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Perception of Spatial Sound PhenomenaCreated by the Icosahedral Loudspeaker
Florian Wendt, Gerriet K. Sharma, Matthias Frank, Franz Zotter, and Robert Höldrich
Institute of Electronic Music and Acoustics
University of Music and Performing Arts Graz
Inffeldgasse 10/3
8010 Graz, Austria
« AUTHOR TELEPHONE (not for publication): +43 316 389 3520 »
Abstract
The icosahedral loudspeaker (ICO) is able to project strongly focused sound beams
into arbitrary directions. Incorporating artistic experience and psychoacoustic research,
this article presents three listening experiments that provide evidence for a common,
intersubjective perception of spatial sound phenomena created by the ICO. The
experiments are designed on the basis of a hierarchical model of spatio-sonic
phenomena that exhibit increasing complexity, reaching from a single static sound object
to combinations of multiple moving objects. The comparison of the hierarchical model to
sculptural theory reveals similarities that pave the way to new compositional
perspectives in spatial computer music.
1
Introduction
The icosahedral loudspeaker (ICO) is a compact, 20-sided, 20-channel playback
device that uses acoustic algorithms to project sound beams into freely adjustable
directions. Beams are not only freely adjustable in terms of their radiation direction, also
different ones can be blended or their beam width controlled. A metaphoric idea behind
employing such sound beams in music is to "orchestrate" reflecting surfaces, yielding
useful effects in the perceived spatial impression.
Figure 1. ICO in the performance setup at the French Pavilion in Zagreb 2014. Picture by Kristi-jan Smok.
How the effects of the ICO work, depends on the sound material, how the sound
beams are configured and mixed, as well as the room situation.
Over the last six years, two basic staging constellations of the ICO have been proven
to be feasible from an artistic point of view: such in typical rectangular rooms, and such
that utilize a concave setup of reflectors behind the ICO. Staging directly affects the
sound propagation paths in the concert situation and thus the number of discretely
localizable directions.
2
In rectangular staging situations, the ICO is placed near the corners of the room,
allowing the orchestration of at least two side walls, see Figure 2(a). For more complex
situations, a set of concave reflective baffles were placed behind the ICO. This permits
more flexibility in setting the number of reflections, see Figure 2(b). For the
controllability of the spatial effects, the ICO’s setup can be fine-adjusted by ear to the
given environment.
(a) Rectangular (b) Concave
Figure 2. Staging constellations of the ICO.
In such configurations, existing compositions were presented at festivals, e.g.,
Insonic2015 Karlsruhe, Darmstädter Ferienkurse für Neue Musik 2014, International
Computer Music Conference 2012 and venues such as Haus der Kulturen der Welt
Berlin, ZKM Karlsruhe, MUMUTH Graz, Forum Alpbach or French Pavilion Zagreb (see
Figure 1).
After many concerts performed with the ICO, listeners reported perceiving auditory
objects that move away from the ICO and which can have various shapes and layerings,
often denoted as sound sculptures or plastics.
3
The appearance of the term sound "sculpture" or "plastic" could be a starting point,
for the research in this field. The term is in use in compositional practice (Arroyo 2012;
Wishart 1996) and can be found in theoretical writings (Emmerson 2000; Peters 2010;
Ihde 2007). It is used in many places in the history of organized sounds and computer
music: Max Neuhaus "Time Square" piece is considered being a sound sculpture (Wilson
2013), Bill Fontana calls his works sound sculptures (fon 2016), Jonty Harrison writes of
sonic sculptures in connection with sound being diffused (Harrison 1998), and
considering the fact that a well known musical software tool is called "AudioSculpt"
(Aud 2004) this clearly hints at a prevalent idea of sound as sculptural material, and the
composition of electronic music as an act that can be linked to this field within fine arts.
Thus the use in the musical context is oscillating between extended sound objects,
loudspeaker constellations and sound as sculptural material itself, reminding of Edgar
Varèses planes, shapes and zones of intensities (Varèse 2004).
