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Road traffic noise is one of the most widespread environmental noise problems globally. It is more than just an annoyance; it is a major health concern. To help reduce noise in our environment, government agencies set pass-by noise emission limits for each category of ground vehicle – critical regulations that must be respected and are a requirement for all automotive OEMs as part of product certification. And these regulations are becoming increasingly tough.

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UN/ECE R51.03 Regulation

UN/ECE R51.03, introduced in 2014, set a timetable for greatly reducing the pass-by noise limit values. For example, by 2024, the new limit for passenger cars (category M1) will be set to 68 dB(A) – a reduction of 6 dB (a factor of 2) over the three phases. Even fully electric vehicles may struggle to meet this limit without attention to noise reduction measures for tyre noise. 

An additional challenge for vehicle OEMs is the constant speed road test, added to the test procedure to reflect more realistic driving conditions, and in recognition of tyre noise contribution becoming increasingly dominant even at low speeds, shifting the noise bias from engine to tyres. The Additional Sound Emission Provisions (ASEP) tests require that manufacturers demonstrate that their vehicles’ sound levels do not significantly differ from the ISO pass-by test result, under typical on-road driving conditions, over a broader range of speeds. The current ASEP definition requires four additional speeds for each gear, a total of 24 additional measurements for a 6-speed car. Future revisions to ASEP that are expected soon are likely to further increase the burden of testing.  

Combine all the additional test requirements with the push for reduced time-to-market requiring more efficient testing, and one wonders how OEMs will find the test capacity to successfully develop the next generation of vehicles in compliance with these international requirements. Indoor pass-by testing is part of the answer. 

Indoor or out?

Traditionally vehicle pass-by noise is measured at an outdoor facility composed of a straight section of road and two measurement microphones, one on either side. The vehicle accelerates between the pair of microphones and the maximum sound pressure level at the two microphones is recorded.  

car pass-by outdoor
Outdoor pass-by noise test track

However, the challenges of field pass-by measurements are many:

  • Are weather conditions acceptable? 
  • Is the background noise low enough?  
  • Is the test surface suitable?  
  • How consistent are the test runs?  
  • Can I minimize my time on the test track? 

And although a field pass-by system can’t control the weather or chirping birds, it does the job of acquiring, processing, storing and reporting data, collecting all required parameters such as engine speed, vehicle speed, weather and acoustic data. For many OEMs, this is still the preferred method used for conformance and homologation.

However, there are many reasons why road vehicle manufacturers are increasingly choosing indoor pass-by testing to help them comply with the new noise limits.

Since 2019, indoor pass-by has been approved for homologation (R51.03 Am4) and conformity of production, and is, therefore, a real alternative to the weather-dependent outdoor method. Measuring a vehicle’s interior and exterior noise during operation is far simpler indoors using a chassis dynamometer in a large hemi-anechoic room. In this controlled environment, measurements are protected from the elements, are highly repeatable, can be made all year round, 24/7, while the stationary vehicle allows easy access for more instrumentation to ascertain the root causes of the emitted noise.

The pass-by test is simulated by driving the stationary vehicle on the dyno as if it were a conventional outdoor pass-by measurement and measuring the emitted sound with a linear array of microphones to either side of the vehicle. The microphone signals are processed together with the vehicle speed to simulate a pass-by result which is directly comparable, including doppler correction, with an outdoor result. The standard provides a method for correcting the tyre noise using data from an outdoor measurement on an ISO-compliant test track.  

How to take advantage of indoor pass-by testing?

Indoor Pass-by

Indoor pass-by testing facility approved for homologation (R51.03 Am4).

There are many factors that determine the pass-by test method but regardless of choice, indoor results can be correlated with outdoor results, ensuring that any decisions made regarding exterior noise are based on correct information.

Development and design

When it comes to development and design there is a long list of possibilities to help vehicle manufacturers investigate and optimize their designs to not only get through the regulations in a cost-effective way, but also address exterior noise earlier in the design process. These include noise source quantification methods, such as array acoustics and Source Path Contribution (SPC) analysis.

Source contribution analysis

One of the biggest advantages of indoor pass-by is the use of noise source contribution analysis to look at the contributing effects of all the different potential sources. This is hugely important for all vehicle manufacturers. Electric vehicles, albeit quieter, still emit noise that is not tyre noise, so it is also important for EV manufacturers to understand the contribution effects of these different/new noise sources. 

SPC is a technique for evaluating contributions from different sources to receiver locations, in this case the pass-by microphones. The dominating sources with respect to a given receiver can be highlighted, then further investigations, using CAE or development testing, can focus on those sources. This method is often used to investigate design solutions for meeting both the pass-by noise regulations and interior noise. 

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Tyre Noise Measurements, Engine Noise Measurement, Exhaust Analysis and Interior Noise Analysis

With the ever-increasing restrictions on vehicle noise and the importance of correcting problems earlier in the design cycle, indoor pass-by is a convenient and efficient way to evaluate exterior noise and avoid the unexpected. From design to homologation, it is becoming a crucial tool in the engineer’s toolbox.