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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
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
Proceedings of the 6th WSEAS International Conference on Applied Informatics and Communications, Elounda, Greece, August 18-20, 2006 (pp241-245)
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).
Figure1. The carry mode and position of mobile phone
A piece of cloth
Skin
Vibration motor
Phone
Vibratio
n m
otor
Vibratio
n m
otor
PocketSpace
Space
A piece of cloth
Skin
Vibration motor
Phone
Vibration motor
Phone
Vibratio
n m
otor
Vibratio
n m
otor
Vibratio
n m
otor
Vibratio
n m
otor
PocketSpace
Space
Proceedings of the 6th WSEAS International Conference on Applied Informatics and Communications, Elounda, Greece, August 18-20, 2006 (pp241-245)
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
Proceedings of the 6th WSEAS International Conference on Applied Informatics and Communications, Elounda, Greece, August 18-20, 2006 (pp241-245)
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.
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Proceedings of the 6th WSEAS International Conference on Applied Informatics and Communications, Elounda, Greece, August 18-20, 2006 (pp241-245)
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Proceedings of the 6th WSEAS International Conference on Applied Informatics and Communications, Elounda, Greece, August 18-20, 2006 (pp241-245)
