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

Towards a soundscape surround index
Tin OBERMAN1, Kristian JAMBROŠIĆ2, Francesco ALETTA1, Jian KANG11

UCL Institute for Environmental Design and Engineering, The Bartlett,University College London, United Kingdom
2
Faculty of Electrical Engineering and Computing,
University of Zagreb

ABSTRACTSound sources are being spread in public spaces during urban design and planning processes, yet little is
known about the effect their spatial pattern can have on soundscape. The aim is to investigate if associations
can be made between the dynamic relation between static sound sources and listener in public space and theperceived soundscape dimensions. An index for measuring spatial variability of sound sources in an urban
acoustic environment is proposed to describe physical features of the investigated phenomena and help topredict its influence on the perceptual outcomes.
Keywords: Apparent source width, Sound source distance, Sound envelopment, Gyroscopic interaction

1. INTRODUCTION
Soundscape is a perceptual construct used for characterisation of urban acoustic environment (1).
It is influenced by a variety of auditory and non-auditory factors (2). While auditory factors analyses
are looking at signal characteristics, sound source preference, perceptual dimensions and acousticalproperties, the site-specific, ‘micro spatial’ and three-dimensional distribution of sound sources has
been somewhat overlooked in soundscape research. This study argues that it is an important perceptual
factor in urban open space settings.
The spatial distribution of sound sources is an inherent part of urban and landscape design tasks
but there is a lack of knowledge on how to optimise position of sound sources, more specifically which
pattern to use and how to design it in relation to locations of other sound (noise) sources and proposed
activities. For example, we might be annoyed more by a busy road in front of us than by the one
behind us, but it is not clear if that is influenced by the visibility of noise sources or perhaps by thehead orientation. Further, we might find a soundscape more vibrant if we experience sounds all aroundus rather than just in front of us, but it has not been explored how strong this effect of spatial
distribution could be.
The recent research conducted by (3) and (4) showed that the spatiality of the auditory stimulus
doesn’t have a significant influence on soundscape dimensions defined by the pleasantness vs arousal
model, which was proposed and validated by (5) and integrated in the ISO/TS 12913 (6). However,the before mentioned recent studies were focused on assessment of certain reproduction techniquesand didn’t report on the spatiality of the recordings used for the experiments in detail. Therefore, it isconsidered that the spatial distribution might be contributing to the overall setting, or very specificdimensions, i.e. perceived eventfulness.
There are still no adequate tools to accurately describe the spatial complexity of an acoustic
environment (7). (8) indicated that the interaural difference caused by location of a sound source
might influence the efficiency of a cognitive task performed. Further, following the research focusedon concert hall acoustics and the intent to control factors such as the apparent source width by design(9, 10), this research aims to investigate the effect of similar concepts with focus on the applicationin design of public spaces. As the current work on the standardization of acoustic measures possibly
moves from monaural to binaural measurements to better characterise all of the site-specificcomplexity (11, 6), this study raises a question of possible value spatial audio data can have incharacterisation of an acoustic environment. Moreover, it aims to provide a conceptual frameworkneeded to characterise the perceptual value of spatial variability of sound sources in public space.

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2. SOUNDSCAPE SURROUND INDEX FRAMEWORKThe soundscape surround index is proposed to describe the spatial quality of the acoustic
environment in public space while keeping the complexity imbedded in soundscape approach. In atypical public space various factors influencing the acoustic environment are present, such as: active
and passive use, temporal and spatial variability of sound sources, different openness resulting indifferent reflections and scattering.
The following framework is suggested to encompass both dynamic and stationary receivers andsources, and both physical properties of an acoustic environment and perceptual attributes, as pictured
in Figure 1.

Spatial auditory data(sound levels, localisation
information in three
dimensions / i.e., 1st orderambisonic recording, Bformat file...)Egocentric physical factors

Main Factors and/orCorrectional Factors

Spatial auditory
factors / Calculated
source width, source
distance andenvelopment
Soundscape surroundindex

Location factors / Allocentric
physical characteristics (size,
shape, distribution of soundsources, distribution ofplanned activities...)Other egocentric data(listener’s position,
head movement...)Perceptual data /Soundscape descriptors

Perceptual experiment using spatial auditory andvisual stimuli and gyroscopic headtracking

