Volume 104, Issue 1, July 1998
Index of content:
- GENERAL LINEAR ACOUSTICS 
104(1998); http://dx.doi.org/10.1121/1.423283View Description Hide Description
The propagation of acoustic or ultrasonic pulses and waves in 1-D media with continuous inhomogeneities due to spatial variations in density, Young modulus, and/or cross section of the propagation medium is discussed. A semianalytical approach leads to a general form of the solution, which can be described by a function, whose Taylor expansion is absolutely convergent. The special case of a periodic inhomogeneity is studied in detail and the dispersion law is found. It is also shown that a finite width pulse is generally not broken down by the inhomogeneity, even though its law of motion is perturbed. A numerical treatment based on the Local Interaction Simulation Approach (LISA) is also considered, and the results of the simulations compared with the semianalytical ones.
104(1998); http://dx.doi.org/10.1121/1.423284View Description Hide Description
The propagation of acoustic waves in immersed waveguides has been previously studied with the help of the finite element method, using the ATILA code [A. C. Hladky-Hennion et al., J. Sound Vib. 200, 519–530 (1997)]. But this method, which is a modal analysis, essentially concerns the case of rectilinear, infinite, and uniform waveguides. Thus this paper deals with another way of solving the problem of wave propagation along waveguides, with the help of a time analysis using finite elements. First, the theoretical formulation is presented for immersed structures. Then, Plexiglas and brass wedge guides, of different apex angles, are considered. When immersed in water, these wedges generate either propagating or radiating wedge waves. The finite element results, using a time analysis, are compared to the previous finite element results, using a modal analysis and to the experiments, leading to a good agreement. Thus the approach can be easily extended to other waveguides whatever their cross sections.
104(1998); http://dx.doi.org/10.1121/1.423305View Description Hide Description
The far-field directivity pattern can be computed using the fast Fourier transform (FFT) algorithm. Numerical implementation of the angular spectrum approach (ASA) is generally used to compute the pressure field. The aim of this paper is to demonstrate that the discrete far-field pressure can be only computed by the calculation of the DFT of the normal velocity. In fact, using the asymptotic expression of the Rayleigh’s integral, which is also the solution of Helmholtz equation, it will be shown that the analytical far-field pressure is given by the Fourier transform of the normal velocity. Guidelines for the selection of sampling interval and the size of the baffle in which the source is mounted will be given. Then this paper shows that the size of baffle influences the angular resolution at which the far field is computed and when the source is oversampled the computed discrete pressure becomes better. Numerical results concerning a transducer that exhibits harmonic oscillations will be considered and discussed.