Collisions and Other Excitations
The category of idiophonics is important in game sound. The majority of casual
sounds are rigid body collisions for simple material objects. Most inventory
items are given a “dropped” sound; footsteps are a collision and crushing pro-
cess (although their complexity means we will study them separately later on).
Movers like doors, train carriages, and boxes are collisions or frictions between
solid structures made of wood or metal. Rolling will be seen as repeated small
collisions, while the noise of stick-slip friction is a kind of small-scale tangential
collision if we wish to look at it that way. Acoustic resonance and the effect of
366
Idiophonics
blowing are not considered here, mainly because in the absence of wind it’s a
human-generated event restricted to musical instruments. However, that said,
steam and air movement in pipes, locomotives, and other vehicles is a topic of
later chapters under the heading “machines.”
Forces
With the majority of idiophonic events being collisions, we will be thinking of
kinetic energy as the source. It may have been given to a body by falling, or by
being pushed by some other actor. We don’t need to think about the primary
instigator (and the preceding energy system), only to know the mass and veloc-
ity of an object at the time we want to create its sound. This is an area where
gravity comes into play. It influences the sound of dragged or pushed objects,
the swinging of chains and ropes, and the decaying motion of a bouncing object.
We also wish to understand friction and fluid viscosity as damping forces. In
real-time physics engines they are used to limit the movement of objects, but
they play an equal role in sound objects to determine decay times and spectral
evolution when things interact.
The Practicals
It would be nice to have room in this textbook to consider a whole range of
processes, especially fragmentation which is a fascinating subject, but we will
limit ourselves to the following studies which cover a useful range of concepts.
•
A telephone bell, as an extended study of a struck object.
•
Bouncing ball, as a decaying series of impacts.
•
Creaking door sound, as a study of friction.
•
Rolling tin can, as regular and irregular movement of an object.
•
Twanging ruler, as a study of plucking, nonlinearity, and discontinuity.
References
Vicario, G.B., Rocchesso, D., Fernstr¨
om, M., and Tekniska Hoegskolan, K.
(2001). “The sounding object.”
<
http://www.soundobject.org
>
. Project jointly
by University of Udine Dipartimento di Scienze Filosofiche e Storico-Sociali,
University of Verona Dipartimento di Informatica, University of Limerick,
Speech, Music, and Hearing centre KTH-Stockholm.
29
Practical 6
Telephone Bell
Aims
In this practical we are going to create the sound of an old style telephone
bell from the era of 1930–1960. As usual we will follow the design pattern that
should by now be familiar. First we will analyse the sound and the nature of
the production mechanism, thinking about the components, structures, and
behaviours. Then we will propose a model, using knowledge about the form
and physical principles of the sound. Next we will use the model, to deter-
mine one or more synthesis methods, and finally we will choose DSP tricks to
approximate the signals we hope to hear.
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