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

Simplification of reflection orders in virtual soundscapes through a
subjective evaluation
Chunyang XU1; Jian KANG2
1, 2Institute for Environmental Design, The Bartlett, University College London, United Kingdom

ABSTRACTSoundscape research stresses the interaction between human beings and sound environments. Virtual reality(VR) technologies provide a vivid approach to boost the virtual sound environment experience for people.
However, the real-time VR audio simulation cannot process the complex acoustic conditions (e.g., boundary
conditions) due to the high computational cost up to now. This study therefore aims to investigate theapplicability of less reflection orders for reproduced sounds in virtual sound environment. To test the effect ofreflection orders perceived by people, a series of subject evaluations was carried out based on audio-visual
VR experience with different scale public squares reproduced in Unity to provide visual scenarios for theparticipants. The participants heard typical sounds (e.g., birdsong, clapping and fountain) in these squares,
and gave their perception evaluation to these sounds. The results show that these reproduced sounds andtypical building spaces can employ less reflection orders which render similar realism and immersion forparticipants in virtual sound environment.
Keywords: Soundscape, Virtual Reality, Sound Reflection

1. INTRODUCTION
The pioneering work to understand of how people perceive sound reflections that arrive within ashort time after the direct sound was explored in the 1850s (1). In room and auditorium acoustics,
sound reflection significantly affects how sounds are perceived by human beings. Clarity,
reverberance (2) and speech transmission quality (3) are all involved with sound reflection in acoustic
design. Both positive and negative effects of sound reflections were found on speech intelligibility inroom acoustics (4). For outdoor sound environment, configurations of urban spaces, sound contexts ,and propagation conditions are more complex. The multiple and moving sources can be additionalvariables in outdoor sound environment research. The numerical simulation and simplifications were
examined in urban squares (5) and urban streets (6–9), and these studies parametrically explored soundfield and sound propagation in different kinds of urban spaces.
The current soundscape research has been stressing more on how people perceive the soundenvironment. Sound reflection is one of the significant components affecting sound propagation in
urban sound environments. To reproduce an ideal sound environment, the reflection order of sounds
should be infinite. The VR technology provides a vivid approach to render virtual built environment
synthesized with sounds. However, real-time auralization is becoming time-consuming with a highreflection order. Meanwhile, the human-environment interaction based on VR experience requires
high synchronization for public participation. It is necessary to auralize sound environment
efficiently and accurately with less computational cost. The previous research explored thesimplifications with the audio-only condition (10). The interactive visual-aural stimuli should be paid
more attention on sound environment evaluation when the current VR technologies have been boosted
with higher visual resolution and more degrees of freedom.
This work therefore aims to investigate the relationship between subjective responses including
immersion, realism and reverberance and different reflection orders under virtual sound environment.
The effects of interactive visual stimulus, the area of squares, and the relationship between
reverberance and reflection orders will be examined with multiple sounds and squares.

1
2

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chunyang.xu.18@ucl.ac.uk
j.kang@ucl.ac.uk

2. METHODOLOGIES
2.1 Scene Selection and Visualization
To investigate the visual-audio coupling under VR experience in urban spaces, four squares wereselected in London: the campus square behind Wilkins Building in University College London,
Paternoster Square south to St. Paul's Cathedral, Cabot Square in Canary Wharf, and Granary Squarenext to Caravan King's Cross. The square behind the Wilkins Building in University College London
was chosen, where the rectangular square was fully surrounded by building facades. Paternoster
square is an urban square next to St Paul’s Cathedral re-developed in the 2000s, and it is enclosed bybuildings with multiple facades and textures. Cabot square is located in Canary Wharf where it is oneof the significant commercial estates in London. This square is surrounded by the buildings over 60 m.
Granary square is a large open square in King’s Cross with the functions of relaxation, commerce andoffice.
Originally, there were fountains in Cabot Square and Granary Square. To investigate to the sounds
of fountains in all squares, extra fountains were virtually placed at the UCL campus square andPaternoster Square. These sites crossed a wide geographical range in London with different functions,areas and built environments. In order to eliminate the undesired scattering and absorption, limited
vegetation exists in these squares, and it was not reproduced during visualization. The visualization offour squares was through the modeling software (SketchUp Pro 2018). These four models consist of
2,733, 4,317, 2,379 and 4,143 edges. The photographs and reproduced scenes of four squares areshown in Table 1. The areas of these four squares are 900 m 2 , 1700 m2 , 6000 m2 , 7000 m2 in order.
Table 1 – Real sites and reproduced scenes

2.2 Sound Selection and Auralization
To investigate the acoustic behaviors of different sounds in these squares, several typical sounds
were chosen including birdsong, clapping and fountain. These three types of sounds have different
acoustic characterizations and soundscape contexts. The sample rate of the three sounds is 44100 Hz,
and the depth is 24 bit. These three sounds represent different kinds of time-frequency
characterizations. Birdsong has the high frequency contents of specific pitches. Birdsong is usuallyconsidered as a natural sound with a positive effect on urban sound environment. Clapping is discretein time domain analogue to an impulse sound, and it is also a sound generated by human activities.
Fountain sound is continuous in time domain, and fountain is a common landscape component in urban
public spaces generating water sound.

