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Comparative studies of perceived vibration strength for commercial mobile phones
Applied Ergonomics 45 (2014) 807e810
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Technical note
Comparative studies of perceived vibration strength for commercial mobile phones Heow Pueh Lee*, Siak Piang Lim Department of Mechanical Engineering, National University of Singapore, 9, Engineering Drive 1, Singapore 117576, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2013 Accepted 5 July 2013
A mobile phone, also known as cell phone or hand phone, is among the most popular electrical devices used by people all over the world. The present study examines the vibration perception of mobile phones by co-relating the relevant design parameters such as excitation frequency, and size and mass of mobile phones to the vibration perception survey by volunteers. Five popular commercially available mobile phone models were tested. The main findings for the perception surveys were that higher vibration frequency and amplitude of the peak acceleration would result in stronger vibration perception of the mobile phones. A larger contact surface area with the palms and figures, higher peak acceleration and the associated larger peak inertia force may be the main factors for the relatively higher vibration perception. The future design for the vibration alert of the mobile phones is likely to follow this trend. Ó 2013 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Mobile phones Vibration alert Vibration perception
1. Introduction A mobile phone, also known as cell phone or hand phone, is among the most popular electrical devices used by people all over the world. A report in 2007 put the total number of mobile subscribers to be more than 3.3 billions (http://en.wikipedia.org/wiki/ Mobile_phone). Ergonomics design of mobile phones is an important subject due to the prolonged and wide usage. For example, an article published in July, 2004 issue of Ergonomics Today pointed out that if the current trend of shrinking mobile phones continued, soon the buttons would only be big enough for three-year olds. The same report also quoted a 2002 survey finding of mobile phone usage in eight large metropolitan areas which resulted in bigger and more muscular thumbs possibly due to pervasive mobile phone use and text messaging. There are some reported studies on the ergonomics design of mobile phones. For example, de Waard et al. (2010) studied the effect of mobile phone usage on the safety of cyclists. Mack and Sharples (2009) used the mobile phones as a case study to demonstrate the importance of usability in product choice. Myung et al. (2003) investigated the importance of design parameters in terms of look and feel of mobile phones based on a consumer survey. Sonderegger and Sauer (2010) also used mobile phones as an example to evaluate the influence of design aesthetics in usability testing. Mobile phones were used in their earlier study (Sauer and Sonderegger, 2009) to examine the influence of
* Corresponding author. Tel.: þ65 65162205; fax: þ65 67791459. E-mail addresses: [email protected], [email protected] (H.P. Lee).
prototype fidelity and aesthetics of design in usability tests. Walker et al. (2009) applied the system thinking to networked interoperable products such as mobile phones. Pattison and Stedmon (2006) studied the importance of human factors in the design of mobile phones for older users. Vibration alert or providing notification through vibration during silent mode or together with normal ring tones is an important function of mobile phones This function was present in the early models of pagers as well as the latest models of mobile phones. The vibration alert function was essential for conveying information to the user in a private manner, especially in meetings, public events, or concert halls, or in noisy environment to augment the normal ring tones. There are however fewer reported studies on the design of vibration alert for mobile phones. Park et al. (2009) studied the characteristics of vibration especially for human perception. They suggested an exemplary design of an authoring tool for vibrations in combination with music and sound effect. Yao et al. (2010) studied the strength perception of vibration signals used in mobile phones using mock cell phones with different phone weight and vibration frequency. Their results showed that for the same measured acceleration on the device, a heavier box was perceived to vibrate with greater strength, probably due to higher inertia force. Furthermore, signals with higher underlying frequency were perceived to be weaker for the same measured acceleration. As pointed out by Yao et al. (2010), Vibration alert was related to human perception of vibrotactile signals. The early studies have covered the aspects of detection threshold (Miwa, 1967; Morioka, and Griffin, 2005; Reynolds et al., 1977; Willes et al., 1991), perception of strength and equal sensation curve (Giacomin et al.,
0003-6870/$ e see front matter Ó 2013 Elsevier Ltd and The Ergonomics Society. All rights reserved. http://dx.doi.org/10.1016/j.apergo.2013.07.006
808
H.P. Lee, S.P. Lim / Applied Ergonomics 45 (2014) 807e810
Fig. 1. A typical setup for the experiment. The analyzers, the charge amplifier, the accelerometer as well as one of the mobile phones are shown in the picture.
