Abstract
Weakly nonlinear sound propagation in a gas where molecular vibrational
relaxation takes place is studied. New equations which govern the sound in
media where the irreversible relaxation may take place are derived and
discussed. Their form depends on the regime of excitation of oscillatory
degrees of freedom, equilibrium (reversible) or non-equilibrium
(irreversible), and on the comparative frequency of the sound in relation
to the inverse time of relaxation. Additional nonlinear terms increase
standard nonlinearity of the high-frequency sound in the equilibrium
regime of vibrational excitation and decrease otherwise. As for the
nonlinearity of the low-frequency sound, the conclusions are opposite.
Appearance of a non-oscillating additional part which is a linear function
of the distance from the transducer is an unusual property of nonlinear
distortions of harmonic at the transducer high-frequency sound
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