Physical Modelling for Sound Synthesis - Game Sound Technology and Player Interaction: Concepts and Developments - page 350

of these modes, and their damping) from this
recording. To enable impact location-dependent
sound synthesis, that is, to realize the subtle dif-
ferences in sound due to an object being struck
on different locations, this process is repeated at
many points on the object's surface. In order to
model friction, a measure of surface roughness is
determined robotically and this is then used for
generating continuous contact sounds.
In the same year, Doel and Pai along with Paul
G. Kry, who specializes in computer graphics
and physics based animations, published Foley-
Automatic: Physically-based Sound Effects for
Interactive Simulation and Animation (Doel, Kry,
& Pai, 2001). In this project the process of sound
synthesis is linked to a dynamic simulation (phys-
ics engine) that allows user interaction. Sound is
generated using modal synthesis, as described in
The Sounds of Physical Shapes (Doel & Pai, 1998),
and modal data is determined as in Scanning Physi-
cal Interaction Behavior of 3D Objects (Pai et al.,
2001). In addition to being linked to a dynamic
simulation, this project is the first to synthesize
continuous contact sounds, that is, rolling and
scraping, in a real-time simulation. The technique
used in this paper could be implemented with any
multi-body dynamic simulation method as the
requisite information, such as object collision
forces, normal forces during continuous contact,
and the position of objects relative to each other,
if not directly available, can be easily computed
from the object velocities and positions which are
available. However, the authors note that methods
which provide smooth surface models are prefer-
able to polyhedral approximations and methods
which model rolling and sliding are also desirable.
In order to create realistic audio, object inter-
actions are simulated at an audio rate which is
much higher than the video rate. The modelling of
object interactions must be carried out quickly and
the authors note a stochastic model that involves
some random element is often appropriate. When
objects make contact, a force signal is calculated
at the audio rate and this is used to drive modal

synthesis. In the case of a single impact, it was
found that the shape of the force profile was not
perceptually important but that the duration of
the force gave a good feel for the hardness of the
objects in contact. For collisions involving hard
surfaces, a burst of impulses at the dominant
modal frequencies of the colliding objects was
found to convincingly produce the sound due to
micro-collisions. To create a scraping impulse,
the phonograph model as described earlier in the
work of Takala and Hahn (1992) is recalled. To
create such a surface profile, noise is filtered to
give a 1 / f shape and a fractal dimension variable is
considered to represent the roughness in the profile
produced. In the case of rolling, it is theorized
that the impulse is similar to scraping but more
“drawn out in time” (Doel et al., 2001, p. 541) and
this is realized by applying a low-pass filter to the
scraping impulse. Suspecting a stronger coupling
between objects during rolling than scraping, it
was found that enhancing the spectrum at the
resonant frequencies of the objects involved gave
better rolling sounds at a higher computational
cost. All of the ideas summarized here have been
implemented in the application FoleyAutomatic
which can “automatically generate high-quality
realistic contact sounds” from physical informa-
tion in order to “increase the feeling of realism
and immersion in interactive simulations” (Doel
et al., 2001, p.543). See Figure 1.
Another, paper published in 2001 by James F.
O'Brien, Perry Cook and Georg Essl, focused on
calculating the sound heard due to a vibrating
object at a relative location. Synthesizing Sounds
from Physically Based Motion (O'Brien, Cook,
& Essl, 2001) describes how to calculate the air
pressure at two points in an environment (for
stereo sound) due to the surface vibrations of an
object. Considering Huygen's principle, the delay
and attenuation due to sound propagation is cal-
culated. However, the technique requires a de-
formable body simulator that calculates the surface
vibrations of objects at audible frequencies and
is therefore not compatible with most real-time