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Touch Is Everywhere: Floor Surfaces
as Ambient Haptic Interfaces
Yon Visell, Alvin Law, and Jeremy R. Cooperstock
Abstract—Floor surfaces are notable for the diverse roles that they play in our negotiation of everyday environments. Haptic
communication via floor surfaces could enhance or enable many computer-supported activities that involve movement on foot. In thispaper, we discuss potential applications of such interfaces in everyday environments and present a haptically augmented floor
component through which several interaction methods are being evaluated. We describe two approaches to the design of structured
vibrotactile signals for this device. The first is centered on a musical phrase metaphor, as employed in prior work on tactile display. The
second is based upon the synthesis of rhythmic patterns of virtual physical impact transients. We report on an experiment in which
participants were able to identify communication units that were constructed from these signals and displayed via a floor interface atwell above chance levels. The results support the feasibility of tactile information display via such interfaces and provide further
indications as to how to effectively design vibrotactile signals for them.
Index Terms—Vibrotactile interfaces, ubiquitous computing, haptic communication design, floor surfaces.

Ç
1

INTRODUCTIONT

ACTILE feedback

has received growing attention as ameans of enhancing or enabling information display indiverse computing applications. As human-computer interaction has extended beyond the desktop computing paradigm, and into every other domain of human activity, tactile
display has grown in importance. This is attributable, inpart, to its ability to overcome the sensory overload andattentional demands that arise in complex, multitasking
environments.
Two new paradigms that have emerged within this context
are those of mobile or wearable information appliances, andof ambient computing. While significant attention has been
devoted to the opportunities for tactile display to enhancemobile applications, less has been given to haptic interaction
with computationally augmented environments. Nonetheless, the fundamental role that floor surfaces play in our
haptic negotiation of everyday environments suggests thatthey hold significant potential for active tactile information
display.
The design of haptic information for ground surfaces has along history, as is evidenced in urban environments. Haptic
markers are commonly used to indicate locations or paths ofinterest to visually impaired people. Similarly, they are
employed to emphasize low-lying features, such as subway
stairs, that need to be highlighted even to sighted individuals. Many everyday ground surfaces could also be profitably augmented with active tactile information displays.
Such displays might find roles that are complementary to

those that have been explored in the mobile computing
domain. Some simple end-user scenarios may be helpful toguide the discussion.
.

. The authors are with the Centre for Intelligent Machines, McGill
University, 3480 University St., Room 424, Montreal H2A 3A7, Canada.
E-mail: yon@cim.mcgill.ca, vinnylaw@gmail.com, jer@cim.mcgill.ca.

A visually impaired pedestrian is traveling to an
appointment on foot and by public transportation.
Reaching a noisy urban crosswalk, she is able toinfer, via a vibrotactile cue received near the curb,that the crossing signal is red. Later, she receives acue indicating that it is safe to cross (Fig. 1). She
enters the subway and is able to feel, via haptic
feedback supplied near the platform edge, that thetrain has not arrived. Once it arrives, similar cues
indicate the locations of the train doors. She reaches
the building of an office, and locates the elevator.
While ascending to her destination, she receives a
vibrotactile cue from the elevator floor, indicating
the floor number that has been reached, andinstantly knows when to disembark.
An elderly person taking a shower in his homereceives subtle vibrotactile feedback to his feet,
unconsciously aiding him in maintaining his balance in response to sensed shifts in his center ofmass and pressure.
A rescue team is engaged in an augmented reality
training simulation that aids them in learning toevaluate and respond to the changing conditionsof a structure during an emergency. They receive
realistic cues through the response of a hapticallyactuated floor delivering signals appropriate to thematerial and local stability of the ground surface
in the simulation.

Manuscript received 16 Nov. 2008; revised 4 May 2009; accepted 21 May
2009; published online 10 July 2009.
Recommended for acceptance by K. Kahol, V. Hayward, and S. Brewster.
For information on obtaining reprints of this article, please send e-mail to:
toh@computer.org, and reference IEEECS Log Number THSI-2008-11-0093.
Digital Object Identifier no. 10.1109/ToH.2009.31.

1.1 Potential Advantages
Floor-based haptic communication interfaces may sharemany positive features that are characteristic of other kindsof haptic communication.

1939-1412/09/$25.00 ! 2009 IEEE.

.

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149

extended to the case of interest. In addition, in large partthrough research conducted over the last two decades, moreis now known about tactile sensation in the feet. Thisbackground knowledge can, by extension, inform thedesign of floor-based interfaces, and it also raises issues
that are uniquely applicable to haptic communication
design for floor surfaces.

2.1

Fig. 1. A possible end-user scenario: pedestrians receive vibrotactile
cues via the ground surface (shown as shading around the feet),indicating the location of the crossing and state of its signal.

.

Complementarity to other modalities: Such displays canfunction well even when visual attention is occupied, or when the environment is noisy.
. Attentional salience: Tactile displays can be aneffective means of directing attention to a significant
event or location, in the presence of task load or
sensory distraction.
. Discreetness: Information received through such adisplay need only be apparent to its user.
Haptic communication via floor interfaces may, in addition,offer some specific advantages, such as those of:
.
.

.
.

.

.

2

Ubiquity and unintrusiveness: Reflecting the near
universal presence of foot-floor contact in humanenvironments.
Acceptability: While people may be averse to touchingcertain objects in public spaces such as restrooms,
floor surfaces are broadly acceptable to touch, via theshoes.
Ready accessibility: Floor-based interfaces need not
require users to possess or wear any special technology, making them accessible to a wider range of users.
Lack of specific demands on visual or manual search: Aninterface might be positioned at a location (such as apedestrian crossing) in such a way that little visualor manual search is required to locate it. This can be
an advantage for users with visual impairments.
Location, navigation, and event salience: Such interfaces
may be well suited for applications that involve
pedestrian navigation, wayfinding, or location-based
information display.
Locomotion-specific effects: As noted below, a floor
might be readily designed to actively enhance
specific sensorimotor aspects of walking.

