Inhomogeneous Acoustic Field Research Articles

Transverse acoustic spin and torque from pure spinning of objects

Acoustic spin has been recently explored for many applications. In particular, transverse acoustic spin was demonstrated for inhomogeneous acoustic fields. In this Letter, we show the emergence of acoustic spin and torque in rotating acoustic objects of the same physical properties as the surrounding, to single out the effects purely due to rotation. The spinning of a cylindrical column of air or water in the same medium possesses intrinsic spin angular momentum, and we study the torque and force it experiences in evanescent acoustic fields. The resulting discontinuity can thus scatter sound in unusual ways, including a negative radiation force, although it has no imaginary part in its parameters.

Particulate aggregation through a modulated annular one-dimensional acoustic field at resonant frequencies

The visualization and analysis of a novel acoustic-particulate system is the objective of this study. The system is composed of rice-husk fired smoke particulates (36.7nm–840μm) and one annular resonant circular-tube waveguide contrarily coupled with two sound sources. The collective interaction behavior process of smoke particulates in an inhomogeneous acoustic field is displayed during an experiment and a simulation. The result shows that the aggregation and fragmentation of particles under a change in resonant frequencies and sound pressure amplitude is extremely complex. This complex process consists of dynamically tuning the particle characteristics to attain stripes shaped like thin-films/umbrellas and clusters with volume-change/fragmentation. The balanced modulation of the acoustic radiation force and secondary radiation force to alter the particle characteristics (size and stack density) is verified to be the control mechanism of the particle system. The intermediate variable of the process control is the acoustic contrast factor (Ф) related to the physical characteristics of the growing particulates. The value plus-minus alternation of Ф results in different particulate processes. This study can enhance the application of aerodynamic acoustic-particulate-fluid systems for environment protection, energy fuel conversion, and industrial production.

Optimization of piezotransducer dimensions for quasicollinear paratellurite AOTF

The method of quasicollinear acousto-optic tunable filters (AOTFs) piezoelectric transducer dimensions optimization is presented. The AOTFs with large interaction length apply the bulk acoustic wave (BAW) reflection from the input optical facet. Optimization is based on spectral approach to simulation of the BAW field in anisotropic media and Raman–Nath equations numerically solved for the inhomogeneous acoustic field. It was found that variation of the transducer dimensions can minimize RF power consumption of the AOTF. Comparison of the optimized transducer dimensions with those commonly used in quasicollinear paratellurite AOTFs showed that it is possible to improve the AOTF energy efficiency in about 2 times. It was shown that acoustic field simulation results obtained for one AOTF geometry can be applied to the other geometries but for equivalent frequency providing the same ultrasound beam ray spectrum width. It was also shown that when choosing the quasicollinear AOTF with reflection geometry, one should not rely only on the AO figure of merit value, since the energy efficiency of such AOTF type will be determined by the product of the AO figure of merit and the AOTF efficiency, which takes into account the change in the acoustic beam width in the reflection process. The results aim at improving the design of AOTFs for ultrashort laser pulse shaping.

Transverse spin and surface waves in acoustic metamaterials

We consider spin angular momentum density in inhomogeneous acoustic fields: evanescent waves and surface waves at interfaces with negative-density metamaterials. Despite being purely longitudinal (curl-free), acoustic waves possess intrinsic vector properties described by the velocity field. Motivated by the recent description and observation of the spin properties in elastic and acoustic waves, we compare these properties with their well-known electromagnetic counterparts. Surprisingly, both the transverse spin of evanescent waves and the parameters of surface waves are very similar in electromagnetism and acoustics. We also briefly analyze the important role of dispersion in the description of the energy and spin densities in acoustic metamaterials.

Theory of acoustooptical filtration of light beams in the inhomogeneous acoustic field

The theory of acoustooptical (AO) interaction between light beams in the longitudinally inhomogeneous acoustic field is constructed with allowance for conditions typical of AO filtration. In the case of smooth acoustic inhomogeneities, analytical expressions for the spatially spectral characteristic relating the spatial spectra of incident and diffracted light beams are derived. The situation where a Gaussian light beam undergoes filtration in the acoustic field excited by a rectangular transducer is discussed in detail. The narrowband quasi-collinear AO filter based on a paratellurite crystal is simulated. It is demonstrated that the maximum diffraction efficiency and transmission-function shape distortion (with respect to the plane-wave model) of the given device depend to the greatest extent on the misalignment between the directions of the group velocities of interacting light beams.

