Nonlinear Distortion of Short Pulses Radiated by Plane and Focused Circular Pistons

Detailed measurements of finite-amplitude pulses radiated by plane and focused circular pistons in water are presented. Comparisons of time waveforms and frequency spectra, both on and off axis, are made with numerical calculations based on the nonlinear parabolic wave equation. Emphasis is on nonlinear distortion of amplitude- and frequency-modulated tone bursts. Use of short pulses enabled resolution of the direct and diffracted waves prior to their coalescence and subsequent shock formation along the axis of the source. Because of its relevance to investigations of cavitation inception, attention is devoted to variation of the peak positive (p+) and negative (p-) pressures along the axis of a focused source. It is shown that with increasing source amplitude, the maximum of each shifts away from the focal plane, toward the source. This effect is more pronounced for p- than for p+.