BACKGROUND

While the literature on haptic interaction design for the feetis quite limited, much of what is known about haptic
communication via other areas of the body may be readily

Tactile Sensation and Psychophysics
of the FootThe tactile sensory physiology and psychophysics of thefoot have been the subject of considerable research, if to alesser extent than in the case of the hand. The foot has, since
the mid-20th century, been acknowledged as one of themost sensitive parts of the body to vibrotactile stimulation[42], [40]. The sensory physiology of the sole of the foot issimilar to that of the glabrous skin of the hand, including
the same types of tactile mechanoreceptors as are present in
the latter [34], namely, fast-adapting type I and II receptors(FA I, FA II) and slow-adapting type I and II receptors (SA I,
SA II). Their nervous responses largely mirror those ofreceptors in the hand [17], although some differences inperipheral vibrotactile information coding have been
identified (see, e.g., Ribot-Ciscar et al. [26]). With regard
to the receptor spatial distribution, Kennedy and Inglis
found those in glabrous skin of the foot sole to be relatively
widely distributed, with (in contrast to the hand) little
preferential accumulation in the toe areas [17]. Receptive
fields were found to be larger than in the hand by a factor of
about three. Physiological activation thresholds were
determined to be higher, on average—by a factor of
approximately eight, in the case of FA II receptors [17].
This latter finding agrees with earlier observations of higher
psychophysical thresholds to vibrotactile stimulation of thefoot compared to the hand [40]. It has been suggested that
this is due, in part, to biomechanical differences between
the skin of the hands and of the feet, and possibly tomechanoreceptor properties [41]. The ball and arch of thefoot have been found to be the areas most sensitive tovibrotactile stimulation [14].
Sensitivity has also been assessed for populations ofdifferent ages, with elderly people demonstrating elevatedthresholds for vibrotactile stimulation at FA II mediatedfrequencies [14] (i.e., those most often targeted by vibrotactile displays [37]). Thus, age is a significant factor inhaptic interaction design for floor interfaces. As in otherareas of haptic communication, such differences may becompensated by learning on the part of users, or byplasticity effects, whereby repeated exposure over time
has been found to improve vibrotactile discrimination [7].
Distinctive functional characteristics of the foot relative tothe hand include the reduced prehensile dexterity of theformer (which is reflected in the kinds of activities in which
it is involved), and the fact that static and dynamic forces on
the feet during stance are higher and more sustained thanthose in the hand (i.e., on the order of 100-1,000 Newtons in
the former case). Thus, while the thresholds measured in thestudies cited above were assessed as subjects were lying
down or otherwise off their feet, when individuals arewalking, those thresholds may be higher, due to adaptation
effects resulting from the large forces involved [8]. As in the

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case of the hand, most of the receptor types of the foot are
simultaneously active during normal motor activities, unlike the more segregated responses that are observed toaccompany simpler cutaneous stimulation by static probes,
vibrators, or electrodes [17], [34]. Ribot-Ciscar et al.
observed that vibrotactile stimulation of the foot can leadto a transformation of physiological messages potentiallyleading to the overestimation of static forces throughcoactivation of SA I afferents [26]. As a result, theapplication of extrinsic vibrotactile stimulation can resultin unintended behavioral modifications affecting postureand gait. Ribot-Ciscar et al., among others, have previously
identified similar, manual sensorimotor impairments affecting workers exposed to high-amplitude vibrations [25]. Rolland Gilhodes have investigated various proprioceptive
illusions that can be induced by vibrotactile stimulation [28].
Humans on foot are implicitly engaged in a sensorimotor
task (e.g., quiet stance or normal walking). The cutaneoustactile channels addressed by the types of interface we
describe here are active in the peripheral regulation ofbalance and locomotion through reflexes coordinating
stimuli felt through the feet to muscles in the leg and foot(see, e.g., Tax et al. [32], Zehr et al. [43]). During locomotion,
the coupling of motor reflexes to cutaneous stimulation has
been found to depend on both stimulus properties and on
the instantaneous gait phase at the time of stimulation. Thus,
if a display is meant to be accessed during locomotion,
careful consideration is warranted of stimulus design andthe timing of presentation. Such entanglements of tactile
sensation in the feet and motor behavior suggest that
constraints on the design of actuated floor displays areneeded in order to avoid adverse effects on gait and stance.
At the same time, certain applications might exploit such
effects. For example, a person entering a dangerous area
might stop more quickly if cued by a suitable vibrotactile
warning signal from the floor.
Finally, there has been much recent interest in theobservation that it is possible to enhance sensation in thefeet, and thereby postural and gait control, by providing
subthreshold noise to the foot soles [23]. This effect is seen
as significant for elderly populations, and for others with
peripheral neuropathies. Sensory aids of this type could be
implemented through the kinds of display described in thispaper, either as a central or complementary feature.

2.2 Prior Research on Interfaces for the Feet
While there has been little research on the design of
haptically actuated floor surfaces, much may be learned
from past work in areas such as the passive haptic designof ground surfaces, tactile feedback in foot-based humancomputer interaction, and locomotion interfaces forvirtual environments.
2.2.1 Haptic Design of Ground Surfaces
Public transit areas, such as urban sidewalks, pose special
risks to pedestrians with visual impairments. This is partly
due to the fact that they cannot make use of visual cues orsigns that are the most common means of marking hazards(e.g., at intersections). Tactile ground surface indicators
consist of regularly textured areas of ground, in the form of
patterns of raised domes, bars, or other bumps, arranged on

VOL. 2,

NO. 3,JULY-SEPTEMBER 2009

the sidewalk to mark significant paths or points of safetyconcern [29]. While international specifications for suchmarkers remain to be established, they must be clearly
identifiable, without being obtrusive. When higher thanabout 5 mm, they have been found to pose risks forstumbling or falling [19]. Alternative means of demarcating
floor areas have been proposed to remedy this. Kobayashi
et al. investigated the discrimination of floor areas byelasticity, in order to determine the feasibility of using thistype of material variation as a substitute for ground surface
indicators [18]. An active vibrotactile cue supplied throughthe ground could provide another approach.

