Proceedings of ISon 2016, 5th Interactive Sonification Workshop, CITEC, Bielefeld University, Germany, December 16, 2016

“SLOWIFICATION”: AN IN-VEHICLE AUDITORY DISPLAY PROVIDING
SPEED GUIDANCE THROUGH SPATIAL PANNING
Jan Hammerschmidt, Thomas Hermann
Ambient Intelligence Group
CITEC, Bielefeld University{jhammers, thermann}@techfak.uni-bielefeld.de
ABSTRACTthe car’s audio system’s sound signal: When a driver has missed a
speed sign and is driving too fast, the sound signal of the car’s audio system will gradually move from a centered position towards
the back of the car. Conveying this information in such a way has
three distinct advantages: a) Panning of a sound signal is rather
easily perceived and rather difficult not to notice, which matches
the importance and urgency of the information. b) The meaningof the sound design should quite intuitively be understood, as you
get the feeling of driving away from “your” sound (which can be
expected to move at the appropriate speed). c) As the compositionof sounds is not changed at all by this auditory display, it is very
unobtrusive and thus should be easily accepted, which is of major
importance when dealing with a sonic environment that so many
people are exposed to as it is the case for automobiles.
Similarly, the driver can be notified by a subtle pan towards thefront of the car, if he or she is driving (significantly) slower thanthe current speed limit would suggest. Such a notification will ofcourse only be triggered if there is no vehicle in front preventing
to drive faster and could also be made dependent on whether there
are any following cars being hindered by the reduced speed.

We present a novel in-vehicle sonification for providing immediate feedback about the current vehicle speed in consideration ofprescribed speed limits and common driving practices. The key
conceptual idea of our “Slowification” auditory display is to assume that the sound of the car (i.e. the car’s audio system) travels
with the allowed resp. expected speed and to virtually position thedriver into this space according to the car’s current speed, resulting in a sound which moves to the back as one drives faster thanallowed and catches up on slowing down. Further changes of thesound for excessive deviations complement this design.
We evaluated the Slowification system in a virtual reality based
car simulator delivering realistic soundscapes of both engine andmedia sound placement, showing that it indeed helps the user todrive within speed limits and additionally provides less distraction
than a conventional visual speed display. Questionnaire results furthermore indicate that users easily accepted this novel auditory display as an unobtrusive in-vehicle user interface.
1. INTRODUCTION
Especially when considering the mostly rather hectic urban traffic, car driving is not only a visually demanding task, but also one
that is safety-critical for both the driver and other road users. Additionally, more and more in-vehicle systems are being integrated
into the car, which almost exclusively rely on visual indicators forinteracting with the driver.
For this reason, recent research efforts have targeted the auditory
domain for in-vehicle interaction (e.g. [1, 2]). The soundscape ofa car, however, is also a difficult environment to deal with, as wehave to take into account a wide variety of background noises coming from the engine, the wind, and the tires. Additionally, many
people are listening to music or utilize a navigation system, which
guides the driver using speech notifications. In consequence, themajority of auditory cues used in the car are of rather salient nature, e.g. the sounds used in parking assistance systems or the
distinct but admittedly fairly unpleasant noise to indicate that thedriver should fasten the seatbelt. Similarly, indication that a driveris exceeding a prescribed speed limit, provided for example by anavigation system, is commonly conveyed by quite salient auditory notifications.
Based on these observations, we propose to use the existingsoundscape as much as possible when developing auditory interfaces in the car, which in this paper will be realized within ourframework of blended sonification [3]. As a concrete application,
we present a novel in-vehicle auditory display for indicating theexceeding of a prescribed speed limit based on spatial panning of

2. RELATED WORK
2.1. Spatial panning to guide users
Although certainly not used in lots of systems, there are a few
instances where spatial panning has been incorporated in user interfaces to inform about an event or point of interest in a certain
direction.
Holland and colleagues, for example, developed a GPS navigation system with the goal to allow users to be engaged in different activities while being guided by the system [4]. To this end,
they decided to use a non-speech audio interface to encode distance and direction of a location. In their prototype, the direction
was represented by spatial panning of a tone based on the current moving direction of the user. Although seemingly coarse, thismethod yielded good enough results to discern the principal direction in an informal user trial.
In the context of automotives, Fagerlönn et al. evaluated different ways of guiding drivers at the early stage of a dangerous driving situation like an imminent collision with another vehicle [5].
In a study with 24 people, they compared using 1) a mild warning sound, 2) reducing the volume of the vehicle’s radio, and 3)
panning the radio’s signal. The authors conclude that panning theradio led to the lowest response times and, at the same time, wassignificantly better rated by users than the volume reduction.