2004; Mansfield and Maeda, 2005), frequency discrimination (Mahns et al., 2006; Tommerdahl et al., 2005), the influence of grip force (Giacomin and Onesti, 1999; Morioka, and Griffin, 2008), level of annoyance (Reynolds et al., 1977) and others. Yim et al. (2007) attempted to quantify the optimal vibration frequency for mobile phones in mobile environment. Subjects were asked to indicate their perception of the randomly given seven vibrotextile stimuli while they performed routine activities on a sidewalk, subway or bus. Their results showed that the optimal vibration frequency in these dynamic states was higher than 180 Hz, considerably higher than the 150 Hz obtained for static environment. Kaaresoja, and Linjama (2005) analyzed the perception of short tactile pulses generated by a vibration motor in a mobile phone. Their results suggested that the optimal duration of the control signal should be between 50 and 200 ms. Instead of using mocked up mobile phones, the present study attempts to analyze and compare the vibration perception of five commercially available mobile phones. The actual phones under the actual operating conditions were used. Two different body sites corresponding to two common manner of holding or keeping the mobile phones were tested in the human subject survey. These are the common hand held posture as well as the right pocket of pants or trousers where the mobile phone is usually kept. 2. Material and methods The majority of mobile phones have the ability to be switched into silent or vibration alert mode. On this setting the phone uses vibration alerts in place of audio alerts. Most mobile phones use an electrically powered motor with a rotating unbalanced mass to achieve this. The rotating mass induces acceleration throughout the motor, which is then transferred throughout the phone. The vibration of the whole phone body is then detectable by the user.
The vibration characteristics of five commercially available mobile phones labeled as product A to product E were first measured by attaching a Brüel & Kjær Accelerometer Type 4367 onto the back flat surface of each mobile phone. The five mobile phone models were from four different manufacturers. The actual identities of these mobile phones were intentionally withheld so as to avoid the risk of commercialization. All the mobile phones were charged fully before the tests. The age of the mobile phones ranged between two months to six years. The acceleration was measured using Hewlett Packard 35670A Dynamic Signal Analyzer with the signals first magnified by a Brüel & Kjær Charge Amplifier Type 2535. The Brüel & Kjær Accelerometer was attached onto the back flat surface of each mobile phone using wax. The mobile phones were placed on a piece of shock absorbing foam to reduce the effect of the impact of mobile phones against the table. A typical setup is shown in Fig. 1. The mobile phones were made to vibrate by calling. Acceleration vs time plots were obtained using Time Capture mode of the analyzer. It should be noted that we would be measuring only the vibration in the axis normal to the main frontal and back surfaces of each mobile phone. The other two directions are much stiffer than this direction of measurement and the vibrational signals were found to be much weaker than the proposed direction of measurement. The second part of the experiments was to characterize the perception of the vibration based on human subject survey. The survey was conducted in an air-conditioned classroom environment with twenty student volunteers aged between twenty and thirty. These students were part of the enrolment for the undergraduate technical elective course on Vibration Theory and Applications conducted at the Department of Mechanical Engineering, National University of Singapore. The average age of the student volunteers was 23.5 years old. Due to the nature of the Faculty of Engineering, there were 19 males and only 1 female student volunteers. Each volunteer was first asked to hold the mobile phone in the palm followed by putting the mobile phone in his or her right side pocket. Each volunteer was asked to assess the strength of the vibration using a 5-point Likert Scale; with 1 indicating the weakest perception and 5 indicating the strongest vibration perception. A 5point Likert Scale was deemed to be able to give sufficient variation in the responses of the volunteers. The five models were tested in random sequence. 3. Results The mass, physical dimensions in terms of length, width and thickness of each mobile phone, the measured dominant frequency of vibration, the measured maximum acceleration as well as the measured maximum inertia force (mass maximum acceleration) are appended in Table 1. Among the five mobile phones labeled as product A to product E, product B has the lowest vibration frequency, which is only 68 Hz. Product A has the highest vibration frequency, which is 229 Hz. However, although Product A has the highest vibration frequency, its acceleration amplitude is the lowest among the five mobile
