INVISCID BURGERS’ EQUATION  1) sound speed in nonlinear acoustics  2) nonlinear distorsion of the wave profile  3) weak shock theory  4) the acoustical.

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INVISCID BURGERS’ EQUATION  1) sound speed in nonlinear acoustics  2) nonlinear distorsion of the wave profile  3) weak shock theory  4) the acoustical potential and the numerical solver

Linear and nonlinear sound speed Linear sound speed Nonlinear sound speed

Nonlinear distorsion of the wave profile

Inviscid Burgers’ equation 1D wave equation Nonlinear sound speed Burgers’ eq.

Poisson’ solution Implicit solution Demonstration + Valid as long as or

Generation of harmonics – Fubini’s solution Harmonic signal Harmonic cascade …

Weak shock theory Weak formulation of Burgers’ equation Weak shock theory Shock at time

Law of equal areas (Landau) Weak shock theory Law of equal areas A + =A -

Example 1: the « N » wave Initial signal Shock position Poisson’ s solution Energy is dissipated through shocks (second principle [s]>0 [P]>0 : compression shocks) Signal energy

Example 1: the « N » wave

Example 2: the sine wave Initial signal Poisson’s solution remains periodic and antisymmetric Poisson’s solution Weak shock

Example 2: the sine wave Saturation : received energy is independant of emitted one !

Nonlinear attenuation The rate of energy dissipation depends on the temporal waveform of the initial signal

The acoustical potential Acoustical potential Poisson’s solution with 1)Shocks are at the intersection of branches + 2)  mono  max  multi Potential is continuous + compression shocks

The acoustical potential (II)

Evolution of complex waveforms Distorsion  HF Shock formation Shock evolution Shock fusion  BF N wave

Conclusion The dominant phenomenon in nonlinear acoustics is the dependancy of sound speed with instanteneous wave amplitude. The evolution of the temporal waveform of a signal as a function of the propagation distance is modelled at 1D by the inviscid Burgers’ equation. Poisson’s solution provides an implicit solution that maybe multivalued. Starting from an initial profile, non- linearities lead to the waveform distorsion, up to the shock formation distance. This is associated to a shift of the frequency spectrum towards high frequencies. Shocks are necessarily compression ones (to satisfy the 2nd principle of thermodynamics). They are determined according to the weak shock theory, or law of equal areas. This implies dissipation of energy, the rate of which depends on the temporal waveform of the signal. Beyond the shock formation distance, waveforms evolve rapidly towards linear profiles (simple waves) matching moving shocks. This generally implies a slow shift of the frequency spectrum towards low frequencies. An efficient way to compute any wave profile at any distance is to use the potential. For potential, the physically admissible solution (that satisfies the entropy condition) is the largest value among the multivalued ones.

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