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Odeon (9.2 Auditorium) was used to characterize the acoustic performance s of these public squares.
Meanwhile, the process of auralization was also based on the impulse response generated from Odeon.
The boundary absorption and diffusion conditions were assigned with different parameters matched
the real sites. The built-in spatial audio plugin in Unity implements the real-time tracing in an assumed
box to achieve real-time auralization with the participant’s spatial displacement. The hybrid method in
Odeon could deal with the complex boundary conditions and generate the point-to-point impulse
responses with acceptable accuracy.
The previous study on simplification through subjective tests validated the reflection orders of 5,
20, and 50 with the aural stimuli only (10). In this work, four reflection orders, i.e., 1, 5, 20 and 1000,
were chosen, and these four different orders were first examined in Odeon. The distance between thesound source and the receiver in the four squares was set to 8 m, and it was a reasonable distance whena participant stood in these squares to observe the events.
2.3 Reproduction and VR Synthesis
The game engine (Unity) was used to synthesize visualization and auralization. The lighting
condition was set to a reasonable solar zenith angle and illuminance according to their geometricallocations. To synchronize the virtual visual-audio environment, a particle system of water splash was
attached to the fountain, and the animation of clapping was given to the characters on the square. Birds
were not rendered in the VR design.
The first-order ambisonic impulse responses were convolved with three dry sounds. The filteredsounds were decoded and spatialized in Unity according to the headset motion and location. The VR
environment was streamed through SteamVR. The high performance graphics card (GeForce GTX
1070) and Central Processing Unit (Intel Core i7-8700K) were used to guarantee the rendering quality.
HTC VIVE was used to provide VR experience. The VR headset was connected with a headphone. The
participants could turn around their heads, but they cannot have the spatial displacement due to thefirst-order ambisonics.
2.4 Subjective Evaluation
The subjective test was divided into two stages. In stage I, the participants were informed to hearthese sounds in the UCL campus square without the VR headset. Since all participants were familiar
with the UCL campus, they could have the corresponding spatial impression on this square. In stage II,
the participants heard these sounds with the VR headset under four reproduced scenes.
Three perceived indicators were selected including reverberance, immersion and realism.
Reverberance was categorized into a technically perceived indicator in the previous study (11). For
different functions of interior spaces (e.g., a lecture room and a concert hall), acoustic performances
are required to render totally different reverberance (12). This discrepancy has been studied in room
acoustics for a long time. Reverberance is more difficult to be perceived under continuous sounds,
compared with sounds of stopped reverberance. Immersion and realism were considered as reproduced
indicators (13). The three adjectives 'reverberant', 'immersive' and 'real' were rephrased into three
indicators: reverberance, immersion and realism. The Likert scale was utilized to describe the rating
scale with the description from 'not at all', 'slightly', 'moderately', 'very' to 'extremely'. The questions
used in the both stages are listed below:(1) How reverberant is this sound environment?
(2) How immersive is this sound environment?
(3) How real is this sound?
For each type of sounds in a scene, a reference sound was given to the participants first. The
reference sound was convolved with the impulse response of the reflection order of 1000. Then, eachsound lasted 10 seconds, and these sounds with different reflection orders were played one by one. The
sequence of these sounds was randomized in each reproduced scene. In addition, 3D Graphical User
Interface (GUI) synthesized in VR designs was shown in front of the participants when they were
doing the evaluation. Whilst it was possible to wear the VR headset, the participants could give theirsubjective perception through a five-button controller. Four scenes were conducted with the sameprocedure. Figure 1 shows the GUI in the reproduced scene and the subjective test by a participant.
Thirty participants with normal hearing and vision took part in the subjective evaluation. The
participants were not informed by which reflection orders were applied in each sound during theevaluation. They all had the basic understanding of perceived indicators used in the formal evaluation.

