Have you ever missed a call while moving? : The Optimal Vibration Frequency for Perception in Mobile Environments Youngmi Baek and Rohae Myung Dept. of Industrial and Information Engineering Korea University Seoul, Korea 136-785 {muse1000mi,rmyung }@korea.ac.kr Jinho Yim Human Interaction Part R&D Team Visual Display Division Samsung Electronics Co. LTD Gyeonggi-Do, Korea 443-742 hci.yim@samsung.com Abstract: The characteristics of mobile phone usage when user is moving are different from those when user is sedentary or working in an office (Baek et al., 2006). This means that the default vibration frequency of current mobile phones may not be suitable for use in a mobile environment. Therefore, this study was designed to investigate the optimal vibration frequency for the perception of mobile phone vibration when the user is moving. To guarantee the validity of this study, subjects were asked to indicate their perception of the randomly given 10 vibrotactile stimuli while they performed routine activities on a sidewalk, subway, or bus for about 2 h. The results showed that the optimal vibration frequency in the dynamic state was 190 Hz, considerably higher than 151 Hz - the optimal vibration frequency obtained in the static state in the previous study. As a result, mobile phone manufacturers should consider this factor when designing the vibration frequency for the vibration mode so that missed calls in mobile environments are minimized. Key-Words: Mobile environments, mobile phone, perception, optimal vibration frequency, missed call, field study 1 Introduction Mobile phones offer multimodal feedback (such as visual, auditory, and vibration feedback) by essentially considering different usage environments so that users can set up a reception mode suitable to their situation in mobile environments. In particular, the vibration mode of a mobile phone allows its users to receive phone calls in noisy environments; the mode also serves to ensure propriety when the user is in a public place, even when the situation does not demand it. However, when people use a mobile phone in mobile environments, calls are frequently missed inadvertently. With regard to this, Baek at al. (2006) reported that a missed call results due to the reception mode settings and the carry mode of a mobile phone when the user is moving; these significantly influence the user s ability to perceive call reception. In addition, Baek et al. make the following statement: Research on the general usage patterns of mobile phone users revealed that in mobile environments, phones were mostly set in the vibration mode and placed in trouser pockets, while the users themselves walked or used some form of transportation. Moreover, users stated that they often missed calls when they were moving. Here, the frequent use of the vibration mode can be attributed to the demands of modern life that require a person to be present in public places for long spells of time. Additionally, it can also be attributed to people not changing their call reception mode to the normal mode once they leave a public place. In short, one of the reasons why users miss calls while moving is that they are unable to perceive the vibrations in the preset vibration mode. This is related to the vibrotactile perception sensitivity of the user and the carry mode of the user s mobile phone, as mentioned previously. In other words, in mobile environments, the phone could either be placed in the user s pocket, belt holder, or bag, or held in the user s hand (Kaaresoja and Linjama, 2005). Moreover, the vibrotactile perception sensitivity of the user diminishes owing to limited attentiveness and inadequate cognitive resources. In addition, there might also be a partial separation between the
vibrotactile output device and the user s skin due to the presence of some material. Currently, the frequency of the vibration motor in mobile phones is approximately between 130 Hz and 180 Hz (these results were obtained after the analysis of vibration motor specifications used by mobile phone manufacturing companies) with an average at 160 Hz (rotation speed: 10000 rpm) (The Electronic Times, 2004). Nevertheless, missed calls do occur inadvertently, and therefore, we expect that a vibration frequency higher than the current default vibration frequency is required. In this paper, we investigate the optimal vibration frequency for perception by a user in mobile environments. (Here, mobile implies a situation where a person or environment moves.) Our study results will provide basic research data for improvements in mobile phones in mobile environments. 2 Background 2.1 Limited attentiveness to mobile phone in mobile environments Mobile environments are very dynamic and unpredictable (Tamminen et al., 2004). When a mobile phone user is moving, his/her attention resources are reserved partly for passively monitoring and reacting to contexts and events and partly for actively constructing them (Oulasvirta et al., 2005). Therefore, since the user is moving and switching his/her attention (generally referred to as dividing attention ) according to the situation, he/she is unable to continuously pay attention only to the mobile phone. As mentioned above, although people display limited attentiveness, their ability for vibrotactile perception enables immediate awareness, even if they are not extremely attentive. However, when people are moving, the vibrotactile perception threshold should be higher than that for the static state because of vibrations that occur spontaneously during movement. Accordingly, by appropriately designing the vibration mode, it should be possible to arrive at a suitable vibration frequency that considers this factor. 