1. Introduction
The wireless earphone market has been showing rapid growth in recent years and was expected to reach 120 million units globally in 2019. The wireless earphone manufacturers, as well as global IT companies (e.g., Amazon and Microsoft) are expected to reach 230 million units in 2020, which is a 90% increase over 2019 [
1]. Wireless earphones can provide a new user experience that traditional earphones cannot provide. Therefore, it is necessary to establish a factor of user satisfaction, among the various user satisfaction factors, wearing comfort could be the main factor affecting the experience of using wireless earphones. Wearable products such as wireless earphones require attention to anthropometric and product design characteristics [
2]. In order to provide a comfortable fit through ergonomically designed products, it is important to measure relevant anthropometric parameters and design the products accordingly [
3,
4,
5,
6].
In order to investigate anthropometric factors affecting the comfort of wearing wireless earphones, it is necessary to consider the user behavior and weight of the product, as well as all contact areas in the ear for various factors related to the wearing comfort. However, previous studies have not fully evaluated user satisfaction or user experience in this aspect, with several limitations regarding the use of existing earphones. First, Jung and Jung (2003) [
7] provided product design guidelines by comparing and analyzing ear sizes and ear-related product sizes. However, anthropometric variables related to the use of earphones were limited to one ear canal area, and research on other ear dimensions related to the use of earphones was insufficient. Moreover, design dimensions were proposed by comparing the size of the ear with the size of the associated product, but no satisfaction with the actual user’s suitability to the proposed design dimensions was identified. Second, Liu (2008) [
6] measured 200 ear dimensions and suggested the design dimensions of the earphones compared to the sample products, but the various human ear dimensions that affect wearing comfort were not considered by limiting the ear dimensions to the ear canal length. Third, Chiu et al. (2014) [
8] conducted a study to evaluate the wearing comfort of Bluetooth earphones for 30 min in a given scenario. The difference in comfort, pain, and weight was investigated depending on the type of product, gender, ear, etc. In addition, the evaluation questionnaire provided detailed footage of ears and identified subjective pain in each area. Although this study evaluated overall comfort according to design characteristics, there is still a lack of research on pain and comfort in terms of ear dimensions and various other factors related to wearing comfort. Firth, Fu and Luximon (2020) [
9] proposed universal criteria for ear-related products to represent human variability using 3D anthropometry, considering different demographic features of an external ear; however, they stated that direct scanning of the ear canal was still difficult. Therefore, it is necessary to explore 3D anthropometric applications in further research to overcome difficulties for practical use. In particular, it could not be applicable in scanning an internal ear for designing ear-related products such as kernel-type earphones. Fifth, Ji et al. (2018) [
10] classified the different ear shapes in 24 groups in terms of the auricular concha, using data from 310 young Chinese people aged 18–28 years. However, participants made up a large percentage of only a small group. As a result, it was difficult to reach an agreement for universal design, providing only a limited number of individualities rather than securing design uniformity. Therefore, for the latest wireless earphones, the findings of the existing research could not be applied because they were obtained on wired earphones or less-used forms of Bluetooth earphones. In conclusion, the design, operation method, and wearing comfort of the latest wireless earphones should be considered.
In general, wireless earphones are heavy in weight because of internal components such as a battery, DAC (digital to analog converter), wireless communication modules, etc. For this reason, the wireless earphones are not fixed enough to ears, causing pain if worn for a long time. Therefore, it is important to identify design factors related to significant ear dimensions for developing well-fixed and comfortable wireless earphones. The purpose of this study is to evaluate the wearing comfort and design appropriateness of wireless earphones in relation to ear dimensions and product design factors. After reviewing existing research, we identified ear dimensions and evaluation items affecting the wearing comfort of wireless earphones with respect to the contact area. Consequently, we tried to identify the relationships between ear dimensions and the wearing comfort of products which were selected based on the market research on wireless earphones. Finally, we propose wireless earphone design guidelines, which can serve as a basis for their future design and universal application.
4. Discussion and Conclusions
Among the factors involved in the use of wireless earphones, the pain of the ear canal area was low in clusters with long ear canal–incisura intertragica lengths, which refers to the distance from the endpoint of the ear canal to the incisura intertragica. Moreover, this is related to the depth at which the product enters the ear, allowing it to fit tightly to the ear and fix in its proper position. The average length of the first cluster (the smallest cluster) for the ear canal–incisura intertragica was 1.17. On the other hand, the average length of the second cluster for that was 1.53. In addition, the depth of the sample products was ranged from 0.8 to 1.5 cm. Therefore, participants in clusters with long ear canal–incisura intertragica lengths can be more comfortable using a variety of products with different depths (lengths) than the participants in clusters with short ear canal–incisura intertragica lengths. Moreover, wearing the product in the correct position can reduce pain by reducing feelings of displacement in the ear area.
