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45º CONGRESO ESPAÑOL DE ACÚSTICA
8º CONGRESO IBÉRICO DE ACÚSTICA
EUROPEAN SYMPOSIUM ON SMART CITIES AND
ENVIRONMENTAL ACOUSTICS
ACOUSTICAVE: AURALISATION MODELS AND APPLICATIONS IN
VIRTUAL REALITY ENVIRONMENTS
PACS: 43.55-Ka
Guilherme Campos1,2, Paulo Dias1,2, José Vieira1,2, Jorge Santos3,4,5, Catarina Mendonça6, João
Pedro Lamas4,5, Nuno Silva4,5, Sérgio Lopes7
1Departmento de Electrónica, Telecomunicações e Informática (DETI), Universidade de Aveiro
Campus Universitário de Santiago, 3810-193 AVEIRO – Portugal
Tel: +351 234 370 355 Fax: +351 234 378 157
2 Instituto de Engenharia Electrónica e Informática de Aveiro (IEETA), Portugal
3 Departamento de Psicologia Básica – CIPsi, Braga, Universidade do Minho, Portugal
4 Centro Algoritmi, Guimarães, Universidade do Minho, Portugal
5 Laboratório de Visualização e Percepção (LVP), Centro de Computação Gráfica (CCG),
Guimarães, Universidade do Minho, Portugal
6 Department of Signal Processing and Acoustics, Aalto University, Espoo, Finland
7 Instituto de Telecomunicações (IT), Aveiro, Portugal
{guilherme.campos, paulo.dias, jnvieira}@ua.pt, jorge.a.santos@psi.uminho.pt
{mendonca.catarina, lamas.jp, nunomiguel06, sergioilopes}@gmail.com
ABSTRACT
This communication is an overview of the FCT-funded three-year research project ‘AcousticAVE
– Auralisation Models and Applications in Virtual Reality Environments’, a collaboration between
the Universities of Aveiro (UA - IEETA) and Minho (UM - CIPsi, LVP). The project involved the
development of auralisation software based on the image-source method accommodating
dynamic scenarios, with real-time tracking of source/listener motion and listener head
orientation. This software supported psychophysical research at the CAVE-like facilities of UM’s
Visualisation and Perception Lab (LVP). This included an investigation on learning effects in
spatial audio perception using non-individualised HRTF sets and distance and time-to-passage
(TTP) perception experiments.
1 INTRODUCTION
The importance of Virtual Reality (VR) has grown rapidly in recent years, with an ever-
increasing range of applications in the most diverse areas. However, most efforts in the design
and development of VR systems have been directed at providing visual immersion. The
development of increasingly convincing models demands that other senses also be considered,
especially hearing – we are still very much in the ‘silent era’ of VR.
A joint initiative of the Universities of Aveiro (UA) and Minho (UM), the research project
‘AcousticAVE – Auralisation Models and Applications in Virtual Reality Environments’ aimed
precisely at integrating visual and aural immersion in VR environments. It received funding from
the FCT (Portuguese Foundation for Science and Technology) for a three-year period which
ended in April 2014 (PTDC/EEA-ELC/112137/2009). Prof. Damian Murphy, from the U. of York
(UK) and, at a later stage, Prof. Juan Miguel Navarro, from UCAM (Spain), acted as external
project consultants.
The project involved essentially two work packages. At UA, an engineering team from IEETA
(Institute of Electronics and Informatics Engineering) tackled the software and hardware
implementation of room acoustic simulation and auralisation models.
At the Visualisation and Perception Lab (LVP) of UM’s Graphics Computing Centre (CCG) in
Guimarães, a team of Psychophysics researchers from CIPsi (Psychology Research Centre)
45º CONGRESO ESPAÑOL DE ACÚSTICA
8º CONGRESO IBÉRICO DE ACÚSTICA
EUROPEAN SYMPOSIUM ON SMART CITIES AND
ENVIRONMENTAL ACOUSTICS
were primarily concerned with the application side of the project, exploring and testing the
developed auralisation systems and software packages at their CAVE-like facilities. These
include a 3m-by-9m continuous projection screen comprising 3 panels with a DLP (Digital Light
Processing) projector per panel for flexible configuration (0°, 90° or 135°), a treadmill
synchronised with the 3D visual scene to allow walking on the virtual environment and an infra-
red motion capture system for user tracking. It was decided that sound presentation should be
binaural, in view of the technical requirements (room acoustic correction, equipment noise
control…) and costs of multi-speaker alternatives such as Ambisonics or Wave-Field Synthesis
(WFS).
