Nonlinear propagation of laser-generated acoustical pulses in granulated media

Abstract

Laser generation of high-intensity broad frequency band acoustical pulses in different fluids and their propagation in water-saturated cobalt-manganese crust (CMC) samples are investigated. Acoustic pulses of amplitude up to 3 MPa and duration about 1 s are generated in water, oil, and benzene by the action of CO2 on a laser pulse. The propagation of the short acoustic pulses through the water-saturated CMC samples is considered. The complexity of the CMC structure leads to specific nonlinear acoustical features of the process—nonlinear distortion of the acoustic pulse and amplitude-dependent attenuation. Abnormal sound attenuation in the high-frequency band is observed with low-energy probing sound pulses. This effect could be connected with the sound energy capture and its localization as oscillating modes of the structure. This effect vanishes with increasing intensity of the probing pulses, apparently due to the changes in crust structure. At the same time, specific nonlinear distortion of the sound pulse takes place—increasing of the acoustic pulse amplitude leads to decreasing of the negative phase of the passed acoustic pulse. [Work supported in part by Russian Fund for Fundamental Research.]