Multimodal Augmented Reality –Augmenting Auditory-Tactile Feedback
to Change the Perception of Thickness
Geert Lugtenberg1,2, Wolfgang Hürst1, Nina Rosa1, Christian Sandor2,
Alexander Plopski2, Takafumi Taketomi2, Hirokazu Kato2
1

2

Utrecht University, Princetonplein 5, 3584 CC Utrecht, Netherlands
Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma, Nara, 630-0192, Japan
geert_lugtenberg@hotmail.com, {huerst|N.E.Rosa}@uu.nl,{sandor|plopski|takafumi-t|kato}@is.naist.jp

Abstract. With vision being a primary sense of humans, we often first estimate
the physical properties of objects by looking at them. However, when in doubt,
for example, about the material they are made of or its structure, it is natural toapply other senses, such as haptics by touching them. Aiming at the ultimate
goal of achieving a full-sensory augmented reality experience, we present an initial study focusing on multimodal feedback when tapping an object to estimate
the thickness of its material. Our results indicate that we can change the perception of thickness of stiff objects by modulating acoustic stimuli. For flexible objects, which have a more distinctive tactile characteristic, adding vibratory responses when tapping on thick objects can make people perceive them as thin.
We also identified that in the latter case, adding congruent acoustic stimuli does
not further enhance the illusion but worsens it.
Keywords: Augmented reality, multimodal AR, multimodal perception.

1

INTRODUCTION

Today’s hardware for Augmented Reality (AR), such as see-through head-worn displays, allows us to project computer-generated imagery (CGI) into our surroundings
at a quality of sometimes stunning realism. Hardware to stimulate other senses thanvision and research on perception of these modalities is however relatively lacking,although we do experience our environment based on the integration of multiple senses; for example, auditory-tactile [1, 3, 4, 10], sound [16], smell and taste [13]. An
obvious example is the exploration of objects and their material. A table might look
like being made of sturdy, solid wood, but when we touch it, we realize that it is justthin synthetic material with the visual texture of wood applied on it. Likewise, picking
up an object allows us to feel its weight and texture and thus make a better estimate ofits volume. Haptic exploration of real objects, such scratching their surface or tapping
them, results in vibrations in the object’s material. These vibrations are dependent on
certain properties of the material, some of which we can estimate when we feel or2hear the vibrations. We know from elasticity theory [9] that the displacement of a rod
or plate that force acts upon depends on the force, elastic modulus of the material, andits thickness. Previous work investigated stiffness perception (i.e., the inverse of elasticity) [3, 17] for virtual objects. In this research, we expand this by looking at real
objects in an AR setting and investigate the influence of sound and touch feedback
upon haptic exploration of thickness. Our concrete aim is to verify if we can change
the perception of thickness when tapping by modulating auditory-tactile feedback.
To get a better understanding of the relative influence of acoustic and haptic feedback when tapping an object to determine its thickness, we first start by augmenting
the auditory modality. In a first experiment, described in Section 3, we measure thecorrectness of discriminating between different thicknesses under varying auditory
conditions. In Section 4, we discuss a second experiment were we first augment thetactile modality before finally studying multisensory integration, that is, the combined
influence of tactile and auditory augmentations for the perception of thickness. Starting with studying sound is motivated by the fact that, compared to haptic hardware
solutions, auditory hardware is easier to obtain, can create a rich, realistic sound experience, and is simple to utilize using headphones. Our results, summarized in Section
5, show that we can indeed manipulate how people perceive the thickness of a material by adding acoustic or tactile feedback. We also show that when both modalities arepresent, manipulation only one of them is not sufficient, but both should be presented
congruently to achieve the desired effect.

2

RELATED WORK

Previous work in Virtual Reality (VR) aiming to influence the perception of physicalproperties related to haptic sensations is seeking to mimic real sensations in such virtual environments [3, 15]. These sensations in VR can be ‘build from the ground up’
whereas real objects have inherited physical properties that are often perceived in a
multimodal way. For example, Hachisu et al. [5] add or subtract vibrations to a haptic
pen applying force to a material. This augmentation creates the illusion of touching a
different material than the real one. This work was the inspiration for our second experiment presented in Section 4, where we propose an approach for augmenting vibro-tactile feedback when tapping an object with your finger to estimate its thickness.
Research in multisensory integration, that is, the way in which the human brain
combines congruently or incongruently perceived stimuli of different modalities, has
shown that we can, for example, get a desired effect of perception of touch by simulation of another, seemingly unrelated modality. A well-known and robust effect is theso-called ‘Size-Weight illusion’ [2]. Here, the visual indication of size has a crossmodal effect on the perception of weight of two equally heavy objects. We also knowthat when stroking a surface, ‘roughness’ can be changed by visual [7, 8] and auditory[4] simulation. The perception of hardness can be influenced by deforming the CGI ofa texture that is projected on a surface upon pressing it [6, 14]. In our research, we areinterested in similar effects using the modality of sound and tactile stimuli, and if
there is a comparable integration effect for the perception of a material’s thickness.

