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An examination of vehicle noise test procedures - Executive summary

Executive summary

Since the early 1970's new motor vehicles have been subject to type approval test procedures that limit noise emission. Since the regulations were first introduced, the limits imposed have been progressively tightened with the overall objective of reducing the levels of traffic noise and the associated impact of traffic noise on people. However, despite the dramatic reductions achieved in power train noise from vehicles for the conditions imposed by the type approval test, traffic noise levels in the UK and Europe do not appear to have fallen significantly. Several studies suggest that the noise from traffic, i.e. after correcting for traffic flow and speed, has either remained virtually constant, or has, under certain conditions risen, over the past twenty years. The lack of a closer correlation between type approval noise limits and traffic noise emission levels has been attributed, in part, to the test procedure that is used for type approval. It is argued that changes are needed so that the test procedure is made more representative of vehicle operations in real road situations.

It is clear that future vehicle noise test procedures will need to ensure that there is a better degree of discrimination between noisy and relatively quiet vehicles for conditions representative of noise intrusion in residential areas. This may require vehicles to be tested under a variety of operating conditions representative of normal driving. Future testing will also need to ensure that appropriate examination of both power unit and rolling noise components are considered and with the emergence of alternative fuelled and hybrid powered vehicles, any future test procedure should also be appropriate for new and emerging vehicle technologies.

Given the broad range of options that are presently being considered by regulatory and standards authorities, the Department for Transport commissioned TRL to examine the technical issues associated with the range of test procedures that could be considered for future vehicle noise testing. In summary, the objectives of the study are to:

  • To produce a critical assessment of the proposed UN-ECE test procedures and their potential effectiveness in meeting the aim of reducing road traffic noise nuisance.
  • To produce an assessment of test procedures not currently included in these new proposals and to examine each procedure's ability to distinguish between different types of quiet or new technology vehicle from relatively noisy vehicles.
  • To provide indicative noise limit levels on the proposed test.
  • To produce an assessment of the costs to UK industry, the UK consumer and the general public resulting directly or indirectly from the impact of the proposals and any recommendations to change these.
  • Provide vehicle noise emissions data that could be used for the purposes of modelling road traffic noise

This report provides the results of this study. It begins with a comprehensive review of the issues associated with changing the vehicle noise type approval test procedure. It includes an assessment of results from drive cycle and perception studies and provides an overview of both current and proposed test procedures and related research studies. This information has been assessed to help define a range of candidate test procedures that have been examined further as part of a measurement programme using a carefully selected sample of vehicles. The test procedures examined include both the current and proposed new test procedures as well as exploring a range of additional test procedures designed to expose noise emission from different types of vehicle operation that are not currently covered in the proposals. These test procedures have examined noise from typical low speed operations as well as noise from stationary vehicles. Noise from vehicles idling, noise from the operation of the air brakes exhaust noise and noise from ancillary equipment such as hydraulic tailgate operation have been measured as part of this programme.

The vehicle sample selected included five HGV's, four buses, five minibuses, four vans, five cars, two sports cars and one 4x4. The results from the test programme have indicated the limitations and advantages of the various test procedures and exposed questions of interpretation of the test methods, some of which have already been addressed by the relevant standards committees. The results been analysed to investigate a broad range of issues associated with changing the test procedure and have focussed on providing answers to the questions posed by the main objectives of the study. The measurement programme has enabled comparisons to be drawn between the different test methods and issues of repeatability, reproducibility and the degree to which the tests are representative of real situations have been examined.

By combining the data obtained with that of a survey being conducted by the Groupe de Rapporteur du Bruit (GRB) it has been possible to speculate on the limit values that could be employed if the proposed test procedure is introduced. Modelling techniques have been used to investigate the benefits, in terms of controlling traffic noise levels, of reducing the type approval noise limits associated with the proposed test procedure. The overall costs and benefits of the proposed changes have been evaluated as part of a Regulatory Impact Assessment.

