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

wendt@iem.at

« AUTHOR TELEPHONE (not for publication): +43 316 389 3520 »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.

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floor: wood

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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

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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).

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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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sound 1sound 2sound 3sound 4sound 5sound 6

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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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(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.

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1 2 3 4 5 60.4

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sound

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(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

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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

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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

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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).

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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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