Properties

The source of the sound determines the characteristics of the sound wave
(frequency and amplitude).
All the energy of the sound wave is received from the source of the sound.

The external forces, which create a pressure gradient in a gas,
created Pressure Gradient Elastic Waves.

The sound waves propagate out from the sound source.
In the case of a point sound source placed in homogeneous infinite space, the surface of the front of the sound wave is an expanding sphere (full solid angle).

Pressure Gradient Elastic Waves propagate along the vector of the pressure gradient.

If the sound source pulsates or carries out oscillatory motion, the pressure in a point of space changes periodically and the molecules of gas carry out oscillatory motion

During the PGEWs propagation, the oscillatory motion of the gas molecules is absent.
The pressure in a point of space can changes periodically (when sound create the PGEWs)
or non periodically
(when turbulence create the PGEWs)

In sound wave, the compression and rarefaction zones alternate and move together in the same direction, moving away from the sound sours.

In the Pressure Gradient Elastic Waves,
the waves of compression are directed
toward the pressure increasing,
while the waves of rarefaction
are directed in the opposite direction
toward the pressure decreasing.

The process of “sound” waves propagation in a gas is an isentropic process.

The process of Pressure Gradient Elastic Waves propagation is adiabatic
(no heat supply or removal)
but is not an isentropic process.
The field of forces did the work.
(This field creates the pressure gradient.)
It compresses or expands
the density fluctuation area.

The energy of sound waves
in gases consists
of two components:
the potential energy,
which is due to the magnitude
of the relative elastic strain,
and the component of the kinetic energy of the oscillatory movement of gas molecules. Adiabatic compression and rarefaction have to change the gas temperature in the wave disturbance areas.
However, since in the sound waves these zones alternate,
the net effect is zero.

The component, which is due to
the kinetic energy of the oscillatory movement
of gas molecules, is absent
in the energy of PGEWs.

The energy of the PGEW
consists of two components:
the energy of starting sound disturbances, including the component associated with the change in temperature and pressure
of the disturbance area,
and the energy equivalent of the work done by pressure forces creating pressure gradient,
which compresses and rarefies the wave fronts.

Inside a bonded space,
the sound waves are reflected from the walls.

Inside a bonded space, the compressed
and rarefied fronts of PGEWs
are reflected from the walls
but immediately are extinguished
by next fronts du to interference.
The effect is equivalent to absorption,
and the entire energy of the wave is transferred to the walls in the form of heat or cold.

The PGEW cannot pass
through an extremum point,
thus if the function of the pressure gradient has an extremum (the centre of rotation),
the PGEW is dissipated in this place.