2.2 Vibrotactile perception Most of the literature available on vibrotactile perception focuses on direct contact with the skin (particularly the hand or the fingers) in the static state. Bliss et al. (1974) reported that human skin is very sensitive to vibrating stimuli at 230 Hz, regardless of the contact area, while it is insensitive to vibrations below approximately 100 Hz or above 600 Hz. In other research on the absolute sensitivity of the hand toward vibrotactile stimulus, Lee (1998) experimented with six levels of vibration frequency at regular intervals (24~600 Hz), a vibration contactor, and five stimulus regions in the hand. Through this experiment, Lee verified that the frequency at which the hand was most sensitive was around 240 Hz, regardless of the contact area of stimulus and the region of the hand. Further, Lee (1999) showed that 120 Hz was the most effective frequency for transmitting information to the hand by vibrations. Subsequently, Lee et al. (2004) conducted an experiment using a handheld phone to determine the optimal vibration frequency of a mobile phone in the static state. They reported that a frequency of 140~160 Hz (around 151 Hz) was sufficient to enable psychophysical recognition. In addition, they mentioned that a frequency of 151 Hz was more suitable than 120 Hz for discerning mobile phone vibrations. However, in most actual usage environments, unlike the environments studied in the abovementioned researches, mobile phones are used in the dynamic state with indirect contact with the skin. Generally, a mobile phone is localized on a piece of cloth on the skin (Linjama et al., 2003), and a space is formed between the phone and skin because people are usually standing, as shown in Fig. 1. Moreover, in mobile environments, the user is constantly surrounded by noise made by vehicles, construction work, and so on, and street noise increases or decreases dynamically (Baek at al., 2006). Phone Space Space Skin A piece of cloth Figure1. The carry mode and position of mobile phone Pocket
Thus, it can be explained that mobile phones are not always in direct contact with the skin, and therefore, the vibrotactile contact characteristics can vary according to the user s usage condition (such as standing, walking, sitting, and running) 2.3. Location of vibrotactile stimulus with respect to the body The vibrotactile sensitivity varies on different parts of the body. It is important to consider the location of the phone with respect to the body when studying the vibrotactile interface since different locations have different levels of sensitivity and spatial acuity (Brewster & Brown, 2004). The skin on the fingertips and the lips is the most sensitive, while the leg is a relatively insensitive part of the body. Naturally, differences exist from person to person. 3 Methodology Through this experiment, we aimed to simultaneously achieve two specific goals; first, to determine the optimal vibration frequency for vibrotactile perception in mobile environments; next, to prove whether the vibrotactile perception threshold of mobile phones is higher in the dynamic state than in the static state. 3.1 Subjects Ten paid subjects (4 male, 6 female) participated in this experiment. All the participants were healthy and did not report any known neuropathologies that could affect their vibrotactile perception. The ages of the participants ranged between 24 and 45 years (with a mean age of 29.6 years). 3.2 Apparatus The phone used for the experiment was a general folding type model. For controlling the vibration frequency, we developed a program using C++ and a prototype for vibration generation (hereafter referred to as VibGen ) that can operate in the stand-alone mode. (With this device, it is possible to preset parameters such as the vibration frequency and the time of sending a stimulus.) Also, the portion where the vibration motor was located was connected to the VibGen (See Fig. 2). The vibration motor was a small coin-type DC motor (Fig. 3). The frequency of the vibration motor ranged between 150 Hz and 240 Hz. The frequency bandwidth was selected because it was usable with the vibration motor specification being currently developed. Further, the 150 Hz were chosen a minimum frequency to be used in experiment because approximately 150 Hz was the optimal vibration frequency of a mobile phone at static state in previous study. Figure 2. The VibGen and test mobile phone Figure 3. A coin-type vibration motor 3.3 Experimental design and procedure In this experiment, the independent variable was the frequency (ranging from 150 Hz to 240 Hz at intervals of 10 Hz) and the dependent variable was whether or not the vibrotactile stimulus was perceived. The ten frequencies were tested in random order and the trials were repeated twice. At each vibration frequency, stimulus was given at irregular time intervals preset by the experimenter. The total experiment time was approximately 2 h and a rest time of approximately 15 min was given to the subjects between the experiments. Figure 4. Experiment in mobile environments This experiment was carried out in mobile environments such as a sidewalk, subway, bus, or
