EU Directive 70/157/EEC and UN-ECE Regulation No. 51 specify the sound level that is allowed for road vehicles. Since the old test method was considered to no longer reflect real-world driving, a new draft test method was made available to automotive manufacturers and authorities in 2007. The old test method underestimated the effect of tire rolling noise. The new test method rectifies this situation. For a period of three years, the new test method was monitored alongside the old test method and recommendations were then made concerning what test method should be used and what the noise emission limit values should be.
The old test method was developed under worst case urban conditions, meaning full throttle in urban areas. For passenger cars, the old test method is completed on a test track that is 20 m long and one that had a very smooth and quiet road surface. Halfway along the test track, on either side, a microphone is placed at a distance of 7.5 m from the centerline of the track and 1.2 m above the ground. The vehicle is required to enter the test area at 50 kph and then accelerate at wide open throttle, usually in 2nd or 3rd gear. The minimum required tire thread depth is 1.6 mm. The pass-by noise level is then measured. For other types of vehicles the test is similar, but the test speed and gear selection may vary.
The new test method was developed to be “design independent” and was developed to improve its representation of urban driving conditions. To achieve this improvement, the new method includes a constant speed section as well as a more complicated wide open throttle acceleration test section. During the new test method for passenger cars, the vehicle must enter the test track so that a speed of 50 kph is reached where the microphones are placed (halfway along the test track as in the old test method). The minimum required tire thread depth is at least 80% of the full depth. The pass-by noise of the acceleration section of the test is measured, and if necessary a weighted average sound level is calculated based on each gear that is used during the runs. For the constant speed section of the test, the vehicle is operated at a constant speed of 50 kph along the test track in the same gear that was needed for the acceleration section of the test. The pass-by noise for the constant speed section is then recorded as the maximum level measured by each of the microphones. The total urban sound level is finally calculated as the weighted summation of the sound levels from the acceleration and constant speed tests. A weighting factor is used. For other types of vehicles, the test procedure is less complex and does not include the constant speed section of the test.
During the time period that the two test methods were monitored, the European Commission requested DG Enterprise and Industry, Unite Automotive Industry, TNO to complete a study into what the new limit values for noise emissions should be, among other assessments. TNO was to select a solution from five different options: (1) no change; (2) new method – old values; (3) new method – new limit values equivalent to the old ones; (4) new method – new limit values with noise reduction potential; and (5) new method – new limit values with noise reduction in two steps. TNO was also tasked to recommend whether or not special vehicles, such as ones with a direct-injection diesel engine, high-powered vehicles, or off-road vehicles, should be given special allowances to the noise limit values. These vehicles currently are given allowances to the old test method and the current permissible noise emission levels. TNO was also asked to provide an assessment of the social and economic impact of its selected recommendations.
TNO recommended option 5. In other words, they recommended that the new test method should be used and that the limit values for light and medium vehicles should be lowered in two steps each of 2 dB(A), starting in January 2013. For heavy vehicles, the limit should be lowered by only 1 dB(A) during the first step. If the recommendations are passed, all vehicles will have to comply with the new noise emission levels by January 2017. TNO also recommended that the allowance for direct-injection diesel engines should be removed and that the high-powered passenger car and off-road vehicle allowances should be maintained but with some modifications.
One of the reasons that the European Commission is looking to bring in tighter controls for vehicle noise limits is because of the road traffic noise issues that have been emphasized by a number of recent studies. Since 2000, researchers have been investigating the annoyance level and health effects of the public to road traffic noise. These studies have found that the problem needs to be addressed immediately. People that are exposed regularly to traffic noise can suffer from annoyance, sleep disturbance, concentration loss, speech intelligibility issues, and a general reduction in their quality of life. The researchers found that there has been a continuous growth in traffic volume for all road types. As a result, more citizens are affected by the noise generated by automobiles. The studies also found that the benefits that will be achieved by reducing the noise emission level far outweigh the cost the automotive manufacturers will incur to implement solutions to meet the reduced noise limits. The European Commission also took into consideration technology advancements that have taken place in the areas of powertrain noise and tires when it requested the investigation.
The new test method results in a shift between measuring pure powertrain noise to measuring a mixture of powertrain noise emission and rolling noise. This new mixture is deemed to be more representative of the average driving condition experienced by vehicles than the one simulated with the old test method. Overall, the new text method results in lower noise emissions level than the old test method which is why TNO recommended that the new test method be used with revised noise limits. It was also found that the new test method was more sensitive to environmental factors such as background noise, temperature, humidity, wind speed, etc. However, it was determined that these sensitivities were manageable and that the benefits of the new test method outweighed these negatives.
TNO did site that although the new test method is more representative that it still only encompasses a small portion of the possible operating conditions of a vehicle. They, therefore, recommended that off-cycle provisions be added in conjunction with the new test method which will limit the ability of manufacturers to optimize the vehicles for the test cycle. They recommended using the methodology for Additional Sound Emission Provisions (ASEP) which is being developed by UNECE GRB’s Informal Group ASEP for vehicles falling into the categories of M1 and N1. TNO recommended that no additional off-cycle controls were necessary for vehicles falling into other categories.
TNO stated in their findings to the European Commission that a 1 to 2 dB(A) reduction in automotive noise levels was already achievable with the current technology available to the automotive industry. They also provided some ideas on the areas to target for noise reduction improvements in the future. They found that short-term solutions that would be available in the three to five year period would likely include engine tuning and speed control, engine part damping, reductions in engine exhaust and air induction noise levels, shielding, and enclosure absorption. They stated that these types of improvements were all feasible with the existing automotive components and that they could realistically result in an exterior noise reduction level of 1 to 4 dB(A).
The reason that TNO recommended that the 1 dB(A) allowance for direct-injection diesel engines be removed is that when they tested diesel engines the average test results for the diesel engines were actually slightly lower than the results for gasoline engines. They also found that vehicles considered high-powered actually had an average noise level that was 1.7 dB(A) higher than other passenger cars using the new test method which is why they recommended keeping this allowance in place. They did, however, propose that the criterion to be able to qualify as a high-powered vehicle be changed. The proposed criterion is that the vehicle shall have a power to mass ratio greater than 150 kW/ton (based on the new test method).
Unfortunately, if TNO’s recommendations listed above are enacted without a plan to modify them in the future, TNO themselves admits that the new test method and the new proposed limit values will not drastically reduce the number of annoyed people affected by traffic noise. This result is because the traffic density is expected to continue to increase in the near future. Because of this finding, TNO also recommended to the European Commission that a continuous strategy be put in place to regularly monitor and adjust the noise limit values. The organization believes that by announcing such a strategy in advance the automotive industry will be able to anticipate future requirements more easily and be able to begin to develop a long-term design strategy that is able to cope with these requirements.
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