The main conclusions are as follows:

Assessment of the proposed UN-ECE test procedure

The proposed test condition which involves the measurement of the maximum A-weighted level at moderate acceleration (typically 1 to 2 m s-2) and at a cruise-by speed of 50 km/h appears to be appropriate for assessing the noise impact of vehicles under typical urban driving conditions. The current test involves full throttle acceleration in a low gear so acceleration rates and engine speeds are relatively high. Such test conditions are more likely to be representative of the worst case driving conditions than the average condition represented in the proposed test. Consequently the proposed test should more fairly represent the noise impact of a vehicle under urban driving conditions where the vehicle operation is largely constrained by speed limits, presence of junction controls and other road users.

However, it is clear, that with the introduction of the new test, which is intended to expose propulsion and rolling noise, there is a need to consider revising the specification of the International Organisation of Standardisation (ISO) so that it is a more representative surface. It has been noted that a subgroup of ISO WG42 is planning to revise the current test surface specification. It is hoped that a new surface will emerge from the group that better represents the rougher surfaces found in the UK. In the event that the ISO surface is replaced by a test surface that represents average UK road conditions then the proposed test method becomes appropriate for controlling both propulsion and tyre/road noise. To meet lower limit values manufactures will then need to consider reducing both of these main sources of vehicle noise.

However, adopting the proposed method and imposing limit values at levels which would pass most vehicles that meet the current standard may lead to poor control of noise levels under worst case driving conditions. It has been shown that three M1 vehicles that failed the current test would actually pass the proposed test if the Lurban value was set above 69 dB(A), i.e. a level where 70 % of current vehicles would fail. This argues for retaining the current test (or imposing a similar test) in order to control noise at high engine speeds. Annex 10 would be the appropriate place in the regulation to require additional testing to ensure noise levels were acceptable under more extreme conditions than the proposed test condition.

Assessment of other test procedures

Generally all the tests examined have indicated a good degree of repeatability. However, for a test designed to represent a vehicle accelerating from a stationary condition, such as might occur at a junction or traffic light, there were some problems obtaining consistent results. Of particular interest was the fact that vehicles tested using this low speed test were ranked differently than when tested using the current or proposed pass-by test. This suggests that this type of low speed test is exposing some aspects of noise generation that is not being picked up by the other test conditions. A review of earlier studies examining the subjective assessment of the noisiness of different vehicle operations has indicated the importance of this operating condition. However, if a low speed test of this type is to be included in future revisions of the standard test method then further work is needed on the specification of the driving condition and, of course, additional data would be needed to define appropriate limit values.

Exhaust test results show that there is no real advantage in carrying out a slow or rapid change in the engine speed as in the current ISO 5130 exhaust test. Overall the ranking of exhaust noise levels for the vehicles tested did not correspond closely with either stationary or moving tests. Therefore, the use of a slow sweep type of test for use in controlling noise emission in the environment under normal driving conditions is limited apart from identifying faulty exhaust systems.

Generally with the slow whole vehicle engine sweep test results for light vehicles and buses were well correlated with the corresponding results for the moving tests. For the heavy vehicles tested, the correlations were less good although the sample size available was small. It was found that measurements taken close to the vehicle (i.e. 2m) are reasonably representative of the total noise from the vehicle measured at 7m. Therefore the test could potentially be carried out under restricted space conditions as part of in-service testing of vehicles.

When the results for vehicles idling were examined and for the heaviest vehicles large differences of the order of 15 dB(A) were noted in the results. One vehicle was particularly noisy at idle but performed reasonably well on the pass-by tests. The use of an idle test as a means of licensing heavy vehicles for delivering to noise sensitive areas would be worthy of further consideration in view of the prevalence of this operating condition.

The measurements taken of air brake noise revealed that of the nine vehicles tested, three gave results that exceeded the proposed limit value set for air brake noise measured at 7m. Low values of air brake noise are particular relevant to quiet operations at depots and supermarkets especially if deliveries are made in the evening or at night. As for noise at idle a case could be made for controlling this source of noise through an appropriate licensing system.