Using the terminology derived from sculptural theory, we still lack a specific
denotation for types of sonic sculptures that best represents their perception. This raises
the question whether such entities are perceived intersubjectively as intended by the
composer. Strictly speaking, an objective evidence about the qualities of perceived
sculptural sound objects can only be accessed systematically through listening tests, but
still they are only seldom utilized (Landy 2007; Sharma et al. 2015).
In order not to exclusively leave the experience of the ICO’s auditory objects to its
small concert audience, considering it being a unique prototype instrument, so far, this
article presents several results from formal listening experiments using the ICO.
Moreover, doing so resolves the question of whether (and which of) the ICO’s auditory
objects and sculptures are intersubjectively perceivable. Finally, the article comes up
with a classification of complexity levels concerning sculptured auditory objects, and
categories of plastic sound objects. These can be seen as composition elements and
maybe provide a basis for common verbalization.
4
Experimental Framework and Setup
A general approach to the spatial perception of sound can be found in
psychoacoustic literature. A comprehensive riview of this issue is delivered Blauert
(1983). More specifically the work of Rakerd and Hartmann (Hartmann 1983; Rakerd
and Hartmann 1985, 1986; Hartmann et al. 1989) examines the localization of sound in
reverberant environments, such as rooms. A fundamental phenomenon thereof is the
precedence effect. It refers to a group of phenomena that are thought to be involved in
resolving competition for perception and localization between temporally delayed
sounds with partial coherence, such as a direct sound and a reflection. Comprehensive
reviews approaching the precedence effect were conducted by Litovsky et al. (1999) and
Brown et al. (2015). In addition, localization effects of the ICO in rooms can be partly
deduced by the work on localization in surrounding loudspeaker arrays at off-center
listening positions by Frank (2013); Stitt (2015). More specific studies dealing with the
properties of auditory objects created by variable directivity in a room are still fairly
young cf. Schmeder (2009); Zotter et al. (2014); Sharma et al. (2014); Zotter and Frank
(2015); Frank et al. (2015); Laitinen et al. (2015).
As considered in this article, sculptural sound objects as artistically designed
entities can consist of several time-variant spatio-spectral elements. Consequently, due
to the combinatorial diversity, an exhaustive investigation appears infeasible. To
overcome this problem of complexity, based on our aural experiences with the ICO, we
propose a hierarchical model of spatio-sonic phenomena consisting of three levels:
• Phenomena of first order consist of a single static percept, i.e. a shape/object, that
is triggered by simple element in the aforementioned sense by time-invariant
spatial projection.These fundamental phenomena are easy to explain or investigate
on the basis of psychoacoustic research. Listening experiment 1 evaluates the
perception of first-order phenomena.
5
• Phenomena of second order consist of time-variant spatial projections with similar
excitation signals. Instances of such projections can be trajectories such as turns,
pendulums, or more complex movements. Their perception could be approached
by the "auditory scene analysis" (Bregman 1994). Listening experiment 2 evaluates
the perception of second-order phenomena.
• Phenomena of third order superimpose several phenomena of first and second
order and lead to complex spatio-sonic objects: sound sculptures as artistic entities.
Listening experiment 3 investigates the discriminability of various sculptural types.
In contrast to experiments that examined localization effects of a virtual realization
of the ICO with simplified settings (Zotter and Frank 2015) the experiments we present
here in this work were conducted in a real room, a 6.8m 7.6m 3m large lecture room
with mean reverberation time of 0.57 s to be specific.
−6
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0
1
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20−2−4x position in m
y po
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P1
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Pa
Pb
Pc
Pd
Pe
Pf
melamin resin absorbers on gypsum board
metal board on gypsum board
ceiling: acoustic tiles
floor: wood
glas
s w
indo
ws
and
gyps
um b
oard
glas
s w
indo
ws
Figure 3. Room layout of listening experiment with indicated directivity pattern of ICO.