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2.2. Dynamic Speed Assistance Systems
Although currently the vast majority of speed limits are static (i.e. consist of fixed signs that do not change in terms of position or limit),
there are efforts to introduce more dynamic Speed Assistance Systems, which take into account road geometry and vehicle characteristics [6], or upcoming traffic signal information [7].
These systems will make the use of a traditional visual speed
display far more difficult, as the drivers will have to deal with constantly changing and non-standarized speed limits, which, in turn,would require the drivers to use another (or additional) interface
such as the one presented in this paper.
Figure 1: Picture of the car simulator.
3. INTERACTION DESIGN
Keeping the speed is an important issue when driving and too often the visual focus of attention is shifted to the speedometer andthus distracted from the outside traffic situation where it should
remain. However, speed limits are frequent: in cities, on country roads, close to railway crossings, and speeding is controlled
and penalized. Obviously, the existing visual means for providing feedback about the speed via a speedometer is not an optimal
choice, as it leads to frequent visual distractions. An interaction
design for providing this non-critical yet highly relevant information needs to take the drivers’ primary task and required focus into
account.

3.2. Conceptual idea
The preceding analysis provides the ground for our new innovation: a sonification that works with the existing in-car audio system as source sound to be modified according to the available information. The fact that, in most cases, a car’s audio system isquadrophonic in order to allow a fine balance of sound between
left/right and front/rear to meet the driver’s preferences and thatmost users listen to music, audiobooks or radio while driving is
the technical and conceptual basis for our sonification.
Imagining that the sound of your audio system is not fixedwithin the car, but instead travels at its own speed, the central idea
is that, unlike the car itself, the sound travels exactly as fast as
allowed (resp. as recommended), while still being elastically attached to the car’s center of mass. One would further assume thatthe sound would be represented as a “sound bubble”, which naturally encompasses the car and the driver. With this (metaphorical)
setup, the following conditions can arise:

3.1. Auditory Displays
Using an auditory display would be an intuitive choice to approach
this monitoring task. And indeed, some navigation systems already signal the exceeding of a speed limit by auditory alerts.
These can, however, be experienced as annoying and don’t add tothe driver’s satisfaction (at least subjectively, according to one of
the author’s experience). Furthermore, these sounds don’t represent details about the amount of deviation or significance. Finally,
they can’t inform drivers about the opposite condition (i.e. driving
too slow), for instance when the following traffic is unnecessarily
delayed.
Symbolic auditory displays generally require a cognitive processing of information, which in most situations should not pose a
problem, since the task of driving can become quite automated andwould not require permanent cognitive control. Symbolic communication, however, is necessarily interrupting and risks to be annoying and to create resistance or reactance, which might result inusers experiencing these cues as disturbing or paternalizing.
Analogous representations, in contrast, keep users informed at
all times, provide an, in most cases less accurate, yet continuous
cue about the underlying condition and leave the decision making
in the hands of the user/driver. The reason why continuous auditory displays (or sonifications) have not yet been considered forthe speedometer is that a continuous sound would most likely berather annoying in itself (even if we readily accept permanent engine sounds and would even object if they were removed). One
might also argue that we already have such a (physical) auditory
speedometer in form of the rolling sounds of the wheels. These,
however, are not gauged and depend on the street surface. Furthermore, they are masked by other sounds like the car’s audio systemand the sound of the engine and don’t provide information relative
to the context, i.e. the prevailing speed limits.

