Index of content:
Volume 130, Issue 1, July 2011
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
130(2011); http://dx.doi.org/10.1121/1.3592225View Description Hide Description
Resonance localization of waveenergy in two-dimensional (2D) waveguides with obstacles, known as a trapped mode effect, results in blocking of wave propagation. This effect is closely connected with the allocation of natural resonance poles in the complex frequency plane, which are in fact the spectral points of the related boundary value problem. With several obstacles the number of poles increases in parallel with the number of defects. The location of the poles in the complex frequency plane depends on the defect’s relative position, but the gaps of transmission coefficient plots generally remain in the same frequency ranges as for every single obstacle separately. This property gives a possibility to extend gap bands by a properly selected combination of various scatterers. On the other hand, a resonance wave passing in narrow bands associated with the poles is also observed. Thus, while a resonance response of a single obstacle works as a blocker, the waveguide with several obstacles becomes opened in narrow vicinities of nearly real spectral poles, just as it is known for one-dimensional (1D) waveguides with a finite number of periodic scatterers. In the present paper the blocking and passing effects are analyzed based on a semi-analytical model for wave propagation in a 2D elastic layer with cracks or rigid inclusions.
A comparison of stochastic and effective medium approaches to the backscattered signal from a porous layer in a solid matrix130(2011); http://dx.doi.org/10.1121/1.3598461View Description Hide Description
This paper reports a study of the backscattering behavior of a solid layer containing randomly spaced spherical cavities in the long wavelength limit. The motivation for the work arises from a need to model the responses of porous composite materials in ultrasonic NDE procedures. A comparison is made between models based on a summation over discrete scatterers, which show interesting emergent properties, and an integral formulation based on an ensemble average, and with a simple slab effective medium approximation. The similarities and differences between these three models are demonstrated. A simple quantitative criterion is established which sets the maximum frequency at which ensemble average or equivalent homogeneous medium models can represent echo signal generation in a porous layer for given interpore spacing, or equivalently, given pore size and concentration.
Experimental investigation on reversal of secondary Bjerknes force between two bubbles in ultrasonic standing wave130(2011); http://dx.doi.org/10.1121/1.3592205View Description Hide Description
The direction of the secondary Bjerknes force between a free bubble and an attached bubble was experimentally investigated. The behavior of the two bubbles in an ultrasonic standing wave of 27 kHz was observed using an imaging system with a high-speed video camera. It was demonstrated experimentally that the direction of the force reversed at a specific separation distance between the two bubbles, which was defined as the threshold distance. The threshold distance changed with the radius of the attached bubble. In addition, a theoretical calculation was performed using a previously derived model that coupled the vibrations of two free bubbles [Ida, Phys. Lett. A 297, 210–217 (2002)]. The experiment data for the threshold distance qualitatively agreed with the theoretical predictions, except when the separation distance was very small. Then, it was discovered that the free bubble became trapped near the attached bubble when the separation distance between the two bubbles was very small. This indicated that a stable equilibrium point for the separation distance exists that cannot be predicted by the theoretical model.
Theoretical prediction of the onset of thermoacoustic instability from the experimental transfer matrix of a thermoacoustic core130(2011); http://dx.doi.org/10.1121/1.3592227View Description Hide Description
The aim of this paper is to propose a method to predict the onset conditions of the thermoacoustic instability for various thermoacoustic engines. As an accurate modeling of the heat exchangers and the stack submitted to a temperature gradient is a difficult task, an experimental approach for the characterization of the amplifying properties of the thermoacoustic core is proposed. An experimental apparatus is presented which allows to measure the transfer matrix of a thermoacoustic core under various heating conditions by means of a four-microphone method. An analytical model for the prediction of the onset conditions from this measured transfer matrix is developed. The experimental data are introduced in the model and theoretical predictions of the onset conditions are compared with those actually observed in standing-wave and traveling-wave engines. The results show good agreement between predictions from the model and experiments.