Psychoacoustic Examination of Feasible Level of Additional Warning Sound for Quiet Vehicles

The reduced noise of electric and hybrid electric vehicles has been of particular concern because of the potential danger that these vehicles pose to pedestrians when their approach is inaudible against background noise. To address this issue, the use of additional warning sounds in such vehicles is being considered in various countries. The aim of this study was to examine the feasible level of the warning sounds in some urban environments. The levels of three candidate warning sounds (sound of car horn, engine sound, and band-pass noise) were adjusted by the study subjects against four types of urban background noise presented in a laboratory environment. The subjects were asked to adjust the level of the warning sounds so that they were reliably audible or just detectable. The results showed that the level of background noise and type of warning sound significantly affected the perception of the warning sounds, but there was no significant crosscultural difference between the German and Japanese subject groups. The observations showed that a warning sound that was reliably audible in a particular environment might be inaudible in another environment approximately 10 to 20 dB noisier. The results were also compared with current recommended levels of warning sounds for quiet vehicles.