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Disparity in Horizontal Correspondence of Sound and Source Positioning: The Impact on Spatial Presence for Cinematic VR

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This study examines the extent to which disparity in azimuth location between a sound cue and image target can be varied in cinematic virtual reality (VR) content, before presence is broken. It applies disparity consistently and inconsistently across five otherwise identical sound-image events. The investigation explores spatial presence, a sub-construct of presence, hypothesizing that consistently applied disparity in horizontal audio-visual correspondence elicits higher tolerance before presence is broken, than inconsistently applied disparity. Guidance about the interactions of subjective judgments and spatial presence for sound positioning is needed for non-specialists to leverage VR’s spatial sound environment. Although approximate compared to visual localization, auditory localization is paramount for VR: it is lighting condition-independent, omnidirectional, not as subject to occlusion, and creates presence.
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Disparity in horizontal correspondence of sound and source
positioning: The impact on spatial presence for cinematic VR
Angela McArthur
Media & Arts Technology (MAT), Queen Mary University London, UK
Correspondence should be addressed to Angela McArthur (a.mcarthur@qmul.ac.uk)
ABSTRACT
This study examines the extent to which azimuth disparity between sound-cue and image-target can be varied
in cinematic virtual reality (VR), before spatial presence is broken. Exploring conscious self-reporting and
autonomic arousal via galvanic skin response, it varies displacement of sound source along the horizontal plane,
in five otherwise identical sound-image events, for stimuli of human and object types. The displacements are
applied either consistently (at 5°, 10° or 15° offset to image) or inconsistently (two orders of randomness using
, 10° and 15° offsets) at uniform distance from the user/ camera position. It hypothesizes that consistently
applied disparity in audio-visual lateral correspondence elicits higher tolerance before presence is broken.
Stimuli of a bell alarm clock, and separately, a human actor, were produced and rendered for headset viewing
and dynamic binaural headphone listening. Content was presented to participants under controlled conditions
including familiarization trials. For both experiments the visual targets were associated with the sound cues
through synchronous corresponding visual movement. Thirty-five participants were tested during 70 trials, using
the BBC R&D 360TV player and BBC Spatial Sound Renderer. No support was found for the hypothesis
relating consistency to presence. Consistency as a determinant of arousal proved significant, but inversely to the
expected direction. Presence ratings were high throughout conditions, and though some participants verbally
discriminated precision in localization judgments, this did not impact ratings. The novice status of participants
for both VR and dynamic binaural sound, could account for high ratings, despite familiarization trials. The
multimodal environment of VR has many potentially confounding factors which affected the current study;
ultimately, azimuth sound-image disparity did not noticeably affect novice experience. Further work in study
design, and on the impact (both subjective and objective) of exposure on presence in VR is required at this time,
to better analyze the perceptual correlates of its spatial sound technologies.
McArthur Disparity in horizontal correspondence of sound & source positioning
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1 Introduction
Virtual reality technologies, evolving at speed,
promise the ultimate user experience. Employing
more of our senses, more powerfully, the need to
understand the perceptual correlates of such
technology is vital, as the medium becomes closer
to, and less distinct from, the recipient.
Presence the sense of being in an environment,
whether real or virtual [1] is an indicator of
involvement, affecting the richness of experience.
The importance of audio for presence has been
demonstrated in numerous studies [15]. Older
consumer-level VR technologies lacked many of the
current advantages to make best use of spatial audio
for VR. Even now, rendering is often achieved using
subjective aural judgments, and is affected and
constrained by hardware and system configurations,
process, and timescales. Understanding perceptual
tolerance for disparity in sound-image relations may
help professionals avoid breaking user presence.
Research has supported the idea that cross-modal
consistency is a main determinant of presence,
leading tosynergetic effects […which] enhance the
perception of information as compared to unimodal
conditions[2]. How then, do notions of parity or
disparity in sound-image relations interact with
temporal sequencing and the establishment of
expectation for a coherent and compelling virtual
experience? How far from the simulation of real-
world correspondence is feasible before presence is
compromised? These questions bear relevance to
practitioners who may be apprehensive about the
imprecision of their spatial sound rendering in the
absence of any end-to-end solution, as well as for
those wishing to employ sound design that aims not
just to replicate reality, but to apply sound
imaginatively, leveraging the unique multimodal
affordances of the medium.
