SELF-DEMODULATION OF AMPLITUDE-MODULATED AND FREQUENCY-MODULATED PULSES IN A THERMOVISCOUS FLUID

The self-demodulation of pulsed sound beams in a thermoviscous fluid is investigated experimentally and theoretically. Experiments were performed in glycerin at megahertz frequencies with amplitude- and frequency-modulated pulses. The theory is based on the Khokhlov-Zabolotskaya-Kuznetsov (KZK) nonlinear parabolic wave equation. Numerical results were obtained from an algorithm that solves the KZK equation in the time domain [Y.-S. Lee and M. F. Hamilton, Ultrasonics International 91 Conference Proceedings (Butterworth-Heinemann, Oxford, 1991), pp. 177-180]. A quasilinear analytic solution, which describes the main features of the waveform at all axial locations, is developed in the limit of strong absorption. Theory and experiment are in good agreement throughout the near- and far fields.