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UN R51 Supplement 11 (03 series) · UNECE Regulation No. 51 (ECE R51) — Uniform provisions concerning the approval of motor vehicles having at least four wheels with regard to their sound emissions.

UN R51 — Supplement 11 to the 03 series of amendments

Supplement 10 → 11

Adopted at the WP.29 199th sessionEntry into force not yet known
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What changed
  • New definitions, new test mass rules and a new Annex 10 cover vehicles equipped with an Automated Driving System (ADS).
  • ASEP (Annex 7) now applies to vehicles of categories M1 and N1, not only to those with an internal combustion engine.
  • For EVs and HEVs in the operating conditions of Annex 7, paragraph 1.1., any Enhanced Exterior Sound (EES) must be deactivated at switch-on.
Why
  • The ADS changes aim to make it possible for Automated Driving Systems to meet the requirements of UN Regulation No. 51.
  • The ASEP changes are intended to bring EVs and HEVs that can be propelled with no internal combustion engine operating into ASEP. They are also intended to specify requirements for EES.
  • The calibration text was ambiguous, so OICA proposed to describe the check procedure as ISO 362-1 does.

Q1What changed159 changes

Definitions for ADS and bidirectional vehicles (paras 2.4., 2.11.3., 2.29. to 2.34.)

New paragraphs 2.29. to 2.34. define the ADS, the Dynamic Driving Task, bidirectional vehicles, forward direction, accelerator and categories X and Y. Category X and category Y vehicles are equipped with an ADS and cannot be driven manually at a speed above 6 km/h. Category X vehicles are designed to carry occupants, but category Y vehicles are not designed to carry occupants at any time. For bidirectional vehicles, the reference point must be determined separately for each forward direction (para. 2.11.3.). For motor vehicles, the mass in running order now gives the driver mass as 75 kg and excludes it for category X or Y. The footnote that refers to R.E.3 now cites ECE/TRANS/WP.29/78/Rev.8 instead of Rev.6.

“Category X vehicles” are vehicles of categories M, N, L and T meeting all of the following conditions:
The mass of the vehicle, with its fuel tank(s) filled to at least 90 per cent of its or their capacity/ies, including the mass of the driver, of the fuel and liquids, fitted with the standard equipment in accordance with the manufacturer's specifications and, when they are fitted, the mass of the bodywork, the cabin, the coupling and the spare wheel(s) as well as the tools; and the mass of the driver (75 kg) except in the case of vehicles of category X or category Y.
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Testing vehicles with an ADS (new Annex 10 and test mass)

New Annex 10 adapts Annexes 3 and 7 to vehicles equipped with an ADS and does not add or remove any requirement. For a vehicle with an ADS, other than category X or Y, a test in one mode is sufficient when two conditions are met. The manufacturer declares that the modes should not differ for this Regulation, and the Type Approval Authority and its designated Technical Service agree. A new table translates the actions on the accelerator into the required actions from the ADS. The test mass table adds M1 and N1 rules for vehicles with an ADS that depend on whether a person is inside. For N2 and N3 vehicles with an ADS, md = 0 if there is no driver, and a test person inside then counts as extra load.

For vehicles equipped with an ADS, other than those of category X or Y, it is sufficient to test either in manual operation or ADS operation, when the two following conditions are fulfilled:
For vehicles of category X, 0.9 mro ≤ mt- 75 kg ≤1.2 mro in case a person is inside the vehicle to perform the test. For vehicles equipped with an ADS, other than those of category X, 0.9 mro ≤ mt +75 kg ≤ 1.2 mro in case there are no occupants in the vehicle. For vehicle in category X and Y, 0.9 mro ≤ mt- 75 kg ≤1.2 mro in case a person is inside the vehicle to perform the test. For vehicles equipped with an ADS, other than those of category X and Y, 0.9 mro ≤ mt +75 kg ≤ 1.2 mro in case there are no occupants in the vehicle If the test mass mt is equal to the target mass mtarget, the test mass mt shall be 0.95 mtarget ≤ mt ≤ 1.05 mtarget. If the centre of gravity of the extra loading cannot be aligned with the centre of the rear axle, the test mass, mt, of the vehicle shall not exceed the sum of the front axle in an unladen condition, mfa load unladen, and the rear axle load in an unladen condition, mra load unladen plus the extra loading, mxload, and the mass of the driver md, except in the case of vehicles of category X or category Y where md = 0. mro = mchassisM2M3 + mxloadM2M3 .
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Enhanced Exterior Sound (EES) (paras 2.35., 2.36., 6.2.3.3., 11.19.)