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Figure 1 – GUI in Unity 3D and subjective test through the VR headset

3. RESULTS
3.1 Effect of VR experience
The two-stage subjective test was carried in the UCL campus square. Figure 2 shows thecomparison between audio-only and VR experience conditions under different reflection orders. Threetypes of sounds present the different subjective ratings. For birdsong, the rating difference between the
two approaches is significant under the reflection order of 1 for all three perceived indicators. The
rating of clapping is lower than the other two sounds in immersion and realism. Meanwhile, thevariation of reverberance is the most distinct. When the reflection order is 1, the rating difference
between two evaluation approaches was found to be significant. Compared with the other two sounds,
the clapping sound presents relatively low ratings on immersion and realism for both approaches, andthe variation of reverberance is the most distinct. The clapping sound is discrete in time domain, andthe stopped reverberance is easily perceived by the participants. The two approaches were found tohave a significant difference on immersion and realism under the reflection order of 1.

(a)

(b)

(c)
Figure 2 – Comparison between audio-only [a] and VR experience [v] conditions under different reflection
orders (1, 5 and 20). (a) birdsong, (b) clapping, (c) fountain.
When the reflection order was set to 1, VR experience during the subjective test can increase theimmersion and realism of sounds. All participants were required to evaluate sound impre ssion during
the subjective test rather than VR impression. Thus, the rating gain on immersion and realism ofsounds was generated from the interactive environment.
An unusual point occurs in clapping sound. When the reflection order is only 1, the mean ra ting ofVR experience is lower than the rating of audio-only condition. However, the mean rating difference6435between two approaches is not statistically significant under a reflection order of 1. Compared withother two sounds, clapping is the only discrete sound in time domain, and for each clapping, the signal
analogue to an impulse response includes all acoustic information of a space. Clapping sound withstopped reverberance therefore presents a rating range wider than the other two sounds.
The comparison results between audio-only and VR experience conditions reveal that both
evaluation approaches present the similar results on three indicators when the reflection order reaches
20. The VR experience can improve subjective ratings of immersion and realism when the reflection
order is set to 1.
3.2 Effect of sounds and urban spaces
In stage II of the subjective test, the results in the four squares are shown in Figure 3. The foursquares present different results on three types of sounds and three indicators involved with different
reflection orders.
In UCL campus square, the mean ratings of the three sounds present a significant increase when thereflection order is changed from 5 to 20 for reverberance. In Paternoster square, except clapping sound,a reflection order of 5 is acceptable to render birdsong and fountain with similar results compared withthe reflection order of 20. For Granary square, the mean ratings of immersion and realism for the threesounds are approximate within a narrow range. In Cabot square, the birdsong was found to have asignificant variation when the reflection order is changed from 5 to 20. The clapping and fountainsounds show no notable tendency on different reflection orders for three perceived indicators. It is
feasible to use only a reflection order of 1 to render these two sounds in Cabot square.

(a)

(b)

(c)

(d)

Figure 3 – Subjective ratings under different reflection orders (1, 5 and 20) among four squares. (a) UCL
campus square, (b) Paternoster square, (c) Granary square, (d) Cabot square.
For outdoor sound environment, the large square area (e.g., Cabot square and Granary square)
reduces the rating difference among multiple reflection orders. The sound is more likely to bedissipated in a large space, and the subjective result is also in accordance with the sound propagation
law. In addition, reverberance of clapping sound significantly depends on the acoustic behaviors ofurban spaces. It is feasible to reduce the reflection order when the space is large enough. For a small

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public square, a reflection order of 5 is acceptable to reproduce fountain sound.

4. CONCLUSIONSThis project examined subjective responses of immersion, realism and reverberance on multiple
sounds with different reflection orders through a VR subjective test. The results of the subjective test
revealed that:(1) Compared with the audio-only results, the VR experience was found to increase realism andimmersion of sounds when the reflection order was set to a relatively low range (e.g., 1 and 5).
(2) When the public square was large enough (e.g., >5000 m 2 ), the reflection order was found tohave a weak impact on immersion, realism and reverberance under VR experience for various sounds
including birdsong, clapping and fountain. When the square was enclosed within a relatively small
area (e.g., <2000 m 2 ), a reflection order of 5 or a higher order was required, and the VR exp erience stillcannot fill the gap of realism and immersion generated by the reflection order from 1 to 20.
(3) The sound of stopped reverberance (e.g., clapping) significantly depend ed on the reflection
order, and it needed to be carefully auralized during audio processing and reproduction with theconsideration of spatial characteristics and acoustic behaviors.
Overall, it is acceptable to employ less reflection order during auralization to render sounds under
VR experience with similar realism and immersion. The results on perceived indicators could makecontribution to fast auralization with less reflection order and reasonable accuracy through VR
experience.

ACKNOWLEDGEMENTS
The authors are grateful to Huan Tong and Tingting Yang for their help in modeling built
environment. The authors also thank Andrew Mitchell and Dr Francesco Aletta for valuable comments.
The project was supported by the European Research Council (ERC) Advanced Grant (No. 740696).

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