Volume 118, Issue 5, November 2005
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
118(2005); http://dx.doi.org/10.1121/1.2041331View Description Hide Description
The possibility of determining the location of an acoustic source in the presence of internal waves is investigated. Source localization problems require environmental information as inputs. Internal waves cause uncertainties in the sound speed field. In previous work [J. Acoust. Soc. Am.90, 1410–1422 (Year: 1991)], it was found that a source can often be localized in an uncertain environment by including environmental parameters in the search space and tweaking them, but usually not determining their true values. This is possible due to a parameter hierarchy in which the source position is more important than the environmental parameters. The parameter hierarchy is shown to also apply to uncertainties associated with internal waves. Due to differences in the nature of the parameter space, this problem is solved with a statistical approach rather than a parameter search technique such as simulated annealing.
118(2005); http://dx.doi.org/10.1121/1.2048927View Description Hide Description
An imaging algorithm is presented based on the standard assumption that the total scattered field can be separated into an elastic component with monopolelike dependence and an inertial component with dipolelike dependence. The resulting inversion generates two separate image maps corresponding to the monopole and dipole terms of the forward model. The complexity of imaging flaws and defects in layered elastic media is further compounded by the existence of high contrast gradients in either sound speed and/or density from layer to layer. To compensate for these gradients, we have incorporated Fermat’s method of least time into our forward model to determine the appropriate delays between individual source-receiver pairs. Preliminary numerical and experimental results are in good agreement with each other.
118(2005); http://dx.doi.org/10.1121/1.2042987View Description Hide Description
The treatment of time-reversal imaging of multiply scattering point targets developed by the present authors in Gruber et al. [“Time-reversal imaging with multiple signal classification considering multiple scattering between the targets,” J. Acoust. Soc. Am., 115, 3042–3047 (2004)] is reformulated and extended to the estimation of the target scattering strengths using the Foldy–Lax multiple scatteringmodel. It is shown that the time-reversal multiple signal classification (MUSIC) pseudospectrum computed using the background Green function as the steering vector yields accurate estimates of the target locations, even in the presence of strong multiple scattering between the targets, and that the target scattering strengths are readily computed from the so-determined target locations using a nonlinear iterative algorithm. The paper includes computer simulations illustrating the theory and algorithms presented in the paper.
118(2005); http://dx.doi.org/10.1121/1.2082687View Description Hide Description
Near field acoustic holography is usually based on measurement of the pressure. This paper describes an investigation of an alternative technique that involves measuring the normal component of the acoustic particle velocity. A simulation study shows that there is no appreciable difference between the quality of predictions of the pressure based on knowledge of the pressure in the measurement plane and predictions of the particle velocity based on knowledge of the particle velocity in the measurement plane. However, when the particle velocity is predicted close to the source on the basis of the pressuremeasured in a plane further away, high spatial frequency components corresponding to evanescent modes are not only amplified by the distance but also by the wave number ratio . By contrast, when the pressure is predicted close to the source on the basis of the particle velocitymeasured in a plane further away, high spatial frequency components are reduced by the reciprocal wave number ratio . For the same reason holography based on the particle velocity is less sensitive to transducer mismatch than the conventional technique based on the pressure. These findings are confirmed by an experimental investigation made with a sound intensity probe produced by Microflown.
Reflection and time-reversal of ultrasonic waves in the vicinity of the Rayleigh angle at a fluid-solid interface118(2005); http://dx.doi.org/10.1121/1.2082727View Description Hide Description
When sending a plane ultrasonicwave toward a fluid-solid interface, the reflected wave is affected, depending on the incident angle. Around the Rayleigh angle the reflection coefficient has a strong and rapidly varying imaginary part. This has the effect of distorting the reflected wave front. If this reflected wave is time-reversed and sent back toward the interface, the reflected wave of this time-reversed wave should not present any distortion, as the time-reversal process restores the original phases. A theoretical and experimental study of these phenomena has been done. The distortion of the reflected waves around the Rayleigh angle is observed and as expected this distortion is canceled by the time-reversal process. However a significant loss of energy in the time-reversed signal is observed for incident angles around the Rayleigh angle, as part of the energy contained in the Rayleigh wave escapes the time-reversal mirror and is lost for the time-reversal process. In a second part, it is shown by simulations and experiments how this signal distortion by reflection around Rayleigh angle influences spatial focusing of waves by time-reversal or simple time-delay methods.