Using binaural and spherical microphone arrays to assess the quality of synthetic spatial sound fields

Recently, we completed the Collaborative-Research Augmented Immersive Virtual Environment Laboratory (CRAIVE-Lab) with a usable floor area of 12 × 10 m2 at Rensselaer. The CRAIVE-Lab project addresses the need for a specialized virtual-reality (VR) system for the study and enabling of communication-driven tasks with groups of users immersed in a high-fidelity multi-modal environment located in the same physical space. For the acoustic domain, a 134-loudspeaker-channel system has been installed for Wave Field Synthesis (WFS) with the support of Higher-Order-Ambisonic (HoA) sound projection to render inhomogeneous acoustic fields. An integrated 16-channel spherical microphone array makes the CRAIVE-Lab an ideal test bed to study different spatial rendering techniques such as Wave-Field Synthesis, Higher-Order Ambisonics and Virtual Microphone Control (ViMiC). In this talk, sound-field measurements taken with a traditional binaural manikin will be compared to spherical microphone recordings to assess the quality of the different rendering techniques for large-scale labs. A focus will hereby be set on assessing the sweet spot area for different rendering techniques. [Work supported by NSF 1229391, NSF 1631674, and the Cognitive and Immersive Systems Laboratory (CISL).]

Acousto-optic interaction in an inhomogeneous acoustic field

Light diffraction in an inhomogeneous acoustic field of the near-field zone of a piezoelectric transducer has been investigated for two versions of acousto-optic interaction: collinear diffraction (at a Bragg angle of 90°) and quasi-orthogonal diffraction (at small Bragg angles). The amplitude, angular, and spectral characteristics have been numerically calculated for calcium molybdate and paratellurite crystals. It is established that the acoustic-field inhomogeneity reduces the diffraction efficiency, broadens the angular and spectral interaction ranges, and thus negatively affects characteristics of acousto-optic devices.

Hybrid acousto-optic Fourier processor for imaging spatially inhomogeneous acoustic fields

A new method is proposed for imaging spatially inhomogeneous acoustic fields. The approach is based on the Fourier transform of a coherent light field formed as a result of the Bragg diffraction of a probing laser beam on an ensemble of quasi-plane acoustic waves in a lithium niobate crystal. These waves appear as a result of the transformation of an imaged acoustic field by a spherical acoustic lens. The proposed method has been experimentally verified using model multielement acoustic sources.

Experimental Study of Acoustic Flows in a Focused Ultrasound Field

The development and structure of steady-state acoustic flows in the focal region of a spherical ultrasonic concentrator is first studied using the particle image velocimetry (PIV ) method in order to improve the technology of rapid growth of salt single crystals. By comparing the results of rheological and acoustic measurements, it is shown that the PIV method can be effectively used for rapid diagnostics of a spatially inhomogeneous acoustic field.

Pulse signal detection in a focal area of a convergent wave front

Diffraction effects, taking place during nonlinear transformations in inhomogeneous acoustic fields, are experimentally investigated. The case of a convergent spherical wave front propagating in a uniform nonlinear medium, detection of an acoustic field in a focus, and receiving of the detected signal in the region of the initial wave front aperture are considered. A spherical piezoceramic transducer is used in the experiments as a focusing device. Broad-angle “nonlinear scattering” signals have been recorded at the experimental facility where a pulsed mode of focused transducer operation in water is implemented. The dependence of the amplitude of the signal, detected in the focal area, and its shape on the scattering direction, as well as on the distance between the focus and the receiving point, are studied.

10.1007/s11445-008-2027-x

The growth of the isolated (100) face of a KDP crystal at exposure of the phase boundary to the initial ultrasound field and a standing acoustic wave has been investigated. A significant growth response of this face, exposed to sound normally along the acoustic axis in the near zone of a piston-like half-wave vibrator with f = 20 kHz and tangentially in the pulsed two-frequency (f = 600 and 900 kHz) standing-wave mode, has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are different. The results obtained show a fundamental possibility of controlling crystal growth and dissolution by varying the parameters of inhomogeneous acoustic field.