The ICO was placed near the corners of the room, which corresponds to a
rectangular performance situation. To investigate the influence of the listening position
6
on spatial sound phenomena, the subjects performed the tests at different listening
positions. Figure 3 shows the layout of the room with indicated positions for ICO and
listeners (experiment 1, 2: P1 - P2, experiment 3: Pa - Pf). In addition, the materials of the
6 main boundary surfaces are given and a the directivity pattern of a third order sound
beam is indicated.
Listening Experiment 1
The first listening experiment evaluates the perception of static sound beams with
various azimuth angles.
Sound projection was achieved by 4 different spherical beams of third order at
azimuth angles of 0°, 90°, 180°, and 235°(delineated in Figure 4). Sounds 1 . . . 4 consisted
of a pink noise burst with combinations of two different durations for the onset and
release time (tshort 10ms, tlong 500ms). Each of these sounds is represented by a
marker whose outline indicates the appropriate envelope, i.e. sound 2 exhibiting a slow
onset and short release is represented by \, see Figure 4. Sound 5 (represented by +)
consisted of a sequence of irregular bursts and sound 6 (represented by a) was a chain of
regular fine grains. For each run, subjects were seated at one of the two listening
positions (P1 and P2), facing the ICO with the possibility to move their heads. The
listeners were asked to specify azimuth angle and distance of the auditory objects within
an ICO-centric coordinate system. Seven selectable stimuli, thereof six belonging to
different sounds with random-selected direction, were shown together on one screen to
allow comparative responses. The seventh stimulus was a random-selected repetition of
one of the other six on screen. With 4 such screens per listening position, each of the 15
participants gave 2 . . . 4 responses per stimulus, in total all listeners gave 420 responses
per listening position, as shown in Figure 4. Sounds are coded as marker shapes and
beam direction as marker colors (see Figure 5 for color coding of beam direction).
7
Results
The distinct coloration of the marker cloud determined by x and y values in Figure 4
indicates already the intersubjective perception of different projections. This is
supported by the two-dimensional median values shown in Figure 5.
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(a) Listening position P1
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sound 1sound 2sound 3sound 4sound 5sound 6
(b) Listening position P2
Figure 4. Collected answers for both listening positions.
A pairwise analysis of variance (ANOVA) of all azimuth angles for each beam
direction confirms this assumption. For both positions, all 6 sounds yield at least to 3
different directions (p $ 0.05), whereas for some conditions, neighboring directions are
perceived to be statistically identical (p % 0.95). According to the ANOVA for P1, beam
directions 0` and 90` tend to coincide, whereas for P2 the directions 180` and 235
`
coincide.
The distance of auditory objects to the ICO is the second parameter analyzed in the
experiment. Figure 6(a) shows the 95% confidence intervals of distance for all
conditions. The perceived distance of sounds 1 . . . 4 (S1 . . . S4) depends on their onset
duration. (ANOVA: pS1S2©S3S4 0.0017). Similarly, sound 6 (S6) is localized closer to the
8
ICO than sound 5 (S5). This can be explained by a higher proportion of transient signal
components within S6. Both dependencies support findings on the existence of the
precedence effect for transient signals (Hartmann 1983; Rakerd and Hartmann 1985).
Moreover we see that auditory objects perceived at P2 are closer than for P1 (p 0.002).
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(a) Listening position P1
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sound 1sound 2sound 3sound 4sound 5sound 6
(b) Listening position P2
Figure 5. Indicated beam directions and median values of all subjects.
A combined presentation of azimuth angle and distance as dependency of the onset
(long onset: S1-S2; short onset: S3-S4) can be found in Figure 6(b), where the angular
distribution of the median distance (including interquartile ranges) is shown for
listening position 1. Except for the zone behind the ICO, S3 and S4 are localized being
closer to the ICO.
These results show that the ICO is able to trigger single auditory objects in space by
using spherical beamforming algorithms. Depending on its staging and the listening
position, different zones around the ICO can be "orchestrated". The perceived distance,
and therefore reachable spatial extent of the auditory scene, is signal-dependent. We
have shown that the more transient signals, i.e. signals with short onset durations, tend
to be localized closer to the ICO than smooth signals.