Auditory perception has been considered a
hypothesis generation and testing process […]
constructed from the available information and
tested against subsequent experience (often over a
very short time interval)”[6]. This could prove
useful for creative compositional strategies - VR
content is currently almost exclusively short-form,
providing limited exposure times. Further, given the
relative novelty of the medium, expectations of users
are uninformed by pre-existing bodies of work
(though may be informed by expectations transposed
from other media or real-world spatial sound).
2 Background
Presence
Under discussion here is spatial presence as the most
popular construct of presence [7]. Spatial presence is
a concept developed in psychology [8] and
communication science [9] with various measures of
assessment available [10,11]. A theoretical two-
stage model of its structure was proposed [12] in
which users need to develop a mental model of the
space depicted, then accept this model as their own
(egocentric) locus of experience. If both stages are
passed, spatial presence is assumed. The later stage
can be considered unconscious and automatic.
Accordingly, users will activate the most convincing
(consistent and error-free) mental models from
alternatives to define and maintain their egocentric
position. This infers that spatial presence increases
with consistency.
Consistency between sound and image is of key
concern for VR, which is necessarily a multimodal
environment. With no distinct ‘frame’ within which
to focus, users have potentially more information to
process this processing involves complex
interactions. Sensory information received is not
processed independently. Each modality initially
encodes environmental information differently, but
ultimately we are presented with a coherent
perceptual experience [1316].
Take, for example, the way in which we resolve
conflicting sensory data from different modalities, to
a near-optimal (though inaccurate) combination.
The ‘Ventriloquist Effect’ [17,18] demonstrates this
in terms of localization: sensory information is
constructed in such a way to support an illusion.
Cinemas provide an example of our willingness to
accept dialogue not coming from the image source.
Spatially at least, vision over audition seems to exert
a dominant influence on the resolution of cross-
modal, biasing perceived location [19]. This raises a
concern about the regular sequencing of the sound
events, as the expectation of (particularly a visual)
target may influence attentional shifts in other
modalities (herein sound) [20,21].
Audio and presence
Understanding how sound positively impacts
presence in VEs is important. Improved realism and
quality may seem key, indeed they exert a powerful
effect, but we should be cautious in speculating that
ever-increasing fidelity and refinement produces
McArthur Disparity in horizontal correspondence of sound & source positioning
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proportionate returns in presence. Work to assess
the influence of perceived quality of audiovisual
reproduction when either audio or video is varied, is
largely inconclusive [2225]. A direct link between
perceived quality and presence cannot be inferred
however. In fact, the uncanny valley effect [26],
where artificial renderings that come close to realism
are seen as more uncanny (invoking different criteria
for evaluation) than those which are less realistic,
may apply to sound as much as image [27,28].
Certainly, advances in head tracking and motion
tracking, the convolution of sound with head-related
transfer functions (HRTFs) and room acoustic cues,
as well as binaural synthesis [29], improve
externalization [2,30] (where sound is located in the
environment, rather than within the head) thus also
localization. They also enable presence, though as
stated, not always realism [31].
Auditory localization
This study focuses on horizontal plane localization
which, in contrast to the vertical plane, provides
more precise localization acuity. It also attests to the
importance of the auditory processing of spatial
information in dealing with omnidirectional
information, a unique concern for VR content
creators.
The human ability to localize sounds depends on the
central auditory processing of binaural and monaural
acoustic information. Binaural inter-aural time
difference (ITD) and inter-aural level differences
(ILD) are the primary cues for localization in the
horizontal dimension. Generally, we are better able
to localize sounds frontally than laterally - the
minimum audible angle (MAA) [32] needed to
discriminate sources is lowest at central positions
(with reported accuracy of about 1°). This
declines at lateral positons, to about 10° [33].
Methodological background
In the present study, margins of displacement (5°,
10° and 15°) are sufficiently large to allow for
broadband source discrimination, particularly given
users’ freedom of head movement (which, as for
real-world sound cues, crucially aids disambiguation
of sound localization). This does however make
experimental design problematic, though more
ecologically valid.
