Importantly, electroacoustic music is not limited to the
source-environment model. Then too, there are
profound differences in the sound material. This is
especially important for two reasons: spatial percepts
are shaped in part by the content of the sound sources
[10, 11] and electroacoustic composers are not limited to
pre-existent acoustic sources. Not only do
electroacoustic composers have the freedom to design
sounds that specifically support spatial effects, but they
can also explore acoustic realms that break down
ecological validity!
2.2 Spatial Attributes and Scene Analysis
In a particularly useful discussion, Rumsey [15]
considers spatial sound attributes within the framework
of auditory scene analysis [3]. He states that spatial
attributes "should be unambiguous and preferably
unidimensional (in other words, they should represent a
single perceptual construct)." Spatial attributes have to
do with the tangibly three-dimensional aspects of sound
including such properties as width and distance. Most
importantly Rumsey' separates spatial attributes from
attributes of spaces, that is, from the properties of the
rooms (or other environments). The attributes of spaces
have traditionally included properties such as
reverberance and liveness. The confluence of these
categories had been unexamined in the subjective
evaluation of concert halls and audio reproduction
systems where sound sources are assumed to be
contained within environments, and, in fact, Rumsey's
discussion is itself directed toward such typical audio
reproduction settings. This confluence of categories is
essentially a misalignment of physical and perceptual
acoustics. So, for example, if we shift our orientation
completely to the side of the listener's auditory
organization and observe that the indirect sound of early
reflections influences the perception of the auditory
source, then that indirect sound is essentially part of the
auditory source's signal just as much as the direct sound.
And, from this point on we must be particularly clear
about the separation between the acoustic source signal
(with all of its acoustic constituents) and the source's
perceived image (with all of its perceptual spatial
attributes). The separation of the categories is
particularly useful in electroacoustic music where the
source-in-environment model is only one of many%
possible spatial treatments.
Having achieved a clear separation of spatial
attributes from other properties of sound in space,
Rumsey goes on to examine spatial attributes within the
context of auditory scene analysis. In creating an
organizational framework for evaluating the kind of
sound reproduction that interests him, he proposes four
levels of organization that are common in the experience
of listening to recorded music: source, ensemble, room
and scene. These labels are meant to be more general
categories of nested organization than types of acoustic
sources. How these four levels of organization interact
with the spatial attribute of width is illustrated in Figure
1. At the lowest level of organization an individual
source has width. At a next higher level of organization
1 All subsequent references to Rumsey relate to [15].
and grouping, an ensemble of sources has ensemble
width. Such a grouping depends on the particulars of the
scene and, for example, might variously be composed of
the violin section, the string section or the whole
orchestra. At the next highest level of organization for
recorded music we can speak of room width and beyond
that the width of the entire auditory scene. This scenebased approach isolates the 'what' from the 'where' and
removes the conceptual confusion inherent in
considering spatial attributes without addressing auditory
grouping.
We can now propose spatial terminology to be
applied to the relevant nested levels of auditory
organization. Rumsey proposes that all spatial attributes
be limited to five, three 'dimensional' attributes and two
'immersive' attributes. His three dimensional attributes--width, distance and depth---can be instanced by an
individual sound source, an ensemble, room or scene
(although it is unlikely that an individual sound source
Source Width
Ensemble Width
Room Width
Scene
Figure 1. Spatial attributes of nested levels within an
auditory scene (after Rumsey [15]).
could instance depth). Rumsey mentions that he is
omitting height, we assume because his focus is on
reproduction systems without elevation, but he also
omits any other aspect of direction (even though he has
contributed to its discussion in other research [11,16]).
We will add direction to our list of spatial attributes2
Rumsey's category of immersive spatial attributes
covers the domain of auditory spatial perception
described by such interrelated terms as spatial
impression, spaciousness and listener envelopment.
Within this framework of scene analysis, Rumsey
recommends the adoption of only two terms:
envelopment and presence. He proposes that the term
'envelopment' be applied to the experience of being
surrounded by sound either both from multiple sound
sources and from a diffuse field such as reverberation.
These are called 'source-related' and 'environmental'
envelopment respectively. Rumsey has newly proposed
that the term 'presence' be applied to the "sense of being
inside of an (enclosed) space." We will adopt all of
Rumsey's terminology (with our small addition) for
consistency's sake and defer judgment on the use of the
term 'presence' to future literature. Our resulting
terminology can be summarized as follows:
2 We will treat direction as a single, unambiguous perceptual attribute
even though its geometrical representation requires two dimensions.
The dominant role of the left-right axis in the play of perceptual
grouping is considered later on.
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