New paragraph 2.35. defines EES as any sound from active systems installed to enhance the exterior sound, if another UN Regulation does not cover it. For EVs and HEVs in the operating conditions of Annex 7, paragraph 1.1., EES must be deactivated at switch-on by the vehicle master control switch. For these vehicles, each activation of EES needs an intentional action of the driver, and it must be possible to deactivate the EES. For these vehicles, the sound at standstill and at 20 km/h or less must meet paragraph 6 of UN Regulation No. 138. Their sound from 20 km/h to 80 km/h must meet the requirements of this Regulation. Until 24 months after Supplement 11 enters into force, Contracting Parties can extend 03 series approvals granted before it without taking paragraph 6.2.3.3.1. into account.

When the vehicle is switched on by the vehicle master control switch, any EES shall be deactivated.
"Enhanced Exterior Sound (EES)" means any sound
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ASEP for electric and hybrid vehicles (para. 6.2.3. and Annex 7)

ASEP in Annex 7 now applies to vehicles of categories M1 and N1, not only to those equipped with an internal combustion engine. Vehicles electrically propelled without internal combustion engines operating must comply with Analysis method 1 (Slope assessment method) only. A new Table 1 gives the ASEP control range for vehicles under paragraph 1.1. and for all other operating conditions. For vehicles under paragraph 1.1., the sound is evaluated as a function of vehicle speed with a constant slope of 0.25 dB/(km/h). New paragraph 3.4. gives a sound level increase ΔLASEP,j for tests of purely electrically propelled vehicles, and it is 0 dB for all other tests. Paragraph 3.6. now uses Lκ,j - ΔLASEP,j ≤ LASEP,κ.j + x and lists the vehicles for which x = 3 dB + (limit value - Lurban).

The Additional Sound Emission Provisions (Requirements of Annex 7 ASEP) apply only to vehicles of categories M1 and N1 equipped with an internal combustion engine.
For vehicles in operating conditions in accordance with Annex 7, paragraph 1.1. For vehicles in all other operating conditions Vehicle speed vAA_ASEP vAA > 20 km/h vAA ≥ 20 km/h Vehicle acceleration aWOT_ASEP aWOT ≤ 5.0 m/s2 Engine speed nBB_ASEP not applicable nBB ≤ 2.0 * PMR-0.222 * S or nBB ≤ 0.9 * S, whichever is the lowest Vehicle speed vBB_ASEP for vehicles tested in Annex 3 with locked gear vBB ≤ 80 km/h If the vehicle, in the lowest valid gear does not achieve the maximum engine speed nBB_ASEP below 70 km/h, increase the vehicle speed in that gear to reach the maximum engine speed nBB_ASEP, but no more than 80 km/h. For any other gear, the maximum vehicle speed is 70 km/h. non-locked gears vBB ≤ 80 km/h Gear selection only if applicable, e.g. for hybrid electric vehicles: gears κ ≤ gear i as determined in Annex 3 gears κ ≤ gear i as determined in Annex 3 Transmission condition for vehicles tested in Annex 3 with locked gear Geari, geari-1,… Geari, geari-1,… non-locked gears Non-locked gears Non-locked gears
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Check of the acoustic measurement system (Annex 3, para. 1.2.)

The Technical Service must set the duration of a measurement session, and one session must not be longer than one day. At the start of every session, the entire acoustic measurement system must be checked with a sound calibrator and, if the equipment allows it, adjusted. At the end of the session, the same calibrator is used again, and without further adjustment the readings must differ by 0.5 dB or less. If the difference is greater than 0.5 dB, the results of the whole measurement session are discarded. Before this supplement, two consecutive checks had to differ by 0.5 dB(A) or less, or the results after the previous satisfactory check were discarded. The paragraph now names Section 6 of ISO 362-3:2022 as the description of a qualified calibration method (electrical calibration).

Calibration Verification and adjustment of the entire Acoustic Measurement System acoustic measurement system for a Measurement Sessionevery measurement session
At the beginning and at the end of every measurement session the entire acoustic measurement system shall be checked by means of a sound calibrator that fulfils the requirements for sound calibrators of at least precision Class 1 according to IEC 60942:2003. Without any further adjustment the difference between the readings of two consecutive checks shall be less than or equal to 0.5 dB(A)., and – if the equipment allows it – adjusted to the reference values given by the calibrator.
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ISO 362-3:2022 for indoor tests (para. 11.20.)

The provisions for indoor tests now cite ISO 362-3:2022 instead of ISO 362-3:2016. Two cited parts of the standard change: Annex B, paragraph B.6 becomes B.7, and Annex E becomes Annex F. From the entry into force of Supplement 11, ISO 362-3:2022 must be accepted for all approvals under this Regulation. Until five years from the entry into force of Supplement 11, ISO 362-3:2016 must also be accepted for all approvals. The item “Evaluation of free rolling sound” no longer refers to Appendix 3 of Annex 3.