In terms of the sense of fixation to the ear canal, it was high in clusters with long incisura intetragica–antitragus length. The area between the incisura intetragica and the antitragus is the area in contact with the bottom of the earphone, and for clusters with relatively long areas, the lower part of the earphone was sufficiently supported so that the product could be worn in the correct position. Considering the length of the incisura intertragica–antitragus, the first group had an average length of 0.7 cm, and the second group had an average length of 1.1 cm, which is shorter than the length of the sample product (1.7 cm). However, when the actual product is worn, the lower part of the product can be supported because the product is placed inside the ear, and the lower support part of the earphone was wider in a cluster with a longer length. In addition, in the clusters with short and protruding anitragus, the product was held at the anitragus area, resulting in the product not being worn in its proper position of the inside of the ear canal. Therefore, clusters with long incisura intertragica–antitragus lengths can increase the fixation in the ear canal area by making it easier to wear and support the product.
Ear sizes vary by age and gender [
17]. Moreover, these variations were noticed in diverse ethnic groups such as Italian, European, Caucasian, Turkish, Dutch-German, North American, and other countries [
6,
9,
13,
18,
19]. In general, Koreans usually have larger ears [
6], and Lee et al. (2018) [
20] revealed that ear measurements in Korean were significantly larger and more varied than Caucasians in most ear dimensions. In this study, we divided 38 participants into two clusters by small and large ears (cluster one and two) and found that the cluster of small ears (cluster one) had more wearing discomfort and less fixation in the ear canal and incisura intertragica (related ear dimensions to earhole). Moreover, this study indicates that people with small ears tend to experience more discomfort in earholes because various ear dimensions are usually higly correlated with each other [
7]. Hence, other ethnic groups that had relatively smaller ears than Koreans could also be more uncomfortable in wearing wireless earphones. Although participants with small ears (the cluster one) showed little dissatisfaction in this study, this could be a still problem because Westerners generally have smaller ears than Asians including our data from the 38 participants. Therefore, an appropriate solution should be suggested for designing smaller wireless earphones, considering earhole sizes.
In general, kernel-type products (sample product one and sample product three) had more pressure on the earhole than open-type products. Due to the characteristics of kernel-type products, the product contacts the inside of the ear canal, causing more pressure than open-type products that do not reach the inside of the ear canal. In addition, participants responded that they felt more pressure on the earhole area when the size of the earhole was large, even with the same kernel-type product. The average length of the participants’ ear canal was 0.73 cm, and the average width of their ear canal was 0.59 cm, while sample product three had the largest size of 0.9–1.2 cm, which caused the highest pressure on the ear canal. According to responses to the adequacy of the design factors, there were many responses that the size of the earhole contact area was appropriate for products between 0.6 and 0.7 cm in size, and the larger the size, the lower the ratio. Therefore, when designing kernel-type products, it is necessary to reduce the feeling of pressure by considering the size of the ear tip part that contacts the ear canal area.
The Kernel-type products, on the other hand, scored a high level of sense of fixation. Since kernel-type products are designed to reach the inside of the ear canal and are fixed to the ear canal, providing a strong sense of fixation. However, in the case of sample product three, the size of the ear tip (the part that contacts the ear canal) was designed to be larger than the size of the ear, so the ear tip could not properly be held to the inside of the ear canal. Thus, the sense of fixation was lowered even it was a kernel-type product.
The touch-type products (the sample product one and two) caused less pain to the participants. In general, the wireless earphone is operated by touching the product or pressing a button, causing pain due to pressure applied to the area where the earphone is worn. In the case of the button-type products (sample products three and four), the pressing force is stronger than that required for the touch-type products, which can cause more pain. In the case of sample product three, the pain in the earhole area was the highest because the product was in contact with the inside of the earhole, causing more pain in the earhole area while pressing the button. In the case of sample product four, the pain in the concha area was also high while operating because the button-type method was used to cover the concha area. Moreover, the pain in the area was caused by contacting the product while pressing the button.
The comfort of the ear canals was higher in the open-type products than the kernel-type products. In terms of the earhole, the open-type ones had a higher comfort rating than the kernel-type ones. These results indicate that the open-type ones were designed to be worn without contacting the inside of the earhole, providing a greater level of comfort in the earhole compared to the kernel-type products. Regarding the comfort in the concha area, however, there was a difference in the results between both open-type products. The comfort of the concha area was the highest in sample product two (concha area size: 1.7 cm × 1.7 cm) with the smallest main body of earphone (concha contact area), and it was lowest in sample product four (concha area size: 2.2 cm × 2 cm). The average width of the participants’ concha area was 1.8 cm, indicating that the main operation part size (head size) of sample product four is designed to be larger than the ear size. According to the evaluation on the appropriateness of earphone sizes, the larger the size of earphones, the less appropriate they are. This shows that although both products are the same as open-type, the level of comfort in the ears varies depending on the size of the product.
The earphones used in the experiment weighed between 4–7 g (g), and most participants responded that the lower the weight, the more appropriate. For example, they replied that the lightest weight of 4 g was the most appropriate, but the heaviest weight of 7 g was the least appropriate. Since the ear area is more sensitive than other body parts [
21], the difference between weights can be accurately recognized [
8]. 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 examined the 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.