Providing
a smooth, quiet environment for drivers and passengers is
one of the many challenges faced by automobile manufacturers,
and mathematical tools are required to meet this challenge.
These tools are used both for modeling, to predict the performance
of a specific design, and for data analysis, to assess the
performance of real vehicles throughout their service lives.
Techniques based on Fourier transforms, such as modal analysis
and statistical energy analysis, have proven indispensable
for both performance prediction and assessment. However,
it is becoming increasingly clear, as these Fourier-based
techniques mature, that there is an important class of sounds
and vibrations for which the Fourier transform is simply
not very useful. The archetype of this class is the rattle,
which is a repetitive series of mechanical impacts between
two or more objects. Rattle sounds are characterized by
a low duty cycle (the time duration of the individual event
is short relative to the period of silence between successive
events) and the lack of a perceptible musical pitch (the
resonant vibrations that result from the impacts are heavily
damped). These two characteristics of rattles make them
especially suited for analysis with wavelet transforms,
since the individual wavelet basis vectors share these same
properties. This is in contrast to humming or whistling
sounds, which have a high duty cycle and a definite musical
pitch, much like the sines and cosines that make up the
Fourier basis. To illustrate the utility of this approach,
a new technique for wavelet-based transient waveform visualization
developed at Ford Research Laboratories will be discussed
and applied to examples of realistic automotive rattle sounds.
The "Industrial Problems" to be discussed arise in two different
areas. First, there are some specific problems prompted
by the waveform visualization algorithm mentioned earlier.
Second, and more generally, there is the question of how
far the analogy between the engineering applications of
the Fourier Transform and the Wavelet Transform can be extended.
The wavelet-based data visualization algorithm mentioned
earlier is analogous to the calculation of Fourier-based
energy spectral density. Do any of the many other Fourier-based
engineering algorithms have wavelet-based analogs, still
waiting to be discovered?