2.2.2 Haptic Locomotion Interfaces
One area of recent research has concerned the engineering of
locomotion interfaces for virtual environments, as recently
reviewed by Iwata [15] and Hollerbach [12]. However, thisresearch has predominantly focused on the challenging
problems of high-fidelity force-reflecting haptic interaction
for omnidirectional virtual walking experiences. The displayof vibrotactile information underfoot for the purpose ofincreasing immersion during locomotion in virtual environments has only recently begun to be addressed [20].
2.2.3 Tactile Communication with the Feet
Vibrotactile displays for presenting information-bearing
stimuli to the soles of the feet have been little explored todate. Shoes for this purpose have, for example, been
investigated for information conveyance via nonintrusive
or hands-free interfaces [31], [36]. Rovers et al. found thatusers were able to identify several families of haptic icons,consisting of moving patterns on the foot sole presented
through an array of small vibration motors in the sole of ashoe-like apparatus [31].
Despite the limited research that has addressed the feet,there is ample evidence that information can be transmitted
via diverse body surfaces, devices, encodings, and undermany different conditions; extensive reviews of related
considerations from the literature in these areas areprovided in [7], [37], [21]. Beginning in the 1960s, Geldard
and others systematically studied the use of tactile displays
for communicating symbolic information via different partsof the body. Later, research on sensory substitution aimed atconveying information about shape, spatial configuration,or environmental conditions near a user of a distributed
tactile display; such displays were designed for body parts
such as the tongue, forehead, thigh, or back [37]. Basicguidelines for stimulation by vibrotactile feedback are now
being formulated [16]. Trends in recent research aim at
uncovering central capacities for, and limitations on, tactile
information processing [7], and at establishing a foundation
for the design of large sets of structured vibrotactile
messages, based on perceptual and usability criteria [21].
Although such guidelines necessarily depend on the display device, application, and user community addressed,
basic strategies have been successfully applied to manydifferent interfaces and sensory modalities. We profit fromprior research on musically inspired tactile communication
design discussed in Section 3.7.

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2.2.4 Human-Machine and Human-Computer Interfaces
There are many control interfaces for machine operation by
foot (car accelerator pedals, dental equipment, and sewing
machines), and somewhat fewer for human-computer interaction (foot-controlled computer mice, sensing floors, andshoes). Few of these have profited from active haptic
feedback. Systems providing haptic warning cues via an
automobile’s accelerator pedal have been researched formany years as a means of improving driving safety [22], andimplementations have now reached the market (e.g., Infiniti’s Distance Control Assist). Ferber et al. studied haptic
communication during a human control task conducted on a
haptically augmented stair climbing machine. They foundsome simple haptic cues supplied to the feet via an exercise
machine to be effective at aiding participants in maintaining
a target exercise level. The cues consisted of regularly spaced
tapping sequences encouraging the person exercising toexert more effort when he or she slowed down [6]. Rovan andHayward developed vibrotactile interfaces for furnishingadditional feedback during computer music performance,
including augmented floor tiles and in-shoe stimulators [30].
While they reported the feedback to be subjectively effectiveat conveying spatial and temporal information, no systematic
evaluation was performed. We are not aware of any priorwork on the design of vibrotactile information displays forpedestrians via the actuation of floor surfaces.

3

AMBIENT HAPTIC FLOORS: DISPLAY DESIGN

Since ambient haptic floor displays have only recently
been developed, the potential design space has been
little explored to date. In this section, we highlight roles
that such interfaces might profitably take on in futureapplications and discuss design issues, including prototyping methods. We present a prototype floor platformand describe several design strategies that we have
implemented for it.

3.1 Roles for Haptic Communication
Although application requirements depend on factors thatare difficult to anticipate outside of a case-based discussion, a number of possible roles for a haptic floor interface
can be identified.
Location-based information display. Due, in part, to thecommonality of foot-ground contact, such a device may besuited for providing notifications about a current or future
event associated with a location (e.g., the arrival of a bus atits stop), or of informing about an ongoing process.
Display of navigation-salient information. Such a
device could be highly suited for demarcating a location
or region of interest to pedestrians (e.g., a crosswalk
location, or an in-store promotion), a direction or directionsof interest, or for indicating a pedestrian path or passage(e.g., the route to a nearby emergency exit).
Functional augmentation of features or devices. In
interactive settings, floors as interfaces may prove useful for
supplementing the functionality of existing architecturalfeatures (e.g., steps, entrances, stairs, and tactile groundsurface indicators) or interfaces (e.g., foot switches, haptic
locomotion interfaces, and exercise machines [6]).

151Creative, entertainment, or emotional communication.
They may also be used for diversion, comfort, entertainment, or in the context of artistic creation, for example, bysilently providing a common tempo to musical performers,
or by communicating information to a computer music
performer [30].
Sensorimotor aids. Cutaneous tactile stimulation via thesoles of the feet can be designed to improve locomotion
during rehabilitation or normal activities [24], [10]. It could
also be used to provoke reflexes meant to avert a pedestrian
from stepping in the path of danger.
Virtual reality display. Augmented floor surfaces can beused for simulating natural ground surface features (e.g.,
earth, sand, and stone) for virtual reality or augmented
reality simulations that involve movement on foot (see [38]and Section 3.5).

