ISO/FDIS 16254

This document is derived from ISO362-1^[2] and specifies an engineering method for measuring the sound emitted by M and N category road vehicles at standstill and low speed operating conditions. The specifications reproduce the level of sound which is generated by the principal vehicle sound sources consistent with stationary and low speed vehicle operating conditions relevant for pedestrian safety. The method is designed to meet the requirements of simplicity as far as they are consistent with reproducibility of results under the operating conditions of the vehicle.

The test method requires an acoustic environment which is only obtained in an extensive open space. Such conditions usually exist during the following:

measurements at official testing stations.

The results obtained by this method give an objective measure of the sound emitted under the specified conditions of test. It is necessary to consider the fact that the subjective appraisal of the annoyance, perceptibility, and/or detectability of different motor vehicles or classes of motor vehicles due to their sound emission are not simply related to the indications of a sound measurement system. As annoyance, perceptibility and/or detectability are strongly related to personal human perception, physiological human condition, culture, and environmental conditions, there are large variations and therefore these terms are not useful as parameters to describe a specific vehicle condition.

Spot checks of vehicles chosen at random rarely occur in an ideal acoustic environment. If measurements are carried out on the road in an acoustic environment which does not fulfil the requirements stated in this document, the results obtained might deviate appreciably from the results obtained using the specified conditions.

In addition, this document provides an engineering method to measure the performance of external sound generation systems intended for the purpose of providing acoustic information to pedestrians on a vehicle’s operating condition. This information is reported as objective criteria related to the external sound generation system’s sound pressure level, frequency content, and changes in sound pressure level and frequency content as a function of vehicle speed.

This document adds a metric related to the human perception of tonal loudness, the psychoacoustic tonality. The psychoacoustic tonality can be used to estimate audible frequency shifts of the sounds by identifying the most audible component in each auditory frequency band (critical band), as well as to determine if the band(s) so identified meet audibility criteria.

AnnexA and AnnexC contains background information relevant in the development of this document.