Characteristic properties of Pressure Gradient Elastic Waves

 

We single out the main differences between Pressure Gradient Elastic Waves from conventional “sound” waves (here the term “sound” combines the elastic oscillations regardless of their frequency).

 

To compare the properties, two types of waves are presented in the table. 

The normal  “sound” waves

Pressure Gradient Elastic Waves

 The “sound” waves always arise in compressible fluids when there is a sound source, creating density fluctuations.
 Three conditions are necessary for Pressure Gradient Elastic Waves arising: 1. The substance must be a compressible fluid.
2. A pressure gradient must exist inside a space or a volume filled with a compressible fluid.
3. Density fluctuations must be present. These fluctuations may be a result of sound.
 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”.
 Pressure Gradient Elastic Waves created by external forces, which create a pressure gradient in a gas.
 The “sound” waves propagate out in the direction of the sound source. In the case of a point “sound” source placed in homogeneous infinite space, the surface of the front of the acoustic wave is an expanding sphere (full solid angle).
 Pressure Gradient Elastic Waves propagate along the vector of the pressure gradient.
 The process of “sound” waves propagation is periodically process. The reason of this is the fact that “sound” source always pulsates or carries out oscillatory motion. 
 The oscillations are absent in Pressure Gradient Elastic Waves since they do not exist in the field of force, which generates the pressure gradient. 
 In “sound” wave the compression and rarefaction zones alternate and move together in the same direction.
 The compressed front of the Pressure Gradient Elastic Waves is directed toward the higher pressure zone, while the rarefied front is directed in the opposite direction toward the lower pressure zone.
 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 force field did the work. 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; a component of the kinetic energy of the oscillatory movement. 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, is absent in the energy of Pressure Gradient Elastic Waves since there are no oscillations in the PGEW.The energy of the PGEW consists of two components: the energy of starting “sound” disturbances, including the component associated with the change in temperature of the disturbance area, and the energy equivalent of the work done by pressure force, which moves the wave front.
 Inside a bonded space, the “sound” waves are reflected from the walls.
 Inside a bonded space, Pressure Gradient Elastic Wave is absorbed by the walls. The PGW reflection and its movement in the opposite direction is impossible. This is prevented by the force, creating a pressure gradient. As the result of Pressure Gradient Elastic Waves absorption, 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, it is dissipated in this place.
 The “sound” wave transfers the energy obtained from the sound transmitter. Their absorption usually has very small changes the thermodynamic characteristics of the system.
 Pressure Gradient Elastic Waves take energy over all space, and carry it to the direction of increasing pressure.The heat transfer increases the temperature in the high pressure zone and reduces it in the low pressure area.