3.2 Physical Interaction Design Considerations
Good design demands consideration of the device’s physicalconfiguration and other factors affecting the interaction.
Some relevant considerations include: Whether the interface
is to be integrated with an existing feature or device (e.g., anarea of tactile ground surface indicators), or if it is toconstitute a new artifact, such as a custom-built floor
platform; Whether the interaction is to be afforded to a single
person or to a group; The required spatial resolution; Thelevel of interactivity that is needed; The level of independence of stimulation to each foot. Such considerations canonly be addressed in detail through a particular design.
3.3 Prototyping Technologies and Methods
Many of the technologies needed for prototyping vibrotactile floor displays are readily available and require comparatively little engineering to deploy. The necessary electronic
components consist of vibrotactile transducers, amplifiers,
and signal generation circuitry (typically some form ofcomputer with associated digital to analog converter).
Among the distinguishing technical requirements are therelatively higher power demands, owing to both theelevated vibrotactile sensory thresholds in the feet relative
to those in the hand, and to the requirement of actuating afloor surface bearing a large mass.
Among other commercially available actuation technologies, one suitable choice is the linear voice coil motor. It
consists of a metallic mass suspended on an elastic structure
between the coils of an electromagnet, which, when driven,
exerts a force directly against a structure to which it is
attached (see the recent review article [9] for a discussion).
An advantage of these actuators is that they provide
independent control over amplitude and frequency, which
is not the case for simpler devices, such as the eccentricmass motors commonly used in mobile phones. A second
advantage is that the entire actuator may be concealed
within the device. Several commercially available voice coil
actuators exist that may be suitable for prototyping floor
surface applications such as those described here.1 The
main technical requirements are that they possess a usable
frequency bandwidth overlapping the domain of sensitivity
1. Examples include the IBEAM VT200 (Sonic Immersion LLC, Hürth,Germany) and the TST239 (Clark Synthesis, Inc., Littleton, Colorado).

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Fig. 3. A 2 ! 2 tile prototype seen from above. Force sensing resistors
are positioned as shown.

Fig. 2. (a) Interactive, four-tile prototype described in the text.
(b) Diagram of same, with PC and rendering software.

of FA II receptors in the feet and are capable of supplying
sufficient power to actuate the surface. Such actuators canreadily be driven with normal audio amplifiers.
Some technical simplifications are made possible bypresenting vibrotactile stimuli via a rigid, actuated floor
surface. First, this approach facilitates the protection of allelectronic components beneath the surface of the floor (seeSection 3.4). Second, the size of the surface being actuatedmeans that if it is driven at frequencies extending into the
audible band, usable auditory feedback can be supplied
through the same interface, allowing for cross-modal
information display. Since vibrations above about 1,000 Hz
are not felt, higher frequency signal components may, inpart, be shaped so as to improve the accompanying auditory
feedback. While we have found this feedback usable inpractice, there are significant limitations arising fromstructural resonances and emission characteristics of the
foot-floor system.
In some cases, a stimulus may be supplied through thefloor in a way that is conditional upon, or computed as afunction of, forces supplied by a pedestrian stepping ontothat area of the floor. For example, a floor surface may be
configured to provide a response resembling a (virtual) footswitch, controlled by foot pressure. Interactivity may beenabled via an array of force sensors embedded in the floor.
A detailed discussion is omitted here. One example isdescribed below.
We would like to address a few concerns that may arisein the mind of the reader regarding such interactions. First,
despite the coupling between actuator and sensor, theforces supplied by the former are often negligible, because
the magnitudes involved (on the order of a few Newtons
or less) are much smaller than the low-frequency forces

exerted by pedestrians against the ground (i.e., as large as
1,000 Newtons). Second, although actuator size or otherconstraints may dictate a coarse spatial arrangement of
actuators (distributed with a spacing of one or several feetin distance) this need not necessarily be apparent to a user.
A stimulus is generally felt through each area of the foot in
proportion to the force it exerts on the vibrating surface.
Thus, even though the spatial density of actuators may bevery coarse, the resulting experience is that the stimulus
originates at the locus of foot-floor contact. The effect issimilar to that used to present virtual tactile buttons ontouchscreen displays. For similar reasons, boundaries
between individual tiles are less noticeable than might be
expected, provided similar responses are supplied byadjacent tiles.

3.4 Prototype Vibrotactile Device
The device used in our work has evolved through a series of
earlier prototypes [38] to pedestrians standing or walking
upon it. It is assumed that they are wearing their accustomed
footwear, and no special equipment is required to be worn.
In addition to the actuators, force sensing capabilities havebeen integrated to enable interactivity, along the lines
described above. We have previously used this interface tointeractively synthesize vibrotactile signatures similar to
those that would normally be generated by walking on
natural materials, such as snow or gravel [38].
The device is simple and robust in construction. A
prototype is shown in Fig. 2. The four tiles are rigid
polycarbonate, of dimensions 30:5 cm ! 30:5 cm ! 1:25 cm.
They rest on an aluminum substructure. A force sensing
resistor (Interlink model 403) encased in foam rubber 0.5 cm
thick is positioned under each of the four corners of eachtile (Fig. 3).
A vibrotactile actuator is rigidly attached to the underside of each tile via a steel mounting bracket. In applications
that are interactive, data from the force sensors are digitizedand transmitted via a serial USB link to a personal computerthat interactively generates the vibrotactile signals. The
actuators are inertial voice coil motors (Clark Synthesis
model TST-239). They are capable of driving the floor
surface with a greater power (by a factor of about four) thanis required for the tiles used here. However, they are also
used to prototype applications in which much larger tile
surfaces are driven.

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3.5 Temporal Interaction Design Considerations
A floor-based haptic communication display, such as that
described above, can also be characterized in terms of
temporal properties of the stimuli involved, and the userinteractions, if any, that generate them. Conceptually, one
can group these qualities into those describing short-time
properties of the vibrotactile signals being displayed, themanner in which they are patterned on longer time scales,
and the type of interactivity afforded by the display andapplication.
For the purpose of this paper, short-time features arethose related to time scales (smaller than about 200 ms) overwhich the stimulus may be thought to be relatively
stationary or unitary. Such signals can be thought of as
building blocks from which more complex stimuli can bebuilt. They may be characterized by properties related totheir frequency content, temporal extent, temporal amplitude envelope, or modulation that is applied to any of these.
Alternatively, they might, as discussed below, be designedby analogy to a physical event, such as an impact transient(IT), with properties resembling hardness or resonance.
Low-level signals can be profitably organized into
structured temporal patterns. A motivation for doing so is toimprove identifiability or to increase perceptual information
content in a set of stimuli. The frequency bandwidth andtemporal resolution of the tactile channel limit the amount of
information that can be encoded via short-time features. In
recent research on vibrotactile communication, musicallyinspired structures (motifs or rhythms) have been profitably
used to design larger sets of vibrotactile icons that areperceptually well distinguished from each other [3], [35],[33], [21]. An alternative approach, discussed further below,
is to arrange low-level signals to resemble the temporal
pattern of a physical process, such as bouncing or breaking(for an example from an auditory perspective, see [39]).
Finally, properties related to the interaction itself refer tothe manner in which vibrotactile signals are generated in
response to user actions, as captured through sensors of thedevice. Here, one can also allow for the possibility that thedevice is noninteractive. Properties of the interaction aredetermined by the sensing method used, and the way inwhich the resulting data are used to control the synthesis of
the vibrotactile signal. According to the nature of thiscontrol mapping, the interaction may be characterized as
discrete or continuous. A continuous interaction gesture canbe described as effective (in the gestural typology suggested
by Cadoz [4]) if the vibrotactile signal is generated in a waythat energetically increases with the energy of users’
actions. A related notion is that stimuli may be synthesized
as if generated by a virtual physical system, such as a bed ofgravel, which is meant to be stepped upon.
3.6 Three Interaction Cases
To illustrate these design considerations, we present threedifferent interaction case examples (Fig. 4) developed forthe interface described in Section 3.4.
1.