A ‘customer-supplier’ relationship was established between the two teams, inasmuch as the
model development work at UA aimed at meeting the requirements of the VR experiments
designed by the UM team. The main research focus was on the role of aural cues in the
perception of human motion, commonly referred to as Biomotion (see Figure 1). Biomotion
perception experiments imply dynamic scenarios; the models must provide control over virtual
source movement and respond to changes in listener head orientation and position, tracked in
real-time. Also, since perception cues are obviously not only aural but also visual, it must be
possible to integrate the two; in fact, their interdependence and sensitivity to synchrony are
points of particular research interest. For these reasons, controlling the relative latency of the
two channels (visual and aural) is also crucial. These two requirements (real-time tracking and
accurate audio-visual synchronisation) posed the most significant engineering and signal
processing challenges.
Figure 1 – A ‘point-light walker’: biomotion perception experiments are often
based on video projections using simplified human representations (avatars)
of this kind. Integration of aural stimuli (e.g. avatar step sounds) requires not
only correct spatialisation (so that they can be perceived to have been
emitted at the desired points in space) but also accurate synchronisation with
the corresponding visual stimuli.
2 AURALISATION MODEL DEVELOPMENT
2.1 Virtual Microphone Positioning
The first auralisation tool developed in the context of this project [1] was designed to simulate
the process of microphone positioning in audio recordings. The application is based on a dense,
regular grid of impulse responses pre-recorded on the room region under study for a given
sound source position – see Figure 2.
The desired microphone trajectory is specified using the mouse cursor on a diagram
representing that region. Each block of output sound is obtained by convolving the anechoic
stream representing the sound source with the room impulse response (RIR) corresponding to
the position currently selected. The convolution engine uses a very efficient block-based
variation on the overlap-add method, especially suited to accommodate the change of RIR filter
on successive audio output blocks; a short cross-fade is applied to suppress audible RIR
transition glitches.
45º CONGRESO ESPAÑOL DE ACÚSTICA
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This program allows real-time operation with room impulse responses of virtually unlimited
duration. However, being based on in-situ measurement of monaural RIRs, it is does not lend
itself directly to model-based binaural auralisation.
Figure 2 – Mechanical platform used for recording a 2-dimensional RIR grid.
2.2 LibAAVE Auralisation Library
The development of the main software package for auralisation in interactive virtual reality
environments built on a line of research on audio-visual VR initiated at IEETA with the MEng
dissertation project ‘Virtual Hall’ [2]. A demo from this project using a low-cost head-mounted
display (HMD) system was awarded the first prize at the 2007 Audio Technology contest of the
Portuguese section of the Audio Engineering Society (AES). An important potential application
envisaged for this kind of systems was explored in [3].
In order to ease the gradual refinement of the room modelling and auralisation functions and the
integration of new features, the software package was shaped as a library named LibAAVE and
made freely available under a public license (https://code.ua.pt/projects/acousticave). LibAAVE
is presented in some detail in [4] and [5]. It is designed to take a geometry model of the virtual
room in .obj format and allow arbitrary movement of both sources and listener. Its basic
operation principles are illustrated in Figure 3. Two functional blocks can be distinguished:
acoustic model processing and audio processing.
Figure 3 – Overall LibAAVE operation structure.
... ... ...
AUDIO PROCESSING
Listener
Primary
Sound Sources
Room
3D Model
Wavefront
.OBJ File
Anechoic
Audio
Streams
Positions Head Orientation
(inertial head tracker)
Binaural
Audio Stream
(headphones)
source 1
sound path
source 1
anechoic
audio
source n
anechoic
audio
source 1
binaural
audio
source n
binaural
audio
...
...
∑
Position
(sensors/controls)
source n
sound path
source 1
velocity
... source n
velocity
source 1
azimuth &
elevation
... source n
azimuth &
elevation
Delay, Attenuation Re-sampling HRTF
Delay, Attenuation Re-sampling HRTF
ACOUSTIC MODEL PROCESSING
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ENVIRONMENTAL ACOUSTICS
The acoustic model processing block is based on simple, well-established geometric acoustic
models: the direct sound and reflections from each primary source are computed dynamically
by the mirror-image source (MIS) method, taking into account the acoustic properties of the
room (extracted automatically from its 3D geometry model), the sound source trajectories and
listener position, tracked in real time. The result is a set of sources (primary sources plus
respective mirror-images up to a certain order specified by the user). Source ‘visibility’ tests are
applied to determine which of these need be considered.