3

3

EXPERIMENT 1: AUGMENTED SOUND

If we assume that there is no tactile indication of thickness, that is, no noticeable difference when touching a thick or a thin object, then perception of thickness of two
visually indistinguishable objects is entirely dependent on what we hear. Therefore,
we hypothesize that we can change the perception of thickness solely by modulationof the auditory feedback when tapping it. We summarize this assumption in our firstresearch question:
RQ1: Can we achieve a different perception of thickness (solid or hollow) of an
object when tapping it by solely modifying auditory feedback and otherwise fixedphysical properties?
We approach this problem by measuring the correct identification of a cube-shaped
object as ‘solid’ or ‘hollow’ in a yes/no-type psychophysical experiment. Figure 1
shows the sound spectrograms of the typical impulse responses of a thick and thinmaterial; plastic, as used in this experiment, and wood, as used in Section 4. They
illustrate that a long decay of low frequencies is characteristic for thin material, as
opposed to a short decay of higher frequencies for bigger, more solid ones. Thus, forour experiment, we generated the needed sounds by resynthesizing the original realtime sound of a tap on a cube and transforming it according to the desired characteristic (Table 1). By resynthesizing, we take the acoustic properties of the material andobject (modal models [16]) and the velocity and duration of the tap into account.

Fig. 1. Spectrograms of impulse responses of plastic cubes (left) and wooden plates(right) show characteristics of thick (top row) and thin (bottom row) material.
Sounds originating from thin objects have a high amplitude, shown in white, in thelower frequency range. Furthermore, we see that these high amplitude low frequencies have a longer decay time.

4
Table 1. Characteristics of thickness in vibrating material
Avg. amplitude
Thick
Thin

low
high

3.1

Experimental design and setup

Frequency range ofhigh amplitudes
1000 Hz
0 Hz – 1000 Hz

Decay time
short
Long

Used platform. Figure 2 shows the platform used in the first experiment. A Mogges
piezoelectric sensor [12] is connected to the Mogees Virtual Studio Technology(VST) plugin running in Cycling’74 Max 6 on a MacBook Pro. This VST plugintransforms the vibrations measured by the piezoelectric sensor in real-time and outputs it as audio signal to the headphones worn by the participant. We used Sehnheiser
CX3.00 in-ear headphones, so that earmuffs could be worn over the headphones tocancel out external sounds during the experiment. The used objects are two 3Dprinted cubes of a styrenic plastic material that have a Young’s Modulus of 2.0-2.6
GPa. The cubes’ dimensions are 70x70x70mm. The hollow one has walls with athickness of 2mm.

Fig. 2. Setup for experiment 1: (left) Participants tap the top of a solid or hollow
cube. There is visually no distinction between the two thicknesses (during the experiment, the opening in the right figure was facing down). The cube is placed in aholder that is firmly attached to a table surface. A piezoelectric sensor is attachedto the side of the cube (right) to capture vibrations in the material.

Method and procedure. Before participants enter the examination room, one of the
cubes (either hollow or solid) is placed in the holder on the table. Test subjects thenread an information sheet before an examiner explains the procedure again verbally.
Each participant tests three conditions: no sound augmentation (control), white noisesound (no auditory cues), and resynthesized sound (simulating thin or thick material).
The resynthesized sounds are pitched down and up and decay time increased anddecreased for the modulation of thin or thick, respectively. The values for pitch anddecay are determined subjectively to create a big contrast between sounds for thickand thin.