A floor surface displays structured vibrotactile
signals composed of abstract elements (such as asum of sinusoids with a certain temporal envelope)
resembling musical notes and arranged in a rhythmic pattern (i.e., a tacton).

153

Fig. 4. Representations of three interaction paradigms. 1. A vibrotactile
message designed using waveform-level properties (i.e., a tacton).
2. One derived from a model of a physical process, such as an object
tapping on the tile. 3. A virtual physical material interaction, such as astep onto a bed of gravel.

2.

Such a surface presents stimuli that resemble thevibrational signature resulting from a physicalobject, such as a hammer, tapping with a rhythmic
pattern on the underside of the tile. These representationally coded signals may be thought of as
tactile icons.
3. A tile surface responds to the applied force exerted
by a footstep with a signature approximating that ofa natural ground material, such as gravel or snow. A
computer simulation is responsible for rendering
this feedback in a way that is highly dependent upon
the measured force profile with which the step is
executed in time [38].
Case 3 has been explored in an earlier publication by theauthors [38]. Methods for the design of stimuli for cases 1
and 2 are described in detail in Section 3.7. They areevaluated in the experiments described in Section 4.

3.7 Vibrotactile Stimulus Design
Methods for the presentation of structured vibrotactile
stimuli corresponding to case examples 2 and 3 ofSection 3.6 were prototyped. The approach is based, inpart, on a musical phrase metaphor that has been evaluatedpositively in recent literature on vibrotactile display [33],[3], [35]. In it, a stimulus is encoded in a rhythmic phrase,characterized by a set of notes each having a certain onset
time, duration, and amplitude. A phrase is constrained toconsist of two or more repetitions of a musical bar. The time
domain of a single bar is quantized into 24 unit steps. A
tempo is set, determining the total duration of a bar (inseconds). Notes can begin at any step and possess durations
given by integer numbers of steps. Only a single note isallowed to play at any time. The amplitude of each is
specified at its onset. Notes within the pattern of a given
vibrotactile signal differ only in their amplitudes anddurations, so that all possess the same short-time parameters (e.g., frequency and roughness). This limits thenumber of parameters that must be specified for each

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affecting the frequency content, duration, amplitude modulation, and amplitude temporal envelope of the signal, asprescribed in case example 1 of Section 3.6.
A basic harmonic signal sðtÞ is composed of a sum of afundamental sinusoidal component s0 ðtÞ ¼ sinð!0 tÞ at angular frequency !0 , and N harmonic components at
frequencies !k ¼ k!0 . The desired waveform can be
efficiently generated by a standard technique from audio
synthesis, known as Chebychev waveshaping [1]. In it, s0 ðtÞ
is passed
P through a static nonlinear transfer functionwðxÞ ¼ k ak Tk ðxÞ, where ak are the desired harmonic
amplitudes and Tk ðxÞ is the kth Chebychev polynomial.
The result gives the desired harmonic signal sðtÞ asXsðtÞ ¼ wðs0 ðtÞÞ ¼
ak Tk ðs0 ðtÞÞ
ð1Þ
k

¼

Fig. 5. The eight rhythms (R1-R8) from which both sets of stimuli werebuilt. Time runs left to right. The bold horizontal lines show the temporalextent of notes. The vertical bars coincident with the onsets indicate
amplitude. Bar duration and number of repetitions are shown.

stimulus. The algorithms used to synthesize the note-levelsignals are described in the sections that follow.
The vibrotactile signals used in the experiment describedin Section 4 were built from a set of eight rhythmic patterns
generated using this method. The corresponding bars areshown in Fig. 5. The duration of each bar is between 450
and 750 ms, and each is repeated a fixed number of times(either three or four). As shown in the figure, an accented
note marks the beginning of each bar, serving as an“anchor” for the pattern [33]. The waveform of a stimulus
from the experimental set, corresponding to rhythm R1
from Fig. 5, is shown in Fig. 7.

3.7.1 Waveshaping Synthesis
The first approach we adopted to the design of short-timestimuli is based on the specification of basic signal propertiesN%1
X

ak sinð!k tÞ:

k¼0

ð2Þ

Using this method, it is simple to design a waveform whoseenergy lies within the target frequency band, centered on
approximately 250 Hz.
The complete short-time waveform yðtÞ is obtained bymultiplying the harmonic waveform sðtÞ by an amplitudeenvelope function eðtÞ with desired starting time ts ,duration T , amplitude A, and attack and decay times ta
and td . Finally, it is subjected to form of roughening, via
amplitude modulation with a signal rðtÞ ¼ 1 þ mr sinð2!fr tÞof frequency fr and modulation depth mr . That is,yðtÞ ¼ sðtÞ eðtÞ rðtÞ.
o generate the stimuli used in the experiment (Section 4),
N ¼ 10 harmonics were used, with fundamental frequency
f0 between 30 and 70 Hz, and a modulation frequency of
fr ¼ 25 Hz. The harmonic amplitudes ak and the roughness
mr , were set heuristically by the designer, as was the basefrequency f0 . Waveforms used in the experiments are shownin Fig. 6a.