Table 1 Measured characteristics of the mobile phones. Mobile phone models
Mass (kg)
Length (mm)
Width (mm)
Thickness (mm)
Frequency (Hz)
Peak acceleration (m/s2)
Peak inertia force (kg m/s2)
Product Product Product Product Product
0.134 0.314 0.085 0.116 0.135
104 128.2 99.8 109.8 115
50.0 55.66 50.6 45.0 61.0
15.50 22.64 13.80 11.00 11.60
229 68 172 184 184
0.26012 0.32422 1.32926 2.02368 3.40785
0.0323 0.1018 0.1130 0.2347 0.4601
A B C D E
H.P. Lee, S.P. Lim / Applied Ergonomics 45 (2014) 807e810
809
Table 2 Summary of survey outcome. H (mobile phone held in palm). P (mobile phone held in pocket). F (Female), M (Male). Phone models (Age)
(2 months) product A (6 years) product B (2.5 years) product C (8 months) product D (1.5 years) product E Gender Age/yrs
Likert scale
H P H P H P H P H P
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Average
2 1 2 3 5 4 3 2 4 4 M 27
2 1 2 2 4 3 3 2 5 4 M 30
2 1 2 2 4 4 2 2 5 4 M 22
2 1 2 2 4 4 2 2 4 4 M 23
2 1 3 2 4 3 2 2 5 4 M 23
2 2 1 1 4 4 3 2 5 5 M 24
2 1 2 1 4 3 3 2 4 4 M 24
2 1 3 3 4 4 2 1 4 4 F 21
3 1 1 1 5 5 2 2 5 4 M 24
1 1 1 1 5 4 2 2 4 3 M 20
3 1 4 2 5 4 2 1 5 3 M 24
2 2 1 1 5 4 2 1 3 3 M 22
2 2 1 1 4 4 3 3 5 5 M 22
4 3 1 1 5 3 3 2 5 4 M 22
3 2 3 2 5 4 3 1 5 3 M 22
1 1 1 1 3 1 2 1 2 2 M 23
1 1 1 1 3 2 1 1 3 2 M 25
1 1 1 1 3 2 1 1 4 2 M 25
2 1 1 1 4 2 2 1 4 3 M 23
2 1 1 1 4 3 2 1 4 3 M 24
2.05 1.3 1.7 1.5 4.2 3.35 2.25 1.6 4.25 3.5
phones, which is only 0.2601 m/s2. Product E has the highest peak acceleration, which is 3.4079 m/s2. The outcome of the vibration survey based on the twenty student volunteers was presented in Table 2. 4. Discussion Among the five mobile phones for the survey, most of the student volunteers or participants felt that product E and product C had better vibration perception than the remaining phone models, be it holding in palms or putting in their pockets. Product A had the weakest vibration perception level when putting inside the pocket. Product E had the highest peak acceleration as well as inertial force among the five phone samples. This was probably the reason why it was consistently ranked as the phone which gave the strongest vibration alert. The student volunteers in general felt that mobile phone placed inside pocket would result in lesser vibration perception as compared to mobile phones held in palms. This was probably due to the layer of cloth that separated our skin from directly contacting with the vibrating mobile phones. Part of the vibration could have been absorbed by the garment of our pants. The vibration perception of Vibration of Product B was in general weak although it had the largest surface area among the five mobile phone models. This was probably due to the low vibration frequency and its relatively low peak acceleration value. Also, the relatively old age of this mobile phone might affect its strength of vibration. Both product A and product B had the lowest peak acceleration as well as inertia force values among the five mobile phone models. This was the reason why both of these mobile phones had the lowest vibration perception among all five mobile phones. Yao et al. (2010, 2007) reported that a larger weight of the mock up mobile phone would require smaller acceleration to produce the same perceived strength. Product B was about three times the weight of product E as it was a much older model before the current design trend of miniaturization. The much lower vibration frequency of product B was probably the cause of relative low acceleration and also the resulting low perception of vibration. Product A was about the same mass as product B. The higher peak acceleration of product A was due to the relatively higher vibration frequency of product A which could be the cause of the peak acceleration to be of the same order of magnitude of product A. The relatively higher vibration frequency of product A and consequently the higher peak acceleration and inertia force was probably the cause for the vibration perception to be higher than product B. The number of student volunteers participating in the survey was twenty. The number was still considered to be small but it was much larger than the twelve participants reported in Yao et al.