The audio processing block generates binaural output sound, suited for headphone or earphone
presentation, by adding the contributions from all the ‘visible’ sources. The anechoic sound
streams from each source are filtered according to their propagation path characteristics. In
particular, directional cues are obtained by applying head-related transfer-function (HRTF) filters
according to the propagation angles relative to the listener head, worked out in real time with
the help of a head-tracking device. An Intersense InertiaCube BT was used during the
development stage at IEETA. Different HRTF sets can be selected, taken from public-domain
databases, namely the KEMAR-based set from MIT Media Lab [6] and CIPIC [7]. As in the
virtual microphone positioning system [1], cross-fading is applied to avoid audible HRTF
transition glitches.
The MIS method is only suited to real-time simulation of the very early part of the RIR. In other
words, for real-time operation the user must specify a sufficiently low maximum reflection order
(typically n<6 for a single primary source on the platforms tested); this is because the
computational cost of checking source visibility increases exponentially with reflection order [5].
The late RIR must therefore be simulated by other means. The reverberation tail solutions
adopted in LibAAVE are described in [8]. Acceptable sound quality was obtained with the
Datorro algorithm, but a feedback delay network (FDN) was preferred, as it allows frequency-
dependent RT60 configuration.
LibAAVE includes functions to support visualisation of the virtual room based on its .obj
geometry model. Figure 1 illustrates one of the various interactive applications developed to
demonstrate its operation.
Figure 4 – LibAAVE demonstration GUI: the movements of the virtual source (loudspeaker) and listener
(face) within the room are controlled using the mouse; head orientation can be controlled using the mouse
or a head-tracking device (Intersense InertiaCube BT). The lines between represent the sound paths from
‘visible’ sources up to the reflection order specified by the user. The binaural output at the listener position
is played in real time on headphones.
Significant effort was put into documentation, to ease future use and refinement. Along with the
code, a report including examples of how to build auralisation models is available at UA’s
software repository (https://code.ua.pt/projects/acousticave). Demonstration code and videos
are available at http://sweet.ua.pt/paulo.dias/AcousticAVE/.
45º CONGRESO ESPAÑOL DE ACÚSTICA
8º CONGRESO IBÉRICO DE ACÚSTICA
EUROPEAN SYMPOSIUM ON SMART CITIES AND
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2.3 User Tracking and Auralisation with iOS Devices
The integration of LibAAVE with an ultrasonic indoor localisation system to track listener
position was tested in [9]. The localisation system can be based on mobile devices (e.g.
smartphones) and, in contrast with the Vicon motion capture system used at LVP (see section
3), requires only a few small, inexpensive ultrasound beacons installed in the room, making it a
much more portable alternative, especially useful for audio augmented reality applications. A
prototype was built and tested using an iOS smartphone equipped with accelerometer,
gyroscope and magnetometer, which allow inertial head-tracking with automatic drift correction.
LibAAVE is currently being ported to iOS, since the available memory resources also allow the
auralisation software to be run from the device itself [10].
2.4 3D Data Acquisition for Room Acoustic Modelling
Practical usage of an auralisation package demands efficient tools of feeding its acoustic model
with the relevant room configuration data, namely 3D geometry and acoustic properties of the
boundary materials. In the LibAAVE case, this means creating an appropriate .OBJ room
model.
The problem of acquiring real room data – especially important for validation purposes – was
addressed in [11] using the Microsoft Kinect sensor and the Kinect Fusion application to
generate polygonal models of the room boundary surfaces. Tools to help automate the
identification of surface materials and assignment of acoustic absorption/reflection coefficients
to each polygon, as required in geometric modelling, were developed with the Visualisation
Toolkit (VTK). A voxelisation algorithm was also developed to generate, based on the surface
polygonal model, a 3D node grid covering the volume of the room. This is useful for physical
room acoustic modelling, whose application is envisaged in future LibAAVE developments. With
the information on room surface absorption (from the polygonal model) and volume (from the
3D grid), it is possible to estimate reverberation time (RT60) automatically using Sabine’s
formula and shape the reverberation tail accordingly. A summary of this work can be found in
[12]. The data acquisition and modelling algorithms were tested on a meeting room at IEETA,
as illustrated in Figure 5.
Figure 5 – Polygonal model of room boundary surfaces (left) and corresponding voxelisation (right).