3.7.2 Synthesis of Virtual Impact Transients
The second method used to synthesize short-time stimulus
components is based on virtual impact events, consisting oftransient forces generated by the collision of two virtual
elements: an exciting, but nonresonant object, termed theFig. 6. Waveforms used to generate stimuli used in the evaluation, ordered as reported on in the experiment. (a) Those generated using
waveshaping synthesis. (b) Those generated using virtual impact transient model. The final stimulus sets result upon superimposing the rhythmic
patterns described in Section 3.7.
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VISELL ET AL.: TOUCH IS EVERYWHERE: FLOOR SURFACES AS AMBIENT HAPTIC INTERFACES

155

Fig. 7. Example of a stimulus used in the experiment, resulting from the model settings that produced the first “impact” waveform of Fig. 6 incombination with the first rhythmic pattern of Fig. 5.

hammer, and a resonant object, termed the sounding object[27]. The impact force F ðtÞ is generated by an efficient,physically based synthesis model obtained from a simplified version of Hertz’ law, known as the Hunt-Crossleymodel [13]. It is obtained by numerically integrating thedifferential equation_
F ðtÞ ¼ kxðtÞ" % #xðtÞ" xðtÞ:ð3Þ

_Here, xðtÞ is the compression displacement and xðtÞis thecompression velocity. The impact force has parameters
governing stiffness k, dissipation #, and contact shape ".
This force is provided as an input to the resonant sounding
object. The latter is modeled as a set of N resonant filters
operating in parallel, with a combined impulse response
given byyðtÞ ¼NX
i¼1

ai e%bi t sinð!i tÞ;

ð4Þ

and determined by a set of parameters governing the modal
amplitudes ai , decay rates bi , and resonant angularfrequencies !i . An impact event is synthesized by initializing (3) with the desired velocity vI of impact, andsubsequently, integrating the composite system in time.
See [27] for a more detailed discussion, including an
overview of the numerical implementation we have used,which is based on the open source Sound Design Toolkit
from the University of Verona.
The impact event transients used in the evaluation partof this paper were synthesized with two resonant modes(N ¼ 2). The transients themselves exhibit more complex
frequency spectra than this might suggest, due to dynamic
coupling of the nonlinear impact model with the modal
resonant object [27]. Exemplary waveforms synthesized forthe evaluation using this method are shown in the bottomof Fig. 6.

4

EVALUATION

An evaluation of the methods for vibrotactile information
display via floor surfaces that were described above has
been conducted. The aim was to determine whether
vibrotactile signals designed in the two ways described
could be distinguished when presented via a floor surface.
Two sets of eight signals were designed, one based on the
waveshaping method and the other based on the virtual
impact transient method. (We refer to these as sets WS andIT, respectively.) Both sets were designed using the sameeight underlying rhythmic patterns (shown in Fig. 5) as
noted in Section 3.7.

The vibrotactile stimulus sets differed, therefore, only inthe note-level signals from which they were composed. Eachtacton from the waveshaping synthesis (WS) set was
assigned distinct values of the fundamental frequency f0 ,ten harmonic amplitudes ak , and amplitude modulation
depth mr . Stimuli from the impact transient set were each
designed with different values of the hardness k, contact
shape ", and of the frequencies !i and decay factors bi of thetwo modes. The latter were chosen so that the frequency
spectrum of the stimulus had most of its energy near 250 Hz.
Another significant difference between the two sets ofstimuli is that the notes from the waveshaping synthesis set
differ in duration according to the length of the note in thebar, while in the impact transient set, the amplitudeenvelope of each note decays exponentially as determinedby the modal decay factors bi . However, because therelevant decay times were generally short, as can be seen
in row 2 of Fig. 6, the durations were shorter, in nearly allcases, than in the WS set, and they were independent of thenote duration specified in the rhythmic pattern.
The experiment assessed both the rate of correct
identification and learning for the two sets. In descriptivecomments received during pretesting, we observed atendency for people to describe the IT stimulus set in termsof familiar impact events. As a result, we hypothesized thatthese stimuli might prove more recognizable, despite thesmaller number of parameters that were used to distinguishthem, and the relative lack of note duration information in
the patterns.

4.1 Methodology
A total of 24 people aged between 20 and 39 years took partin the experiment. All gave their informed consent inagreement with university ethics guidelines. Twelve ofthem were male and 12 were female. Fourteen of them were
university students. The experiment was designed with asingle independent factor, resulting in each group of
12 participants being presented with the task of identifying
stimuli from one set. A between-groups design was usedbecause the rhythmic content in the two stimulus sets is
identical, so participants exposed to one set would be
expected to perform better than otherwise expected withthe second set.
Each participant was given a pair of hard soled men’s
dress shoes in his or her size to wear during the experiment.
Apart from size differences, all the shoes were identical. The
amplitude of vibration of the tile was adjusted as needed foreach participant, depending on his or her ability to detect areference vibrotactile noise signal. At the beginning of thesession, participants received instructions, together with an
explanation and demonstration of the experimental interface

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and the operation of the display. At all subsequent stages(except during pauses), participants were required to standon the actuated floor surface, and to wear closed-ear
headphones playing pink noise at a loudness sufficient tomask the (generally low-level) sounds produced by theapparatus. The software application used during the experiment ran on a personal computer, implementing therespective design method from Section 3.7. In addition to
the floor tile, the interface consisted of a graphical userinterface with numbered buttons, one for each icon,presented on a computer monitor. Participants selected
items in the graphical user interface using a mouse.
The experiment was based on absolute identification of
the eight stimuli from the respective set, with a uniquecorrect response required for each (the numerical identifierof the icon, ranging from 1 to 8). The same stimuli were used
for all participants within a given group, but they were
presented in random order in each session of the experiment,with a different ordering presented to each participant.
After an introduction to the device and interaction
method, participants were given five minutes of self-guided
learning. During this time, they could select a numerical
identifier and be presented with the stimulus corresponding to their selection. The rest of the experiment was
divided into six sessions. During each session, all stimuli
were presented twice. Thus, overall, each participant was
asked to identify every vibrotactile stimulus from their set a
total of 12 times. Each session took less than about four
minutes to be completed. Participants were allowed a short
break between sessions, but in most cases, preferred tocontinue so as to avoid forgetting the learned associations.
Participants were presented with stimuli sequentially. At
each presentation, they could press a button to play thestimulus up to four times before supplying a response.
Feedback, in the form of the correct stimulus ID, wasprovided after each response was given. As in previous
studies [11], the reason for providing feedback was tofacilitate the assessment of recognition after learning andrate of learning throughout the experiment.

