“Natural Selection: a SteThoscopic Amphibious
Installation”
Mr T.J.Davis, BSc,
MA.
Sonic Arts Research
Centre, Queen’s University Belfast, UK
e-mail:
tdavis01@qub.ac.uk
Dr P.Rebelo, BMus,
MMus, PhD.
Sonic Arts Research
Centre, Queen’s University Belfast, UK
e-mail:
P.Rebelo@qub.ac.uk
Abstract
This paper discusses emergence as a
complex behaviour in the sound domain and presents a design strategy that was
used in the creation of the sound installation Natural Selection to encourage the perception of sonic emergence.
The interactions in Natural Selection
are based on an algorithm derived from an innately sonic emergent ecological
system found in nature, that of mating choices by female frogs within a calling
male frog chorus. This paper outlines the design and implementation of the
installation and describes the research behind its design, most notably the
notion of embodiment within a sonic environment and its importance to the
perception of sonic emergence.
1. Introduction
A fascination for models derived from natural organisms has
a long history of influence within the arts. In recent years, there have been
numerous implementations of algorithmic systems that model the phenomena of
emergent behaviour. In an attempt to reproduce the necessary conditions for
emergent behaviour to occur, these systems employ bottom-up strategies through
the definitions of simple rules for the behaviour of local agents.
In the context of sound design, a potential problem with
many of these systems is that the algorithms they are based on do not derive
from sound but typically from a system that exhibits perceived emergence
through the application of graphical (Boids) [13] or evolutionary (Genetic
Algorithm)[1] models. The implementation of these algorithms to a sound world is
then based on a more or less arbitrary mapping procedure between a graphical
and a sonic model.
The installation described here attempts to sidestep issues
pertaining to the mapping of graphical to sonic emergent systems by designing
the work through a carefully selected natively sonic emergent environment with
special consideration given to the notion that the phenomenon of emergence is
manifest in the dynamic listening system of a perceiver embodied within the
environment.
We have argued elsewhere [3] that the modeling of emergence
is linked to how we perceive it. In the context of emergent sound we feel it is
important to model behaviour that features intrinsic aspects of emergence in
the acoustic domain, in contrast to arbitrary mapping between domains.
'In
the same way that behaviours such as flocking are better understood from a
distance, we argue that sonic emergence can only be perceived when considering
the listener as an agent of that very behaviour. The ear does not act as a
stethoscope, listening in from the outside, but rather as a participant in a
space in which it takes the role of one, of many agents.' (Davis and Rebelo)
[3]
2. Ecological Model
This
installation is based on the listening ecology found in the model of female
frog mate selection within a calling male frog chorus. Through a study of
current research into the mating of frogs we have found that female frogs
select a mate from within the male frog chorus according to information
afforded to them through the temporal and spectral characteristics of their
calls [5]. Frogs have a complex auditory system that is designed to help them
recognise and respond to calls of their own species. They have a variety of
different calls for such situations as mating, distress, release, warning, rain
and definition of territory. The calls for different species are distinct in
temporal and spectral characteristics. This helps the frogs recognise calls of
their own species within a dense chorus [11].
The mating
calls of frog species under study can be characterised by four main parameters:
dominant call frequency (the frequency with the highest spectral intensity),
pulse rate, call rate and call duration. With dominant call frequency relating
to frog size, pulse rate relating to the ambient temperature of the
environment, for example the water temperature, and call rate and call duration
relating to the preference of each individual animal [8]. The characteristics
that were found to have the most effect on female choice of mate, were dominant
call frequency and call length [8]. It was found females preferred longer lower
calls, the pitch of the calls having a strong correlation to the body size of
the males and thus their
successfulness in mating. It has been found by Wollerman that a “female frog
could detect a single male’s calls mixed with the sounds of a chorus when the
intensity of the calls was equal to that of the chorus noise” [14]. Given that
there is a 6dB fall off in the intensity of the signal with each doubling of
distance away this means that for an average spaced chorus (0.08 males per
metre2 [14]), the female can only distinctly ‘hear’ between three
and five males at any one time
This
installation is based on a simple model of the interactions found between the
male frogs in the chorus with the user taking on the role of the female. Each
male frog is represented by a mechanical ‘sound object’, which consists of a
resonator excited by a motor. These sound objects have been designed to
represent the four main characteristics of the male frogs call: the size of the resonator being linked to
the dominant call frequency of the frog; the pulse rate is variable dependent
on lighting conditions (each “frog” is fitted with a light sensor the output of
which is affected by the user’s location causing disturbances to the light
source); the call duration is fixed but different for each frog.
2.1.
The Algorithm
The rules that
govern the local interactions between the males are based on research that
suggests that female frogs prefer 'leading males', i.e. males that are
perceived by the female to be calling in front of the other frogs. [7]. To produce this effect the male frogs listen
to their nearest neighbours (the others being masked by the noise of the
chorus) and consciously alter their call rate so as not to coincide with other
males. To model this interaction each “frog” has a set call rate which is
modified through the application of an algorithm based on a resettable
oscillator. Each of the frogs is wired to its two nearest neighbours to
resemble the listening conditions in the wild [11]. If a “frog” 'hears' a
neighbour making its call, by virtue of receiving a current down a wire it
resets its oscillator to 1 thus inhibiting its own call and lengthening or
shortening its own call rate. The local interaction between the male frogs is
the algorithmic structure that has been set up to exhibit emergent results. The
temporal form of the piece is not only governed by the frogs own set call rate
but by the interactions of the frogs with their nearest neighbours.
'Form is a dynamic
process taking place at the micro, meso and macro levels. When properties not
explicitly determined by specific parameters emerge at different levels, we
witness a pattern-formation process. In this case form is not defined by the algorithmic
parameters of the piece but results from the interaction among its sonic
elements. in a general sense higher level forms or behaviors resulting from
interaction of two or more systems' [9]
Fig 1 Resetable Oscillator Model
2.2 Design Considerations
This
installation attempts to promote a ‘learning through interested interaction
approach’. [12], thus there are no instructions or rules set out for the user
to follow and there are no prescribed levels or modes of interaction. If this
approach is to work it is important that the installation compels the user to
interact through generating interest in the sound objects, it is also important
that the user feels free to approach these sound objects and that the spatial
setup of the installation allows for a liberated mode of enquiry. Since there
is no statement of artistic intention for the piece the mode of communication
between the sound objects has to become apparent through interaction and
investigation. To make this
communication explicit a network of wires suspended between the objects
suggests links and interactions within the installation space. This suspension is
well-over head height and suggesting an enclosed but accessible space,
hopefully encouraging visitors to “enter” the space inhabited by the sound
objects.
The sound
objects were first laid out in a circular form, [Fig 2], so as connections to
their two nearest neighbours is easily obtained. The order of the sound objects
is then shuffled [Fig 3] to generate a more non-linear spatial structure, so
that it is now possible to stand within the proximity of a small cluster of
sound objects, or to move to a more open space dominated by fewer objects. We
will argue later that this facility for the user to define their level of
engagement and embodiment within the piece is important in leading the ear in
the perception of emergence.