• If the driver exceeds the speed limit, the sound bubble would
fall back and be dragged by the car behind the user bymeans of the elastic attachment. This situation would naturally lead to the perception of the audio system’s soundpanning to the rear.
• On the other hand, if the driver goes slower than the allowed
tempo and there is both traffic behind and no traffic in front(which certainly can, yet only with additional sensors, beregistered), then the sound bubble would travel faster thanthe driver and lead to a spatial shift of the sound towards
the front.
• Finally, if the car’s speed is the same (or within tolerance)
as recommended, the bubble would be perfectly centered,leading to no audible modification of the sound.
The metaphor would not only allow to determine the spatial
location (which, in terms of feedback signals, is an analogous relative corrective cue). It would also allow to coherently manage
a number of coupled features, such as decreasing the sound level
as the car’s distance to the sound bubble’s location increases, orto add reverberation, delay or other filtering plausible for distantsound sources. Such subtle cues might add to an enhanced senseof realism in this auditory display and thus improve its perception
and also lead to an increased acceptance.

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3.3. Prototype implementation
As a first prototype, we implemented a rather straightforward version of the concept described in the previous section. For this, wefirst defined a measure for driving faster (or slower) than a recommended speed:

d(

✓
,
v
,
⌧
)
=
max
↵·|
v
ref

v | + (1

↵) ·

| v|
· vn
vref

◆
⌧, 0 ,

where v = v vref is the (absolute) difference between the current and a reference speed, ↵ is a weighting factor balancing relative and absolute speed difference, and vn is a predefined neutral
speed, where the (unweighted) relative and absolute speed differences would be the same. In our study (cp. Section 4), we used
↵ = 0.8 and vn = 70 kmh. ⌧ is a measure for the tolerated
deviation from the reference speed and is used to define a ‘speed
channel’ around vref , with a lower and and upper bound for going
too fast (⌧u ) or too slow (⌧l ). In our current implementation, we
have defined ⌧l = 3 and ⌧u = 5.
Driving faster than vref would lead to a gradual spatial shift ofthe sound towards the back, while driving slower to a shift towards
the front of the car. The amount of panning is determined byp
Pu/l =d( v , vref , ⌧u/l )with(d) = ⇢ · d,Figure 2: Hardware setup for the study. Two additional loudspeakers (not seen in the picture) were placed behind the participant.
The computer monitor on the right was used only for controllingthe application and could not be observed by the participants during the experiment. The head tracking sensor of the Oculus Rift
can be seen between the two loudspeakers in the front.

where (d) leads to a more noticeable spatial shift after crossing
the threshold. In our quadrophonic speaker setup, we pan eachstereo channel separately with Supercollider’s1 Pan2 UGen. Furthermore, if P > 1, the volume of the audio signal will be reducedby ⌫db ·(P 1), indicating a further movement of the sound bubble
towards the respective direction (cp. Section 3.2). For the study,⌫db = 25 and ⇢ = 0.2.
Finally, when dealing with changing speed limits or even traffic lights, the bounds of the speed channel further deviate: As it iscommon practice for a driver to ‘coast’ (i.e. only slowly decelerate)
when encountering traffic lights or a slower speed limit, the lowerlbound vref
will drop by a deceleration constant ad = 0.1 kmh/m
well before passing the sign, meaning that there will be no panningto the front if the driver chooses to do so. In contrast, the upperubound vref
will drop rather near the sign by a braking constantab = 0.8 kmh/m to indicate the upcoming speed limit, if the driver
has not reduced the speed by then.

in three.js3 (i.e. it can be run in any browser), which makes thesystem a very portable one.
The car simulator features a physics based engine model, including a torque map to model the engine’s varying torque responses depending on the input throttle. Furthermore, it has a dedicated interface to SuperCollider via OSC4 , which is also used tocreate the engine sound. For the study, we implemented a way tostream (internet) radio into Supercollider via a virtual soundcard inorder to simulate listening to the radio while driving and as input
for our Slowification system.
4.2. Study Design
With the help of our simulator environment, we conducted a study
to evaluate the prototype implementation of the Sonislowcation
system discussed in Section 3.3. To reduce the number of necessary participants, we employed a within-subject design. For eachcondition, the participants had to drive the same test track threetimes in order for them to familiarize with the the respective display. Controlling for ordering effects, we employed a counterbalanced measures design, where both condition sequences were
evenly distributed among the study participants.
For the study, we designed a circular track, with speed limits
ranging from 30 kmh to 130 kmh. The lengths mi of the individual segment belonging to a particular speed limit li were chosen
in such a way that the time needed to drive through them was approximately the same, i.e.

