## 4 Discussion and Conclusions
As the use of wireless earphones increases in the future, the wearing time and usage are expected to increase, so designs that reduce the weight of earphones are required. Most participants responded that touching the entire head of the earphone was the most appropriate to operate wireless earphones. Operating while wearing the wireless earphones is difficult because they could not see the operating part of the product. Therefore, in order to intuitively operate the product without visually identifying the operating part of the product, it is necessary to design a form that can be operated even when touching any part of the head. This study investigated the effect of ear sizes and product design features on the wearing comfort of wireless earphones, however, the following limitations can be listed. First, as the number of sample products for the experiment was four, the product attributes of wireless earphones were limited to revealing all discomfort factors in a systematic approach. In other words, the effects of the specific characteristics of the product were not fully considered. By increasing the number of sample products, a clearer relationship between ear dimensions and characteristics of the product can be identified in further studies when products that have diverse characteristics are acquired. Second, wireless earphones generally have a longer wearing time in real life than in an experimental setting (20 min in this study), so they should be evaluated for a longer time in future studies. Third, this study examinedthe impact of the size of ears on the wearing comfort of wireless earphones. Therefore, only two groups (with small and large ears) were targeted to distinguish them by identifying very significant correlations between ear dimensions and product attributes. However, we could not obtain various types of ear measurement data because of the limited number of participants. In particular, the size of the ears varies with age [22], and it tends to grow as the cartilage in the pinna hardens [7]. Since the participants in this study were limited to those in their 20s, additional research considering other age groups should be performed. Although we presented eight ear dimensions for the evaluation and derived the results from the wearing comfort evaluation based on the four major characteristics (size, operating method, weight, shape), this was not enough to suggest specific dimensions and forms for the wireless earphone as universal design guidelines. Based on the results of this study, further studies should be conducted on various attributes of ear size or shape for optimal design characteristics of comfortable wireless earphones, and more participants should be tested with balanced demographic characteristics such as age, gender, and race. Compared to conventional earphones such as wired earphones and headphones, wireless earphones are usually heavier because of the inclusion of various modules, and users can be more active while wearing them. For these reasons, this study systematically evaluated the wearing comfort of the wireless earphones and identified factors that had a great influence on the wearing comfort. In particular, compared to previous studies, we identified differences in two groups of ear sizes considering specific ear dimensions (e.g., ear canal-incisura intertragica and incisura intertragica-antitragus lengths) for the wireless earphones. Therefore, in the future, it will be possible to design wireless earphones based on the differences in the relevant ear dimensions from this study. In addition, the results of this study can form a major basis for further investigating the correlation between the ear dimensions and features of wireless earphones for improved wearing comfort. Conceptualization, H.S. and S.B. ; methodology, H.S. and S.B. ; investigation: H.S. analysis, H.S., Y.Y. and S.B. ; writing--original draft preparation, H.S. and G.W.S. ; writing--review and editing, G.W.S., Y.Y. and S.B. ; supervision, S.B.