In the absence of specific requirements in UNECE regulation No 28 and as provided in p
oint 1.3 of Part 1, audible warning or additional devices installed on vehicles which are propelled by means of one or more electric motors may incorporate a feature enabling the device to be intermittently activated in such a way that it operates at a significantly lower sound pressure level than required for audible warning devices, emitting a continuous, uniform
 sound with a sound spectrum that does not vary perceptibly during operation, with the 
  ...[+++]aim to for instance alert pedestrians about the vehicle approaching.