4. STUDY
In order to assess the efficacy of our design in terms of a) drivers
adhering to the prescribed speed limit, b) the subjective and measured distraction by the panning, and c) the acceptance of the general design, we have developed a simulator environment specifically tailored to evaluate in-vehicle auditory displays.
4.1. A Virtual reality car simulator
The core of our evaluation system is a car simulator conveying avirtual reality 3D environment with the help of an Oculus Rift2
for a realistic driving experience (also cp. Figure 1). It is writtenti ⇡ tj , i, j 2 [1..n],with ti =mili

Furthermore, the curve radius was adjusted depending on the respective speed limit so that segments with a high speed limit have a

1 Supercollider:A real-time audio synthesis language (http://supercollider.github.io)
2 Oculus Rift: A virtual reality headset (https://www.oculus.
com)

3 three.js: A JavaScript 3D Library (http://threejs.org)

4 OSC: Open Sound Control (http://opensoundcontrol.org)

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NVt Ht HSS

AbsVSuteSy
SSVwificHtiVn
VisuHS dispSHy VnSy

HVw much did yVu hHve tV
cVncentrHte Vn the feedbHck?
HVw distrHctinN wHs
the feedbHck?
HVw cVmfVrtHbSe cVuSd yVu
Httend tV the feedbHck?
HVw stressfuS wHs
HttendinN tV the feedbHck?
HVw heSpfuS did yVu
find the feedbHck?

Figure 3: Main results from the questionnaire of the study. Answers could be given on a 7-point Likert-type scale indicating the level ofagreement with the statements that were given. For this visualization, only the responses that were not “neutral” are displayed.
wider radius than segments with a lower one. The time to complete
one lap is approximately 2 minutes.

For the actual experiment, all participant were told to first familiarize with their “real-world” environment in order for them tobe able to easily reach the pedals and the steering wheel. Only insome cases it was necessary to adjust the position of the pedals.
Moreover, the participants were told that they could select any(internet streamable) radio channel so that they could adjust theirsoundscape to what they were accustomed to when driving a car.
All of them, however, were satisfied with the default selection of
1Live5 , which is a quite popular and known German radio channel.
Then, after familiarizing with the Oculus Rift and the car simulator, the participants had two driving sessions – one with and one
without the Sonislowcation system – where they would independently complete three laps of the track (also cp. Section 4.2).
After each session, they completed a questionnaire about thepreceding driving session, followed by several comparative questions

4.2.1. Attention task
In order to compensate for the comparably distraction-free simulator environment, we also introduced an attention task for theparticipants to simulate the usual distractions (e.g. other cars, bicycles, and a lively surrounding) that are present when driving acar. In the spirit of the time, we designed a Pokémon-themed taskthat was both simple and engaging: While driving on the street,
there will appear different kinds of Pokémon that you can catch– true to the original game – with a Pokéball (also cp. Figure 1).
This works simply by looking at the Pokémon and pressing a button located on the steering wheel.
4.2.2. Hardware setup

4.4. Goals and Hypotheses

In Figure 2, we can see the actual hardware setup used in the experiment. Four loudspeakers (Genelec 8020A) were placed in aquadrophonic setup around the user. As a virtual reality headset,
we used the consumer version of the Oculus Rift. As input devices, we used a consumer-grade steering wheel (Logitech Wingman Formula GP), which also has pedals included.

The primary goal of the experiment was to evaluate the described
design under the following aspects:• Adhering to the prescribed speed limit: As the participants are given the secondary task of catching Pokémonand the speed limit changes several times while driving the
track, it can be expected that there is a certain amount oftime where the respective speed limit will be exceeded.
Our main hypothesis is that the Slowification system will
help the participants to better adhere to the prescribed speed
limits than without it (H1).

4.3. Procedure
At the beginning, all participants signed a written consent that thedata obtained during the experiment could be used in this study andcompleted a short introductory questionnaire dealing with generalquestions about personal preferences and previous experiences.
They were also given a short written introduction explaining
the basic concept behind the feedback provided by the Slowification system and telling them what they were expected to do during
the experiment.
Specifically, they were told to 1) keep on their lane, 2) not todrive through red traffic lights or ignore stop signs, and 3) to comply to the speed limits – i.e. to follow the common traffic rules. As
the last (secondary) assignment, they were told to capture as many
Pokémon as possible, including how to do so (cp. Section 4.2.1).

