On the signal amplitude asymmetry in nonlinear propagation

Nonlinear propagation of high intensity ultrasound beams combined with diffraction generates signal amplitude asymmetry. To try to explain this phenomenon we model the nonlinear pulse as a sum of harmonic signals. The phase of each harmonic signal differs from that of a plane wave of same frequency and same origin by a unique phase angle: the phase parameter Psi. For physical pulses, Psi varies between -pi/2 for no amplitude asymmetry and 0 for maximum asymmetry. Using numerical simulations we compute the amplitude of the harmonic components of a signal propagating nonlinearly and generated by a piston source. As the input pressure level increases, Psi gets closer to 0 at the shock distance leading to a larger peak-positive to peak-negative pressure ratio. Beyond the shock distance Psi tends faster towards -pi/2 with increasing input pressure. The variations of Psi are also linked to the 90 degree phase shift of the fundamental signal from near to far field. This phase jump contributes to the emergence of...