Biomedical Engineering Reference
In-Depth Information
The Natural Interactive Walking EU project, active until fall 2011, has put major
emphasis on the audio-tactile augmentation of otherwise neutral floors through the
use of active tiles as well as instrumented shoes. Both such interfaces, detailed in
Sect.
12.3.3
, were designed based on the fundamental hypothesis that a credible,
however informative augmentation of a flat, solid floor could be realized via the
superposition of virtual audio-tactile cues. As noted in Sect.
12.2.3
, in practice these
cues had to guarantee an especially “strong” characterization to walkers having nor-
mal sensory abilities, mainly to counterbalance the unavoidable bias caused by the
visual appearance of a ground surface: silent floors, then, were augmented so to
sound either as aggregate grounds, or strongly coloring (such as wooden) surfaces.
Effective audio-tactile simulations of aggregate and resonant ground categories
have been obtained through physically-based sound synthesizers, whose low-level
core made use of the same dynamic impact model as that used by Fontana and
Bresin [
30
]. In phenomenological sense, physics-based models have the fundamental
advantage to provide a coherent multimodal feedback: since they reproduce force
and velocity signals, then their response can be inherently used to mechanically
excite the resonant body, in our case a floor; once this excitation is known, along
with the resonance properties of the same floor, then it is not difficult to get sounds
as well as vibrations from it. Specifically, a footstep sound can be considered to be
the result of multiple microimpacts between a shoe and a floor. Either they converge
to form a unique percept consisting of a single impact, in the case of solid materials,
or conversely they result in a more or less dispersed, however coherent burst of
impulsive sounds in the case of aggregate materials.
An impact involves the interaction between two bodies: an active exciter, i.e.,
the impactor, and a passive resonator. Sonic impacts between solid surfaces have
been extensively investigated, and results are available which describe relationships
between physical and perceptual parameters of the objects in contact [
52
,
103
].
The most simple approach to the synthesis of such sounds is based on a lumped
source-filter model, in which a signal
s
(
t
)
modeling the excitation is passed through
a linear filter with impulse response
h
modeling the resonator, and resulting in an
output expressed by the linear convolution of these two signals:
y
(
t
)
.
A more accurate reproduction of the contact between two bodies can be obtained
by simulating the nonlinear dynamics of this contact: a widely adopted description
considers the force
f
between them to be a function of the compression
x
of the
exciter and velocity of impact
(
t
)
=
s
(
t
)
∗
h
(
t
)
x
, depending on the parameters of elasticity of the
materials, masses, and local geometry around the contact surface [
3
]:
˙
−
kx
α
−
λ
x
α
˙
x
,
x
>
0
(
,
˙
)
=
f
x
x
(12.1)
0
,
x
≤
0
where
k
accounts for the material stiffness,
λ
represents the force dissipation due
to internal friction during the impact,
α
depends on the local geometry around the
≤
contact surface. When
x
0 the two bodies are not in contact.
Friction is another crucial category at the base of footstep sound generation [
36
].
This phenomenon has been synthesized as well, by means of a dynamic model in
