Biomedical Engineering Reference
In-Depth Information
features of sound reproduction, an issue that raises questions of 3D audio, a research
and application field whose specific links to the rendering of walking sounds will be
treated later in Sect.
12.2.3
.
High-level cues are intuitively not too difficult to be rendered, as soon as a suffi-
ciently large collection of data is put available for inferring a convenient statistical
model for the walking cycle of a homogeneous population. More interesting are
the constraints among instances of such cycles taking place in collective contexts,
giving rise to
entrainment
effects [
104
]: for these effects the exact role of sound
is currently unknown, in spite of a conspicuous number of works dealing with the
relationships existing between gait cycle and rhythmic (especially musical/dance)
sonic patterns [
93
].
Low-level cues represent an even more challenging design issue. By bringing
information on the interactions taking place during the contact between the foot and
the ground, they mainly report about the materials the floor and the shoes are made
of. For this reason, the accuracy of their reproduction depends on the ability to embed
this information within a sound synthesis model. Normally, these models must keep
parametric control of the temporal as well as spectral features of the synthesis: as
we will see in Sect.
12.2.2.2
, the former are especially important for determining the
correct
particle density
during the reproduction of aggregate grounds such as those
made of crumples, ice, snow, creaking wood; conversely, the latter provide a unique
color
to the contact events, hence becoming crucial in interactions with solid floors,
where the entire footstep sound is represented by one or very few contact events.
Further information, concerning several characteristics of a walker (weight,
height, age, sex) results from the interplay of low- and high-level cues, and the
information they provide about foot gesture, postural habits and locomotion style of
the walking person: a credible rendering of footstep sounds must account also for
this interplay, for which a comprehensive collection of kinematic and biomechanical
data is not available yet [
22
]. This and other knowledge gaps currently make the
design of interactive walking sound synthesizers a difficult task.
12.2.2.1 Early Models
The first systematic attempt to synthesize walking sounds was proposed by Cook
in 2002 [
14
]. In this pioneering system, engineered on an STK-based sound engine
known as Bill's Gait, the author introduced research elements that are still stimu-
lating nowadays. In particular, Bill's Gait successfully implemented a number of
solutions that are still largely state-of-the art in the realm of real-time sound process-
ing: he detailed an analysis procedure which included Linear Predictive Coding for
the extraction of footstep color, a Wavelet analysis for estimating the particle density,
and an envelope following of the gait sequence for informing the higher-level sta-
tistics on amplitude and frequency of the walking cycle. His model could store data
on footstep signatures from sound signals, which were recorded during foot interac-
tions with diverse floors. The same signatures could be reproduced online essentially
by reversing this procedure, i.e., by mapping the predictor onto the coefficients of
