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Perception of Spatial Sound Phenomena Created by the ... ... intersubjective perception of spatial sound phenomena created by the ICO. The experiments are designed on the basis of

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    « AUTHOR TELEPHONE (not for publication): +43 316 389 3520 »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