1 Introduction

Sensory marketing has advanced significantly over the past decade, and has evolved to become an important tool for predicting the success of products and services in the market [1]. Subjective sensory factors have a stronger influence on consumers than objective factors [2]. Among various sensory stimuli, this study focused on sound. In durable consumer goods, the sound emitted from the product (hereinafter referred to as product sound) contributes to product competitiveness [3]. Product sounds are classified into two categories: sequential and intentional sounds. The sequential sounds result from moving parts when consumers use a product, while the manufacturer's marketers or engineers provide the intentional sounds for specific purposes [4]. Both sounds are used in the product design process to intuitively communicate information to the user. Therefore, sound designers, or communication engineers,” needed to make design decisions related to product sound for effectively conveying the product’s characteristics [5].

For automobiles, product sound can be one of the purchasing decision factors [6]. Recently, by switching from the engine of a gasoline vehicle to the motor of an electric vehicle (EV), the quietness and degrees of freedom of generated sounds have increased. For example, Porsche has developed artificial driving sounds for EVs and improved their product appeal [7]. However, in academic research on product sounds, the perspective of the task differs. Noise reduction has been challenging that has been discussed for several years [8,9,10]. For EVs, significant research has focused on protecting pedestrian safety, which is a new problem because of quietness [11,12,13,14,15,16].

Regarding the driving sound of cars, companies are shifting their focus to value creation represented by the communication of product concepts; though, academic research is still limited to discussions on noise reduction and safety. Accordingly, this study fills this gap. We target the Japanese automobile market to evaluate the effect of the engine sound of gasoline-powered vehicles on the attractiveness of the products and motor sounds of EVs. The verification method used was a randomized controlled trial (RCT) in an online survey environment. For verification, we prepared four types of cars with three types of driving sounds (gasoline-powered vehicle engines, gasoline-powered sports vehicle engines, and EV motors) from 12 movies. Academic research on EVs has increased dramatically over the last 10 years, but little research has been conducted from a marketing perspective [17]. This study supplements the existing knowledge on the value-creation perspective of EV driving sounds. This study goes beyond noise reduction and safety assurance and shows the importance of sound design in improving product value.

2 Literature Review and Hypotheses

2.1 Changes in Issues Introduced by the Conversion to EVs

Academic research on product sound has long focused on noise reduction. The reason for this is that the noise problem has become more serious due to the development of industries such as automobiles and aircraft [18]. In particular, the noise generated by aircraft has an impact on health [19], and complaints from local residents are still being received, especially in urban areas near Heathrow (United Kingdom) and Haneda (Japan) Airports [20, 21]. In cities, cars account for 85% of the noise [22]. Noise in the car affects consumer annoyance [8]. Hence, there is a great need to reduce driving noise [23], which is a serious issue for automobile manufacturers [9]. However, that situation is now changing. The driving force for change is the shift to EVs. Governments around the world are rapidly moving forward with this move [24,25,26]. As a result, automakers are also changing their business strategies and accelerating investment in EVs [27, 28]. Owing to these trends, the number of EVs (including plug-in hybrids) sold in 2022 is expected to exceed 10 million, accounting for approximately 14% of global vehicle sales [29]. This trend is expected to continue.

2.2 Research Themes Targeting EVs

Research on EVs has been divided into three main themes. The first is noise reduction. Although it is much quieter than a gasoline vehicle [30], the sound does not completely disappear. Hence, several studies have been conducted to reduce the noise in EV driving sounds [31,32,33].

The second is the development of warning sounds for pedestrians. EVs are quiet when driven at low speeds, which raises pedestrian safety concerns. It is estimated that the traffic safety risk for pedestrians is 30% higher than that for gasoline vehicles [12]. Many drivers are aware of the risks and are more alert while driving an EV [34]. European Union law takes effect that requires electric and hybrid four-wheeled vehicles to emit an artificial noise when traveling at low speeds by “Acoustic Vehicle Alert System” in 2019 to prevent collisions between EVs and pedestrians; hence, from July 2021, the regulation applied to all new electric and hybrid vehicles [35]. Based on this background, sound design that enhances safety is being actively researched while considering new noises in the city [11, 14, 15].

The third aspect is the appeal of product features through the product’s sounds that communicate information to consumers sensuously [4]. For example, the sound of closing a vehicle door creates an impression of functional value, such as durability, and emotional value, such as relief [36]. Products' startup sounds create perceptions of being “cute” and “cool” [37]. In other words, the product’s features are embodied by the product’s sound. Even in EVs, protecting pedestrian safety and appealing emotional features are not contradictory but compatible [38]. In addition, consumer values are changing because of the shift to EVs. In the past, the large driving sounds of sports cars attracted consumers; however, the quietness of EVs has been gaining consumer acclaim [39]. In addition, by adopting EVs, consumers demonstrate their identities such as an environmentally friendly lifestyle [40] and innovation [41] that incorporates advanced technologies. We inferred that driving sound, which is the focus of this research, may convey the attractiveness of driving performance. Accordingly, we propose the following hypotheses:

H1: The driving sound of the EV enhances product attractiveness more than the driving sound of the gasoline-powered vehicles.

