Stephen Tashi said:
As I said in the original post, it is obvious that the defect in defining a wave to be a solution to "the" wave equation is that there more equations than "the" wave equation which are also called wave equations. Do I need to explain that further?
Here is an excerpt from the introduction to G.B. Whitham's textbook, Linear and Nonlinear Waves.(pages 2-3)
"
1.1 The Two Main Classes of Wave Motion
There appears to be no single precise definition of what exactly
constitutes a wave. Various restrictive definitions can be given, but to
cover the whole range of wave phenomena it seems preferable to be guided
by the intuitive view that a wave is any recognizable signal that is
transferred from one part of the medium to another with a recognizable
velocity of propagation. The signal may be any feature of the disturbance,
such as a maximum or an abrupt change in some quantity, provided that it
can be clearly recognized and its location at any time can be determined.
The signal may distort, change its magnitude, and change its velocity
provided it is still recognizable. This may seem a little vague, but it turns
out to be perfectly adequate and any attempt to be more precise appears to
be too restrictive; different features are important in different types of
wave.
Sec 1.1 THE TWO MAIN CLASSES OF WAVE MOTION 3
Nevertheless, one can distinguish two main classes. The first is formu-
lated mathematically in terms of hyperbolic partial differential equations,
and such waves will be referred to as hyperbolic. The second class cannot
be characterized as easily, but since it starts from the simplest cases of
dispersive waves in linear problems, we shall refer to the whole class as
dispersive and slowly build up a more complete picture. The classes are not
exclusive. There is some overlap in that certain wave motions exhibit both
types of behavior, and there are certain exceptions that fit neither.
The prototype for hyperbolic waves is often taken to be the wave
equation
...
although the equation
φ_t + φ_x = 0 (1.2)
is, in fact, the simplest of all. As will be seen, there is a precise definition
for hyperbolic equations which depends only on the form of the equations
and is independent of whether explicit solutions can be obtained or not.
On the other hand, the prototype for dispersive waves is based on a type of
solution rather than a type of equation. A linear dispersive system is any
system which admits solutions of the form
φ = acos(κx — ωit), (1.3)
where the frequency to is a definite real function of the wave number k and ω
the function ω(k) is determined by the particular system. The phase speed
is then ω(k)/k and the waves are usually said to be "dispersive" if this
phase speed is not a constant but depends on κ. The term refers to the fact
that a more general solution will consist of the superposition of several
modes like (1.3) with different k. [In the most general case a Fourier
integral is developed from (1.3).] If the phase speed ω/k is not the same for
all k, that is, ω≠c_0 k where c_0 is some constant, the modes with different k
will propagate at different speeds; they will disperse. ..."