INFLUENCE OF SELF-ACTION EFFECTS ON THE FIELD OF A NONLINEAR ACOUSTIC EMITTER
Abstract
The aim of the work is to experimentally study the role of self-action effects accompanying
the propagation of ultrasonic beams of the biharmonic pump wave on the formation of spatial
characteristics of difference frequency wave field. The article studies the changes in spatial distributions
of the wave amplitude of the difference frequency under the combined action of quadraticnonlinear
effects (amplitude-dependent nonlinear attenuation), acoustic flow and heating of the
medium by finite amplitude pump waves. The experiments were carried out in a 211 m3 laboratory
pool with fresh water at room temperature, where a flat piezoceramic radiator with a diameter
of 20 mm horizontally emitted a pump wave beam with an average frequency of 2 MHz. The
biharmonic pump wave generated a difference wave in the water with a frequency of 80 kHz or
160 kHz. A small hydrophone was moved horizontally along and across the acoustic axis of the
pump beam to record axial and transverse distributions of the difference frequency wave amplitude. The amplitude of the pump waves, duration, repetition period and duty cycle of the emitted
pulses were used as the parameters of the study. At constant amplitude of the emitted waves, the
pulse duty cycle determines the average pump intensity, on which the development of self-action
effects depends. In contrast, the quadratic-nonlinear effects are determined by the amplitude of the
pump wave and do not depend on the duty cycle. Increasing the amplitude of the pump waves and
the constant duty cycle shows a joint increasing effect of quadratic-nonlinear effects and selfaction
effects on the spatial characteristics of the difference frequency wave field. For water at
room temperature, the acoustic flow and thermal self-action led to the pump beam defocusing,
which significantly accelerated the spatial loss of the difference frequency wave amplitude with the
distance from the emitter along the acoustic axis. It is shown that the greatest influence of selfaction
effects takes place in the continuous mode of radiation. The obtained results are of interest
for understanding the features and degree of influence of self-action effects on the field of difference
frequency waves, which is important when using a nonlinear acoustic emitter in measuring
and diagnostic tasks, for information transmission and remote search of objects.
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