40. GRBP noted that the secretariat had received a letter from the Civil Association Anti-Noise Environmental Monitor on the possible incompatibility between the sound limits for buses of UN Regulation No. 51 and the guidelines of the World Health Organization (WHO) (GRBP-78-25). Stressing on its constant efforts to reduce the sound limits in UN Regulation No. 51, GRBP pointed out that the noise level figures in UN Regulation No. 51 and the WHO guidelines were not directly comparable for the following reasons:

1. different distances from the sound source;
2. the WHO guidelines operate with average noise levels while UN Regulation No. 51 refers to peak noise levels;
3. the tests of UN Regulations No. 51 include acceleration at wide open throttle which is uncommon in real bus traffic.