5Participants are asked to tap the cube in front of them on the top side only with theindex finger of their dominant hand. They can repeat this tapping action as often as
they want before writing down on an answer sheet if they think the cube is hollow orsolid. Then, the cube is exchanged with the other model (out of sight of the participant, so they cannot see the thickness of either of the cubes). The order of cubes aswell as audio conditions was counterbalanced among all participants.
Thus, given the two independent variables cube thickness (solid, hollow) and tapping sound (real, static noise, thin, tick), each participant repeats all permutations ofthese variables one time, resulting in 2*4 = 16 trials per participant. The duration of
the whole experiment for one person is about 30 minutes.
Participants. Eight students with ages ranging from 20 to 31 participated in our experiment. They volunteered and were not reimbursed in any way. Their number was
motivated by the number of unique permutations of the independent variables. While
this sample size is not enough to gain statistical significance, it is sufficient to identify
a trend confirming or rejecting our research question RQ1, that is, if sound augmentation is sufficient or if tactile augmentation is needed as well.

3.2

Results and discussion

Figure 3 shows the results of the experiment. We see that in the control condition,
where real sound is heard (real.hollow and real.solid in the diagram), on average
participants estimated the thickness of the cube correctly in 81.25% of the cases (75%for hollow, 87.5% for solid cube). In the conditions where any sound cue is removed
by generating static white noise (static.hollow and static.solid), the correctness for thesolid cube stays high (93.75%), but for the hollow one, it drops to 18.75%. This already indicates a strong impact of the hollow tapping sound on thickness perception,
whereas the relevance of the solid sound seems less apparent. Looking at the simulated hollow sound (hollow.hollow and hollow.solid), as expected, we see a high correctness when the hollow sound matches the hollow material (87.5%), but a strong
drop when the hollow sound is played upon tapping a solid cube (12.5%). This provesthat peoples’ perception of thickness can indeed be manipulated by changing theacoustic stimuli. Yet, results for the simulated solid sound (solid.hollow and solid.solid) are close to chance level (56% for hollow cubes and 63% for solid ones)
suggesting that subjects were just guessing. This shows that this sound also has an
impact on the perception, because people were not able to make the right choices
anymore, but that it is not enough to consistently create the target scenario.
There are two possible explanations for why simulated solid sounds did not have asimilar manipulating effect as hollow ones. First, the sound of tapping a solid object isalready less significant in real life, whereas real hollow sounds are much more distinctive and meaningful. The fact that we observed a strong drop when comparing realsound feedback to white noise in case of the hollow cube, but none in case of the solid
one seems to confirm this. If this assumption is correct, our results would show that
we can indeed use audio to manipulate peoples’ perception, but only for certain sub-6
sets of materials, characteristics, and audio signals. Another reason why results forsolid cubes are around chance level could be that the sound sample used in our experiment is just not indicative or distinctive enough to cause a better effect. We canspeculate that with a better sound quality we might still be able to achieve the desired
effect. Also, if people would compare the solid sound to the hollow one directly, it islikely that they discriminate between the two correctly. To cope with this, we will
modify the experimental design for our next experiment by using a two-alternative
forced choice task. We will also use pre-recorded audio samples instead of synthesized ones. Further experiments with improved sound quality for solid sounds are left
for future work.
In conclusion, we can answer our research question positively, that is, we can indeed achieve a different perception of thickness solely by auditory feedback. Yet, thisdoes not seem to be true for all situations, and the concrete conditions and possible
limitations need to be specified by further experimentation.

Fig. 3. Boxplot of results of the thickness discrimination task in experiment 1. Meanscores of correctly identifying the cube’s real thickness are indicated in red.

4

EXPERIMENT 2: AUDITORY-TACTILE FEEDBACK

In the introduction, we discussed that human perception is generally based on a combination of simultaneously perceived multimodal stimuli, such as visual, acoustic, andhaptic feedback when tapping an object. Experiment 1 showed that acoustic stimuli
might be sufficient to change perception under certain conditions. In our second experiment, we want to further focus on the integration between two modalities – soundand tactile feedback – and verify their influence on thickness perception. We summarize our goal in a second research question:

7
RQ2: Can we achieve a different perception of thickness (solid or hollow) of an
object when tapping it by modifying auditory and tactile feedback and otherwise fixedphysical properties?
For this second experiment, we switched the objects from cubes to larger wooden
plates to make the tactile indication of thickness more pronounced. Because using theterms “hollow” and “solid” does not make sense for such an object, and the terms“thick” and “thin” might be too context dependent and be interpreted differently by
subjects, we changed the experimental design. With a two-alternative forced-choice(2AFC) psychophysical experiment, subjects are now asked to correctly identify
which one of two plates is “thinner” than the other one.
4.1

Experimental design and setup

Fig. 4. Setup for experiment 2: (a) two wooden plates are clamped into two wooden
frames. Vibration-damping material is attached to places where the wood orclamps touch the underlying surface. In the middle of the plates are Force Sensing
Resistors (FSR) that are pasted on top of vibro-tactile actuators as shown in (b).
The actuators are connected to an amplifier and together with the FSRs make aserial connection to a computer. (c) shows the back-view of the frames. Bothframes can be turned-over to change the thickness of the plates.

