Meeting minutes from the GRBP Informal Group RD-ASEP held August 31–September 2026 in Berlin, Germany. The group reviewed the status report to GRBP, an issues list, and draft text for UN R51.04. OICA presented a document on the sensitivity of RD-ASEP results and difficulty of assessing worst case vehicle. The Secretary presented updated terms of reference to GRBP for approval. Experts are requested to submit all documents a minimum of 7 days prior to any IWG meeting date.

Proposal to establish revised terms of reference for the IWG RD-ASEP covering vehicles of categories M₁, N₁, and L₃. The group shall modify current regulations to mandate revised RD-ASEP, update Annex 9, delete Annex 7 and update relevant paragraphs in the main body and Annex 3 of UN R51, develop limit curves for different riding situations in UN R41-05, consider relevant transitional provisions, and develop coordination between UN R51 and UN R138. The IWG shall be chaired by Germany with OICA as Secretary for M/N categories and IMMA for L category, and shall present a working document for M₁ and N₁ vehicles by February 2028.

Proposal to amend UN Regulation No. 51 on the noise of M and N category vehicles, introducing comprehensive provisions for measuring and limiting sound emissions. The proposal establishes testing methods for vehicles in motion and at rest, defines sound level limits by vehicle category and power characteristics, and introduces Real Driving Additional Sound Emission Provisions (RD-ASEP) for M1 and N1 vehicles to ensure compliance during typical on-road driving conditions representative of urban and suburban traffic. Transitional provisions permit Contracting Parties to continue approving vehicles under preceding series until a specified date.

The agenda includes a review of Terms of Reference from GRBP 78th Session (GRBP-78-13), report of Data Analysis Task Force, review of issues list, review of draft text, and discussion of UN R51 matters. Dates and locations of 2027 meetings will be determined.

The 15th RD-ASEP informal group session will be held at Nissan Technical Center Europe Spain in Barcelona. The venue is located 11 minutes walk from Mercabarna Metro Station Line 9S. Participants should make hotel reservations independently; options include areas near metro line L9S or city center near metro line L5, which connects with L9S. Participants must provide last name, first name, phone number, company name, and email in advance.

The IWG RD-ASEP is developing a mandatory test procedure to assess compliance with paragraph 6.2.3 of UN R51.04, ensuring vehicle noise emissions at various speed, load, and acceleration conditions are linked to vehicle physics without defeat devices. R51.04 RD-ASEP expands testing from full acceleration only to any acceleration position, increases vehicle speed range from 20–80 km/h to 1–100 km/h, adds vehicle performance criteria, requires random test points for approval rather than manufacturer declaration, and can test gears and loads previously unavailable. An updated GRB-68-03 will accompany R51.04. Submission targets include informal draft September 2026 and expected working document adoption February 2028, with TOR extension requested through 2028.

An improvement intended to increase vehicle robustness in Annex 3 creates more difficulties in RD-ASEP assessment. Small variations in Annex 3 parameters may result in unexpected substantial exceedances in Annex 7. Minimal changes in the calculated tyre share at the anchor point may cause vehicles to fail Annex 7. The regulation should ensure realistic test effort and workload for compliance verification, as current requirements may require up to 180 combinations per type across test tracks, temperatures, driving modes, and tyres.

RD-ASEP includes technical background and vehicle application guidance for Real Driving Additional Sound Emission Provisions under UN R51.03 Supplement 7 Annex 9 for the mandatory monitoring phase between 07/2023 and 06/2024. RD-ASEP, referenced in GRB-68-03, applies to vehicles of category M₁ and N₁ equipped with internal combustion engines and covers engine speeds up to 95% of typical driving in speed ranges applicable to sub-urban conditions. A Sound Expectation Model predicts vehicle sound levels at randomly selected operation conditions by splitting measured pass-by test results into tyre rolling sound, mechanical no-load sound, and dynamic sound components, using reference values from Annex 3 type approval tests with logarithmic functions and parameter table slopes. Compliance assessment allows measured sound to exceed calculated expected sound by not more than 2 dB for maximum two test runs out of fifteen. Model validation with 2024 monitoring database confirms the tyre rolling sound and power train sound models represent well approved tyres and engines, with less than 5 dB typically separating pass-by run results from the limit.

