# A Basic Study on Sound Control System for Ultra-Compact Electric Vehicle by Using Masking
## 1 Introduction  
Ultra-compact electric vehicles (EVs) with a short width and wheelbases, which can accommodate one or two seats, are becoming increasingly popular. These vehicles can pass through narrow streets and are expected to be a new mode of personal transport in the city [1]. In addition, because the vehicle uses an electric motor as a power source instead of a gasoline engine or a diesel engine, it not only has a low environmental impact, but also does not generate noise from the engine. Therefore, the noise input into the vehicle mainly consists of a road noise from the tires and a wind noise. This noise causes a problem by deteriorating the comfort in the cabin [2; 3; 4]. A conventional vehicle usually installs a sound absorbing material in the cabin or a speaker for active noise control (ANC) [5]. However, due to the compactness of the ultra-compact EVs, installation of these devices is difficult. This causes the deterioration of the comfort of the vehicle. Therefore, we focused on a giant magnetostrictive actuator (GMA), which is small in size and generates a high output and proposed an ANC system using the wall vibration generated by the GMA, thereby confirming the silencing effect [6; 7]. Furthermore, our studies are not only aimed at \"noise reduction\", but also at \"pleasant sound\" to control a comfortable space for several drivers.  
However, research on the determination of the ride comfort of ultra-compact EVs has been predominantly conducted through questionnaires and sensorial evaluation [8]. Sensuous and subjective evaluation can measure human sensations directly. On the contrary, it is difficult to evaluatequantitatively these results, since these evaluations are self-report which depends on subjectivity and individual preferences of an experimental participant [9]. That is enough scientific and engineering research on human sensitivity have not been performed. In another research field, a masking that adds background music (BGM) to noised space has been proposed to reduce subjectively the noise of a person in the space. According to [PERSON] et al. [10], the discomfort caused by the noise in urban areas was decreased by masking the noise with music and water sounds. Furthermore, [PERSON] reported a study on the subjective evaluation of comfortability attained using some masking sounds for noise [11]. The authors created a system that provided comfortable interior noise environment for ultra-compact EV users. We focused on the masking that uses sounds such as music to control the road and wind noise in the cabin. Even though there have been studies related to noise reduction in the cabins of ultra-compact EV by using the masking, none of them provides a quantitative evaluation based on biologic information.  
In this study, we used a music to mask the interior noise in ultra-compact EVs and to improve the driver comfortability. For providing the driver comfort, we focused on the 1/\(f\) fluctuation in the music [12] and the relationship between the heart rate and tempo of music [13]. Furthermore, it has not been researched about the usefulness of masking in the driving condition even though a driver uses masking during operating steering wheel in actual situation. We used electroencephalography (EEG) for obtaining the biologic information to quantitatively evaluate the comfort attained by masking the interior noise.