Electric or hybrid electric vehicles (EVs/HEVs) have the advantages that they make lesser noise compared to conventional engine vehicles. This quietness is, however, a matter of concern for pedestrians because the sound of EVs/HEVs may be inaudible in urban backgrounds. Hence, the application of additional sound- emitting device has been studied. Gathering wide range of acoustic knowledge on feasible design of the additional sounds is crucial for developing regulations or designing the device itself. Through the authors' re- searches, it has been revealed that the type of warning sounds significantly affected the required sound level as well as background levels. A warning sound stimulus, which had lesser loudness level, had lesser detectabil- ity. In this present study, the authors confirmed that the warning sounds could have similar detectability if they have similar loudness level. The frequency characteristics of the stimuli were varied while having equiv- Alent powers in each 1/3 octave band so that they shall have equivalent loudness level. Moreover, the hearing impression of the stimuli were examined. The results showed that the impression of warning sounds could be varied while having equal detectability.