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    Nonlinear Generation of Airborne Sound by Waves of Ultrasonic Frequencies
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    The paper deals with parametric excitation of nonlinear standing waves in acoustic resonators. There are a number of methods which enable to generate intensive acoustic fields confined to resonators. The acoustic system, which is composed of an acoustic resonator and source, is often bulky and expensive. For this reason we designed a method which is inexpensive and at the same time enables miniaturization of the discussed acoustic system. This method is based on the use of a parametric acoustic piston source which radiates an amplitude modulated ultrasonic waves. The carrier ultrasonic wave is the finite amplitude one hence the nonlinearity of the fluid enables to self-demodulate the amplitude modulated ultrasonic waves. The approximate solutions were found for the used model equation. From the realized analysis it follows that parametric excitation of nonlinear standing waves inside of the acoustic resonators represent a perspective method.
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    Measurement methods aimed at determining material properties through nonlinear wave propagation are sensitive to artifacts caused by background nonlinearities inherent in the ultrasonic generation and detection methods. The focus of this paper is to describe our investigation of nonlinear mixing of surface acoustic waves (SAWs) as a means to decrease sensitivity to background nonlinearity and increase spatial sensitivity to acoustic nonlinearity induced by material microstructure.
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    Abstract : This report discusses five projects all of which involve basic theoretical research in nonlinear acoustics. (1) Pulsed Finite Amplitude Sound Beams are studied with a recently developed time domain computer algorithm that solves the KZK nonlinear parabolic wave equation. (2) Nonlinear Acoustic Wave Propagation in a Liquid Layer is a study of harmonic generation and acoustic soliton information in a liquid between a rigid and a free surface. (3) Nonlinear Effects in Asymmetric Cylindrical Sound Beams is a study of source asymmetries and scattering of sound by sound at high intensity. (4) Effects of Absorption on the Interaction of Sound Beams is a completed study of the role of absorption in second harmonic generation and scattering of sound by sound. (5) Parametric Receiving Arrays is a completed study of parametric reception in a reverberant environment.
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    The oscillations of ultrasound contrast agents are of particular importance to the understanding of the propagation of acoustic waves in the bubbly liquids (suspensions of ultrasound contrast agents). Acoustic waves propagating in bubbly liquids have been investigated extensively. Little has been dedicated to the resonance effects of the microbubbles on the propagating waves. Here a nonlinear partial differential equation for describing one-dimensional acoustic waves propagating near the resonance frequency of the microbubbles in bubbly liquids is obtained. The present equation recovers classical results for propagating acoustic waves with finite amplitudes in liquids and interprets the acoustic localization in bubbly liquids explicitly.
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    Laser techniques enabled generation of very high amplitude pulses with acoustic Mach numbers about 0.01. Such waves drive the medium into the nonlinear elastic regime and shock fronts can be formed during their propagation. As an intense surface acoustic wave (SAW) propagates, the temporal evolution of the wave shape provides information on the nonlinear acoustic parameters and the nonlinear elastic constants of the material. The nonlinear propagation of SAW pulses exhibits different types of nonlinear behavior depending on nonlinear acoustic constants. Changes of a SAW pulse shape were calculated using a nonlinear evolution equation. Measurements of SAW pulses in polycrystalline stainless steel have demonstrated that a compression of the pulse takes place in this material corresponding to a positive parameter of the local nonlinearity, which was evaluated by fitting the parameters of the evolution equation to the experimental data. Numerical simulations of a dispersive propagation of nonlinear SAWs in a system comprised of a substrate with a film were performed. A formation of a relatively stable portion of the waveform was found in agreement with recent observations of soliton-like SAW pulses.
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