An indoor solution to an outdoor problem

Red car in an anechoic chamber for indoor test-by testing of the noise


How indoor pass-by testing helps vehicle OEMs comply with new noise limits


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. These regulations are becoming increasingly tough.


UN/ECE R51.03, introduced in 2014, altered the driving conditions to put greater emphasis on tyre noise, and defined a timetable to greatly reduce pass-by noise limits. By 2024, the new limit for passenger cars (category M1) will be 68 dB(A) – a reduction of 6dB since R51.03 was introduced. Even for fully electrically powered vehicles this will be a challenge.


Not only are the noise limits becoming harder to meet, the amount of testing required to prove compliance has increased. The Additional Sound Emission Provisions (ASEP) require manufacturers to 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. New revisions to ASEP, due in 2021, are likely to increase the volume of testing even more.


With these additional test requirements, and the push for reduced time-to-market requiring more efficient testing, vehicle OEMs face a big challenge to find the test capacity to develop the next generation of vehicles in compliance with the requirements. Indoor pass-by testing is an important part of the answer.


Homologation and conformity

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.


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 get through the regulations in a cost-effective way, including noise source quantification methods, such as array acoustics and Source Path Contribution (SPC) analysis.


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.


With the ever-increasing restrictions on vehicle noise and the importance of correcting problems earlier in the design cycle, indoor pass-by is becoming a crucial tool in the engineer’s toolbox.

Related blog articles