With more narrow sound bandwidth, azimuth
localization worsens [3437] thus an alarm clock
(which allows both ITDs and ILDs to be used for
localization) was chosen. Human stimulus type
served two purposes as a useful comparison to the
clock, and because informational content of sound
stimuli has been shown to be critical for auditory
attentional capture [3841]. A semantically
impoverished utterance (“laaaa”) was chosen to
control out any confounding semantic effect.
Sound events were presented in identical order to
control for primacy (where order of presentation is
privileged) in a non-reverberant room to control for
precedence effect (early reflections being privileged)
[42]. It was necessary to duplicate the object and
human to control for image variation and auditory
spectral variation. Binaural room impulse responses
(BRIRs) were added for realistic reproduction. Such
signal processing has been shown to improve
presence ratings [31] [43] whilst reflecting that
azimuth localization error does not differ
significantly in generic Vs individually measured
HRTFs when head-tracking is utilized [33,4448].
Thus, a steady broadband source in a dry room
offered optimal localization conditions [35].
Participants in this study were not discounted on the
basis of partial hearing loss, which “hardly detracts
[…] from directional hearing in the horizontal
plane….” [49] and is not alone a good predictor of
localization performance [5055].
Including self-reporting measures in this study
reflects the fact spatial presence is considered a
conscious experience [5659]. The Swedish Viewer-
User Presence Questionnaire (SVUP) [60] unusually
includes questions dealing specifically with sound.
In this study, five items were used, relating to sound
localization, sound quality, and spatial presence.
Self reporting can be limited when examining
expectation, which is temporally contingent;
questionnaires are by nature reflexive, and allow
retrospective ‘fitting’ by participants. An autonomic
measure was considered useful to make detail
available on the first potential stage [12] of spatial
presence - the unconscious construction and
acceptance of a mental schema.
Changes in the electrical conductance of the skin,
broadly within the term ‘galvanic skin response’
(GSR), have been correlated to both reported
behavioral presence [11,61] and reported breaks in
presence [62]. GSR captures arousal data from the
sympathetic nervous system which mediates stress
responses, and plays a significant role in motivation,
emotion and orienting response to novelty [6365].
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3 Method
Participants
Of 35 adult participants, 5 were non-BBC, and were
consequently compensated for their time. Three
’non-responder’ participants were discounted. More
males (n24) than females (n11) took part of varying
ages (n21 = 25-34 years, n9 = 35-44 years, n3 = 18-
24 years, n2 = 45-54 years). Most participants (n21)
reported having neither eyesight or hearing
difficulties, with many (n12) reporting eyesight
difficulties (ranging from short sightedness to
astigmatism) and very few (n3) reporting hearing
difficulties (ranging from mild tinnitus to slight
hearing loss). Where possible, participants’ eyewear
was worn under the headset during trials, however in
some cases this was not feasible due to size, fogging,
or discomfort.
Most participants (n13) had never worn a headset
before or had used one for less than an hour (n11).
Some (n7) had used one for 1-5 hours, with very few
(n4) having used one for more than 15 hours, (n2 as
consumers, n2 as professionals). Most participants
(n26) had no specific interest in audio, whereas
some (n7) identified themselves as enthusiasts.
Consequently, the tests reflect novice listener
experience.
Stimuli
Ten unique stimuli (Table 1) of 105 seconds
duration were produced containing five sound events
at equal intervals, each of 3 sec duration to allow for
head-movement disambiguation.
Table 1. The ten experimental conditions
Kodak PixoPro SP3604K cameras and a AKG
C414B ULS microphone recorded the alarm clock
and human actor, separately, in the BBC R&D
listening room, at 0°, 45°, 100°, 250° and 315°
azimuths (Fig.1) at 150cm distance to camera, and
close distance to microphone, sampling at 48kHz.
Fig.1 Stimuli showing sequence of sound-image
events (1 5) and potential sound displacements
The clock was elevated to ensure it was the same
height as the actor’s face. An LED light was
attached to the clock so that - in addition to the
hammer striking the bells on the top of the clock -
there was a clear visual cue to indicate which clock
was sounding. The actor was instructed to
exaggerate mouth opening during sounding, to
produce a similarly clear visual cue. The utterance
frequency was a consistent 278Hz.
