Relationship between fluctuation strength and detectability of alert sounds for hybrid and electric vehicle

The motor sound on hybrid and electric vehicle is quiet at low speeds. Thus, pedestrians have difficulty recognizing those vehicles approaching them. Those vehicles were designed to play an alert 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 those vehicles but also impression of the sound. For pedestrians, it's important to make it easier to recognize those vehicle. Also, alert sounds should not contribute to traffic noise annoyance. Our previous studies found that the fluctuation frequency, non-periodic fluctuation and amplitude envelope are effective to enable people to notice approaching vehicles. Another studies showed that the shape of envelope and the frequency contribute to impression of alert sound. Also, it showed that these characteristics affected hearing fluctuation, or fluctuation strength. This paper describes the unclear points of previous, say, the independent effects of the fluctuation frequency, non-periodic fluctuation and amplitude envelope on detectability and their combined effects. Also, we investigate relationship between detectability and fluctuation strength on warning sounds for those vehicle. Investigations were carried out by using synthesized motor sounds which were designed to have periodic and non-periodic fluctuations. Those motor sound has characteristics of fluctuations in gasoline powered vehicle. Envelopes, i.e., modulation wave in AM, were set for deviations for time and amplitude, and amplitude-modulated motor sounds were synthesized using sine wave, sawtooth wave, and rectangle wave as modulation wave. Then, their effects on detectability by pedestrians were assessed. The results revealed that non-periodic fluctuation enhances the ability with which people detect approaching hybrid and electric vehicles in case of some fluctuation frequencies. Moreover, our results demonstrated that fluctuation strength influences the detectability of approaching those vehicle.