9
1 2 3 4 5 60.4
0.6
0.8
1
1.2
1.4
1.6
sound
dist
ance
in m
P1P2
(a) Median and 95% confidence interval of per-ceived distance over all beam directions.
(b) Median and interquartile range for stimuli S1-S2(long onset) and S3-S4 (short onset) at P2, equallysplit on 12 circular segments around the IKO.
Figure 6. Perceived distance of auditory objects to the ICO.
Listening Experiment 2
The second listening experiment evaluates the perception of time-variant
trajectories of sound beams.
The creation of time-variant spatial projections is a further means of expression and
hence a further step towards the orchestration of space. A narrow beam moving along
the horizontal plane of the ICO is a rather simple realization thereof. Three different
realizations of this trajectory have been investigated in second listening experiment: 2
opposed half turns (clockwise/cw and counterclockwise/ccw) and a full turn, all
starting at 90` (indicated in Figure 7 and 9),
Each trajectory lasted 5 s and the subjects were asked to adjust 10 markers to the
perceived location in successive half-second intervals during playback. Markers were
successively flashing in the associated playback time instant and could be moved by
10
mouse on a graphical interface showing the layout of the test setup. Playback could be
repeated until listeners were satisfied with the match between marker placement and
what they heard. Room and positioning of the ICO and listener remained the same as
for listening experiment 1. Due to the relation between onset duration and perceived
distance, two variants of pink noise bursts were tested. Sound 1 consisted of uniform
pink noise representing an infinite onset duration. Sound 2 consisted of 200ms pink
noise bursts, each one with a 10ms linear fade-in and fade-out and 100ms of silence in
between. Additionally, a sequence of irregular bursts (denoted as sound 3) and a chain
of fine regular grains (denoted as sound 4), known from the previous experiment, were
tested. The experiment was carried out using 15 listeners.
Results
For representation of the collected data, a two-dimensional plot for each time step
presents its mean value within the 95% confidence area (see Figure 7).
1 2 3 40.4
0.6
0.8
1
1.2
1.4
1.6
sound
dist
ance
in m
P1P2
Figure 7. 360` turn; mean and 95% confidencearea for sound 1 and sound 2 at listening posi-tion P2.
Figure 8. Mean and 95% confidence interval ofperceived distance over all trajectories.
11
The circular trajectory is reflected by almost perfect circles around the ICO, obtained
by the mean values of the collected data (see Figure 7). Furthermore, the 95% confidence
areas are almost equally spread around the ICO, which is in contrast to the findings from
experiment 1, where localization could not fully encircle the ICO. Not only the full turn
delivers smooth results, also the half turn trajectories shown in Figure 9 impress and
reflect the idea of the spatial movement.
(a) 180` turn counterclockwise (b) 180
` turn clockwise
Figure 9. Mean and 95% confidence area for sound 1 and sound 2 at listening position P2
In contrast to the almost closed trajectory of the full turn, we see that the
progression of half turns covers more than the expected semi circle. This widening could
be caused by the slower angular speed (half as fast as for the full turn) of the spherical
beam or by a psychoacoustic phenomenon termed auditory representational momentum
(Getzmann and Lewald 2007), which describes the displacement of the final position of a
moving sound source in the direction of motion.
Comparing the perceived distance of auditory objects, Figure 7 and 9 do not show a
signal dependency similar to experiment 1. This is in contrast listening position 1
12
(pP1 $ 0.001, see Figure 8). Furthermore, the distances perceived to be smaller at P2 than
at P1 is not as pronounced in this experiment (pS1...S4 0.200). After removing sound 1,
we reattain statistical significance (pS2...S4 $ 0.001) as in experiment 1.
Finally, a direct comparison of all collected data of experiment 1 and 2 exhibits that
the perception of trajectories cannot be fully explained by extrapolating the perception
of static sound beams. It seems that listeners try to understand the intended motion and
thus get a more comprehensive percept. This process of grouping sensory data into
separate mental representations, called "auditory streams", has been named "auditory
scene analysis" (Bregman 1994).