23.5
(2010). The present study used the actual commercial mobile phone models and not the mocked up models used in some of the reported studies. A consequence or limitation was that the effect of mass, contact surface area, and frequency could not be analyzed independently as in some of these reported studies. However, it would give the vibration perception of actual commercial mobile phone models. The current trend of miniaturization had resulted in mobile phones of smaller and lighter design. For example, the four mobile phone models besides model B which was relatively older were all smaller and lighter. Another design trend was to have mobile phones of smaller thickness but bigger surface area as in model E. For smaller mobile phones, the users will typically hold the phones in their palms which are likely to be in contact with the main front or back surfaces of the mobile phones. Based on the reported studies by Johansson (1976) and Johansson and Vallbo (1979), there are four different receptor types located in the glabrous skin of the fingers and palms which react to mechanic deformation. Among the four types namely the Merkel receptor system (SA-I), the Meissner receptor system (RA-I), the Pacini Corpuscels receptor system (RA II/PC), and Ruffini receptor system, the Pacini Corpuscels (RA-II/PC) located at palms and fingers are responsible for accelerations in the skin deformation with highest sensitivity at about 100e200 Hz The vibration frequencies for the newer mobile phone models tested in this study are found to be all within this range. A larger contact surface area with the palms and figures, higher peak acceleration and the associated inertia force may be the main factors for the relatively higher vibration perception for mobile phone model E. The future design for the vibration alert of the mobile phones is likely to follow this trend. The numbers of mobile phone models as well as volunteers are still too small to do a proper statistical analysis although they are still larger than all the reported studies. The plan for the future is to involve a larger pool of volunteers as well as a more diversified range of mobile phone models for the vibration perception study. 5. Conclusion There are many factors that may affect the human response to the vibration alert of mobile phones. The frequency and amplitude of vibration, surface texture, battery level, physical size, as well as the body parts that are in contact with the mobile phones may all affect the perception of vibration alert. The main findings for the perception surveys of these five commercially available mobile phones were that higher frequency and amplitude of the acceleration would result in stronger vibration perception of the mobile phones. A larger contact surface area with the palms and figures, higher peak acceleration and the associated higher peak inertia force may be the main factors for the relatively higher vibration
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perception for mobile phone model E. The future design for the vibration alert of the mobile phones is likely to follow this trend. Conflict of interest The authors would like to declare that there are no issues related to conflict of interest for this study. Acknowledgment The authors would like to acknowledge the participation in this study of some of the students from the class of Vibration Theory and Applications conducted in the Department of Mechanical Engineering, National University of Singapore. References de Waard, D., Schepers, P., Ormel, W., et al., 2010. Mobile phone use while cycling: Incidence and effects on behaviour and safety. Ergonomics 53 (1), 30e42. Giacomin, J., Shayaa, M.S., Dormegnie, E., Richard, L., 2004. Frequency weighting for the evaluation of steering wheel rotational vibration. International Journal of Industrial Ergonomics 33 (6), 527e541. Giacomin, J., Onesti, C., 1999. Effect of frequency and grip force on the perception of steering wheel rotational vibration. In: Proc. Conf.New Role of Experimentation in the Modern Automotive Product Development Process, pp. 17e19. Nov. 1999. Johansson, R.S., 1976. Receptive field sensitivity profile of meachnosensitive units innervating the glabrous skin of the human hand. Brain Research 104 (2), 330e334. Johansson, R.S., Vallbo, A.B., 1979. Tactile sensibility in the human hand: relative and absolute densities of four types of mechanoreceptive units in glabrous skin. Journal of Physiology 286, 283e300. Kaaresoja, T., Linjama, J., 2005. Perception of short tactile pulses generated by a vibration motor in a mobile phone. In: Eurohaptics Conference, 2005 and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2005 and World Haptics, pp. 471e472. Mack, Z., Sharples, S., 2009. The importance of usability in product choice: a mobile phone case study. Ergonomics 52 (12), 1514e1528. Mahns, D.A., Perkins, N.M., Sahai, V., Robinson, L., Rowe, M.J., 2006. Vibrotactile frequency discrimination in human hairy skin. Journal of Neurophysiology 95 (3), 1442e1450.
