Let denote the transverse displacement, in one plane, of a
vibrating string [60]. The complete linear time-invariant
generalization of the lossy, stiff string is described by the differential
equation

(21) |

(22) |

where . Thus, a general map of versus , corresponding to a partial differential equation of any order in the form (20), can be translated, in principle, into an accurate, local, linear, time-invariant, discrete-time simulation. The boundary conditions and initial state determine the initial mixture of the various solution branches as is typical in, say, the stiff string [19].

In summary, a large class of wave equations with constant coefficients, of any order, admits a decaying, dispersive, traveling-wave type solution. Even-order time derivatives give rise to frequency-dependent dispersion and odd-order time derivatives correspond to frequency-dependent losses. Higher order spatial derivatives can be approximated by higher order time derivatives and treated similarly [10]. The corresponding digital simulation of an arbitrarily long (undriven and unobserved) section of 1D medium (such as a string or acoustic tube) can be simplified via commutativity to at most two pure delays and at most two linear, time-invariant filters. In higher dimensions, such as for the 2D mesh, the per-sample filtering cannot in general be exactly commuted to the boundaries of the mesh. However, an approximation problem can be solved which matches the observed modal frequencies and decay rates using sparsely distributed low-order filters in an otherwise lossless mesh (e.g., around the rim).

In practical physical simulation scenarios, such as for real-world strings
or acoustic tubes, it is generally most effective to identify *experimentally* the attenuation and dispersion associated with wave
propagation at each frequency over the band of interest [108,49].
These data can be
used to design a digital filter which gives an optimal approximation over
the propagation distance desired. If needed, the per-sample filter
identified in this way can be translated into higher-order terms in the
wave equation for the medium. Thus, the wave equation itself can be
``identified'' from measured input-output behavior of the medium (assuming
it is linear and uniform) rather than being derived from physical
principles and physical constants of the medium as is classically done
[61,11].

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