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of cogs dropped onto a table, is the synchronous
and causal relationship between the parts.
This Newtonian correctness can be extended
to acoustic models like vehicle engines, best
suited to waveguide methods, or abstracted to a
control level, such as for the bouncing ball that
loses energy. Application at the control level
should be of particular note to designers of audio
systems linked to game physics engines as this
represents the perfect interface of simple classi-
cal physical models (elasticity, mass, damping,
friction, rolling and other common behaviours)
to audio DSP. Ideally, such parameters should be
exposed at the sub-millisecond refresh rate or at
audio block frequency.
At a design level there are many advantages
to componentised construction. Sound designers
have already dreamed of modular systems in
which they can create objects by combination.
The software systems Cordis Anima , Modalsys ,
and Mosaic were the musical forerunners of newer
systems that allow plug and play modelling. An
advantage, explored in my work constructing game
objects like guns, rocket launchers, and vehicles,
is that one can obtain unexpected behaviours for
free. For example, once a weapon body is con-
structed then reload sounds are available at no
extra computational cost. Likewise construction
of a car chassis and bodywork to get the correct
engine filter implies the availability of door sounds
with little further work.
Newtonian simplifications need more attention
once we enter a real game scenario. Causality is
often represented (in duplicate) at a higher level
of abstraction in games systems. Above the phys-
ics engine, the collision or action system often
maintains a causal trace, an actor-instigator chain,
in order to make logical gameplay decisions.
The “one hand clapping” problem (in which we
have to ask which of two objects, both of which
are mutually excitor and resonator, such as two
colliding billiard balls, is the source of sound) is
a false dilemma imposed by the faulty logic of
non-relativistic representation. Unless one object
is statically coupled to a significant radiator, for the
instantaneous sound excitation we should consider
symmetrically only the respective structures and
the velocity (total kinetic energy) with which they
are brought together.
Good behaviour for structural
and causal systems
Perhaps a way to appreciate the importance of
proper structure and causality is to consider a case
where it is not observed and the consequences.
Often we play with wild settings of a synthesiser
and discover a wonderful result, a subversion of a
tuba that suddenly sounds exactly like a motorbike.
I sometimes call these “ islands ”, because they are
disconnected in behavioural timbre space.
This can be seen from two viewpoints, physical
and psychoacoustic.
Let's concentrate on the psychoacoustic inter-
pretation first, where we have activated a higher
level recognition schema, albeit a false one. This
may be due to a spectral match, an associative
(metaphorical or similar match) or a partial
behavioural (mechanistic) match. Whatever the
basis of the match, once identified and without
further information, the tuba is a motorbike until
we know more. In the analysis of Vicario (2001)
these mismatches are interpreted in a Kantian
phenomenological sense. He describes a typical
causal identification error; the sound of rain on
a window that turns out to be branches rattling
against the glass.
Partial behavioural matches are intriguing as
they form one of the pillars of traditional sound
design, shaking an umbrella for bird sounds,
crunching vegetables to make the sounds of break-
ing bones. An object that displays some subset
of the behaviour of another can often be coerced
to produce signals easily mistaken for the target,
especially when supplemented with confirming
visual stimuli. The trick here, for the sound de-
signer, is to identify those behavioural parameters
which might exaggerate or counterpoint a desired
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