PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

The use of inertial measurement units in virtual reality systems forauralization applications
Kristian JAMBROŠIĆ1; Miljenko KRHEN1; Marko HORVAT1; Tin OBERMAN2
1

University of Zagreb, Faculty of Electrical Engineering and Computing, Unska 3, Zagreb, Croatia
2

University of Zagreb, Faculty of Architecture, Fra Andrije Kačića Miošića 26, Zagreb, Croatia

ABSTRACT
Auralisation finds increasingly more important applications in everyday life with the development and broad
availability of virtual reality technologies and systems. This paper presents a new research project focused on
auralisation implementation in different hardware systems, mainly by using binaural technology with an
added head tracking system. Besides commercially available VR systems, the possibility of using
inexpensive inertia measurement sensors connected to simple embedded systems will be investigated as analternative auralisation system for adding natural aural experience in VR applications. In addition, thepossibility of using widely available smartphones with already integrated movement and direction sensors
for auralisation purposes using a binaural system will be examined. The listed platforms will be measured
and compared considering their precision, latency and quality of the virtual sound field using practicalauralisation examples in laboratory conditions, and in real life situation in-situ. Measurement plans for sensor
parameters and characteristics, such as delay, sensitivity and accuracy, are presented in the paper for both
IMU sensors connected to an embedded system and sensors integrated in smartphones.
Keywords: Virtual Reality, Auralization, Inertial measurement units

1. INTRODUCTION
The range of possible indoor and outdoor noise perception scenarios is so wide that novel
techniques and methods must be designed to predict and model the variety of acoustical situations
under a controlled experimental setup. It has been shown that the combination of virtual acoustic
reality and computer-based acoustic simulation allows the study of sound in any environment, offering
multiple research possibilities for urban designers, architects and acoustical engineers (1).
Auralisation is a growing research field within applied acoustics. It was defined in the 1990s, as theprocess of rendering audible the sound field of a source in both indoor and outdoor environments,using physical or mathematical modelling. This rendering is done by simulating the binaural listening
experience at a given position in the modelled space (2). This concept has been further developed,
having evolved into a research field known as Virtual Acoustics (3). Virtual Acoustics is part of abroader domain called Virtual Reality (VR), which unites sound with vision, as well as with tactile
sensation (to a smaller extent) into the immersive experience.
The discipline of auralisation integrates methods of physics and acoustic engineering withpsychoacoustics, auditory cognition and electroacoustic sound reproduction. The interest inauralisation has been enhanced in recent years, due to the advances in areas such as spatial soundreproduction, sound field analysis, and numerical methods. However, there are still many unsolved
problems, including: modelling real sources for these environments, limitations on methods ofcomputing the acoustic parameters for annoyance assessment, lack of evaluation benchmarks,
uncertainty evaluation of auralisation models, etc. (4). The importance of auralisation and virtual
acoustics has been recognized by governmental organizations and national funding bodies in Europe.
They have funded several research projects related to auralisation and virtual acoustics.