Used platform. Two wooden frames have been constructed to each hold two wooden
plates in place. The wooden plates have a dimension of 200x200x<T>mm with <T>
being a thickness of either T=4mm (thin) or T=10mm (thick). Each frame can hold athin and a thick plate. Two clamps are used to hold the plates in place and preventing
inter-resonance. The clamps and the wooden frames are covered with a 9mm thickpolyethylene foam to stop vibrations in the plates from resonating into the underlying
surface. The setup is illustrated in Figure 4 (a) and (c). To generate tactile vibrations
when users tap on the plates, we have created two tactile sensors/actuators, as shownin Figure 4 (b). They consist of a PVC cylinder with a diameter of 20mm and a heightof 8mm with a cutout in which the vibro-actuators fit precisely. The vibro-actuatorsand connected AL-202H Amulech amplifier are per design of the TECHTILE toolkit[11] and can display a range of 1-20000Hz. A Force Sensitive Resistor (Interlink

8
FSR402) is attached on top of the vibro-actuator and PVC cylinder. Pressure on the
FSR is registered by our serially connected software, which measures the tap force
approximately between 0 and 50N. Based on this tap force, the software plays a prerecorded audio file. Measured latency between moment of impact on the FSR andaudio output ranges from 20ms to 60ms.
Method and procedure. Each participant is tested under the following conditions:tactile stimuli (thin, thick), auditory-tactile stimuli (congruent or discrepant with eachother). Under each condition real tactile feedback of the tapped plate is present. The
participants are asked to determine the thinner one of the two plates by tapping on thesensors. The two plates presented to the subjects are always of opposed thickness; athick and a thin one. The thinner one is on the left side in 50% of the cases, but theorder is randomized. There are no visual cues indicating the thickness of the plates.
In a preprocessing phase, we recorded the actual tapping sounds used later in theexperiment to make them sound more realistic, thus avoiding possible negative influences on the results (cf. discussion of experiment 1 in the preceding section). Whencapturing the sound snippets, we have also recorded the impact force of the tap andsaved it a pair to be played-back during the experiment. In total, we have recorded
twelve sound-force pairs of tapping on thick wood, and nine sound-force pairs oftapping on thin wood. The forces of the taps are approximately equally distributed
ranging from subjectively ‘soft’ touch to a ‘very hard’ tap.
We decided to generate vibro-tactile feedback by modulating a sine wave. Our aim
is not to simulate all touch sensations, as this would arguable result in a virtual reality
problem. Instead, we add vibratory cues to vibrations in the real material. Frequencies, base amplitude, and decay times for thick and thin vibrations are summarized inTable 2. The decay time represents the total time for an exponential fade-out to zeroamplitude of the sine wave. These specific values have been chosen subjectively tohave a large contrast in feedback between ‘thin’ and ‘thick’.
When entering the examination room, subjects are asked to sit on a rotating chairin front of the equipment and read an information sheet. Every participant then undergoes one training round to familiarize them with the auditory and tactile experience,taking particular care on practicing tapping intensity and order. They first had to tap
the left plate three times on the sensor; with a soft tap, (place finger upon the sensor;+/- 1N force), a medium tap (+/- 20N force), and a hard tap (+/- 50N force). During
training, participants tap on the plates without any augmentation and are asked with
one is thinner. They are then told if they are correct or not. Via in-ear headphones,
they are presented with a sample of thick and a sample of the pre-recorded tapping
sounds for wood; a ‘thin’ and a ‘thick’ one, and must indicate which one ‘sounds
thinner’.
During the actual test, participants wear additional earmuffs over the in-ear headphones to eliminate external noise. For every condition, they must tap with the index
finger of their dominant hand, and start with the left plate, followed by taps on theright one. After this process, they must indicate which plate they think is the thinner
one by saying ‘left’ or ‘right’, which was logged by the examiner. Then their chair isrotated around, so they are facing the back wall and cannot see the examiner modify-