4.2 Results and Discussion
A log of the stimuli and responses was recorded by theapplication throughout the experiments. Participants werealso interviewed following the experiment.
The stimulus sets each included eight items, near thelimit of what participants might be expected to retain inworking memory. Training was limited, consisting of a
maximum of, on average, 20 reinforced presentations ofeach stimulus at the end of the final session. The stimuli
were not assigned any mnemonic, other than an arbitrary
numerical index, that could be used to remember them.
Although introducing a semantic association to eachstimulus (for example, the name of a meaningful event)can be used to improve recall, good design of display andstimuli is a prerequisite to good performance [33]. Since ourintention was to assess the suitability of the display
methods behind these two stimulus sets, we wished to
avoid introducing any external effects that might beassociated to the choice of semantic labels.
The mean rate of correct identification after six sessionsof enforced learning, averaged between all participantsVOL. 2,

NO. 3,JULY-SEPTEMBER 2009

Fig. 8. (a) Correct identification rate for each of the 12 participants ingroup assigned to stimulus set IT, after six sessions of assessment with
enforced learning. (b) Similar rates for the WS set group.

(including both stimulus sets) was 61 percent, with astandard deviation of 21 percent. Chance performance
would correspond to 12.5 percent. Postlearning identification rates for participants in the impact transient group andfor those in the waveshaping synthesis group are shown in
Fig. 8. Four of the 12 participants in the IT group were able
to achieve 80 percent or better correct identification after
less than 20 minutes of enforced learning. One of the
12 participants in the WS group was able to reach this levelof performance.
The results obtained appear to be comparable topublished results on absolute identification of vibrotactile
stimuli via manual interfaces after short periods of learning.
For example, Enriquez et al. reported average identification
performance of 73 percent (versus the expected chance
performance rate of 33 percent) after an average of
20 minutes of learning [5].
The confusion patterns for the stimuli in each group of
the experiment are shown in Fig. 9, averaged between all
sessions and participants. The least confused stimulus was
identified at an average rate of 80 percent, while the mostconfused stimulus was identified at an average rate of only
25 percent. A close comparison of the confusions withineach stimulus set with the stimulus properties themselves
did not reveal any easily discernible feature of the rhythms
or the short-time waveforms that caused them to be
confused. Nonetheless, the confusion patterns for the two
sets appear to be relatively distinguished from one another,which suggests that the short-time features were perceived
differently. Performance varied considerably between participants as the experiment proceeded, with a few showing
consistently high improvement between sessions, while

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VISELL ET AL.: TOUCH IS EVERYWHERE: FLOOR SURFACES AS AMBIENT HAPTIC INTERFACES

157

Fig. 11. Box plot summary of the mean correct identification rate for the(a) IT and (b) WS stimulus sets.

Fig. 9. (a) Confusion pattern within the set of eight WS stimuli insession 6, averaged between participants. (b) The same data for the setof eight IT stimuli.

others showed nearly none. A summary of the mean correct
identification rate using each icon set, averaged between all
sessions, is provided in Fig. 11. Mean performance on theimpact transient set was higher than on the waveshaping
synthesis set, but the level of significance is marginal(ANOVA 1-way p ¼ 0:06). Mean correct identification rates
in each enforced learning session are shown in Fig. 10. The
mean correct identification rate after each of the first two
training sessions was significantly higher for the IT set(p ¼ 0:01 and 0.02, respectively), but thereafter, the results
lacked significance (p > 0:1 in all cases).
The IT group approached its peak performance with far
fewer presentations of the stimuli than was the case for the

Fig. 10. Mean correct identification rates in each session. The error bars
show 95 percent confidence intervals.

WS group. Concretely, after session 1, the IT group
achieved more than 80 percent of the performance itreached in session 6, a rate that the WS group would notachieve until session 3. This suggests that stimuli similar
to the IT set might be advantageous for applications inwhich little training would be expected, although further
investigation of this is needed. Examples of such applications could include displays in public spaces, such as
crosswalk indicators.

4.2.1 Subjective Ratings and Comments
Participants completed questionnaires after the experiment,
rating the task difficulty, subjective properties of thestimuli, providing descriptive feedback regarding thestrategies they adopted. Participants were asked to rate
the difficulty of the first task session and the last session ona scale from 1 (very easy) to 5 (very hard). As expected, thefirst task session was rated as significantly harder than thelast session (mean 3.96 versus 3.04, with p ¼ 0:04).
An analysis of the rated difficulty of the first session(Anova one-way), with the stimulus set type as theindependent factor, revealed the WS set was perceived as
initially harder than the IT set (mean 4.4 versus 3.5, withp ¼ 0:02). However, there were no statistically significant
differences between the rated difficulties of the stimulus
sets during the last session. This echoes what was seen in
the performance data: the IT stimulus set was more
identifiable than the WS set at the beginning of theexperiment, but the difference faded before the end of
the experiment. Based on participant comments, cognitive
fatigue may have been a factor here.
The study also aimed to assess the level of comfort tousers of a vibrotactile display employing these stimuli.
Participants were asked to rate the vibrotactile stimuli on a
five-point scale from 5 (comfortable) to 1 (uncomfortable).
The average rating among all participants was 3.5 with astandard deviation of 1.1. No participant rated the stimuli
as uncomfortable, and no significant differences in comfort
ratings were found between the two groups. Some of themnoted becoming uncomfortable with standing in the sameplace and posture during each session.
During subsequent interviews, several participants reported that the association part of the task, which required
them to learn the numerical indices of the stimuli, was thehardest part of the experiment, and that they experienced

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IEEE TRANSACTIONS ON HAPTICS,

cognitive fatigue as the experiment wore on. Some participants suggested the task may have been facilitated by theaddition of semantic information or a nonnumerical mnemonic symbol (e.g., an animal name). As discussed above,
this observation may hold potential for aiding the association part of the task, and, while it would certainly makesense for an application designer to consider, it does notnecessarily speak of the design of the stimuli themselves.