• Distraction: We furthermore assume that, in comparison tokeeping an eye on the visual speed display, the participants
will be less distracted by the panning of the radio’s sound.
We assume that this will, on the one hand, be measurable
by the amount of time the participants will deviate fromtheir lane (H2), but will also lead to the participants feelingless distracted, as should be reflected by the answers in thequestionnaire (H3).
5 1Live:
A German radio channel (http://www1.wdr.de/radio/1live)

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• Helpfulness: Although the helpfulness of the Slowification
system should as well be reflected by H1, we also expect
the perceived helpfulness to be something that can be confirmed by the questionnaire (H4).
• Acceptance: A final important aspect of a user interface
design that is meant to be installed in an automotive context
is the user acceptance.
Although most of the participants can be expected to be accustomed to the conventional speed dial and to the routinely
glance to the dashboard, we hope that the Slowification system will at least be as comfortably to use for the participants
as the speed dial (H5).
5. RESULTS
In total, we invited 22 people to try out the Slowification system
within our simulation environment. Three of them, however, hadto abort the experiment as they were very soon feeling sick because of the VR environment (this is a common problem with VR
Devices such as the Oculus Rift and has nothing to do with the
Slowification system), leaving a total of n=19 fully evaluable data
sets. The participants were 21-30 years old and balanced in termsof gender (9 male and 10 female participants). If not otherwise
noted, we used a conventional t-test for comparing values fromdifferent conditions. For calculating the effect size, Cohen’s d was
used.

(a)

(b)

Figure 4: (a) Percentage of time that a person was driving morethan 15 kmh faster than the prescribed speed limit. (b) Percentage of time that a person deviated too far from the street resp. thecorrect lane.
The whiskers denote the 5% and 95% percentiles of the data, while
the notches represent the 95% confidence intervals of the median.
The mean values of the data are illustrated by the red boxes.
found, i.e. although they were (at least partly) able to process theprovided information (cp. Section 5.1), the participants still tried toconsciously attend to it. However, this seemed to be less stressful(3.32±1.45) than when attending to the speedometer (4.32±1.59)
and further supports H3 (p/2 < 0.05, Cohen0 s d = 0.64).
Finally, as a measure for how well such a system would be
accepted as an additional in-vehicle user interface, the participants
stated that they could attend to the Slowification more comfortably (4.95 ± 1.19) than to the speedometer (3.74 ± 1.37), which
confirms H5 (p < 0.05, Cohen0 s d = 0.92).

5.1. Measured data
In order to evaluate to what extent the prescribed speed limits wereadhered to, we analyzed the percentage of time for each lap that a
participant was driving more than 15 kmh too fast. As can be seenin Figure 4a, this was considerably less the case for the panning
condition (7.5% ± 9.5) than for the baseline condition (12.7% ±15.7), which confirms our hypothesis H1 (p < 0.05, Cohen0 s d =0.39).
Furthermore, as a measure for being distracted, we compared
the amount of time the drivers deviated from their own lane bymore than 40 cm (Figure 4b). Although the differences are notas striking, there is a significant difference when considering our
one-sided hypothesis (p/2 < 0.05, Cohen0 s d = 0.34) between
driving with (53.2%±11.0) and without (56.9%±10.6) the Slowification system, confirming H2.

