United States of America.
Furthermore, the trend for more diesel equipped cars in Europe is being followed in the United States of America. Both of these trends may change progressively the traffic noise conditions.