1

kristian.jambrosic@fer.hr

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2. AURALIZATION TECHNOLOGIESCurrently, auralisation techniques are penetrating three main fields: the virtual world of computer
games and smartphone applications, walk-through modes of architectural design programs, and thedesign of acoustically sensitive spaces. More applications are expected for predictive orientation to theauditory system, indoor-outdoor acoustics measurement, effects of noise exposure and potential social
impacts, involving: urban acoustic design, social interaction, social and health impact, the creation ofquieter environments enhancing speech perception, etc. (5, 6).
The scope of auralisation and audio in virtual reality is an inter- and trans-disciplinary field, so theusual channels for interchanging information are not effective. Contributions are needed from audio
and acoustic engineers, architects and urban designers, computer and social scientists, physiologists,
cognitive scientists, etc. New research in auralisation should focus on the application of innovativetechnologies for monitoring acoustic, psychoacoustic, and psychological parameters in urban areas,
and on modelling these spaces with the aim of developing new solutions for indoor/outdoor
noise-related problems. The combination of new acoustic modelling methods and immersive audio
rendering techniques are expected to provide new auralisation -based tools for the assessment of
acoustic impact in diverse environments. Although one could argue that in-situ measurements are themost realistic testing tool, many non-auditory factors can influence in-situ evaluations that can be
avoided in laboratory experiments, such as excessive low or high temperature, strong daylight or night
conditions, smell, etc. (7, 8).
Binaural hearing is essential for realistic auditory perception because it presumes the ability of thehuman hearing to perceive spatially separated sound sources, which is quite different from theperception of simple mono sound recordings. Head-related transfer functions (HRTF) are essential for
simulating spatial hearing, and they are the basic tool for a wide range of virtual auditory display
applications (guiding systems, game and mobile sound, room acoustic design, etc.). Although many
datasets of HRTFs of acoustic mannequins already exist, different methods have been developed formeasuring personal HRTFs, even in an inexpensive and/or fast manner (9).
2.1 IMU sensors
The binaural method of auralisation is used more and more in Virtual Reality (VR) systems andAugmented Reality (AR) systems (10). Dedicated commercial binaural systems with head movement
tracking are quite expensive today because they are not produced in large batches, and require several
sensors and a computer unit used exclusively for this task. At the same time, there are many
experimental systems developed in recent years that are successfully used for head tracking
applications in the process of auralisation. Such systems include ultrasonic trackers (usually 6 Degrees
of Freedom, 6-DOF), simpler 1-DOF ITD systems for horizontal plane tracking, electromagnetic
devices with 6-DOF and 3-DOF capable sensors, optical analogue devices with 2-DOF and 1-DOF,
optical digital systems with 3-DOF and 2-DOF, inertial systems with 6-DOF (with and without opticalreference), and one- or two-camera face-tracking systems with 6-DOF (11). All these sensors andsystems are not equally suitable for auralisation purposes, but there is no detailed comparison of theiradvantages and disadvantages for binaural synthesis. Not all sensors are necessarily expensive; someof the systems are low-cost solutions connected to well-known embedded computer systems such as
Arduino or Raspberry Pi (12). In all systems and technologies, it is essential that the latency of thehead position measurement is read with a sufficient frequency, and that the data tr ansfer and dataprocessing does not take too much time. Otherwise, the feeling of transparency and non-existing lag isruined for the listener who is experiencing the virtual auditory scene while turning their head (13). The
common name used for many of these sensors is the Inertial Measurement Unit (IMU).
In recent years, a technology that integrates all functionalities in terms of motion sensors has
become widely available – smartphones. Nowadays, they can be found in virtually every home, andtheir number surpasses the number of people using them. Smartphones are already equipped withseveral sensors that can be readily used for real-life calculation of their position and orientation in
space, such as accelerometers, magnetometers and gyroscopes, all built in space- and money-saving
MEMS technology, as shown in Figure 1.
An accelerometer is a sensor for measuring the acceleration, e.g. the change of velocity over time.
In smartphones, it is built as a MEMS sensor (Micro Electro Mechanical System). A gyroscope is a
device that consists of a spinning disc, which can rotate freely around its axis, regardless of theorientation of the housing frame. In smartphones, the gyroscope is again designed in MEMS
technology. Vibration structures are used. A magnetometer measures the magnetic field in space, the2612

compass being its most primitive version. The magnitude of the magnetic field is often required, alongwith the direction of the magnetic field. For this reason, the Hall Effect is used. The Hall sensors are
too big to fit into an average smartphone. Therefore, the magnetometer is also made in MEMS
technology. It uses the action of Lorentz forces on the MEMS element, which causes a conductor withcurrent flowing through it to move when if finds itself in a magnetic field.

IMU data logging andproccessing

Gyroscope(3 axis)Accelerometer(3 axis)Magnetometer(3 axis)Figure 1 – Typical sensor types found in IMUs used for head tracking in auralization
Smartphones are a good choice for head movement detection if they are firmly fastened to thehead(phones). Since they integrate all required sensors, the only task is to extract the positional data
and transfer them to the computer that performs the binaural synthesis. At the same time, smartphones
are already computers with enough processing power to render binaural signals and scenes on the fly.
There can be many applications of these systems, such as multi-party conference systems, pedestriannavigation (12), or even mapping the sound field with the built-in or external microphones for any
acoustic measurement (14).
Many more applications of auralization exist, and many more are coming in near future. This isespecially true for music production that desperately searches for a way -out from the obsolete stereo
system that has been around for over 60 years. Moreover, there has been no real breakthrough with thehome-cinema surround sound systems as a mere upgrade to the old 2-channel stereo audio systems(15). Health application that uses augmented reality for visually disabled persons is also a new field forauralization (16). For people with the need of wearing hearing aids, auralization can be used for manyaugmented reality applications (17).