Mansfield, N.J., Maeda, S., 2005. Equal sensation curves for whole- body vibration expressed as a function of driving force. The Journal for the Acoustical Society of America 117, 3853. Miwa, T., 1967. Evaluation methods for vibration effect: Part 3.Measurements of threshold and equal sensation contours on hand for vertical and horizontal sinusoidal vibrations. Industrial Health 5, 213e220. Morioka, M., Griffin, M.J., 2005. Thresholds for the perception of hand-transmitted vibration: dependence on contact area and contact location. Somatosensory and Motor Research 22 (4), 281e297. Morioka, M., Griffin, M.J., 2008. Equivalent comfort contours for vertical vibration of steering wheels: effect of vibration magnitude, grip force, and hand position. Applied Ergonomics 40 (5), 817e825. Myung, H.Y., Han, S.H., Hong, S.W., Kim, J., 2003. Incorporating user satisfaction into the look-and-feel of mobile phone design. Ergonomics 46 (13e14), 1423e1440. Park, T., Hwang, J.H., Hwang, W., 2009. Facilitating the design of vibration for handheld devices. LNCS 5611. In: Jacko, J.A. (Ed.), Human-computer Interaction, Part II, HCII 2009. Springer-Verlag, Berlin Heidelberg, pp. 496e502. Pattison, M., Stedmon, A., 2006. Inclusive design and human factors: designing mobile phones for older users. Psychnology Journal 4 (3), 267e284. Reynolds, D.D., Standlee, K.G., Angevine, E.N., 1977. Hand-arm vibration, part III: subjective response characteristics of individuals to hand-induced vibration. Journal of Sound and Vibration 51 (2), 267e282. Sauer, J., Sonderegger, A., 2009. The influence of prototype fidelity and aesthetics of design in usability tests: effects on user behaviour, subjective evaluation and emotion. Applied Ergonomics 40 (4), 670e677. Sonderegger, A., Sauer, J., 2010. The influence of design aesthetics in usability testing: effects on user performance and perceived usability. Applied Ergonomics 41 (3), 403e410. Tommerdahl, M., Hester, K.D., Felix, E.R., Hollins, M., Favorov, D.V., Quibrera, P.M., Whitsel, B.L., 2005. Human vibrotactile frequency discriminative capacity after adaptation to 25 Hz or 200 Hz stimulation. Brain Research 1057 (1/2), 1e9. Willes, P.G., Pearce, S.M., Rice, P.J.S., Mitchell, J.M.O., 1991. Vibration perception threshold: influence of age, height, sex, and smoking, and calculation of accurate centile values. Diabetic Medicine 8 (2), 57e161. Walker, G.H., Stanton, N.A., Jenkins, D.P., Salmon, P.M., 2009. From telephones to iPhones: applying systems thinking to networked, interoperable products. Applied Ergonomics 40 (2), 206e215. Yao, H.Y., Hayward, V., Cruz, M., Grant, D., 2007. The effect of weight on the perception of vibrotactile intensity with Handheld devices. In: Proc of Second Joint EuroHaptics Conf. and Symp. Haptic Interfaces for Virtual Environment and Teleoperator Systems,, pp. 551e552. Yao, J.Y., Grant, D., Cruz-Hernandez, J.M., 2010. Perceived vibration strength in mobile devices: the effect of weight and frequency,. IEEE Transaction on Heptics 3 (1), 56e62. Yim, J., Myung, R., Lee, B., 2007. The mobile phone’s optimal vibration frequency in mobile environment. In: Lecture Notes in Computer Science, Usability and Internationalization, HCI and Culture, vol. 4559, pp. 646e652.
Contents lists available at ScienceDirect
Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Technical note
Comparative studies of perceived vibration strength for commercial mobile phones Heow Pueh Lee*, Siak Piang Lim Department of Mechanical Engineering, National University of Singapore, 9, Engineering Drive 1, Singapore 117576, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2013 Accepted 5 July 2013
A mobile phone, also known as cell phone or hand phone, is among the most popular electrical devices used by people all over the world. The present study examines the vibration perception of mobile phones by co-relating the relevant design parameters such as excitation frequency, and size and mass of mobile phones to the vibration perception survey by volunteers. Five popular commercially available mobile phone models were tested. The main findings for the perception surveys were that higher vibration frequency and amplitude of the peak acceleration would result in stronger vibration perception of the mobile phones. A larger contact surface area with the palms and figures, higher peak acceleration and the associated larger peak inertia force may be the main factors for the relatively higher vibration perception. The future design for the vibration alert of the mobile phones is likely to follow this trend. Ó 2013 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Mobile phones Vibration alert Vibration perception
1. Introduction A mobile phone, also known as cell phone or hand phone, is among the most popular electrical devices used by people all over the world. A report in 2007 put the total number of mobile subscribers to be more than 3.3 billions (http://en.wikipedia.org/wiki/ Mobile_phone). Ergonomics design of mobile phones is an important subject due to the prolonged and wide usage. For example, an article published in July, 2004 issue of Ergonomics Today pointed out that if the current trend of shrinking mobile phones continued, soon the buttons would only be big enough for three-year olds. The same report also quoted a 2002 survey finding of mobile phone usage in eight large metropolitan areas which resulted in bigger and more muscular thumbs possibly due to pervasive mobile phone use and text messaging. There are some reported studies on the ergonomics design of mobile phones. For example, de Waard et al. (2010) studied the effect of mobile phone usage on the safety of cyclists. Mack and Sharples (2009) used the mobile phones as a case study to demonstrate the importance of usability in product choice. Myung et al. (2003) investigated the importance of design parameters in terms of look and feel of mobile phones based on a consumer survey. Sonderegger and Sauer (2010) also used mobile phones as an example to evaluate the influence of design aesthetics in usability testing. Mobile phones were used in their earlier study (Sauer and Sonderegger, 2009) to examine the influence of
* Corresponding author. Tel.: þ65 65162205; fax: þ65 67791459. E-mail addresses: [email protected], [email protected] (H.P. Lee).
