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Vehicle noise control: an OECD plan for governments

28th January 1972
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Page 30, 28th January 1972 — Vehicle noise control: an OECD plan for governments
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Which of the following most accurately describes the problem?

by a special correspondent

TOO little study of gearbox and brake noise and a corresponding dearth of detailed information about the noise caused by the interaction between tyres and road surfaces — these are factors which are stressed in a new document on vehicle noise which could have far-reaching consequences for both manufacturers and operators. The report, Urban Traffic Noise has been produced by the OECD's Consultative Group on Transportation Research and represents the work of delegates from 14 countries under the chairmanship of Mr B. T. Price from the United Kingdom.

The Organization for Economic Co-operation and Development has its headquarters in Paris and is the influential parent body of the European Conference of Ministers of .Transport on which Britain is represented; this year's president is in fact John Peyton, Minister for Transport Industries.

The group's report makes eight recommendations to form the basis of a strategy for the control and progressive abatement of traffic noise in cities, and quite a number of the recommendations affect vehicle operation.

Their first recommendation is that Government should enact standards for maximum permissible noise emission and should adopt effective procedures to enforce standards. But the report warns that initially the standards should be set and enforced at levels which are consistent with the available technology. As noise reduction techniques become more advanced, so the standards should be made more stringent.

Enforcement of standards should be carried out through type testing of new vehicles and — an aspect sometimes overlooked — type testing of such important components as replacement silencers. Such initial enforcement through standards should, says the report, be coupled with periodic inspection and roadside spot checks. Simple and inexpensive standard instruments should be used for the noise testing, states the group.

In reading this recommendation, the report draws attention to the practice in the city of Stockholm where for instance buses have to pass a noise emission test and vehicles which exceed a certain limit are rejected. This illustration is used to show the importance of the influence that can be exterted through the purchasing powers and practices of official bodies.

The report also recommends that governments include stringent noise emission standards in specifications for all government vehicles.

Restrict noisy traffic So far as urban planning is concerned, it is recommended that governments should encourage restrictions on noisy traffic and re-routeing away from residential and quiet zones. In this regard, it draws attention to the closure of certain streets to heavy lorries at night as in some Swiss and Danish towns. A number of other recommendations in this context are made ranging from more extensive use of tunnels and open cuts to use of noise screens.

Governments, says the report, should support detailed studies of the cost of noise abatement, particularly studies undertaken in co-operation with the motor industry. One of the points recommended here is that such studies should identify technological improvements in engine and vehicle design to meet a range of more stringent standards. Another would be to explore a method of allocating development costs between taxpayers and the driving public.

So far as research and development into motor vehicles is concerned the report says that three particular areas should be investigated: the intake and exhaust systems on vehicles, the engine and transmission, and the interaction between tyres and road surfaces. It is considered that investigation should be made into the long-term effects of traffic noise on people.

The final recommendation is that an international conference be convened to exchange views and share experience on traffic noise control but the date of this should be considered carefully because of the commitments in regard to the United Nations Conference on the Environment which is to take place in Stockholm this year.

Some of the information included in the survey is rather obvious — the comment that bigger engines are accompanied by higher noise levels, for example. However, there are other more useful items of information. Attention is drawn to the fact, for instance, that from one to two per cent of all trucks make more noise when empty ' than when loaded according to a Swedish investigation. This noise, says the report, is due mainly to loose panels, boxes, rattling chains and so on all of which cause a great deal of clatter and are often the result of carelessness. ,Noise limits in several countries include a rule on the loading of commercial vehicles, the report points out.

With regard to intake and exhaust silencers the report says that the greatest differences amongst silencer types are to be found on trucks. The position and condition of a truck exhaust silencer has a major effect on the noise produced, the report comments, and the difference between no silencer and a stock silencer in good condition is typically 15 decibels. A very good silencer provides a further reduction of only about 3 dbA, approximately to the point at which the noise of the engine covers that of the exhaust.

Standards of noise control with diesel vehicles, says the report, and certainly the mechanical condition and maintenance of the machines, is generally high on trucks. As a truck engine is required to produce more power at certain times, such as on an up-grade, the exhaust pressure rises and as exhaust pressure rises the efficiency of a reactive silencer increases. Thus the increased noise produced by greater engine power is somewhat offset by the greater efficiency of the silencer but unfortunately, the report adds, not enough to lower the noise level.

The report goes on to state that unless replacement silencers are subject to regulation, any benefits gained from better standards in factory installed units are quickly lost. Silencers on heavy goods vehicles must be replaced about every 50,000 miles, therefore better silencers could represent an increase in the repair and replacement overheads for transport firms. A goal for silencer design could be to produce a long-lasting as well as a quieter unit, says the report.

The report 'notes that turbocharging as compared to natural aspiration undoubtedly reduces noise at high power by lowering the exhaust back-pressure. However, it changes the frequency content of the noise, especially at lower levels. Turbocharging softens the low-frequency, high-energy engine noise while the high-frequency components remain. The report finds, as mentioned earlier, hat little applicable experimentation has seen done on either gearbox or brake noise. rhe study of these automotive parts, it says, s certainly important in the prevention of raffle noise since most urban traffic is top-and-go and thus requires the most gear hitting and the brake application of any thase of a vehicle operation. As in other .reas of automotive noise, old or faulty ■ arts and improper use produce the naximum amounts of noise.

Of engine noise, the report says that .irborne noise produced by engine vibration ecomes more and more significant as ,etter intake and exhaust silencing is chievcd. This, it says, also places a ractical upper limit on intake and exhaust ilencine, since further reduction would not e observable due to the, predominance of ngine noise.

The noise associated with internal ornbustion. says the report, can be todified considerably by changes in the ylinder pressure /time pattern. Noise level hanges of five dbA for diesel engines and 9 bA for petrol engines have been observed ith cylinder pressure pattern changes. hese results suggest that further study 3uld provide worthwhile reduction in tgine noise.

■ inging tyres The report deals with the noise produced y tyre and road-surface interaction. elativcly little research has been done on tis or on aerodynamic noise, says the port. Many characteristics of the tyre, the sad and the vehicle suspension are, )wever, important in noise reduction. Tyre anufacturers have developed less noisy cad compounds and patterns such as triations in pattern points around the tyre I break up sound harmonies and aquencies. The singing of tyres on St-moving commercial vehicles is usually Lused by tread-wear that leaves a pattern small holes, says the report, and these iction cups, as it describes them, pop as e tyre rolls. It is suggested that tyre anufacturers might develop tread designs at do not wear in such a way.

A change in road surface from rough to tooth asphalts can lower the noise level )rn tyre and roadway interaction by about dbA. unfortunately, smooth asphalt °vides a substantially lower traction in A weather.

Assuming maximum exhaust silencing, 3derate control of the mechanical noise d normal tyre tread designs, a large diesel ictortrailer combination can be expected record 10 to 15 dbA higher noise levels an a passenger car at the same speed — e to the relative contact areas of the tyres th the road. The report goes on to say it the amount of tread and the pattern of tyre, the condition of the road (wet or y), the stiffness of the tyre casing, the Lding of the tyres and the resonance tween the tyre and the vehicle body are all :tors that must be better understood for tter control of source noise. A full 1-down on the noise rules for vehicles in

14 countries represented on the nnmittec is set out as appendices to the sort.


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