Hybrid and electric powered vehicles (HV/EV) that are much quiet compared to the conventional vehicles are becoming common in the global. Their quietness make it much more difficult to receive sound cues for the pedestrians including the visually impaired. Installation of external acoustic signs for the "quiet vehicles" is being considered in Japan. Not to make a new traffic noise problem by the easygoing counter-plan, it is important to clarify the requirements for the external acoustic sign. This study is an examination on the sound levels for the possible acoustic signs through a psychoacoustic experiment. The environmental sounds that were recorded with Head and Torso Simulator (HATS) on the sidewalks paralleling the roads and the acoustic sign that were recorded with HATS to simulate position and distance between the vehicle and pedestrian were prepared as the stimuli. The participants adjusted the sound levels of the acoustic signs to a suitable level while listening to both the acoustic sign and environmental sound stimuli. The results showed that the adequate sound level for the acoustic sign depended on the condition of environment, and that those signs should not be used in uniform sound level.