The agenda includes welcome and opening remarks, introduction of participants and organizations, adoption of the draft agenda, review of minutes from the 13th session of June 2026, review of Terms of Reference from GRBP 78th Session (GRBP-78-13), UN R51 matters, review of report to GRBP, and determination of the date and place of the next meeting, scheduled for 20–22 October in Barcelona, Spain.

This document provides guidance for interpreting paragraph 6.2.3 of UN R51.03 concerning Additional Sound Emission Provisions for vehicles of category M₁ and N₁. Part A clarifies that typical on-road driving means driving at speeds between 15–100 km/h with accelerations not exceeding 2 m/s² and vehicle performance not exceeding 35 m/s, across any gear except reverse at engine speeds of at least 1.5 times idle speed and not exceeding the maximum specified in Annex 7. Significant deviation is defined as a sound level exceeding the reference value by at least 6 dB(A). Type approval test results derive from Annex 3 or Annex 7, with reference values determined using regression curves for Slope-Assessment or calculation principles for Lurban-Assessment. Part B addresses electric and hybrid vehicles when electrically propelled, extending ASEP scope under Supplement 11, applying comparable principles with modifications regarding active sound enhancement systems and operating speed ranges between 20–100 km/h for electric vehicles.

The agenda includes discussion of Terms of Reference from GRBP-78-13, procedural update from GRB-68-03, Analysis Task Force Report, Editing Task Force Report, R51.04 Issues List Review, draft R51.04 text review, Inside and Outside Boundary Condition discussions, and confirmation of next meeting date and place as August 31–September 1 in Geneva, Switzerland.

Current RD-ASEP derives the regulatory level based on the certified value without accounting for margin against regulatory values of the certified value. RD-ASEP additionally regulates partial acceleration, unlike current ASEP which only stipulates full-throttle acceleration. It is proposed that RD-ASEP should take into account a margin against the regulatory values of the certification values, similar to current ASEP, to prevent quiet vehicles such as BEVs from being unfairly deemed NG.

In vehicles with automatic transmissions, upshifts during testing cause SEM expected values to be exceeded. The proposal addresses three cases: vehicles with non-locked gear ratios, vehicles with locked gear ratios, and vehicles with only one gear ratio. For Case 1, such data does not meet conditions in R51-03 Annex 9 “4.5.2. Non-locked transmissions” and require no SEM correction. For Case 2, the proposal adds text to Annex 9 (7), Appendix 1, paragraphs 3.4. and 3.5., permitting substitution of maximum engine speed nMAX(AA-BB)TEST for nBBTEST when the maximum engine speed between AA’ and BB’ exceeds the engine speed at BB’.

The IWG RD-ASEP informal group is developing a mandatory procedure for UN R51 after the monitoring phase. The Analysis Task Force determined vehicle databases and expectation model are suitable for RD-ASEP use. The Editing Task Force created a consolidated R51.04 draft incorporating R51.03 Supplements up to Supplement 11, with revised manipulation and defeat device provisions and mandatory random testing for approval. R51.04 RD-ASEP expands vehicle speed range to 0-100 km/hr, adds vehicle performance criteria, requires testing for approval, and permits random test points and any acceleration position, compared to R51.03 ASEP.

The RD-ASEP Informal Working Group met April 14-16, 2026 in Berlin. The Analysis Task Force concluded data cleaning and will use data to answer questions and update the parameter table. The original aim of RD-ASEP was to address vehicles with louder partial throttle than full throttle results. The Editing Task Force reviewed draft R51.04 and was directed to provide an updated draft for the June meeting, replacing the old Annex 7 with the current Annex 9. The Chair requested the Editing Task Force prepare a summary report to GRBP for September evaluating the monitoring database, Expectation Model, and draft R51.04.