The Imperative of Urban Tranquility and the Birth of Regulation.

The regulation of motor vehicle noise emissions is no longer merely a technical matter, but rather a critical public health and environmental protection issue. Research shows that chronic exposure to traffic noise is linked to increased stress and significant sleep disturbances. Noise pollution directly and negatively affects quality of life, particularly in densely populated areas. Recognizing the importance of mitigating this impact, technical regulations have been established to control noise at its source, the vehicles themselves.

UNECE R51: The Global Reference Standard

The regulation of motor vehicle noise emissions is no longer merely a technical matter, but rather a critical public health and environmental protection issue. Research shows that chronic exposure to traffic noise is linked to increased stress and significant sleep disturbances. Noise pollution directly and negatively affects quality of life, particularly in densely populated areas. Recognizing the importance of mitigating this impact, technical regulations have been established to control noise at its source, the vehicles themselves.

What Is UNECE Regulation R51?

UNECE R51 applies to vehicles in categories M and N in relation to their noise emissions. The regulation distinguishes a “vehicle type” based on differences such as:

  • engine compartment design and materials

  • type of propulsion system (combustion, electric, etc.)

  • rated net power

  • silencing system

This ensures that each variant is subject to a separate type approval, reflecting its specific acoustic characteristics.

The Concept of Type Approval

Type approval is an administrative and technical process through which a competent authority certifies that a vehicle prototype complies with all regulatory requirements. It is mandatory for commercialization in the 38 contracting parties to the 1958 UN Agreement, including the EU, Japan, and Australia.

Regulated Components

The regulation focuses on the noise reduction system, including exhaust pipes, expansion chambers, and silencers. The air filter is considered part of the system only if essential for compliance. Documentation must include detailed drawings and material specifications.

The “E-mark” Approval

Once approved, vehicles and components bear the E-mark, showing the country (e.g., E57 San Marino) and approval number, with the first digits indicating the amendment series.

From R51.02 to R51.03: A Crucial Transition

Earlier versions of the regulation faced criticism for being ineffective in real-world conditions. The old R51.02 test focused on unrealistic accelerations, creating a gap between lab results and urban reality.

Introduced in 2016, R51.03 represented a paradigm shift, simulating real urban driving and addressing key shortcomings. Main changes include:

  • mandatory use of standard tires

  • inclusion of rolling noise

  • calculation of a single Lurban value, representing average urban noise

The Science of Sound: R51.03 Test Method

The core procedure is the pass-by noise test, with measurements taken under wide open throttle (WOT) and constant speed. Precision is critical: instruments must meet strict accuracy requirements, and test tracks must avoid reflective obstacles.

The Lurban value is calculated by combining WOT and constant-speed results, weighted by the partial power factor (kP). Noise limits are progressively stricter, reaching 68 dB(A) for passenger cars by 2024.

Indoor Pass-By Noise Tests

Amendment 4 to R51.03 introduced indoor testing, using a dynamometer in a semi-anechoic chamber. This method increases predictability, efficiency, and standardization by eliminating weather dependence.

Beyond the Limits: ASEP

The Additional Sound Emission Provisions (ASEP) ensure vehicles are not artificially quiet only during type approval tests. ASEP targets conditions beyond the standard cycle and addresses active exhaust or sound systems.

Manufacturers must provide documentation proving compliance across a wider range of speeds and accelerations.

The Silent Revolution: EVs, Hybrids, and UN R138

With electric and hybrid vehicles, the challenge reversed: they were too quiet, creating risks for pedestrians and cyclists. UNECE R51 (max noise) had to align with UN R138 (minimum noise), requiring AVAS systems that generate artificial sound up to 20 km/h and when reversing.

R51.03 exempts EVs without combustion engines from ASEP, provided AVAS sound levels remain under 75 dB(A) outside R138 scope.

Implications for the Automotive Industry

UNECE R51 deeply impacts vehicle design and manufacturing. Compliance requires R&D investment, technical documentation, and ongoing conformity of production checks to ensure vehicles maintain approved acoustic characteristics throughout their lifecycle.

why choose ats group?

ATS Group is a group of companies operating in the field
of Inspections, Certifications and Type Approvals in the Automotive sector.

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type approval of whole vehicles

ATS Group, through its companies designated by e24 and e5 marks, is able to issue the type approval of complete vehicles.

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type approval of vechicles parts

With the type approval Approval Authority designations (E57), (E5) and (E24) ATS Group is able to carry out approvals of components, systems and technical units of vehicles in general.

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certifications

ATS Europe, as a Certification Body accredited under ISO 17021-1: 2015 as a Certification Body for Quality Management Systems under ISO 9001: 2015, has become a leading company in the field of certifications.

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testing laboratories

The test laboratories affiliated with ATS Group are ISO 17025 accredited, ensuring reliability, accuracy, and compliance with international standards.

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ats app

Our proprietary online platform can manage the whole approval process, fron the quotation till the direct download on the final Type Approval Certificate.

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competitives prices based on market analysis

ATS Group offers competitive prices and fast turnaround times, ensuring an efficient and high-quality service to meet customer needs promptly.

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