Figure 1 –Framework for developing the soundscape surround index
One of the trademarks of soundscape research is finding a balance between quantitative andqualitative data. The proposed index is calculated from location data (stationary and allocentric, i.e.,
planar shape of the stationary sound sources considered, size and shape of the public space itself) andspatial audio data (recorded or simulated) collected from a number of measurement points (egocentric
level, i.e., receiver’s position changes in relation to the sound source analysed). To characterise awhole place with clear physical boundaries, such as square, or even a loosely defined area, the index
would be then calculated following the method similar to a method such as ISO 3382 for measuring
reverberation time of a room (12, 11), where different sound – receiver positions would be used to
calculate the mean value for the space analysed and compensate for relative sound pressure levels,
lateral energy fractions and other site-specific characteristics.
2.1 Location factors
Location factors represent the contextual characteristics of a public space. They include the shapeof the analysed sound sources in relation to the layout of the space analysed, so they inform thepotential number of measurement points needed.
Stationary sound sources in public space are designed in different ‘planar patterns’ or ‘shapes’. For
the purpose of the index, the simplification to a point, line and an area pattern is suggested for twodimensional layouts, as in Figure 2, applicable to both horizontal and vertical planes. The term ‘spatial
pattern’ was preferred to ‘spatial distribution’ since it implies a designed form rather than a random
distribution. Conversely, the term pattern might imply the temporal aspect of the phenomena or eventhe directivity pattern of a microphone, which however was not conceived in that sense within thisresearch.

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Figure 2 – Three basic spatial patterns according to which the stationary sound sources can be designed in a
public space: point, line and area pattern (from left to right)
Most typical use of the classification proposed might be the layout of water fountains in a square,as pictured in Figure 3. They allow a listener to experience it from outside (point and linear patterns)
or from within (areal pattern). In the three sites shown in the Figure 3, the fountains interpreted as
point and line might also be experienced from all sides, which doesn’t always necessarily need to be
the case. The pattern may be applied to planar stationary sound sources, vertical or more spatiallycomplex ones accordingly. Therefore, receiver points could be besides, within, above or below thesound source pattern.

Figure 3 – Examples of water fountains following the three basic spatial patterns featured in three squaresin Sheffield (from top to bottom): Barker’s Pool (point), Leopold Square (line) and Sheaf Square (area)
Further location factors include the size and the reverberation time which contribute to thelistener’s envelopment but also the functional aspect of the design, i.e. accessibility, shape of areas
for staying or passing by.
2.2 Spatial audio factors
Spatial audio factors, derived from spatial audio refer to the data obtained from a measurement
point which contains information on sound energy on the X, Y and Z axes. Following the use of figureof-eight microphone (13) and complex microphone arrays (14, 15) for predicting listener’s
envelopment in concert halls, it is expected that decomposing spatial audio signal may be used for thedevelopment of the surround index.
The objective factors considered to be calculated using the spatial audio information are:831

- source width which describes the angle between a listener and analysed sound source pattern as
illustrated in Figure 4;- source distance in free field which describes the distance between a listener and the analysedsound source pattern;- listener envelopment which describes the influence of lateral reflections;- temporal structure / loudness variability (i.e., N5 and N95) calculated for X, Y and Z axes, which
describes how dynamic is an acoustic environment in time and space in terms of the perceived
loudness.
Each of the before mentioned measures (source distance, width and envelopment) were initiallybased on subjective measures (such as Apparent Source Width, Listener Envelopment, SpecificDistance Tendency)(16, 10) but they also allow for calculating objective indicators which were foundto be highly correlated with subjective results, i.e. the late lateral sound level (13).
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Figure 4 – Source width in three basic settings - the point, linear and areal source