3 MODEL INTEGRATION
Over the course of the project, successive versions of LibAAVE, as well as simpler tools for
offline generation of binaural sound (e.g. a MATLAB application developed early in the project
for synchronised avatar step sound generation) were installed, configured and adapted at LVP
to suit the specific needs of each audio or audio-visual perception experiment.
An important task was to feed LibAAVE with real-time listener/source position and head
orientation data from LVP’s Vicon motion-capture system, which is based on infra-red cameras
tracking reflective stickers fixed to the relevant targets (in this case listeners and sound
sources). Vicon’s software development kit (SDK), a C library allowing communication with
Vicon’s applications (Nexus, Blade and Tracker), was used for that purpose. Although not ideal
45º CONGRESO ESPAÑOL DE ACÚSTICA
8º CONGRESO IBÉRICO DE ACÚSTICA
EUROPEAN SYMPOSIUM ON SMART CITIES AND
ENVIRONMENTAL ACOUSTICS
in terms of latency and precision, head-orientation detection with the Vicon system is possible
using a set of 3 stickers to define the head’s reference axes.
In March 2014, at the second of two ‘Auralisation Models and Applications’ workshops
organised to showcase the project, the LibAAVE-Vicon motion capture integration was
demonstrated with a real-time audio-visual VR simulation of a musical quartet.
As explained in the Introduction, controlling the relative latency of the audio and video chains is
a crucial requirement, not least because audio-visual synchrony perception is a research issue
in its own right – take, for instance, [17] and [18]. With the help of a Brüel & Kjær Pulse data
acquisition system and appropriate instrumentation, a device was implemented to analyse the
degree of synchrony of different signals (electric triggers, motion-capture events, markers on
the auralised sounds and projected images...) and employed in experiments to measure the
processing latencies of the audio, video and motion-capture signal chains of the VR
environment. The results were documented in an LVP internal report and, based on it, a guide
was prepared on how to perform this kind of measurements and assess/adjust synchrony.
4 PSYCHOPHYSICS RESEARCH
4.1 Adaptation to Non-Individualised HRTFs
As any auralisation package based on geometric room acoustics, LibAAVE relies on HRTF
filtering to impart the directional cues that allow 3D source localisation. Since obtaining
individualised HRTF filter sets would pose very serious practical difficulties, HRTF sets
measured on dummy heads with ‘average’ characteristics are used instead. As mentioned
before, two such HRTF sets were adopted in LibAAVE [6][7]. It is therefore extremely important
to assess the perceptual effectiveness of HRTF processing and understand to what extent the
use of non-individualised HRTFs might hinder spatial sound perception.
The issue was addressed from a learning perspective [13][14]. A set of experiments showed
that mere exposure to virtual sounds processed with non-individualised HRTFs did not improve
the subjects’ performance in sound source localisation, but short training periods involving
active learning and feedback led to significantly better results. These findings indicate that using
auralisation with non-individualised HRTF should always be preceded by a learning period. This
work, on the basis of an earlier presentation at the 129th AES Convention, was selected for
publication in the AES Journal [15].
An additional set of experiments were devised to investigate this learning effect in further detail.
The experiments involved three groups of subjects and a careful schedule of azimuth/elevation
localisation training and test sessions over the course of one month. This made it possible to
study the persistence in time (memorisation) of the learning effect, its dependence upon the
type of sound source and its decomposition in terms of azimuth, elevation and their cross-
dependence (i.e. how azimuth localisation training affects elevation localisation performance
and vice-versa). Externalisation effects were also studied. The results evidenced that sound
localisation with altered cues is easily trained and subject to generalisation effects across space
and sound source type: a brief training session with a restricted set of sounds and source
directions is enough to improve localisation performance for trained and untrained sounds in
trained and untrained directions. The learning effects are persistent; they can still observed one
month after training, especially in azimuth localisation. Externalization levels are also increased
by training, although not directly related to localisation accuracy levels [16].
4.2 Sound Presentation
The choice between headphones and earphones is an unresolved debate. In order to obtain
some guidance regarding this issue, a few experiments were carried out to compare localisation
performance with in-ear phones (Etymotics ER-4B) and headphones (Sennheiser HD 650)
already available at LVP. This unpublished work involved training using analogous
methodologies to those employed in the HRTF studies. The experiments were repeated under
45º CONGRESO ESPAÑOL DE ACÚSTICA
8º CONGRESO IBÉRICO DE ACÚSTICA
EUROPEAN SYMPOSIUM ON SMART CITIES AND
ENVIRONMENTAL ACOUSTICS
different noise levels; in both cases, the global average localisation error was lower with
headphones.