H2: The driving sound of the EV enhances product attractiveness more than the driving sound of the gasoline-powered sports vehicles.

3 Method

3.1 Stimulus

The vehicles in the driving movie used for the evaluation were the Honda NBOX (micro), Toyota PRIUS (hatchback), BMW 3 SERIES (sedan), and Honda S660 (sports). As shown in Fig. 1, all the vehicles were filmed on the same test course to match the conditions. Each video was one minute long. Next, we prepared three types of driving sounds: Honda FIT, a gasoline-powered vehicle engine; Ferrari F8 TRIBUTO, a gasoline-powered sports vehicle engine; and Tesla Model 3, an EV motor. The three cars were driven on the same test course, and the driving sound was recorded inside the car. Based on the above, we prepared four types of cars x three types of driving sounds = 12 movies. This makes it possible to evaluate the effects of the driving sounds equally.

Fig. 1
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Video stimulus

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3.2 Survey and Verification

As sound evaluation is sensory, ranking multiple sounds is unreliable. Hence, we adopted RCT, a highly reliable method for evaluating causal effects. An online survey targeting people in their twenties to sixties was conducted in Japan from February 25 to March 3, 2022. The respondents were required to own a car(s). After distributing the survey to 3000 people, responses were collected from 2347 who owned a car(s). Each respondent was randomly assigned to three assignments: Group 1 was presented with a video of a gasoline-powered vehicle engine driving sound, Group 2 was presented with a video of a gasoline-powered sports vehicle engine sound, and Group 3 was presented with a video of an EV motor. The assignments of the driving sounds and cars in the video are listed in Table 1. The five questions were as follows: (1) sex, (2) age, (3) car ownership, (4) driving frequency, (5) attractiveness of the car after watching the video (“How attractive did you feel the car in the video?”) and (6) impressions of driving sounds (free answers). For question (5), the option was on a seven-point Likert scale (1 = not attractive at all, 7 = very attractive). For question (6), we explained to the respondents that this was the first survey on driving sounds (Table 2).

Table 1 Number of respondents for driving sound and car in the video
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Table 2 Respondent attributes of each group
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For verification, the chi-square test was used for pairwise comparisons with Bonferroni-corrected p-values for the three groups. The responses regarding attractiveness were converted into three categories: positive (5–7), neutral (4), and negative (1–3). The null hypothesis is, “There is no difference in product attractiveness between groups.” The analysis environment was R statistical analysis software, version 3.6.3.

4 Results and Implications

4.1 Results

As shown in Table 3, the percentage of people who felt attractive was higher in the given order: Group 2 (gasoline-powered sport vehicle engine) < Group 1 (gasoline-powered vehicle engine) < Group 3 (EV motors). The chi-squared test showed a p-value of 0.018; hence, there was a significant difference at the 5% level in the attractiveness of the products. As shown in Table 4, significant differences were detected only in Groups 2 and 3 as a result of pairwise comparisons. Accordingly, H1 was rejected and H2 was supported.

Table 3 Chi-square test result
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Table 4 Pairwise comparisons with Bonferroni corrected p values
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The moderating effects of sex and age were also evaluated. Table 4 shows the results of the chi-squared test according to sex. In both sexes, EV motor received the highest evaluation, and the gasoline sports engine received the lowest evaluation. However, a significant difference was detected only in men. Table 5 presents the evaluation results by age. Superiority or inferiority in the evaluation of driving sounds was common in both age groups. A significant difference was detected only in the age range of 20–30 s. In other words, younger generations and men had higher evaluations of EV motors.

Table 5 Chi-square test result by gender
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Next, by text-mining answers regarding the impressions of each driving sound, we confirmed the reasons for their attractiveness. Figure 2 shows the word cloud for nouns and adjectives that appeared more than 10 times. It can be confirmed that "quiet" and "noisy" appear equally. Figure 3 shows the results of the correspondence analysis. Looking around the EV, “quiet,” “acceleration,” and “good” are plotted. In other words, the fact that it is quiet, even when accelerated, creates a good impression. Furthermore, the characteristic word for EV is “pedestrian.” There are concerns regarding the safety of pedestrians owing to excessively quiet motor noise. “Uncomfortable” and “noisy” are plotted around the gasoline-powered vehicle engine. As for the gasoline-powered sports vehicle engine, while some said it was powerful, many respondents thought it was a small vehicle, like a micro-car, because of its loudness (Table 6).