3. RECENT RESEARCH IN THE FIELD
In order to understand the auralization process, one must understand the principles of room acoustic
measurements and computer simulation. This was also one of our research topics where ray -tracing
parameters were harmonized in order to achieve realistic room impulse responses in computer
simulations, essential to experiencing an auralized space as realistic (18). Auralization of closed
spaces is essential for many applications, such as making an aural experience of any new orreconstructed room for architects, investors, historians, nonprofessionals, but also for visually
impaired people who rely completely on their hearing rather than on vision.
Two studies showed how the interior acoustic design of spaces directly influences the way weexperience the size of the room and our position in it, the so-called self-localization experience (19,20). This is achieved by choosing various kinds of wall, floor and ceiling materials with different
sound absorption and sound scattering characteristics. The idea was to make some guidelines forarchitects in order to support visually impaired and blind persons entering public buildings.
Another very recent research topic was connected to the use of auralisation as a part of a holistic,
augmented reality experience. Sound is used here as a mechanism for path following and trajectory
tracking of remotely operated vehicles. Using this technique, the visual load for the operators is
reduced. Using more senses together gives the operator a more harmonized cognitive load when being
exposed to tasks that require a long-lasting mental concentration (21).
Finally, a pilot project on how to use head tracking systems in virtual audio environments for areal-time binaural synthesis. This is again essential for receiving a well-balanced and realistic auditory
experience of a simulated reality, or when simply playing back a recorded sound event that alreadyhappened in the past, but we want to experience it now (22).

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4. RESEARCH METHODOLOGYThis project proposal is a continuation of previous research done by the research team. The main
research outcomes expected in the field of auralisation are the precisely defined limitations andconstraints connected to various auralisation systems. Firstly, limitations of head tracking of thelisteners in order to achieve a seemingly transparent aural experience that completely immerses thelistener in the virtual reality surrounding, at least in terms of hearing. Secondly, the comparison of thebinaural systems with implemented head tracking with available loudspeaker-based auralisation
technology with dedicated auralisation software, such as Ambisonics sys tem, or similar.
The main difficulty of audio technologies that are sensitive to head movement today is the speed ofhead tracking, i.e. the latency of the system when freely moving the head and looking around thevirtual surrounding. This means that any latency larger than approximately 20 ms is experienced as a
“drag” of the sound field behind the movement. This is no noted issue for commercial VR systems, butthey are certainly not affordable for a wide group of potential beneficiaries and users. Therefo re, theusability and optimization of using inexpensive IMU units connected to embedded systems as aninterface towards computers rendering binaural scenes is expected in this project, as well as acomparison with commercially available systems.
The final goal is to compare these two systems with smartphones that generally possess all therequired computational power and positioning sensors, but are not widely used for auralisation
purposes. Smartphones are very favorable because of their existence in virtually every home, andcertainly for the great majority of people today. This is already shown in the usage of cheap VR system(like Google Cardboard) where the visual part of VR immersion can be achieved to a certain extent,
but not the aural one, at least not uniting it with head tracking which is essential for good immersion
and proper auralisation, as mentioned before.

Head tracking systems
IMU sensors connected tosimple embedded systems

Smarthphones withintegrated IMU sensors

VR commercial systems
with integrated headtracking capability

Head tracking quality evaluation
Objective measurements:
sensor responsiveness to movement,
sensor delays, parameter drift

Subjective audio quality evaluation:
binaural image stability, head movement
stability, overal sound envelopment

Figure 2 – Basic schematics of the research plan
A lot of research time will be dedicated to measurements of various parameters of the sensors. This
includes measurement of the precision of absolute and relative position and orientation, their pitch,roll and yaw values, and their responsiveness to movements depending on the speed of movement,
travelled distance and time offset, and parameter drift. Existing measurement equipment available atthe Faculty of Electrical Engineering and Computing, and the Auralization Laboratory in particular,
will be used for precise measurements of these parameters. Measurement data will be statisticallyanalyzed and repeatability and uncertainty of the measured data will be calculated . The general
overview of the research plan is shown in Figure 2.
Another key objective is auralisation by computer simulation and numerical methods. For this,several levels of auralisation algorithms will be used – from simple coding of binaural synthesis using
mathematical convolution (for stationary and time-varying signals) to dedicated software forauralisation of virtual sound environments using position data from sensors and advanced algorithms
for binaural synthesis. All this will be done for the available hardware platforms, and when possible,

