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.