If a point of a medium is fixed, that is if it can't change its physical state, the energy can't overstep it so there can't be flux of energy through it.

To simplify the understanding of this phenomenon, we may consider one-dimensional waves as, for example, those on the strings of musical instruments. The extremities of these strings are fixed to the frame of the instrument, they cannot oscillate like the other points of the string, so their energy is constantly null and there's no flow of energy through them.

Like a particle hitting a hard wall with speed perpendicular to the wall itself, the wave reverses its speed, spreads in the opposite direction, hits the other end by which it is again reflected and so on.

Progressive waves and regressive waves mutually interfere. The result is an interference figure,
called ** standing (or stationary) wave**, with nodes at
the ends of the string and, possibly, other nodes equidistant from each other. This result implies that the length
of the string must exactly contain an integer number of half-wavelengths.

Let *L* be the length of a string and *λ* the wavelength

You can see some other maybe useful animations in Waves.

If a string (and, in general, any other elastic medium) carries a standing wave, each of its points behaves like
an harmonic oscillator. These oscillators have equal angular frequencies *ω* but different amplitudes,
depending on their distance from the extremities. If we let 0 and *L* be the positions of the extremities,
with a convenient choice of the time origin, the physical state of a point P, at position *x* between 0 and
*L*, is described by the equation

in which the amplitude *C sin kx* must be 0 when *x* is multiple of the half-wavelength and,
in particular, when it is equal to the half-wavelength:

Therefore we have

The (5.4) is the equation of the n-th standing wave on one-dimensional medium with length *L* and with
nodes at its ends.

The constant *C* depends on the total energy
owned by the medium.

If we call *μ* the linear density of a one-dimensional medium

from (3.9) the energy of a particle with mass *dm* swinging with angular frequency *ω* and amplitude
*A* results

In (5.4) the amplitude is , therefore

The sum of the energies of all the particles of the medium coincides with the total energy owned by the medium

The last expression of *C ^{2}*, used in (5.5), gives

and finally

(5.8) shows that the relative amount of energy located between *x* e *x+dx*
is proportional to the square of

that is: the **energy density** at the position *x* is given by the square of *ψ _{n}*.

Therefore the square of *ψ _{n}* represents the

(5.9) is independent from *μ*, so it is a general property of the waves.

So if we let

and substitute this value in (5.4)

we obtain the equation of the n-th stationary wave, which can be carried by a one-dimensional medium
with length *L*, normalized so that the square of its amplitude would represent the energy density.

More simply the equation (5.11) may be written

with