6. DISCUSSION AND CONCLUSION
The conducted study gives a first indication for the efficacy of the
Slowification concept (Section 5). We are, however, aware that thechosen implementation as well as the subjective choice of parameters (cp. Section 3.3) might not necessarily be the best possible
one. Nonetheless, this study provides a baseline for the efficacyof the concept and space for future refinements of the implementation.
Although the majority of participants (67%) indicated that theywould prefer the Slowification over the speedometer, we argue thatin its current form, it cannot replace the visual display: When actively being attended to, the speedometer offers a rather precise
way to determine the car’s speed and we think that this possibility should remain (besides the legal complications that would arise
when completely removing the speedometer). However, it is one
possible direction of future work to evaluate how well the Slowification works as the only available feedback.
Several users reported that they could barely perceive the spatial shift of the sound while, at the same time, apparently reacting
to it. Although this certainly needs further investigation, it is insofar remarkable, as that, even after only a very short time of getting
accustomed to it, some participants were apparently able to sub-5.2. Questionnaires
This result is supported by the responses to the question how distracting the participants found the respective feedback. As canbe seen in Figure 3, when being supported by the Slowification
system (2.79 ± 1.54), the users felt significantly less distracted(p < 0.05, Cohen0 s d = 1.01) than when not (4.42 ± 1.6), whichclearly confirms H3.
Being asked about helpfulness, however, participants rated thetwo conditions almost the same (p > 0.7), which obviously cannot support our H4. Our interpretation of this result is that theparticipants, in the short amount of time they had to become accustomed to the system, could not consciously “grasp” it in a waythat they could assess it as useful. This is also reflected by theanswers to the question, how much the participants had to concentrate on the feedback, where no significant differences between
using the Slowification system and only the speedometer could be

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consciously perceive and react to the subliminal changes of thesound. Seen from a different perspective, the result of users immediately feeling rather comfortable with the system leaves someroom for making the indication of driving too fast (or too slow)
more distinct, which is something that should be evaluated in future studies. Another way to further evaluate our speed indicator
would be to compare it with a different type of (auditory) display,e.g. an alert-based system, which we would assume to be rated as
far more annoying than the Slowification.
During the study, one participant stated that “the panning isa really good idea” but felt that she needed more time to get accustomed to it and suggested “more time for test drives”. Anotherway to give users more time to get accustomed to it would be to
install the system in a small number of cars for people to experience the feedback over a longer period of time. While certainly
more difficult to evaluate as we would be dealing with a completely
uncontrolled environment, this would give insight into how users
would be using the system after really becoming accustomed to it
and how well it is usable in real-life situations.
Finally, it would be interesting to extend the use cases of thesystem by integrating an adaptive speed assistance system basedon traffic light predictions [7], which we think would make theadvantages of the Slowification even more distinct than with static
speed signs only.

[6] F Jimenez, F Aparicio, and J Paez, “Evaluation of in-vehicle
dynamic speed assistance in spain: algorithm and driver behaviour,” IET Intelligent Transport Systems, vol. 2, no. 2, pp.
132–142, 2008.
[7] Behrang Asadi and Ardalan Vahidi, “Predictive cruise control:
Utilizing upcoming traffic signal information for improving
fuel economy and reducing trip time,” IEEE transactions oncontrol systems technology, vol. 19, no. 3, pp. 707–714, 2011.

7. ACKNOWLEDGMENTS
We thank CITEC’s central lab facilities for providing us with an
Oculus Rift for our study.
This research was supported by the Cluster of Excellence Cognitive Interaction Technology ’CITEC’ (EXC 277) at Bielefeld
University, which is funded by the German Research Foundation(DFG).
8. REFERENCES[1] M. A. Nees and B. N. Walker, “Auditory Displays for InVehicle Technologies,” Reviews of Human Factors and Ergonomics, vol. 7, no. 1, pp. 58–99, Aug. 2011.
[2] Myounghoon Jeon, Jan Hammerschmidt, Thomas Hermann,
Pavlo Bazilinskyy, Steven Landry, and Katie Anna E. Wolf,“Report on the in-vehicle auditory interactions workshop:
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[3] René Tünnermann, Jan Hammerschmidt, and Thomas Hermann, “Blended Sonification: Sonification for Casual Interaction,” in The 19th International Conference on Auditory
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[4] Simon Holland, David R Morse, and Henrik Gedenryd, “Audiogps: Spatial audio navigation with a minimal attention interface,” Personal and Ubiquitous computing, vol. 6, no. 4,
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[5] Johan Fagerlönn, Stefan Lindberg, and Anna Sirkka, “Gradedauditory warnings during in-vehicle use,” in Proceedings ofthe 4th International Conference on Automotive User Interfaces and Interactive Vehicular Applications - AutomotiveUI’12, New York, New York, USA, Oct. 2012, p. 85, ACM Press.

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