Listening Experiment 3
The third listening experiment evaluates the discrimination of various spatio-sonic
objects, in this context denoted as sound sculptures.
The concept of organizing spatio-sonic phenomena hierarchically from first to third
order, and the idea of building different perceivable sculptural categories upon this
scheme yields the stimuli set of this experiment. Three different categories of stimuli
were created, each representing a sculptural shape. The material used for the creation of
the 12 stimuli (3 to 5 per category) was composed from four different sound origins. The
idea was to use easy recognizable idioms, close to the stimuli from preceding
experiments, but with a more musical sound as to bridge the laboratory situation and
experiences with more performative situations. The stimulus composition and grouping
is shown in Table 1. Each of the stimuli had a duration of 30 seconds. To test the ability
of naive discriminability of sculptural categories, neither the hierarchical organization
scheme, nor the composer’s categories were known to the listeners. Hereby, the potential
side effect of interpreted terminology is eliminated.
13
stimulus element 1 element 2sound trajectory sound trajectory
AI bn 180° staticAII lfd 180° staticAIII sms 0° staticAIV lfd low-cut 236 Hz 210° static sms 210° staticBI bn ccw 237°/s rotationBII lfd low-cut 236 Hz ccw 140°/s rotationBIII sms cw 180°/s rotationBIV chog ccw 270°/s rotationBV lfd ccw 120°/s rotationCI bn ccw 180°/s rotation bn high-cut 2426 Hz cw 180°/s rotationCII bn 305° static bn starting after 15 s cw 180°/s rotationCIII lfd low-cut 236 Hz ccw 180°/s rotation sms cw 180°/s rotation
Table 1. Composition of stimuli from elements: constant brownian noise (bn), low-mid-frequencydrone (lfd), multi-layered stretched metal sound with long on- and decay (sms), and chain of fineregular grains (chog). All angels are in the horizontal plane around the ICO.
Discriminability of perceived sculptural shapes was tested by a three-alternative
forced-choice (oddity) method (Kingdom and Prins 2010). Each stimulus triplet
consisted of two stimuli of one category and one stimulus of another and were presented
subsequently in a common listening session. The listeners were asked to write down the
stimulus that differs from the others by its spatial appearance. As a control condition,
triplet 3 comprised stimuli of solely one category. Table 2 shows all tested triplets. The
differing stimulus within each triplet is highlighted in bold.
triplet 1 2 3 4 5 6stimulus 1 AI AIII BII BIV CII CIII
stimulus 2 BI AIV BV BI BIII CI
stimulus 3 AII CI BIII AII BV AII
Table 2. Permutation of stimuli in triplets.
The listening session was conducted twice with 6 subjects each. To monitor possible
impacts of the listening position, the listener were spread within the room (Pa - Pb, see
Figure 3) and changed their position after the first run. Additionally 5 out of 6 subjects
evaluated the same triplets using mono playback over headphones. Since there is no
14
spatial difference within the triplets, subjects were requested to discriminate them on
arbitrary characteristics.
Results
Response frequencies for listening experiment 3 are shown in Table 3.
triplet 1 2 3 4 5 6stimulus A 0 1 0 0 10 0stimulus B 12 0 5 0 2 0stimulus C 0 11 7 12 0 12
triplet 1 2 3 4 5 6stimulus A 0 0 0 4 1 1stimulus B 0 0 0 1 4 4stimulus C 5 5 5 0 0 0
Table 3. Frequency of each stimulus as perceived most different in each stimulus triplet using theICO (left) and mono playback over headphones (right). Bold numbers mark the stimulus that wasdesigned to have a different sculptural shape.
For all test triplets played back by the ICO listeners agreed (' 83%) and detected the
stimulus with different sculptural shape in accordance to the composers intention.
Control triplet 3 did not yield to such a clear agreement and listeners determined 2
stimuli to be perceived differently. For mono playback, similar agreements across all
triplets are found (' 90%), but this time without accordance to the intended different
sculptural shape. Only triplet 2 yields an agreement across playback techniques.