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for multiple sets of HRTFs. The goal is to compare the speed of execution, the latency introduced by
the software itself, and all audible artefacts that occur because of filtering, averaging, and other dataprocessing. Such processing is required because HRTFs are always recorded in discrete angular
positions, while binaural synthesis must provide a smooth aural experience when the head of listeners
is in motion.
The final key objective is to test and compare all hardware and software systems. For this, several
aspects are needed, comparing measurable delays and sound localization precision, where teammembers will evaluate the precision and overall impression of the systems regarding several
measurable parameters and using semantic difference scale in order to obtain usable statistics. This
includes the evaluation of direction (azimuth and elevation), overall satisfaction with the renderedauditory scenes, the feeling of involvement in the scene, sound envelopment, spaciousness, lag,artefacts in sound coloration and distortion, etc. The evaluation of the auditory quality in the VR
systems can be added with the visual scene which changes the overall evaluation, as known fromliterature. In the end, auralisation of binaural synthesis with head tracking will be compared withsimple binaural recordings without head tracking ability, and finally with auralisation us ing
loudspeakers in systems such as Ambisonics. All these systems will be compared using the same
semantic difference questionnaires in order to obtain a relevant conclusion on the quality of thesystem.
Since auralisation with smartphones is a mobile system used in-situ, in many different surrounding
outside laboratories, the final field tests will be done. All parameters of auralisation quality will be
evaluated by the same group, i.e. the team members of the project. Now they will be exposed to othertype of surroundings, including noisy environments where many sound sources emit noise at the sametime, thus changing the overall impression of the auralisation quality and auralisation immersions.
Both evaluations – in laboratory and in the field, will be compared, and the usability of the proposedsystem for field applications will be assessed.

5. EXPECTED RESULTS
In terms of general impact of this project, the outcomes can directly influence a broad communitythat extends far beyond the interested scientific community. VR technologies are currently being
developed in an accelerated pace, and this project focuses on the use of smartphones with all embedded
sensors for real-time binaural auralisation with many potential applications. It is reasonable to expectthat the outcomes will stimulate a lot of follow-up research and development in the field of new games
for smartphones, new possibilities for visually impaired users, improved intelligibility for video andteleconference systems (due to spatial distinction of speaker positions), etc. Real-time auralisation ofindoor and outdoor spaces is expected to become more and more important to architects, urban
designers, urban planners, decision makers, etc. The possibility will be opened for participative
sensing and evaluation of environmental sounds and soundscapes by a large group of people, thus
potentially participating in decision making, mobile television within the VR systems including bothaudio and video immersion, etc.
Thus, the group of potential beneficiaries from the project outcomes goes far beyond the scientists
and researchers in the field. As the research is interdisciplinary by nature, any positive development in
the field opens new possibilities of application to new groups of beneficiaries, from small children to
adults, from educational purposes to leisure applications, from single assisted aids for disabled toparticipative application that are used by many.
The impact of the proposed project can be notable and important, having in mind the potentialusability of auralisation for a wide group of people, i.e. the owners of smartphones. This could involve
many participants in all kinds of subjective assessment tasks, first coming to mind the question ofnoise annoyance level depending on the soundscape, and VR systems are an ideal platform for this.
Participation of citizens in policy‐making and evaluation increases involvement in reaching asustainable European society. It can also be the basis for creating European societies of ci tizens, which
in turn can help strengthen social cohesion. Focusing on the local living environment and on noise canbe a deliberate choice. In contrast to other environmental components, environmental sounds can beeasily heard and appraised by people. It is discussed on many internet forums, and can become a pointof debate: certain sounds are loved by some, and hated by others.

