Psychoacoustic Requirements for Warning Sounds of Quiet Cars

According to upcoming legislative regulations in certain countries, electric and hybrid-electric vehicles (EVs and HEVs) will have to be equipped with devices to compensate for the lack of engine noise needed to warn pedestrians against the vehicles. This leads to the question of appropriate sound design which has to meet specific psychoacoustic requirements. The present paper focuses on auditory features of warning sounds to enhance pedestrians' safety with a major focus on the detectability of the exterior noise of the vehicle in an ambient noise. For the evaluation of detectability, the psychoacoustic model developed by Kerber and Fastl will be introduced allowing for the prediction of masked thresholds of the approaching vehicle. The instrumental assessment yields estimates of the distance of an approaching vehicle at the point it becomes audible to the pedestrians. This way the risk of collision can be assessed assuming that only auditory cues are used by pedestrians to identify vehicles before crossing a street. The corresponding algorithm was applied in a field study to assess the detectability of selected EVs and HEVs as opposed to conventional vehicles. A second focus of this paper will be on the issue of recognisability. It is suggested to design sounds which both are perceptually indicative of the vehicle behaviour and inform about the specific type of propulsion and vehicle make. As a result, the concept of a sound signature specific for vehicles with electric propulsion is proposed which may enhance detectability considerably while recognisability is retained to a large extent.