prototype fidelity and aesthetics of design in usability tests. Walker et al. (2009) applied the system thinking to networked interoperable products such as mobile phones. Pattison and Stedmon (2006) studied the importance of human factors in the design of mobile phones for older users. Vibration alert or providing notification through vibration during silent mode or together with normal ring tones is an important function of mobile phones This function was present in the early models of pagers as well as the latest models of mobile phones. The vibration alert function was essential for conveying information to the user in a private manner, especially in meetings, public events, or concert halls, or in noisy environment to augment the normal ring tones. There are however fewer reported studies on the design of vibration alert for mobile phones. Park et al. (2009) studied the characteristics of vibration especially for human perception. They suggested an exemplary design of an authoring tool for vibrations in combination with music and sound effect. Yao et al. (2010) studied the strength perception of vibration signals used in mobile phones using mock cell phones with different phone weight and vibration frequency. Their results showed that for the same measured acceleration on the device, a heavier box was perceived to vibrate with greater strength, probably due to higher inertia force. Furthermore, signals with higher underlying frequency were perceived to be weaker for the same measured acceleration. As pointed out by Yao et al. (2010), Vibration alert was related to human perception of vibrotactile signals. The early studies have covered the aspects of detection threshold (Miwa, 1967; Morioka, and Griffin, 2005; Reynolds et al., 1977; Willes et al., 1991), perception of strength and equal sensation curve (Giacomin et al.,
0003-6870/$ e see front matter Ó 2013 Elsevier Ltd and The Ergonomics Society. All rights reserved. http://dx.doi.org/10.1016/j.apergo.2013.07.006
808
H.P. Lee, S.P. Lim / Applied Ergonomics 45 (2014) 807e810
Fig. 1. A typical setup for the experiment. The analyzers, the charge amplifier, the accelerometer as well as one of the mobile phones are shown in the picture.
2004; Mansfield and Maeda, 2005), frequency discrimination (Mahns et al., 2006; Tommerdahl et al., 2005), the influence of grip force (Giacomin and Onesti, 1999; Morioka, and Griffin, 2008), level of annoyance (Reynolds et al., 1977) and others. Yim et al. (2007) attempted to quantify the optimal vibration frequency for mobile phones in mobile environment. Subjects were asked to indicate their perception of the randomly given seven vibrotextile stimuli while they performed routine activities on a sidewalk, subway or bus. Their results showed that the optimal vibration frequency in these dynamic states was higher than 180 Hz, considerably higher than the 150 Hz obtained for static environment. Kaaresoja, and Linjama (2005) analyzed the perception of short tactile pulses generated by a vibration motor in a mobile phone. Their results suggested that the optimal duration of the control signal should be between 50 and 200 ms. Instead of using mocked up mobile phones, the present study attempts to analyze and compare the vibration perception of five commercially available mobile phones. The actual phones under the actual operating conditions were used. Two different body sites corresponding to two common manner of holding or keeping the mobile phones were tested in the human subject survey. These are the common hand held posture as well as the right pocket of pants or trousers where the mobile phone is usually kept. 2. Material and methods The majority of mobile phones have the ability to be switched into silent or vibration alert mode. On this setting the phone uses vibration alerts in place of audio alerts. Most mobile phones use an electrically powered motor with a rotating unbalanced mass to achieve this. The rotating mass induces acceleration throughout the motor, which is then transferred throughout the phone. The vibration of the whole phone body is then detectable by the user.
The vibration characteristics of five commercially available mobile phones labeled as product A to product E were first measured by attaching a Brüel & Kjær Accelerometer Type 4367 onto the back flat surface of each mobile phone. The five mobile phone models were from four different manufacturers. The actual identities of these mobile phones were intentionally withheld so as to avoid the risk of commercialization. All the mobile phones were charged fully before the tests. The age of the mobile phones ranged between two months to six years. The acceleration was measured using Hewlett Packard 35670A Dynamic Signal Analyzer with the signals first magnified by a Brüel & Kjær Charge Amplifier Type 2535. The Brüel & Kjær Accelerometer was attached onto the back flat surface of each mobile phone using wax. The mobile phones were placed on a piece of shock absorbing foam to reduce the effect of the impact of mobile phones against the table. A typical setup is shown in Fig. 1. The mobile phones were made to vibrate by calling. Acceleration vs time plots were obtained using Time Capture mode of the analyzer. It should be noted that we would be measuring only the vibration in the axis normal to the main frontal and back surfaces of each mobile phone. The other two directions are much stiffer than this direction of measurement and the vibrational signals were found to be much weaker than the proposed direction of measurement. The second part of the experiments was to characterize the perception of the vibration based on human subject survey. The survey was conducted in an air-conditioned classroom environment