2.3 Allocentric and egocentric factors
Human brain can locate objects in space on the allocentric and on the egocentric level, where theformer is relative to the head and the later relative to the world (17). This is crucial having in mind
that movement of people is a key feature of a public space. It is suggested that the soundscape surround
quality in an urban environment should be regarded in a similar manner:- Allocentric level – following the way the sound sources are organised in space,- Egocentric level – following the (dynamic) position between a listener and sound sources.
Both depend on the physical position of fixed sound sources, their physical accessibility (i.e., thepedestrian path can lead between the sound sources or just tangle them from one side) and contextual
factors influencing the listener’s attention and experience. For instance, egocentric auditory distancelargely depends on the perceived reverberation (16), which is a location factor.
Allocentric factors look at the position of source in relation to the public space. Similar to thelocation factors, the allocentric ones tackle the number and position of measurement points needed.
They take into account whether a stationary source is near the edge of the public space so it can beexperienced only from one side, or is it in the centre so it can be experienced from all sides; or doesit cover a large portion of the public space so it can be experience from within.
Egocentric factors on the other hand describe the position of sound sources in relation to thereceiver, such as front, back or side position. While the narrow front/back localization is often
confusing (18), this might also be true for the third dimension (19). Egocentric factors change as thereceiver moves through public space and as she/he moves her/his head. This is important as by doingso, he/she will experience the same stationary sound source from different angles and distances.
Therefore, the framework pictured in Figure 5 is suggested to test how nine basic scenarios performin terms of objective and perceptual attributes. A1, A3 and B3 might perform very similar as thereceiver sees only a point source. B1 and C1 might be perceived similar as well as the receiver sees
only a line source. On the other hand, B2 and C2 might be similarly perceived as well.

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A3

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B3

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C2*********
C3

Figure 5 The framework showing nine basic scenarios of source – receiver relations. The three basic source
patterns (point, line and area) are pictured in red, while the receiver and the source width in relation to thereceiver are pictured in black.
To characterise a space that features a stationary sound source, a sufficient number of receiverpoints needs to be considered. Further research would need to be carried out to optimise the necessary
pattern of receiver points.
2.4 Perceptual factors
It is well known that a lot of physical acoustic metrics do not necessarily translate to perceptual
dimension in a linear way (20, 21, 16). Perceptual data within this framework refer to usual
soundscape descriptors (5, 6) and the perceptual values of spatial audio factors – source width, source
distance and listener envelopment. For the purpose of the index, they are intended to be used ascorrection factors.

3. SOUNDSCAPE SURROUND INDEX DEVELOPMENT AND APPLICABILITY
The soundscape surround index is conceived within the larger framework of soundscape indicesand its primary role is to adequately address auditory spatiality. Therefore, it doesn’t consider
quantifying non-auditory factors such as the visibility of sound sources and/or other contextual factors,
since those are imbedded within the larger framework (22).
The effect of spatial relations between the sources and the listener has been mainly limited toapplications in concert venues and audio production (23). The idea behind the soundscape surroundindex is to provide a metric derived from recorded or simulated spatial audio data and from location
it describes. It would comprise of physical measures and correctional factors.
The index could be used to predict the effect of a sound source to be introduced but would also be
useful for an assessment of the overall auditory spatiality in existing acoustic environments which
don’t necessarily feature designed stationary sound sources.
The soundscape surround index would enhance urban and design practice in tasks considering
designed stationary sound sources such as water features, noise sources moving along the constantroute, different activities such as sitting or passing by along the designed route. It might also
contribute to spatially informed noise masking.
To develop and validate the index a perceptual experiment will follow, featuring the relevant
sound-receiver scenarios. Egocentric factors need to be tracked and controlled during the listening833

experiments, for example by using gyroscopic tracking of head movements (24).
It is expected that the higher surround value and the higher spatial variability of sound sources
increase perceptual values along the arousal dimension (eventful/uneventful).

4. CONCLUDING REMARKSIn this paper, the framework and key factors for developing soundscape surround index have been
discussed.
The proposed soundscape surround index is aimed to describe perceptual quality of acoustic
environment’s spatiality by combining indicators used in concert hall acoustics (listener envelopment,sound source width, sound source distance), environmental acoustics (psychoacoustical loudness),
neuroscience (allocentric and egocentric aspects) and soundscape research (perceptual dimensions).
It is conceived to enhance the practice of introducing sound sources in public spaces and contributeto spatially informed masking of noise sources, restoration and overall enjoyment in public space.
Within the framework of the research project Soundscape Indices, the soundscape surround index
would figure as one of the fuzzy-logic-indices or as a correction factor for a single soundscape index.

ACKNOWLEDGEMENTS
This project has received funding from the European Research Council (ERC) under the European
Union’s Horizon 2020 research and innovation programme (grant agreement No. 740696, project title:
Soundscape Indices - SSID) and from the Croatian Science Foundation (grant agreement No. HRZZIP-2018-01-6308, project title: Audio Technologies in Virtual Reality Systems for Auralization
Applications - AuTAurA).

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