A test on ancillary equipment involved tailgate operation on a delivery truck, automatic door operation and "kneel" operation on buses. The "kneel" operation on a single deck bus produced the highest level of 73.3 dB(A) at 7m. The findings suggest that ancillary equipment is an important noise source which should be considered when designing vehicles for quiet operation.

Results for alternative fuelled vehicles

There was no consistent pattern in the results for Compressed Natural Gas (CNG) vehicles and their diesel equivalents. This might suggest that for the vehicles studied there is no distinct advantage in preferring these vehicles in order to reduce community noise impact. However, a larger sample and greater examination of existing data is required before definite conclusions can be drawn.

An electric van produced levels of 68.8 and 68.2 dB(A) on the current test and proposed tests respectively. The diesel equivalent van produced significantly higher levels of 75.6 and 71.4 dB(A) respectively. It can be seen that both under average and extreme driving conditions the electric equivalent van has a distinct noise advantage. Currently there are few electric vehicles in use but these results argue for a careful consideration of the merits of promoting their use in the future.

Indicative noise limits for the proposed test procedure

In order to set appropriate limit values for the new test it is appropriate to consider technical equivalence to the current standard. However, it is clear from the scatter plots of Lcurrent against Lurban that, although there is a trend for the proposed test to give lower noise levels than the current method, there is no one-to-one correspondence that could be used to reliably establish a set of new limit values. Since no exact technical equivalence exists it is difficult to decide an appropriate limit value based on a purely statistical analysis of test values. In GRB it was agreed that the way forward was to examine the costs and benefits of applying various limit values and to make a decision on the appropriate limit value informed by a knowledge of the trend between costs and benefits. The benefits in terms of traffic noise reduction of lowering Lurban of light vehicles by a significant amount (3dB(A)) have been discussed in this report and without an appropriate test surface they are predicted to be small (less than approximately 0.5 dB(A)).

Recently GRB data for M2, M3, N2 and N3 category vehicles has become available. It is likely that in order to set limit values for these heavier vehicles a similar approach to the analysis of the data will be taken.

Costs and benefits to UK industry, consumers and the general public of introducing the proposed test method

The Regulatory Impact Assessment (RIA) portion of this report recommends that the proposed new ECE vehicle noise test be adopted by the UK. The new test should start as soon as possible, e.g. in 2007, with noise limits that are equivalent to those currently in use with the old test. A tightening in 2010 by 2 dB should then occur.

The benefit to the UK of each 1dB reduction in noise measured at dwellings was found to be at least £524 million per annum. If the new ECE test were to reduce real noise from road traffic by 2dB, this would mean an annual benefit of £1048 million and a minimum benefit cost ratio of 116. This figure is far higher than that from most public investment projects available to governments. However, a reduction of 2dB in the limits for the new ECE test is likely to result in a much smaller reduction in real noise from road traffic than 2dB, because the test surface that is currently use in the test is unrepresentative of real roads in the UK. Although work is in hand to specify a new test surface that is more representative it is likely to be some time before the specification is available as a standard. Given this concern over the surface, it can be stated that the minimum benefit cost ratio of the proposed new test therefore lies somewhere in the range of 11-116.

This RIA should be completed when the GRB sets numerical values for the noise limits to be used with the new test. In addition, data needs to be collected on the proportion of the road network that is coated with each type of thin surface course.

Vehicle noise emission data for modelling purposes

The data collected in the current project will be used in the follow-up project to Harmonoise called IMAGINE (Improved Methods for the Assessment of the Generic Impact of Noise in the Environment). The project started in December 2003 and was funded under the 6th EC framework programme (CT-2003-5033549) and by Department for Transport (DfT). TRL are playing an active part in refining the source model which forms a major part of the overall project.