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Index: Physical Audio Signal Processing
Physical Audio Signal Processing
Traveling-Wave Solution
Wave Velocity
Converting String-State to Traveling-Waves
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Traveling waves propagate (i.e. travel) through space. — Click for http://ccrma.stanford.edu/realsimple/travelingwaves
The displacement of an object describes how far away and in what direction it has been moved, or displaced, from its rest position. Displacement is a commonly chosen wave variable in physical modeling. — Click for http://ccrma.stanford.edu/~jos/Mohonk05/Ideal_Plucked_String_Displacement.html
In physics, a wave is an oscillation that propagates through a medium (space-time, gas, fluid, or solid) — Click for https://en.wikipedia.org/wiki/Wave
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A sinusoid is any function of the form A sin(ω t+φ), where t is the independent variable, and A, ω, φ are fixed parameters of the sinusoid called the amplitude, (radian) frequency, and phase, respectively. Sinusoidal motion is produced by any 'pure' vibration, such as that of an ideal tuning fork or mass-spring system. — Click for https://ccrma.stanford.edu/~jos/mdft/Sinusoids.html
The wave equation refers to the partial differential equation governing wave phenomena in elastic media such as air and isotropic solids. — Click for https://en.wikipedia.org/wiki/Wave_equation
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The Fourier theorems establish important, elementary time-frequency relationships. The theorems are largely the same whether stated for the case of the Fourier Transform (FT), Discrete Fourier Transform (DFT), Discrete Time Fourier Transform (DTFT), or Fourier Series (FS). — Click for https://ccrma.stanford.edu/~jos/mdft/Fourier_Theorems.html
Setting
, and extending the summation to an integral,
we have, by Fourier's theorem,
(C.14)
for arbitrary continuous functions
and
. This is again the traveling-wave solution of the wave
equation attributed to d'Alembert, but now derived from the
eigen-property of sinusoids and Fourier theory rather than
``guessed''.
An example of the appearance of the traveling wave components shortly
after plucking an infinitely long string at three points is shown in
Fig.C.2.
Figure C.2:
An infinitely long string,
``plucked'' simultaneously at three points, labeled ``p'' in the
figure, so as to produce an initial triangular displacement. The
initial displacement is modeled as the sum of two identical triangular
pulses which are exactly on top of each other at time 0
. At time
shortly after time 0
, the traveling waves centers are
separated by meters, and their sum gives the trapezoidal
physical string displacement at time
which is also shown. Note
that only three short string segments are in motion at that time: the
flat top segment which is heading to zero where it will halt forever,
and two short pieces on the left and right which are the leading edges
of the left- and right-going traveling waves. The string is not
moving where the traveling waves overlap at the same slope. When the
traveling waves fully separate, the string will be at rest everywhere
but for two half-amplitude triangular pulses heading off to plus and
minus infinity at speed
.