Video was stitched in PixPro 360 at maximum
resolution, before editing in Adobe Premiere, to
create five identical clocks (Fig.2) and five identical
actors. It was then down-sampled for smoothness of
playback (1920 x 1080 resolution, h.264 format).
Sound was post-processed in Cockos’ Reaper as a
multi-channel WAV, transcoded into uncompressed
multichannel PCM audio, and placed into an MKV
container, with the video file.
Fig.2 An equirectangular view of the object stimulus
Human stimuli
Consistent Condition
Human stimuli Random
Condition
5° displacement
5 - 15° displacement - first
order
10° displacement
5 - 15° displacement - second
order
15° displacement
Object stimuli
Consistent Condition
Object stimuli Random
Condition
5° displacement
5 - 15° displacement - first
order
10° displacement
5 - 15° displacement - second
order
15° displacement
McArthur Disparity in horizontal correspondence of sound & source positioning
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Procedure
Upon arrival, participants were asked to complete a
questionnaire about their experience of VR and
audio. They then began trials in an enclosed room,
sitting on an office swivel chair for ease of (limited)
movement. They were connected to a BioTrace
NeXus 10mkii, an Oculus Rift DK2 headset, and to
Beyerdynamic DT 770 PRO, 250 ohms headphones.
The stimuli were loaded into the BBC R&D 360TV
player, which directly decoded video, and output
audio to the BBC Spatial Sound Renderer (a data-
based dynamic auralization system) [66] which
processed it with BRIRs using the SOFA
MultispeakerBRIR convention [67]. BRIRs were
previously measured with a dummy head
microphone on a rotational mount [68] at head yaw
rotations in steps in the BBC R&D listening room
[69]. Orientation data from the DK2 was relayed to
the 360TV player (Fig.3). Volume was kept at a
consistent 72dB SPL across trials.
Fig.3 Experimental playback configuration
Participants each experienced 4 trials, in one 30-
minute session (Fig.4). Trials were randomly
ordered and equally balanced so that each participant
had two trials of human stimuli, two of object
stimuli, and so that each stimulus was presented an
equal number of times across the 140 total trials.
The first two trials acted as familiarization trials and
were discounted from analyses.
After each trial, participants verbally scored (SVUP)
presence ratings, on a Likert scale 1-7. No feedback
was given during any of the sessions.
The BioTrace native software captured GSR at a rate
of 32 samples per second (sps).
Fig.4 A participant during trials
4 Results
Three trials were discounted due to technical
malfunctioning. Data was analyzed in Matlab - GSR
measurements were normalized and differences in
peak amplitude windows (pre and post stimulus)
were derived. These were processed using a moving
average filter of 64sps. To create skin conductance
responses (SCR) - the phasic component of the
signal [70] - linear detrending at breakpoints of 30
seconds was performed. This removed any negative
trends due to cumulative effects between the skin
and sensor, of charge over time [71].
Linear regression analyses for each dependent
variable, and linear mixed modelling, tested the
relationship between arousal and presence ratings, as
a function of experimental condition (consistency).
Presence scores from each question were analyzed
as separate variables [72]. Arousal and presence
ratings were also tested for correlation.
Condition (consistency) was not a significant
determinant of presence for either human or object
types. Arousal was not a function of condition in the
predicted direction. As such, the hypothesis was
unsupported. Further, there was no significant
relationship between arousal and presence.
Fig 5 shows all data for all 5 conditions / stimuli (c1
c5) Note: arousal and presence data have been
overlaid for comparison but are represented by
different Y values. Presence bar (1-7) coloration
denotes that <4 infers lack of presence.
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Fig.5 Presence ratings with arousal data
The null hypothesis that presence and condition are
independent, was tested using the Pearson’s chi-
squared test. This showed that the null hypothesis
could not be rejected, and was in fact supported.
A relationship was found between condition and
arousal (p=0.01894) with an inverse line of fit to the
expected direction (Fig.6).
Fig.6 Arousal as a function of condition for human
stimuli type
Linear mixed modelling took GSR as independent
variable, and introduced presence scores as
potentially random elements in turn, assessing their
relationship to condition using an ANOVA. The
same was done GSR as a potentially random factor.
No significant results were yielded.
