General solution
The general solution of the two behavioral equations is written, for the pressure,
where
is the wave number defined by
with
the speed of sound. The velocity is deduced from the pressure by considering the equation which expresses the phenomena of inertia,
The term
is called the characteristic impedance or surge impedance of a wave guide.
The volume velocity through a surface
is written,
and in this case, the characteristic impedance is written
.
