Working draft proposal for a 04 series of amendments to UN R51, establishing specifications for sound emission measurement and limits. The regulation sets sound level limits varying by vehicle category, mass, and power output, ranging from 68 to 79 dB(A). It includes methods for measuring vehicle sound outdoors and indoors, specifications for test conditions including vehicle mass, speed, and acceleration, requirements for sound reduction systems and exhaust silencing, transitional provisions, and Real Driving Additional Sound Emission Provisions (RD-ASEP) for M₁ and N₁ vehicles with specific exemptions based on engine capacity, payload, mass, and power-to-mass ratio.