Fig. 2
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A word cloud on driving sound impressions

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Fig. 3
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Correspondence analysis of frequent words in impressions to driving sounds

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Table 6 Chi-square test result by age
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4.2 Theoretical Implications

The importance of product sounds has been actively discussed with increasing interest in sensory marketing. In this marketing field, product sound design is one theme that is attracting particular attention [4]. For example, its effectiveness has been proven in home appliances [3] and furniture [5]. Incorporating product sound into the customer experience provides consumers a sense of product quality and image [42,43,44], which can have a positive impact on customer satisfaction and willingness to pay [45]. However, EVs rapidly developing worldwide are limited to noise reduction [46,47,48,49] and safe sound design [11,12,13,14,15,16]. Although businesses demand the value of EV driving sounds, academic research surprisingly lacks this knowledge. To fill this gap, this study demonstrates the effect of enhancing product attractiveness by comparing gasoline vehicles and EVs. Based on this knowledge, EVs should be studied from the perspective of value creation, rather than negative elimination. In our previous study [50], we found that the driving sound of an EV enhanced the perceived driving performance of a product more than that of a gasoline vehicle. This study was extended to sports-type gasoline vehicles, and the effect of enhancing the attractiveness of the product was verified.

4.3 Practical Implications

The results of this study have three practical implications. First, practitioners should be aware of the risk of negating the effectiveness of product sounds if we simply pursue quietness. Noise reduction is a priority in this era of health-damaging noise [18, 19]. However, even if the situation is eased, if companies stick to quietness, there will be divergence from consumers' perception of value. Research on product sound has focused on noise reduction, but it should be recognized as providing an emotional experience and utilizing product sound to embody the product concept. Second, marketers and engineers should be aware that recent consumers tend to be more attracted to EVs over gasoline gasoline-powered vehicles. This is probably because consumer values are changing due to the influence of innovative products centered on Tesla. Consumers have a beautiful and sophisticated impression of the EV category and the electrified future, which Tesla has been the driving force behind [51, 52]. The results of this study also confirm that consumers find the engine of a sports car confusing but find the quietness of EV attractive. The trend toward a higher evaluation of EV motors is more pronounced among younger generations and men. Traditional automakers still value the sound of big gasoline engines. In fact, companies currently developing EV driving sounds embody powerful driving sounds, such as sports cars, even though they are EVs [7]. However, from the consumers’ point of view, the direction they aim for may differ.

5 Limitations and Future Work

This study has some limitations. First, the results do not account for the differences in effects due to differences in countries. In addition to sex and age [53, 54], existing research has confirmed differences in the evaluation of sounds by culture [55]. Therefore, country-specific differences in EV driving sounds should also be considered. Second, elucidation of the mechanism for designing an effective EV driving sound was beyond the scope of this study. Sound has three dimensions: loudness (dB), pitch (Hz), and timbre [56]. However, it has no objective index [57, 58]. Unfortunately, the complexity of human auditory processing systems makes it difficult to assess consumer perception of the acoustic behavior of automobiles with simple physical indicators [59]. Therefore, the purpose of this study was limited to verifying the difference between a gasoline vehicle and EV using the driving sound. Third, the video-viewing environment was left to the survey respondents. Although we warned respondents to set the audio before watching the video, we could not guarantee that everyone did it faithfully. It is unclear if participants used the built-in loudspeakers of their devices (smartphone or laptop) or headphones. Depending on the device used and the surrounding environment at that time, there may be differences in the sounds that people hear. However, owing to verification by RCT, the number of respondents who could not watch properly occurred with the same probability in each group. In other words, by adopting the RCT method, the device’s effects and the environment’s effects are removed, and the pure sound effect can be extracted. Giving participants an audio test before the promotional video helps conduct a more detailed survey. Furthermore, it is necessary to prepare a survey venue and provide the respondents with a driving-sound stimulus in the same environment. This experimental design is expected to yield more reliable results. Fourth, the sound pressure in this experiment is slightly different for each driving sound. The reason is that the collected data of each driving sound was used as is to bring it closer to the actual product. Normalizing all sound files to the same average level is useful for removing these effects. Fifth, the vehicles treated in this study were limited to one condition each for a gasoline vehicle and an EV. Therefore, to generalize the conclusions, increasing the number of vehicle types and verifying under different conditions such as design, driving performance, fuel efficiency, and usability is necessary. Sixth, the results of this study are limited to the context of video marketing communications. Note that the subject did not enter the vehicle. Therefore, it is desirable to verify the same in the physical driving environment of a car. These are future research topics.