Perception of Spatial Sound Phenomena
Created 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
[email protected]
« AUTHOR TELEPHONE (not for publication): +43 316 389 3520 »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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