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Indoor Pass-by: A solution to an outdoor problem

Indoor pass-by testing helps vehicle OEMs not only to comply with new noise limits but also investigate and optimize their designs in a cost-effective way.

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.
UN/ECE R51.03

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? 

pass-by brochure icon
LEARN MORE BY DOWNLOADING OUR BROCHUREAnd 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.