## 1 Introduction  
The automotive industry is shifting its focus from vehicles with internal combustion engines to electric vehicles, powered by advancements in technology and increased environmental awareness of global warming. It is predicted that the use of eco-friendly vehicles such as electric vehicles (EVs) will continue to expand in the future. One of the advantages of vehicles using electric motors is that there is no noise pollution, which is in contrast to those using internal combustion engines [1]. However, the low noise level of an EV running at a low speed can lead to a situation where pedestrians cannot be acoustically alerted to the approaching vehicle, thus increasing the risk of accidents. Vehicles generate noise owing to driving speed, powertrain, tire-to-road contact, and aerodynamic sound. When a vehicle is driven at a high speed, the noise generated by the aerodynamic sound and tire-to-road contact dominates over other noise sources, resulting in a sufficiently high noise level that can be easily heard by pedestrians and people in other vehicles. In the case of a vehicle with an internal combustion engine, pedestrians can recognize the movement of the vehicle by the noise generated by the combustion of air and fuel inside the engine even when it is driven at a low speed [2]. However, an electric vehicle running at a low speed (<30 km/h) has a low noise level, making it difficult for pedestrians to hear [3]. Moreover, when the background noise is high, pedestrians often fail to perceive the approaching vehicle early enough to avoid the risk of an accident [4; 5]. In 2008, the University of California, Riverside, tested the time for a pedestrian or cyclist to acoustically perceive a vehicle running at a low speed. Consequently, it was observed that background noise, such as the noise generated by an idling vehicle motor, makes it impossible for pedestrians to recognize and cope with the approaching vehicle [6; 7; 8]. The US National Highway Traffic Safety Administration announced in its 2009 statistical report that when moving at a low speed, an EV is more prone to cause accidents than an internal combustion engine vehicle [1; 9]. The US and EU countries mandate the installation of an acoustic vehicle alerting system (AVAS) on an electric or hybrid vehicle so that pedestrians and visually impaired people can perceive its movement from a minimum level of noise against road traffic noise and ambient background noise [10; 11; 12; 13]. An AVAS for hybrid or electric vehicles is a useful device for cyclists and pedestrians, especially for the blind. However, there are arguments against the installation of AVASs because they can generate additional noise pollution. In this context, many studies have been conducted to minimize the side effects of AVASs and generate an effective alarm sound. [PERSON] and [PERSON] measured and analyzed the AVAS warning sound of several EVs and confirmed the difference in noise measured from the left and right of each vehicle [14]. [PERSON] and [PERSON] assumed that the manufacturer should comply with the AVAS minimum sound pressure level on all sides of the vehicle, making the warning sound excessive in a specific direction depending on the AVAS speaker installation method [15]. To address this problem, studies are underway, and it has been verified that high-quality warning sounds can be generated by creating a sound field controllable with a loudspeaker-based system [16; 17]. However, although a loudspeaker can effectively emit sound, its installation requires vehicle design modification, resulting in increased production costs and interference with other vehicle systems. In addition, since the loudspeaker cone is exposed to external environments, such as wind, dust, and moisture, the durability of the system may be a problem [18]. Mounting a standard round loudspeaker inside the engine room in consideration of exposure to the external environment poses the problem of a warning sound transmitted only in a specific direction [19].