Volume 127, Issue 1, January 2010
Index of content:
- NONLINEAR ACOUSTICS 
An iterative method for the computation of nonlinear, wide-angle, pulsed acoustic fields of medical diagnostic transducers127(2010); http://dx.doi.org/10.1121/1.3268599View Description Hide Description
The development and optimization of medical ultrasound transducers and imaging modalities require a computational method that accurately predicts the nonlinear acousticpressure field. A prospective method should provide the wide-angle, pulsed field emitted by an arbitrary planar source distribution and propagating in a three-dimensional, large scale domain holding a nonlinear acoustic medium. In this paper, a method is presented that is free of any assumed wavefield directionality. The nonlinear acoustic wave equation is solved by treating the nonlinear term as a contrast source. This formulation leads to an iterative scheme that involves the repetitive solution of a linear wave problem through Green’s function method. It is shown that accurate field predictions may be obtained within a few iterations. Moreover, by employing a dedicated numerical convolution technique, the method allows for a discretization down to two points per wavelength or period of the highest frequency of interest. The performance of the method is evaluated through a number of nonlinear field predictions for pulsed transducers with various geometries. The results demonstrate the directional independence of the method. Moreover, comparison with results from several existing methods shows that the method accurately predicts the nonlinear field for weak to moderate nonlinearity.
Dislodgement and removal of dust-particles from a surface by a technique combining acoustic standing wave and airflow127(2010); http://dx.doi.org/10.1121/1.3268507View Description Hide Description
It is known that there are many fine particles on the moon and Mars. Their existence may cause risk for the success of a long-term project for NASA, i.e., exploration and habitation of the moon and Mars. These dust-particles might cover the solar panels, making them fail to generate electricity, and they might also penetrate through seals on space suits, hatches, and vehicle wheels causing many incidents. The fine particles would be hazardous to human health if they were inhaled. Development of robust dust mitigation technology is urgently needed for the viable long-term exploration and habilitation of either the moon or Mars. A feasibility study to develop a dust removal technique, which may be used in space-stations or other enclosures for habitation, is reported. It is shown experimentally that the acoustic radiation force produced by a sound-level standing wave between a -aperture tweeter and a reflector separated by is strong enough to overcome the van der Waals adhesive force between the dust-particles and the reflector-surface. Thus the majority of fine particles ( diameter) on a reflector-surface can be dislodged and removed by a technique combining acoustic levitation and airflow methods. The removal efficiency deteriorates for particles of less than in size.
Finite amplitude method for measuring the nonlinearity parameter in small-volume samples using focused ultrasound127(2010); http://dx.doi.org/10.1121/1.3268602View Description Hide Description
On the basis of finite amplitude and comparative methods, the acoustic nonlinearity parameter of a liquid sample of as small as is measured using an focused Gaussian beam. The sample fills the space between a polystyrene plate and a tungsten reflector set about apart from each other within the focal region. The sound speed and attenuation coefficient are determined using the time of flight and the insertion loss of the sound passing through the sample, respectively. The density is estimated from the reflection coefficient at the interface between the polystyrene plate and the sample, where the transformation from longitudinal to transverse waves is considered. To compensate for the effect of velocity dispersion on the second harmonic generation, the relative phase of the second harmonic sound is also measured using dual-frequency sound. By summarizing all the linear properties and amplitude data of the second harmonic component in the sound transmitted through the sample, the value is finally determined. The measurement is validated through the experiments on nondispersive liquids and weakly dispersive biological samples with known values.