Overall the results are astonishing, and it is shown that an intersubjective
perception of complex spatio-sonic objects exists. Although none of the listeners was
aware of hierarchical spatio-sonic phenomena or the composer’s categories, they almost
fully agreed.
As it is far from evident how to integrate a term from another discipline into music,
but an excellent match under the given results, the subsequent section provides insight
about denomination and delineation of the three categories of the composer.
15
Matter and Space
If we examine some of the historical body-space-relations that have been
distinguished in sculpturing theory (Klant 2003), several relationships between matter
and space have been defined that could be useful in spatialized computer music. What
we can axiomatically say is that "a body" or "matter" is opposed to an infinite space
(Krämer 2011). Both exist in a reciprocal relationship. We can observe that historically
the sculptured body volume opens step by step towards space, trying to invade it until
finally almost dissolving into it. That means that space is not just a surrounding shell or
an envelope but since modernity it is an active co-creator of sculpture. Without empirical
musicological study we might assume that we have a similar idea of spatial sound
composition especially in the cause of the last 20 years in electronic music, when space
became an equitable parameter to e.g. timbre or rhythm (Smalley 2007; Nyström 2013;
Bayle 2007).
In sculpture theory we find two main categories of matter-space relations (Krämer
2011):
1. Kernel-Plastic/Body Plastic1 with the attribute: space superseding 2.
2. Spatial Plastic3 with the attributes: space encompassing or space binding4.
And a compositional strategy that is often called:
3. Kernel-Shell-Concept5 with the attribute: space constituting 6.
1Translated from German by the authors: Kernplastik/Körperplastik: e.g. Auguste Rodin, The Thinker, 19022raumverdrängend3Raumplastik: e.g. Naum Gabo, Linear Construction in Space No.2 (conceived 1949, exhibited version executed
1959-60)4raumumfassend, raumbindend5Kern-Schale-Prinzip: Henry Moore, Mother and Child: Egg Form, 1977, LH 7176raumbildend
16
The latter is a concept creating space by establishing a tension between two entities,
e.g. a focused inside and environmental coordinates at the same time.
The above categories have the advantage that they integrate the handling of space
systematically and have been developed and proven in an artistic practice over
centuries. Moreover, they were confirmed by the perceived discrimination in the
blind-category experiment 3, the results of which match the composer’s intended
sculptural stimuli categories, cf. Table 1:
Kernel-Plastic/Body Plastic: AI...IV ,
Spatial Plastic: BI...V ,
Kernel-Shell-Concept: CI...III .
Conclusion
In this paper we could successfully provide a comprehensive experimental
evaluation of sound phenomena evoked by the ICO. A hierarchical model of spatio-sonic
phenomena was proposed and validated by extensive listening experiments.
Listening experiment 1 examined single static percepts evoked by spatial
projections of the ICO. Depending on the staging of the ICO, different zones around
could be orchestrated. The results revealed where knowledge from psychoacoustic
research is applicable to auditory objects, e.g. distance to the ICO is highly onset- and
envelope-dependent. This relation is known from studies on the precedence effect
(Litovsky et al. 1999; Brown et al. 2015).
Listening experiment 2 examines more complex phenomena evoked by time-variant
spatial projections. The perception of such trajectories is approached by a more
comprehensive mental representation that is known from the auditory scene analysis
(Bregman 1994).
17
Listening experiment 3 investigated the superposition of several phenomena of
static and dynamic nature. The results remarkably support the composer’s terminology
derived from sculptural theory, which delivers a compositional basis and a classification
of complex auditory objects. We could prove that spatio-sonic miniatures using the
categories of the sculptural theory could be intersubjectively distinguished into the
categories intended by the composition. Three categories have been proposed, each of
them describing a matter-space relation, which can be translated into musical context.
Our work was funded by the Austrian Science Fund (FWF) project nr. AR 328-G21,
Orchestrating Space by Icosahedral Loudspeaker.
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