Study of the dissolution and growth of salt single crystals in inhomogeneous acoustic fields: II. A focused acoustic field

The growth and dissolution of the isolated (100) face of a KDP crystal at exposure of the phase boundary to focused acoustic fields have been investigated. Visualization tools have been developed and the scale and dynamics of the arising acoustic flows have been established. A quadratic dependence of the dissolution rate on the sound pressure has been obtained; such a dependence is in agreement with the theoretical concepts about the dependence of the mass exchange rate in the acoustic boundary layer on the field parameters. A significant growth response of the face under study, normally exposed to ultrasound along the acoustic axis in the prefocal and postfocal planes of a spherical concentrator (f = 1.4 MHz), has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are radically different. The results obtained show a possibility of controlling growth and dissolution of crystals by varying the parameters of an inhomogeneous acoustic field.

Study of the dissolution and growth of salt single crystals in inhomogeneous acoustic fields: I. A standing acoustic wave

The growth of the isolated (100) face of a KDP crystal at exposure of the phase boundary to the initial ultrasound field and a standing acoustic wave has been investigated. A significant growth response of this face, exposed to sound normally along the acoustic axis in the near zone of a piston-like half-wave vibrator with f = 20 kHz and tangentially in the pulsed two-frequency (f = 600 and 900 kHz) standing-wave mode, has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are different. The results obtained show a fundamental possibility of controlling crystal growth and dissolution by varying the parameters of inhomogeneous acoustic field.

Light diffraction in an inhomogeneous acoustic field excited by wedge-shaped and parabolic transducers

Light diffraction in acoustic fields created by wedge-shaped and parabolic piezoelectric transducers is theoretically investigated. It is shown that the acoustic fields have a complicated amplitude-phase structure changing with the acoustic frequency. Principal characteristics of acousto-optic interaction are examined. It is demonstrated that the diffraction efficiency in the Bragg regime can approach 100% in spite of a noticeable phase mismatch.

Analysis of Bragg diffraction of light in periodically inhomogeneous acoustic field

Peculiarities of the Bragg diffraction of light in a periodically inhomogeneous acoustic field are studied. Different variants of amplitude and phase spatial modulation of the acoustic field are analyzed. Amplitude, angular and frequency characteristics are calculated for case of strong acousto-optic interaction.

Effect of inhomogeneous acoustic fields on the noise immunity of a planar receiving antenna

Expressions are obtained for determining the noise immunity of a receiving antenna with a planar aperture under the action of different types of inhomogeneous acoustic fields. The inhomogeneity of these fields is shown to considerably affect the noise immunity of the antenna, which should be taken into account in estimating the antenna efficiency.

Energy paths in edge waves

In this paper the energy streamlines, energy paths, and energy streak lines in a steady or unsteady inhomogeneous acoustic field next to an unstable oscillating boundary, such as a vortex sheet or shear layer, are determined. The theory in the paper applies also to an evanescent wave produced by total internal reflection, and to any other type of edge wave, e.g. a coastally or topographically trapped wave in geophysical fluid dynamics. The idea of the paper is that energy velocity, i.e. energy flux divided by energy density, is defined at every point in space and time, not merely when averaged over a cycle. Integration of the ordinary differential equation for energy velocity as a function of position and time gives the energy paths. These paths are calculated explicitly, and are found to have starting and finishing directions very different from those of cycle-averaged paths. The paper discusses the physical significance of averaged and non-averaged energy paths, especially in relation to causality. Many energy paths have cusps, at which the energy velocity is instantaneously zero. The domain of influence of an arbitrary point on the boundary of a steady acoustic edge wave is shown to lie within 45° of a certain direction, in agreement with a known result on shear-layer instability in compressible flow. The results are consistent with flow visualization photographs of near-field jet noise. The method of the paper determines domains of influence and causality in any wave problem with an explicit solution, for example as represented by a Fourier integral.