4.2.2 Description of the Cause of the Stimulus
In a separate study, 10 people who did not participate in theforgoing experiment were presented with the 16 stimuli
from the sets used above, in random sequential order, andwere asked to provide, using their own words, a short label
describing what caused the vibration. Sensory conditions
were the same as in the first experiment. The most common
response was “unknown” (about 18 percent of IT set labels
and 40 percent of WS set labels). One hundred and thirteen
other responses were obtained, 72 of which were unique, if
often very similar (e.g., “door knock” and “angry door
knock”). Among these labels, most were related to familiar
impact events, including: “door knock,” “horse gallop,”“hammer,” “drum,” “foot falls overhead,” and “tapping.”
To give one example, the second IT stimulus was describedby seven of 10 participants as caused by a “knock” or“knocking,” and by another two as originating from a“hammer.” There appeared to be a greater tendency for IT
set stimuli to be attributed to such events than was the casefor WS set stimuli (mean 58 percent versus 18 percent, withp ¼ 0:0002). Here, however, we used our own judgement indeciding whether a label referred to an impact. Several
labels were associated only to the WS set, including: “bassguitar,” “cellphone,” “machinery,” and “car start.”
While it seems likely that there are differences in causal
interpretation between the two groups of stimuli, more
research is clearly needed. It is tempting to suggest that suchdifferences, if established, might be linked to the effectiveness of the stimuli at communicating information. However,analogous research in the auditory display domain, comparing the effectiveness of melodically designed “earcons” withrepresentational “auditory icons,” has shown that this issue
is far from simple or straightforward [2].

5

CONCLUSION

Floor surfaces already play roles that intimately link tactile
sensation in the feet to our everyday activities. The use ofsuch surfaces as displays for active haptic communication
appears to be a valuable idea, and one that naturally
extends accepted areas of practice in the design of haptic
information for ground surfaces. We have highlighted therange of roles that such an interface might play in futureapplications, and have pointed to a few potential end user
applications that illustrate these roles.
Distinguishing advantages of these displays include theirapplicability to a wide range of settings and environments,
their ready accessibility to anyone on foot, and theirpotential for seamlessly presenting dynamic information
linked to a specific location, navigational, or locomotor task.

VOL. 2,

NO. 3,JULY-SEPTEMBER 2009

We have outlined design considerations for haptic
communication through floor surfaces and detailed a device
and set of interaction studies that have been developed toembody these. The technological feasibility of such devices
is high and the cost of prototyping them is low.
A characterization in terms of temporal interaction
properties was used to organize different possible interaction designs presented through the floor interface, andthree demonstrations that we have developed were
described and situated within it. Stimulus design methods
based on signal-level specifications were highlighted, byclose analogy to prior work on vibrotactile information
display. Design methods based on physical metaphors,such as virtual impact events or structured materials, werealso presented.
The experimental results support the identifiability ofvibrotactile messages conveyed through floor surfaces andpoint toward the adaptation of design guidelines developed
for other modes of haptic display to this setting. They alsosuggest that physical metaphors may hold promise for
improving the design of stimuli for such an information
display, although further research is needed on this subject.

ACKNOWLEDGMENTS
The authors gratefully acknowledge support from the
MDEIE of Quebec for the EU FP7 project NIW (no. 222107).
The first author acknowledges support from the EU FP6
project CLOSED (no. 29085) and the ESF COST Action on
Sonic Interaction Design (no. IC0601).

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Yon Visell received the BA degree in physics
from Wesleyan University and the MA degree inphysics from the University of Texas at Austin in
1999. Currently, he is working toward the PhD
degree in electrical and computer engineering at
the Centre for Intelligent Machines, McGill
University, Montreal, Canada. His researchconcerns physically-based haptic synthesis andinterface design for vibrotactile display. He is a
Canadian national delegate to the ESF COST
Action on Sonic Interaction Design, and was a local coordinator for theEuropean project CLOSED at the University of Applied Arts, Zurich. For
two years, he was at Loquendo, San Francisco, California, conducting
research on automatic speech recognition, and was engaged in sonar
signal processing at ARL, Austin, Texas. Subsequently, he was
employed at Ableton AG, Berlin, Germany, where he led audio signal
processing development for an award-winning professional music
production suite. In 2004, he co-founded the art technology organization
Zero-Th, Pula, Croatia.
Alvin Law received the BASc degree in engineering physics in 2007 from the University of
British Columbia, Vancouver, Canada. Since late2007, he has been working toward the MASc
degree at McGill University, in the area of haptic
interaction through augmented ground surfaces.
In 2004, he was a research assistant at the UBC
SSP Laboratory on the Solar Canopy project.
The following summer, he was employed at ATR
Institute International in Japan on the Robovie
project. During 2006, he was a research assistant at the National
University of Singapore studying electromagnetic metamaterials.
Jeremy R. Cooperstock received the PhD
degree in electrical and computer engineeringfrom the University of Toronto in 1996. He iscurrently an associate professor in the Department of Electrical and Computer Engineering, amember of the Centre for Intelligent Machines,
and a founding member of CIRMMT at McGill
University. He directs the Shared Reality Laboratory and leads the technical development of
the Ultravideoconferencing system, for which he
was recognized with an award for Most Innovative Use of New
Technology from ACM/IEEE Supercomputing and a Distinction Award
from the Audio Engineering Society. His research interests focus oncomputer mediation to facilitate high-fidelity human communication andthe underlying technologies that support this goal. He chairs the AudioEngineering Society (AES) Technical Committee on Network Audio
Systems and is an associate editor of the Journal of the AES.

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