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6. PILOT STUDY MEASUREMENTS
Measurements of the quality of head tracking systems have already started. A simplified system has
been used built out of an Android-based smartphone mounted on top of headphones, thus serving as ahead tracking device, an open access software for collecting head movement data, and a PC running a
Matlab-based application for real-time binaural synthesis based on the listener’s head direction. For
smartphone (head) position measurements, the following IMU sensor data collection apps were used:
Wireless IMU and freePIE. All used applications are released under ISC and GNU licenses (22).
6.1 Wireless IMU, freePIE and opentrack
Wireless IMU sends data from sensors using UDP (User Datagram Protocol) to the computer,
Figure 3 left. The program supports the collection of data from three sensors: accelerometer,gyroscope and magnetometer. The UDP packages are sent as numerical values. The speed of sending
packages can be set from the slowest 380 ms to the fastest 5 ms. The best results were achieved with 20
ms because at 5 ms, only every other magnetometer value was sent. The UDP packages are read by a
Matlab function, and the collected data was further used for calculating the position of the smartphone,e.g. the head orientation.
A screenshot of freePIE (Programmable Input Emulator) is shown in Figure 3 middle. It alsocollects sensor data from the smartphone and sends it to the computer. The freePIE sends packages
with a speed of 20 ms to 3 ms. The packages can be collected on the computer using the opentrackprogram that processes the data and sends them to Matlab for computing the binaural audio signal.

Figure 3. Wireless IMU interface (left), freePIE interface (middle) and opentrack interface (right).
The opentrack program is used for head tracking and sending the data to video games, flight
simulators, etc., Figure 3 right. It supports PointTracker, Oculus Rift, Arduino, Android, and otherprotocols. Depending on the input types, the data are processed to calculate yaw, pitch and roll of thedevice. These parameters can be sent to specific software (e.g. Microsoft Flight Simulator), they canemulate some devices (joystick or mouse), or can be sent further as UDP packages. The opentrack
sends a package every 100 ms.
Binaural synthesis is done in Matlab using a code written by the authors for this purpose ( 22). The
first step is to choose the required HRIR’s for the binaural synthesis. The CIPIC HRTF database wasused.
6.2 Head tracking quality evaluation
For testing the time required for the execution of the code, a 5-second pure sine signal with 44.1
kHz sampling frequency was used, generated with MATLAB. The average speed is shown in Table 1.
From the measurements, it can be concluded that freePIE has a shorter processing time by about 20 ms.
This is not unexpected because the UDP packages already include information about the orientation,
so additional processing is not needed. In contrary, the concept of Wireless IMU slows down the2616

execution of the synthesis because of additional processing time required to calculate the position
parameters.
The testing of the subjective quality of head tracking was done using short music excerpts sampled
at a frequency of 44.1 kHz. During the testing, the overlap percentage was varied, the size of the frame,
the refresh rate of the UDP packages and the speed of head movement. It was found that the soundartefacts became inaudible at approx. 15 % overlap for music excerpts.

Table 1. Average execution time.
Average time

Wireless IMU [ms]5140.3

freePie [ms]5121.9It became clear that the main advantage of the Wireless IMU application is that there are no
problems in making a full circle, e.g. crossing the azimuth 180°. This is due to the way that only the
relative change of orientation is coded in the application. At the same time, the freePIE algorithm
tracks the head movement much better. Even at fast head movements there are no delays in the soundimage. The problem occurs only at sharp turns of the head. Then it becomes obvious that the soundfield needs some time to position in its new direction.

7. CONCLUSIONSThis project will focus on research of auralisation implementation in different hardware systems,
mainly by using binaural technology with an added head tracking system. The focus will be on headtracking systems, namely to investigate the possibilities, limitations, shortcoming s and delay of threevery different hardware platforms that can be used for auralisation in the binaural field. These
platforms are 1. simple IMU units connected to inexpensive embedded platforms, 2. smartphones that
already have integrated all needed sensors, and the required computational power, and 3. commercial
virtual reality systems.
It is expected that the project will contribute to the scientific community, but also to generalcommunity and wide beneficiary groups. By showing the possibility to use auralisation with alreadywidely available devices, a number of new applications can be expected in many different fields.
The research is highly interdisciplinary, so any positive development in the field will lead to new
possibilities of application by new groups of beneficiaries.

ACKNOWLEDGEMENTS
The authors acknowledge financial support by the Croatian Science Foundation, (HRZZ
IP-2018-01-6308, "Audio Technologies in Virtual Reality Systems for Auralization Applications(AUTAURA)".

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