As participants had been observed discriminating
some sound-image correspondence precision during
trials, the SVUP questions relating specifically to
sound quality and localization were regressed to
condition, using a linear model. No statistical
significance was shown (p = 0.30648 for the
question “To what extent were you able to localize
sounds?” and p = 0.86395 for the question “How
much did the sound add to the perceived realism?”).
Fig.7 Presence ratings for human stimuli
Fig.7 shows consistently high presence ratings;
though c3 and c4 offer more variance, c5 offers
least. Looking at variance in responses more
broadly, we see (Fig.8) a greater range for human
stimuli (high informational content) than object,
particularly for presence ratings.
Fig.8 Variance in data by stimulus type
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5 Discussion
The decline of arousal with condition type could
reflect participants loosening their schema once
beyond a certain threshold of cross-modal conflict
(revising or relaxing evaluative schema as in the
uncanny valley effect). The displacements, if
exaggerated further, may have overcome this.
Skin conductance measures were more problematic
than anticipated, and may require VR-specific
methodological consideration. Expectation studies
in music center around a corpus of material and
conditioned responses, where VR is a truly novel
medium for most people.
The inclusion of only five SVUP measures may have
contributed to the results. Inclusion of a greater
number of measures would have introduced more
samples though also more potentially random factors
(participants conflated visual awareness in ratings
e.g., the realization they were without legs). Overall,
a more robust study design would have mitigated
against such ‘leakage’ into presence ratings.
Presence scores were a particularly problematic,
being high throughout trials. Participants used
comparative judgments across trials which may have
impacted absolute ratings, and thus results. It was
also observed that participants sometimes initially
rated stimuli highly, being then unable to rate
subsequent stimuli higher, which they conceded they
would like to. Their novice status may be key here.
Familiarization trials aimed at controlling for
novelty (as well as ‘training’ participants for the
non-individualized BRIRs [48]) may have been
insufficient. Novices could be assumed to have
greater cognitive involvement - being more
concerned with sense-making - in the experience,
which is considered a determinant of spatial
presence [12]. In prior work exploring disparity in
sound-image correspondence, naïve participants
were far less critical than research engineers in
ratings when asked to score their ‘annoyance’ at
different configurations of speaker-screen setups as
a measure of disparity [22]. It may be unsurprising
that experts are sometimes able to create situational
models in circumstances where novices were not
[73]. This demonstrates a capacity to construct a
schema which may require more discrimination than
a novice can apply.
However, attention is a prerequisite for presence
[12]. Freedom of head movement meant, in nearly
all instances, users’ attention was oriented towards
the visual target. The ventriloquist effect may not
depend on deliberate visual attention [18], but sound
cueing may have enabled it, essentially contributing
to its own ‘capture’ by image.
The compound effect of naïve listening and viewing
should not be underestimated. If gains in spatial
sound rendering do not address visual biasing in VR,
and imprecise and/or inconsistent sound-image
correspondence have no bearing on presence, a
useful question might be for how long? Both in
terms of the individual, and the market.
6 Conclusions
In novel experiences, users may have an elevated
willingness to suspend disbelief. This willingness to
engage and be spatially present, may be the very
obstacle to its assessment. Participants had
experienced neither VR nor dynamic binaural sound,
perhaps tolerating disparity in sound-image cues
whilst still motivated to maintain a plausible
schema. Prior studies, despite encountering high
presence ratings when varying spatial sound
rendering in VEs [74], have make much progress in
assessing how technical rendering correlates to
perceptual measures. Yet research examining human
factors’ impact on presence is still needed [75,76]
and the current study supports this.
Overall, this study concurs with research showing
how spatial presence and perceptual realism does not
affect enjoyment, underling the need for presence
dimensions need to be treated separately during
analyses [7] and perhaps developed further to
separate out the perceptual correlates of spatial
sound, from image.
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... Dynamic binaural synthesis, utilizing head-related transfer functions (HRTFs), virtual loudspeakers and headset orientation data, provides a compelling experience (Fig. 2) which can be delivered (minimally) over standard headphones, a smartphone and cardboard headset. This affords a sounding world where distance, location and environmental cues remain independently static (or dynamic) when we move and it serves to reinforce presence [4,5] which in turn can 'uplift' potentially presence-breaking features of image [6,7]. Yet the ease of consumption does not reflect the challenges faced by content creators. ...
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