| UN R51 Vehicle Noise | Vehicle Noise Emissions | Uniform Provisions Concerning the Approval of Motor Vehicles having at least Four Wheels with regard to their Noise Emissions Vehicle Noise Vehicle Noise Emissions UN Regulation No. 51 UN R51 | UNR | 51 | |
| UN R59 Replacement Silencing Systems | Replacement Silencing Systems | Uniform Provisions Concerning the Approval of Replacement Silencing Systems Replacement Silencing Systems Replacement Silencing Systems UN Regulation No. 59 UN R59 | UNR | 59 |