Acoustic bullets: Generation and propagation.

摘要

In theory, acoustic bullets are: (1) localized wave solutions to the free space scalar wave equation; (2) acoustic waves whose shapes and/or amplitudes are maintained no matter how far they propagate.;The localized wave (LW) phenomena are studied from new perspectives. The localized wave (LW) solutions are treated as a special class of source signals in the context of understanding the physical process in launching an acoustic field from an axisymmetric planar radiator.;Two of the most important methods in evaluating the acoustic field from an axisymmetric planar radiator (i.e. the modal impulse response method) and the wave vector-time domain method are investigated. In order to establish connection between the two methods, the wave vector-time domain method is used as an alternative to derive the modal impulse response. A novel geometric interpretation of the on-axis response (the acoustic field at the pulse center) is presented, by combining the Rayleigh surface integral and a simple geometric focusing scheme.;Among the vast variety of the acoustic bullet signals, two major families, i.e., Acoustic Directed-Energy Pulse Trains (ADEPT) which uses the Focus Wave Mode (FWM) as building blocks, and X waves which uses the Bessel beams as building blocks are of particular interest. Modified Power Spectrum (MPS) pulses and X waves are studied as two special cases in interpreting the localized wave (LW) phenomena. Theoretical behavior and finite source aperture realization are presented for these two kinds of sources; particularly, their source distribution patterns are related to the localized wave behaviors in terms of on-axis response (the acoustic field at the pulse center) and the near to far field transition. Acoustic bullet sources are characterized by the dynamic focusing mechanism.;Several projection methods including the numerical evaluation of the Rayleigh surface integral, the modal impulse response method and the wave vector-time domain method, are used to forward project the acoustic field for both an MPS pulse and an X wave, in order to have a comprehensive picture of the propagation of the acoustic bullets. Systematic development has been made to explore the methods of localization enhancement. The advantages, disadvantages and limitations of such kind of solutions are revealed.