Effect of amplitude envelope on detectability of warning sound for quiet vehicle

The motor sound on quiet vehicle, such as hybrid and electric vehicle, is quiet at low speeds. Thus, pedestrians have difficulty recognizing quiet vehicles approaching them. A quiet vehicle was designed to play a warning sound to solve this problem. However, it has not been solved yet. When the sound is designed, it should be concerned not only detectability of approaching quiet vehicles but also impression of the sound. For pedestrians, it's important to make it easier to recognize quiet vehicle. Also, warning sounds should not contribute to traffic noise annoyance. Our previous studies found that the fluctuation frequency and non-periodic fluctuation are effective to enable people to notice approaching vehicles. Another studies showed that the shape of envelope and the frequency contribute to impression of warning sound. This paper describes the unclear points of previous, say, the effect of the envelope on detectability of warning sound. Investigations were carried out by using synthesized motor sounds which were designed to have periodic and non-periodic fluctuations, and their effects on detectability by pedestrians were assessed. The results revealed that the shape of envelope influences the ability with which people detect approaching quiet vehicles in case of slow fluctuation frequency.