9
ing the thickness of the plates for the next round. Every permutation of the independent variables is repeated ten times, resulting in 60 trials per participant and a total
duration of about 45 minutes per subject.
Participants. Ten male students participated in this experiment. Participation was
voluntary and subject have not been reimbursed in any way. Due to their length, one
participant’s nails touched the sensor when tapping it, which creates a very different
haptic and auditory feedback. Thus, he was asked to tilt his finger slightly in a waythat the nail would not touch the sensor. Data for this subject does not show a noteworthy difference compared to the others. Therefore, we do not expect any impact for
the results.
4.2

Results and Discussion

Figure 5 illustrates the results of experiment 2 by showing the mean correctness
scores for determining the thinner of the two plates. Labels on the x-axis indicate thedifferent conditions: First, pure tactile where haptic feedback does match the realsituation (Hm) or does not (Hn). They represent the tactile counterpart to the conditions hollow.hollow, static.static and hollow.static, static.hollow in experiment 1.
These conditions are followed by four others where we have both haptic and acoustic
feedback; first where both are congruent and either match the real situation (HmSm)or not (HnSn), then where they are not congruent, that is, either the sound (HmSn) or
the tactile feedback (HnSm) diverge from the real signals.
Like in experiment 1, we want to verify if a non-matching signal can be used tomanipulate the perception of users. We see that this is clearly the case for the pure
haptic situation. Like with pure audio feedback in experiment 1, there is a huge dropin correctness from 94.9% to 23.0% when the haptic feedback does not match the realone (Hn and Hm). A similar decrease appears when we have matching modalities for
haptics and sound; correct answers drop from 85.2% when both signals match the realsituation (HmSm) to 23.9% when they both do not (HnSn). Thus, as expected, byrepresenting an opposed signal for both modalities, we can manipulate people’s perception in a way to experience the ‘wrong’, that is opposite characteristic of the material. Yet, if only one of the two modalities do not match (HnSm, HmSn), results are atchance level (55% and 59.4%). Running a two-way ANOVA with repeated measuresand an error term on differences between participants over the data showed a significant effect (p < 0.1) of sound on correct discrimination of thickness (p = 0.0959) as
well as of tactile feedback (p = 0.0441). However, an interaction effect between thetwo was not significant (p = 0.492). Normality of the mean data was confirmed usinga Shapiro-Wilk test and QQ-normals plot.
Thus, we can positively answer our research question by concluding that perception of thickness can indeed be manipulated by providing opposed haptic and auditory
feedback. Yet, this is only the case if both signals are congruent, as our results showthat one of the two is not enough to achieve this effect.

10
Table 2. Tactile vibratory properties
Thick
Thin

Base amplitude
0.3
1.0

Base frequency
500Hz
100Hz

Decay time
100ms
500ms

Fig. 5. Boxplot of results of experiment 2 with means shown in red. Labels below the xaxis specify the related conditions.

5

CONCLUSION AND FUTURE WORK

We presented two experiments verifying that we can influence how people perceive
certain characteristics of material when tapping on them by manipulating auditory andhaptic feedback. In a first initial test, we showed that we could ‘trick’ people into
perceiving the volume of a solid cube as hollow by just changing the auditory feedback. Despite the small sample size, the trend clearly suggests the potential of such an
approach. Yet, the results also show that there are limitations to this, since a similareffect could not be achieved for a solid, non-hollow cube. A second experiment confirmed these observations in a similar scenario (estimating the thickness of a wooden
plate) and haptic feedback. Again, using different stimuli than the real situation
changed the users’ perception and experience of it. Yet, it also showed that in a more
realistic scenario involving both senses, manipulating one of the stimuli is not sufficient. When manipulating one of them, results were on chance level; neither was thematching one strong enough to let people experience the situation as it is in the real

11
world nor was the non-matching one strong enough to convince them of the opposedcharacteristic. But when both modalities matched, we are indeed able to manipulate
users’ perception again.
Our results are a small but important step towards creating a richer, more multimodal augmented reality environment that are not just focused on visual stimuli but
embrace all our senses. Alternative applications of our research include a scenario
where a single 3D-printed object is presented to users with different characteristics,
such as hollow or solid by purely manipulating the multimodal haptic and acoustic
feedback. Important areas to explore in future research include studying further characteristics of different materials and utilizing richer and more complex audio andhaptic signals. Furthermore, one could explore how adding visual cues could further
help in creating these augmented experiences; promising examples include visualizations of different materials, such as wood, marble, porcelain, and combining themwith different audio cues or haptic feedback.

6

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