public place. The subjects placed the test mobile phone and the VibGen in each of their trouser front pockets and moved around in heavily or sparsely populated areas in the city (Fig. 4). All the actions of the subjects appeared extremely natural so that they did not pay particular attention to their mobile phones. Whenever the subjects sensed the suddenly transmitted vibrotactile stimulus, they responded to the experimenter by saying Received. At this instant, the experimenter recorded on paper the time of response, subject behavior, and context. 4 Results Figure 5 shows the results for the frequency, which had a significant effect on the ability to perceive reception while moving. At shown in Fig. 5, the perception rate was the lowest at 150 Hz and highest at 190 Hz. Furthermore, the figure indicated that the perception rate had a tendency to decrease beyond 200 Hz and increase marginally at 240Hz. Figure 5. Vibrotactile perception probability at each frequency In Fig. 5 two peaks are observed at 190 Hz and 240 Hz. We sought to determine whether there were any significant differences between the perception rates at 190 Hz and 240 Hz. Therefore, we performed the Mann-Whitney test as a nonparametric t-test because the sample size used was small. As a result, the result of the Mann-Whitney test revealed no significant differences between the perception rates at 190 Hz and 240 Hz (p = 0.3398 at a significance level of <0.05). 5 Discussions In previous studies on vibrotactile perception, the optimal vibration frequency of a mobile phone was determined to be 151 Hz, which was sufficient for psychophysical recognition (Lee et al., 2004). However, it was not applicable to actual usage environments because the experiments were conducted in the static state and with direct contact with the skin. Meanwhile, the result presented in this study can be identified as the optimal frequency for perception while moving. This is because; it has validity in that the experiment was conducted in the dynamic state in actual mobile environments and not in a laboratory. The results revealed that the perception rate peaked twice at 190 Hz and 240 Hz. In addition, the Mann-Whitney test did not reveal any statistically significant differences between the perception rates at 190 Hz and 240 Hz. However, mobile phone manufacturers usually do not use a vibration frequency greater than 217 Hz. This is because of technical reasons such as the occurrence of noise at 217 Hz when TDMA (time division multiple access), one of the data transmission methods, is used. Hence, even though statistically significant differences between the two frequencies were not observed, it is possible that 190 Hz, in comparison to 240 Hz, is more appropriate as the optimal vibration frequency while moving. 6 Conclusions This study was initiated because of a usage problem in mobile devices in mobile environments wherein calls are missed when users are unable to perceive the vibration in the preset vibration mode while they are moving. From this perspective, the results shown in this paper will have a serious impact on improving the vibration interface in the dynamic state. The conclusions from this study can be summarized as follows. 1) The optimal vibration frequency for vibrotactile perception in mobile environment is found to be 190 Hz. 2) The vibrotactile perception threshold of a mobile phone appears to be higher in the dynamic state (190 Hz) than in the static state (151 Hz). ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 Project in 2006. References: [1] Baek, Y., Myung, R., and Yim, J., A Study on
Interaction between Multimodal Feedback Setting and Portable Patterns through Behavior Study of Mobile Phone User in Mobile Environments, HCI 2006 Special Interest Groups of Human Computer Interface of Korea Information Science Society, No.1, pp. 579-586, 2006 Personal and Ubiquitous Computing, Vol.8, pp.135-143, 2004 [2] Bliss, J.C., Katcher, M.H., Rogers, C.H. and Shepard, R.P., Optical-to Tactile Image Conversion for the Blind. IEEE Transactions on Man-Machine Systems, MMS-11: pp. 58-64, 1974 [3] Brewster, S. and Brown, L.M., Tactons: Structured Tactile Messages for Non-Visual Information Display, Australasian User Interface Conference AUIC2004, pp. 15-23, 2004 [4] Kaaresoja, T. and Linjama, J., Perception of Short Tactile Pulses Generated by a Vibration Motor in a Mobile Phone, IEEE, 2005 [5] Lee, S., Human Sensitivity Reponses to Vibrotactile Stimulation on the Hand: Measurement of Absolute Thresholds, Journal of Ergonomics Society of Korea, Vol. 17, No.2, pp. 1-10, 1998 [6] Lee, S., Human Sensitivity Reponses to Vibrotactile Stimulation on the Hand: Measurement of Differential Thresholds, Journal of Ergonomics Society of Korea, Vol. 18, No. 3, pp. 1-10, 1999 [7] Lee, B., Park, H., and Myung, R., JND-based mobile phone optimal vibration frequency, Journal of the Korean Institute of Industrial Engineers, Vol. 30, No.1, pp. 27-35, 2004 [8] Linjama, J., Puhakka, M., and Kaaresoja, T., User studies on tactile perception of vibrating alert. HCI International 2003, Vol 3, pp. 280-284, 2003 [9] Loomis, J.M., Tactile Pattern Perception, Perception, Vol. 10, pp. 5-27, 1981 [10] Oulasvirta, A., Tamminen, S., Roto, V., and Kuorelahti, J., Interaction in 4-Second Bursts: The Fragmented Nature of Attentional Resources in Mobile HCI, In Proceedings of SIGCHI Conference on Human Factors in Computing Systems, pp. 919-928, 2005 [11] Tamminen, S., Oulasvirta, A., Toiskallio, K., and Kankainen, A., Understanding Mobile Contexts,