with twenty student volunteers aged between twenty and thirty. These students were part of the enrolment for the undergraduate technical elective course on Vibration Theory and Applications conducted at the Department of Mechanical Engineering, National University of Singapore. The average age of the student volunteers was 23.5 years old. Due to the nature of the Faculty of Engineering, there were 19 males and only 1 female student volunteers. Each volunteer was first asked to hold the mobile phone in the palm followed by putting the mobile phone in his or her right side pocket. Each volunteer was asked to assess the strength of the vibration using a 5-point Likert Scale; with 1 indicating the weakest perception and 5 indicating the strongest vibration perception. A 5point Likert Scale was deemed to be able to give sufficient variation in the responses of the volunteers. The five models were tested in random sequence. 3. Results The mass, physical dimensions in terms of length, width and thickness of each mobile phone, the measured dominant frequency of vibration, the measured maximum acceleration as well as the measured maximum inertia force (mass maximum acceleration) are appended in Table 1. Among the five mobile phones labeled as product A to product E, product B has the lowest vibration frequency, which is only 68 Hz. Product A has the highest vibration frequency, which is 229 Hz. However, although Product A has the highest vibration frequency, its acceleration amplitude is the lowest among the five mobile
Table 1 Measured characteristics of the mobile phones. Mobile phone models
Mass (kg)
Length (mm)
Width (mm)
Thickness (mm)
Frequency (Hz)
Peak acceleration (m/s2)
Peak inertia force (kg m/s2)
Product Product Product Product Product
0.134 0.314 0.085 0.116 0.135
104 128.2 99.8 109.8 115
50.0 55.66 50.6 45.0 61.0
15.50 22.64 13.80 11.00 11.60
229 68 172 184 184
0.26012 0.32422 1.32926 2.02368 3.40785
0.0323 0.1018 0.1130 0.2347 0.4601
A B C D E
H.P. Lee, S.P. Lim / Applied Ergonomics 45 (2014) 807e810
809
Table 2 Summary of survey outcome. H (mobile phone held in palm). P (mobile phone held in pocket). F (Female), M (Male). Phone models (Age)
(2 months) product A (6 years) product B (2.5 years) product C (8 months) product D (1.5 years) product E Gender Age/yrs
Likert scale
H P H P H P H P H P
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Average
2 1 2 3 5 4 3 2 4 4 M 27
2 1 2 2 4 3 3 2 5 4 M 30
2 1 2 2 4 4 2 2 5 4 M 22
2 1 2 2 4 4 2 2 4 4 M 23
2 1 3 2 4 3 2 2 5 4 M 23
2 2 1 1 4 4 3 2 5 5 M 24
2 1 2 1 4 3 3 2 4 4 M 24
2 1 3 3 4 4 2 1 4 4 F 21
3 1 1 1 5 5 2 2 5 4 M 24
1 1 1 1 5 4 2 2 4 3 M 20
3 1 4 2 5 4 2 1 5 3 M 24
2 2 1 1 5 4 2 1 3 3 M 22
2 2 1 1 4 4 3 3 5 5 M 22
4 3 1 1 5 3 3 2 5 4 M 22
3 2 3 2 5 4 3 1 5 3 M 22
1 1 1 1 3 1 2 1 2 2 M 23
1 1 1 1 3 2 1 1 3 2 M 25
1 1 1 1 3 2 1 1 4 2 M 25
2 1 1 1 4 2 2 1 4 3 M 23
2 1 1 1 4 3 2 1 4 3 M 24
2.05 1.3 1.7 1.5 4.2 3.35 2.25 1.6 4.25 3.5
phones, which is only 0.2601 m/s2. Product E has the highest peak acceleration, which is 3.4079 m/s2. The outcome of the vibration survey based on the twenty student volunteers was presented in Table 2. 4. Discussion Among the five mobile phones for the survey, most of the student volunteers or participants felt that product E and product C had better vibration perception than the remaining phone models, be it holding in palms or putting in their pockets. Product A had the weakest vibration perception level when putting inside the pocket. Product E had the highest peak acceleration as well as inertial force among the five phone samples. This was probably the reason why it was consistently ranked as the phone which gave the strongest vibration alert. The student volunteers in general felt that mobile phone placed inside pocket would result in lesser vibration perception as compared to mobile phones held in palms. This was probably due to the layer of cloth that separated our skin from directly contacting with the vibrating mobile phones. Part of the vibration could have been absorbed by the garment of our pants. The vibration perception of Vibration of Product B was in general weak although it had the largest surface area among the five mobile phone models. This was probably due to the low vibration frequency and its relatively low peak acceleration value. Also, the relatively old age of this mobile phone might affect its strength of vibration. Both product A and product B had the lowest peak acceleration as well as inertia force values among the five mobile phone models. This was the reason why both of these mobile phones had the lowest vibration perception among all five mobile phones. Yao et al. (2010, 2007) reported that a larger weight of the mock up mobile phone would require smaller acceleration to produce the same perceived strength. Product B was about three times the weight of product E as it was a much older model before the current design trend of miniaturization. The much lower vibration frequency of product B was probably the cause of relative low acceleration and also the resulting low perception of vibration. Product A was about the same mass as product B. The higher peak acceleration of product A was due to the relatively higher vibration frequency of product A which could be the cause of the peak acceleration to be of the same order of magnitude of product A. The relatively higher vibration frequency of product A and consequently the higher peak acceleration and inertia force was probably the cause for the vibration perception to be higher than product B. The number of student volunteers participating in the survey was twenty. The number was still considered to be small but it was much larger than the twelve participants reported in Yao et al.