4.3 Depth and Time-To-Passage (TTP) Perception Cues
Significant work was dedicated to the investigation of distance (depth) perception [17][18]. As
mentioned before, the interplay between aural and visual cues highlights the importance of
controlling audio-visual synchrony. The selective control of reflection orders implemented in the
auralisation tools was an important feature in the experimental work leading to [19].
The work on the perception of “time to passage” (TTP) and “time to collision” (TTC) of looming
sounds involved experiments with sources of various types travelling different distances at
different velocities and with different occlusion rates [20][21]. This required the auralisation tools
to be configured with an HRTF database including near-field measurements; the choice fell on
the database from the TU of Berlin which comprises measurements at 0.5m, 1m, 2m and 3m
[22].
5 FUTURE WORK
Possible refinements to the acoustic modelling algorithms used in LibAAVE include, for
example:
Adoption of frequency-dependent acoustic absorption coefficients;
Dynamic adjustment of the maximum reflection order;
Quantisation of source path delays so that only primary source FFTs need be
calculated;
Optimisation of source visibility checking algorithms;
Partial pre-calculation and/or less frequent updating of source visibility;
Combination of MIS with ray-tracing or beam-tracing techniques;
Since the goal is to maximise model accuracy while ensuring real-time operation, any
modification must assessed in terms of both perceptual and computational impact.
LibAAVE can benefit enormously from parallel processing through functional decomposition into
two threads (room model and audio) and/or data decomposition within each thread. Both are
highly parallelisable, since sources can be processed independently from one another.
We also plan to test potentially more accurate techniques (namely physical modelling) and
develop novel hybrid models through combination of techniques. One possibility along those
lines is adapting the Virtual Microphone Positioning algorithm to use a grid of Ambisonics RIRs
(obtainable in a single run of a DWM model) instead of a grid of measured monaural RIRs.
HRTF processing can then be applied to perform Ambisonics-binaural conversion and allow
headphone presentation. The MEng dissertation [23] represented an initial step in that direction.
The PhD project already underway to build on the work developed on audiovisual perception
[24] is an example of the numerous threads that can be pursued on the Psychophysics research
front. The results on HRTF learning effects (arguably the most significant novel contribution
from AcousticAVE) call for larger-scale experiments to allow further investigation on the
underlying mechanisms and influential factors. Pursuing the (yet unpublished) work on
headphone vs. earphone sound presentation (section 4.2) could be very valuable in this regard.
On the application front, we intend to create a permanent demo with the prototype mentioned in
2.3, which shows huge practical application potential. Porting LibAAVE to iOS is the most
immediate task; an MEng dissertation was proposed to tackle it.
Developing room model configuration tools along the lines discussed in 2.4 is essential to
promote practical applications. Acoustic archaeology and walkthrough auralisation in cultural
heritage sites are among the most promising.
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REFERENCES
[1] Barker T, Campos G, Dias P, Vieira J, Mendonça C, Santos J (2012) ‘Real-Time Auralisation
System for Virtual Microphone Positioning’. 15th International Conference on Digital Audio
Effects (DAFx-12), York, UK, September 17-21, pp. 137-143.
[2] Casaleiro R (2008) ‘Sala de Espectáculos Virtual’. MEng dissertation. Dept. of Electronics,
Telecommunications and Informatics, University of Aveiro.
[3] Dias P, Campos G, Casaleiro R, Seco R, Santos V, Santos B S (2008) ‘3D Reconstruction and
Auralization of the “Painted Dolmen” of Antelas’. Electronic Imaging Conference 2008 (EI 2008),
SPIE Vol. 6805, 6805OY, Three-Dimensional Image Capture and Applications 2008, San Jose,
California, USA, January 28-29.
[4] Oliveira A, Campos G, Dias P, Vieira J, Santos J, Mendonça C (2013) ‘Aplicação de Auralização
em Tempo Real’. 11th Congress of AES Brasil, S. Paulo, Brasil, May 7-9, pp. 98-101.
[5] Oliveira A, Campos G, Dias P, Murphy D, Vieira J, Mendonça C, Santos J (2013) ‘Real-Time
Dynamic Image-Source Implementation for Auralisation’. 16th International Conference on
Digital Audio Effects (DAFx-13), Maynooth, Ireland, September 2-6, pp. 368-372.