From the entry into force of Supplement 11, ISO 362-3:2022 shall be accepted for all approvals granted under this Regulation. Until five years from the entry into force of Supplement 11, ISO 362-3:2016 shall be accepted for all approvals granted under this Regulation.
Indoor tests shall be performed according to paragraph 3.1. using the specifications of ISO 362-3:2016362-3:2022, variant A. For indoor application, the manufacturer shall provide to the technical service, documentation according to Annex 8, paragraph 1. Variant A is a combination of indoor testing (power train sound) and outdoor testing (tyre/road sound).
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Other changes and corrections (paras 2.8., 2.8.3., 11.17., 11.18.)

Paragraphs 2.8. and 2.8.3. now refer to UN Regulation No. 177 instead of UN GTR No. 21. New paragraphs 11.17. and 11.18. say that Supplements 10 and 11 do not apply to existing approvals and extensions granted before their entry into force. For vehicles with only one gear ratio, the acceleration awot_test used for the partial power factor kp is now limited to 2.0 m/s². The text no longer says that the constant speed test is not required for vehicles with a PMR < 25. The rules for gear α now start with the heading “For vehicles tested in locked condition”, and the words “tested in locked position” are removed. Three formulas lose the arrow sign “➔”, and a temperature correction reference changes from “3.2.3” to paragraph 3.3.3.

"Maximum net power, Pn" means the declared engine power available for propulsion expressed in kW and measured dependent on the drive train concepts pursuant to UN Regulation No. 85 or UN GTR Regulation No. 21177.
The achieved acceleration awot_test , but no more than 2.0 m/s², is then used for the calculation of the partial power factor kp (see paragraph 3.1.2.1.3.) instead of awot ref.
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Q2Why it changed

Proposed by
Experts from the International Organization of Motor Vehicle Manufacturers (OICA), the Task Force on Automated Vehicles Regulation Screening (TF AVRS), the Subgroup on UN Regulations Nos. 51 and 138, and the International Organization for Standardization (ISO)
Working party
GRBP, 82nd session (September 2025) and 83rd session (February 2026)

Reasons

  • TF AVRS has the objective to make it possible for Automated Driving Systems to meet the requirements of the UN Regulations of GRBP. TF AVRS states that it will not add new requirements to these Regulations in their current scope. The approach is to keep the text for manually driven vehicles intact and to add ADS provisions where needed. Annex 10 is introduced to add further guidance on how to test vehicles with an ADS. The definition of “accelerator” is intended to make the text usable for vehicles with an ADS wherever the word occurs.

    Sources (5)
    “The objective of TF AVRS is to amend the UN Regulations in the purview of GRBP, where necessary, to make it possible for Automated Driving Systems (ADS) to meet the requirements of these Regulations.”
    “TF AVRS will not add new requirements to the UN Regulations in their current scope.”
    “The approach is to keep the text for the manually driven vehicles intact, and add provisions for vehicles equipped with an ADS where needed in the main body of the text.”
    “To add further guidance on how to test these vehicles, Annex 10 is introduced.”
    “This is introduced to make the main body of the text, wherever “accelerator” is mentioned, usable for vehicles equipped with an ADS.”

  • The proposal of the Subgroup on UN Regulations Nos. 51 and 138 is intended to introduce a “default mode” and requirements for EES, with transitional provisions. A limit curve is also intended to bring EVs and HEVs that can be propelled with no internal combustion engine operating into ASEP.

    Sources (2)
    “The proposed amendments are intended to introduce a “default mode” and specify the requirements to be applied to the emission of Enhanced Exterior Sound (EES), with the corresponding transitional provisions”
    “to draw a limit curve to introduce electric vehicles (EVs) and hybrid electric vehicles (HEVs) which can be propelled with no internal combustion engine operating in the scope of ASEP (Annex 7)”

  • The calibration text caused ambiguity, because it specified the calibration as a check of the system. The proposal is to describe the check procedure the way ISO 362-1 does. After a session, a re-check looks for a possible drift of the microphone calibration, to validate the measurements.

    Sources (3)
    “Ambiguity did raise on the way of calibration, as the calibration is specified as a check of the system.”
    “It is proposed to describe the check procedure the way it is written in ISO 362-1.”
    “After a measurement session, a potential drift of the microphone calibration is re-checked.”

  • ISO prepared its proposal to reference the latest version of ISO 362-3 (Indoor testing). ISO 362-3:2022 has provisions for test rooms when the required track length is more than 20 m, for example to measure backfire noise. In ISO 362-3:2022, the vehicle test speed range for free rolling noise is adjusted to be consistent with Annex 3, Appendix 3 of this Regulation.