23.5
(2010). The present study used the actual commercial mobile phone models and not the mocked up models used in some of the reported studies. A consequence or limitation was that the effect of mass, contact surface area, and frequency could not be analyzed independently as in some of these reported studies. However, it would give the vibration perception of actual commercial mobile phone models. The current trend of miniaturization had resulted in mobile phones of smaller and lighter design. For example, the four mobile phone models besides model B which was relatively older were all smaller and lighter. Another design trend was to have mobile phones of smaller thickness but bigger surface area as in model E. For smaller mobile phones, the users will typically hold the phones in their palms which are likely to be in contact with the main front or back surfaces of the mobile phones. Based on the reported studies by Johansson (1976) and Johansson and Vallbo (1979), there are four different receptor types located in the glabrous skin of the fingers and palms which react to mechanic deformation. Among the four types namely the Merkel receptor system (SA-I), the Meissner receptor system (RA-I), the Pacini Corpuscels receptor system (RA II/PC), and Ruffini receptor system, the Pacini Corpuscels (RA-II/PC) located at palms and fingers are responsible for accelerations in the skin deformation with highest sensitivity at about 100e200 Hz The vibration frequencies for the newer mobile phone models tested in this study are found to be all within this range. A larger contact surface area with the palms and figures, higher peak acceleration and the associated inertia force may be the main factors for the relatively higher vibration perception for mobile phone model E. The future design for the vibration alert of the mobile phones is likely to follow this trend. The numbers of mobile phone models as well as volunteers are still too small to do a proper statistical analysis although they are still larger than all the reported studies. The plan for the future is to involve a larger pool of volunteers as well as a more diversified range of mobile phone models for the vibration perception study. 5. Conclusion There are many factors that may affect the human response to the vibration alert of mobile phones. The frequency and amplitude of vibration, surface texture, battery level, physical size, as well as the body parts that are in contact with the mobile phones may all affect the perception of vibration alert. The main findings for the perception surveys of these five commercially available mobile phones were that higher frequency and amplitude of the acceleration would result in stronger vibration perception of the mobile phones. A larger contact surface area with the palms and figures, higher peak acceleration and the associated higher peak inertia force may be the main factors for the relatively higher vibration
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perception for mobile phone model E. The future design for the vibration alert of the mobile phones is likely to follow this trend. Conflict of interest The authors would like to declare that there are no issues related to conflict of interest for this study. Acknowledgment The authors would like to acknowledge the participation in this study of some of the students from the class of Vibration Theory and Applications conducted in the Department of Mechanical Engineering, National University of Singapore. References de Waard, D., Schepers, P., Ormel, W., et al., 2010. Mobile phone use while cycling: Incidence and effects on behaviour and safety. Ergonomics 53 (1), 30e42. Giacomin, J., Shayaa, M.S., Dormegnie, E., Richard, L., 2004. Frequency weighting for the evaluation of steering wheel rotational vibration. International Journal of Industrial Ergonomics 33 (6), 527e541. Giacomin, J., Onesti, C., 1999. Effect of frequency and grip force on the perception of steering wheel rotational vibration. In: Proc. Conf.New Role of Experimentation in the Modern Automotive Product Development Process, pp. 17e19. Nov. 1999. Johansson, R.S., 1976. Receptive field sensitivity profile of meachnosensitive units innervating the glabrous skin of the human hand. Brain Research 104 (2), 330e334. Johansson, R.S., Vallbo, A.B., 1979. Tactile sensibility in the human hand: relative and absolute densities of four types of mechanoreceptive units in glabrous skin. Journal of Physiology 286, 283e300. Kaaresoja, T., Linjama, J., 2005. Perception of short tactile pulses generated by a vibration motor in a mobile phone. In: Eurohaptics Conference, 2005 and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2005 and World Haptics, pp. 471e472. Mack, Z., Sharples, S., 2009. The importance of usability in product choice: a mobile phone case study. Ergonomics 52 (12), 1514e1528. Mahns, D.A., Perkins, N.M., Sahai, V., Robinson, L., Rowe, M.J., 2006. Vibrotactile frequency discrimination in human hairy skin. Journal of Neurophysiology 95 (3), 1442e1450.
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