[6] Gardner B, Martin K (2000) ‘HRTF Measurements of a KEMAR Dummy-Head Microphone’.
URL: http://sound.media.mit.edu/resources/KEMAR.html.
[7] Algazi, V. R., Duda, R. O., Thompson, D. M. (2001) ‘The CIPIC HRTF database’. IEEE Workshop on
Applications of Signal Processing to Audio and Acoustics, New York, October 21-24, pp. 99-102.
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[8] Silva N, Oliveira A, Dias P, Campos G, Vieira J, Santos J (2014) ‘Auralização em Tempo Real para
Ambientes Virtuais Dinâmicos’. 12th Congress of AES Brasil, S. Paulo, Brasil, May 13-15.
[9] Lopes S, Oliveira A, Vieira J, Campos G, Dias P, Costa R (2013) ‘Real-Time Audio Augmented
Reality System for Pervasive Applications’. 11th Congress of AES Brasil, S. Paulo, Brasil, May 7-9.
[10] Lopes S, Vieira J, Campos G, Dias P (2014) ‘Sistema de Realidade Aumentada Áudio 3D para
Dispositivos iOS’. 12th Congress of AES Brasil, S. Paulo, Brasil, May 13-15.
[11] Pereira J (2013) ‘Aquisição e tratamento de dados 3D para modelação acústica de salas’. MEng
dissertation. Dept. of Electronics, Telecommunications and Informatics, University of Aveiro.
[12] Pereira J, Silva N, Dias P, Campos G, Vieira J (2014) ‘Aquisição e tratamento de dados 3D para
modelação acústica de salas’. 12th Congress of AES Brasil, S. Paulo, Brasil, May 13-15.
[13] Mendonça C, Santos J, Campos G, Dias P, Vieira, J (2012) ‘On the Adaptation to Non-
Individualised HRTF Auralisations – a Longitudinal Study’. AES 45th International Conference,
Helsinki, Finland, March 1-4.
[14] Mendonça C (2012) ‘Audiovisual Perception of Biological Motion’. PhD thesis. School of
Psychology, University of Minho.
[15] Mendonça C, Campos G, Dias P, Vieira J, Ferreira J, Santos J (2012) ‘On the Improvement of
Auditory Accuracy with Non-Individualized HRTF-based Sounds’. J. Audio Eng. Soc. 60(10), pp.
821-830, October.
[16] Mendonça C, Campos G, Dias P, Santos, J (2013) ‘Learning Auditory Space: Generalization and
Long-Term Effects’. PLoS ONE 8(10). doi: 10.1371/journal.pone.0077900
[17] Silva C (2011) ‘Perceiving Audiovisual Synchrony as a Function of Stimulus Distance’. MSc
dissertation. School of Psychology, University of Minho.
[18] Silva C, Mendonça C, Mouta S, Silva R, Campos JC, Santos J (2013) ‘Depth Cues and Perceived
Audiovisual Synchrony of Biological Motion’. PLoS ONE 8(11).
doi:10.1371/journal.pone.0080096
[19] Mendonça C, Lamas J, Barker T, Campos G, Dias P, Pulkki V, Silva C, Santos J (2013) ‘Reflection
orders and auditory distance’. 21st International Congress on Acoustics (ICA 2013), Montreal,
Canadá, June 2-7 (POMA Vol. 19, 050041).
[20] Silva R (2013) ‘Judging Time-to-Passage of looming sounds’. MSc dissertation. School of
Psychology, University of Minho.
[21] Silva R, Mouta S, Mendonça C, Lamas J, Silva C, Santos J (2013). ‘The role of acoustic cues in
time-to-passage judgments: Judging time-to-passage of looming sounds’. 5th Iberian
Conference on Perception (CIP), A Coruña, Spain.
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[22] Wierstorf H, Geier M, Raake A. Spors S (2011). ‘A Free Database of Head-Related Impulse
Response Measurements in the Horizontal Plane with Multiple Distances’.
URL: https://dev.qu.tu-berlin.de/projects/measurements/wiki/2010-11-kemar-anechoic
[23] Santos F (2012) ‘Auralização Binaural com HRTF e Descodificação de Ambisonics’. MEng
dissertation. Dept. of Electronics, Telecommunications and Informatics, University of Aveiro.
[24] Silva, C. (2012). ‘Audiovisual Perception in a Virtual World: An Application of Human-Computer
Interaction Evaluation to the Development of Immersive Environments’. PhD Thesis Proposal.
School of Psychology, University of Minho.