    Sources (3)
    “prepared by the International Organization for Standardization (ISO) in order to reference the latest version of standard ISO 362-3 (Indoor testing)”
    “ISO 362-3:2022 includes provisions for test room dimensions in cases where the required track length exceeds 20 m — for example, when measuring backfire noise.”
    “The vehicle test speed range for the free rolling noise component has been adjusted to ensure consistency with Annex 3, Appendix 3 of UN Regulation No.51, 03 series of amendments”

  • Supplement 10 changed the calculation of temperature and track corrections, so transitional provisions are needed to avoid unnecessary measurements for extensions. Supplement 11 is considered to include the content of the earlier supplements up to Supplement 10, so it also needs transitional provisions.

    Sources (2)
    “Supplement 10 introduced changes in the calculation of temperature and track corrections introduced in Supplement 7. Therefore, transitional provisions are needed, to avoid unnecessary measurements for type approval extensions.”
    “because it is considered to include the content of previous Supplements up to Supplement 10 as well, transitional provisions are needed.”

Document trail

  1. 2025-06ProposalECE/TRANS/WP.29/GRBP/2025/20OICA proposal for clarifications and corrections
  2. 2025-06ProposalECE/TRANS/WP.29/GRBP/2025/25TF AVRS proposal for vehicles with an ADS
  3. —RevisedGRBP-82-07OICA text to replace GRBP/2025/20
  4. —RevisedGRBP-82-15-Rev.1TF AVRS text to replace ECE/TRANS/WP.29/GRBP/2025/25
  5. —ProposalGRBP-82-22OICA correction of the free rolling tyre noise test in Annex 8
  6. —Working party adoptsECE/TRANS/WP.29/GRBP/8082nd session report, paras 8 and 22
  7. 2025-11ProposalECE/TRANS/WP.29/GRBP/2026/13OICA proposal to refer to UN Regulation No. 177 instead of UN GTR No. 21
  8. 2025-11ProposalECE/TRANS/WP.29/GRBP/2026/4Subgroup proposal for EVs, HEVs and EES
  9. 2025-11ProposalECE/TRANS/WP.29/GRBP/2026/5ISO proposal to refer to the latest version of ISO 362-3 (indoor testing)
  10. 2026-01Amended in sessionGRBP-83-28ISO presentation of ISO 362-3:2022, with transitional provisions
  11. —Amended in sessionGRBP-83-09-Rev.1Subgroup changes to ECE/TRANS/WP.29/GRBP/2026/4
  12. —RevisedGRBP-83-41OICA consolidation of all adopted changes
  13. —Working party adoptsECE/TRANS/WP.29/GRBP/8383rd session report, paras 8, 10 and 11
  14. 2026-04Sent to WP.29ECE/TRANS/WP.29/2026/89Text that GRBP sent to WP.29 and AC.1
  15. —Amended in sessionWP.29-199-10Editorial change to the M1 row of Table 2, reproduced in Annex VI of ECE/TRANS/WP.29/1192
  16. 2026-06-24WP.29 adoptsECE/TRANS/WP.29/1192Adopted at the 199th session (session report, para. 103)
  17. PendingIn forceDate not yet known

Q3Who is affected

Vehicle manufacturer

  • Fill in the new item 0.4.2. that asks if the vehicle is equipped with an ADS.
  • Check the test mass of vehicles with an ADS against the new 75 kg rules.
  • Under paragraph 6.2.3.3., make sure that EES is deactivated when the vehicle master control switch switches on the vehicle.
  • Make sure that EES activation under paragraph 6.2.3.3.1. needs an intentional action of the driver.
  • Give the new EES information that items 1.1.10., 1.1.10.1. and 7.2. ask for.
  • Include M1 and N1 vehicles without an internal combustion engine in your ASEP test plan.

Component supplier

  • Give the vehicle manufacturer the EES details of item 7.2., for example the number and position of actuators.
  • Give the vehicle manufacturer a description of how the EES can be activated and deactivated.
  • If you supply measurement hardware, compare your calibration method with Section 6 of ISO 362-3:2022.

Technical service / Approval authority

  • Set each measurement session to one day or less.
  • Make sure that the results of the whole session are discarded if the readings differ by more than 0.5 dB.
  • Accept ISO 362-3:2022 for all approvals, and ISO 362-3:2016 until five years after Supplement 11 enters into force.
  • For vehicles with an ADS tested in ADS operation, use Table 1 of Annex 10 to translate the accelerator actions.
  • Make sure that the test report describes any additional preparation that you approve for vehicles with an ADS.
  • Use the new Table 1 in Annex 7 to find the control range for each operating condition.

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ⓘ This report gives advice only. It is not legal or type-approval advice.

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