Direction Of Acoustic Propagation Research Articles

GaN surface acoustic wave filter with low insertion loss

Surface acoustic wave (SAW) filter with a low insertion loss (IL) of 4.415 dB has been demonstrated on Carbon-doped semi-insulating c-plane bulk GaN without external lumped element matching. The center frequency, 3 dB bandwidth, out-of-band attenuation, return loss of the filter are 477.05 MHz, 0.308 MHz, 32.5 dB, and −9.72 dB, respectively. The electromechanical coupling coefficient (Kt2), and temperature coefficient of frequency (TCF) of the filter are 0.21 % and −26.0 ppm/°C, respectively. The impact of the number of interdigital transducers (NIDT) and acoustic propagation direction on filter performance has been studied. The IL of filters reduces from 16.07 dB to 4.415 dB with the increase of NIDT from 50 to 150 due to the enhanced acoustic superposition. The numerical distribution of elastic stiffness ([cij]), and piezoelectric constants ([eik]) of GaN has been calculated in Euler angle space, indicating that they are isotropic on c-plane. The small performance difference of filters along the m- and a-direction on c-plane bulk GaN can be attributed to the small offset angle of 0.5° of the bulk GaN wafer or IDT quality variation.

Bulk GaN-based SAW resonators with high quality factors for wireless temperature sensor

Surface acoustic wave (SAW) resonator with outstanding quality factors of 4829/6775 at the resonant/anti-resonant frequencies has been demonstrated on C-doped semi-insulating bulk GaN. The impact of device parameters including aspect ratio of length to width of resonators, number of interdigital transducers, and acoustic propagation direction on resonator performance have been studied. For the first time, we demonstrate wireless temperature sensing from 21.6 to 120 °C with a stable temperature coefficient of frequency of –24.3 ppm/°C on bulk GaN-based SAW resonators.

An Experimental and Theoretical Study of Impact of Device Parameters on Performance of AlN/Sapphire-Based SAW Temperature Sensors.

The impact of device parameters, including AlN film thickness (hAlN), number of interdigital transducers (NIDT), and acoustic propagation direction, on the performance of c-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (λ) of 8 μm, was investigated. The results showed that resonant frequency (fr) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in hAlN/λ favored the improvement of TCF. The acoustic propagation direction and NIDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher fr, Q and than those along the m-direction, and sensors with NIDT of 300 showed higher Q and values than those with NIDT of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher fr of the sensor along the a-direction than that along the m-direction can be attributed to its larger elastic stiffness c11, c22, c44, and c55 values.

Research on Factors Affecting the Imaging Resolution of Magneto–Acousto–Electrical Tomography

As the conductivities of healthy tissue and cancerous tissue are different, methods for detecting conductivity variations of biological tissue are potential medical imaging modalities for early diagnosis of cancer in the future. Magneto-acousto-electrical tomography (MAET) is a noninvasive and hybrid conductivity imaging technology that integrates the advantage of high contrast. It has been demonstrated to have an excellent capability to distinguish conductivity variations along the direction of acoustic propagation and extremely promising as an alternative medical imaging technology for early detection of cancer. However, the existing MAET has a low resolution, and the factors affecting the imaging resolution of MAET are rarely studied systematically. Therefore, we firstly designed an MAET detection system and performed several verification experiments. A B-scan algorithm was then proposed for enhancing the system resolution. Subsequently, two uniform phantoms with different intermediate gaps in the middle and a B-mode imaging experiment on pork tissue were used for testing the performance of detection resolution. Finally, comparative analyses were performed to verify whether the frequency, the cycle number of the excitation signal, and the B-scan algorithm can influence the detection resolution. We obtained the following results: 1) The longitudinal resolution of the conductivity B-scan image increases as the number of cycle excitation decreases. 2) Vertical resolution using a 2.5 MHz probe excitation is significantly better than using a 500 kHz probe excitation. 3) The B-scan algorithm can improve conductivity resolution, and it reveals that our proposed detection system's longitudinal resolution can reach 1mm. 4) we obtained the conductivity profile of pork tissue. The results showed that our detection platform could accurately distinguish the target sample's interfaces of conductivity variations, increasing the excitation frequency, reducing the cycle number, and adopting the B-mode imaging algorithm could improve its conductivity detection resolution, which laid the foundation for the design of a high-resolution MAET detection system.

Thermally Induced Magnetic Anisotropy in Nickel Films on Surface Acoustic Wave Devices

The combination of magnetostrictive thin films with surface acoustic wave (SAW) devices enables the magnetic sensors that exploit the AE-effect (magnetically induced change in modulus) without the need for free-standing structures that are susceptible to vibration and damage. The performance of these sensors is strongly influenced by magnetic anisotropy and, therefore, the state of stress in the magnetostrictive film. We report the fabrication and characterization of magnetic SAW devices comprised of a magnetostrictive layer and interdigital transducers on a piezoelectric substrate. The state of stress in the magnetostrictive layer is varied by annealing. Vibrating-sample magnetometer measurements indicate that the annealed devices show a strong uniaxial anisotropy with the easy axis parallel to the direction of acoustic propagation. The induced magnetic anisotropy is attributed to the anisotropic thermal expansion in the substrate. The change in frequency of one-port resonators and two-port SAW oscillators is measured as a function of dc bias field and ambient temperature. The normalized change in frequency (Δf/f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) is on the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . These results demonstrate that the film stress can be used to optimize the performance of magnetic SAW devices and the SAW devices can be used to characterize the state of stress and magnetic anisotropy of magnetostrictive films.

Large eddy simulations of a transcritical round jet submitted to transverse acoustic modulation

This article reports numerical computations of a turbulent round jet of transcritical fluid (low temperature nitrogen injected under high pressure conditions) surrounded by the same fluid at rest under supercritical conditions (high temperature and high pressure) and submitted to transverse acoustic modulations. The numerical framework relies on large eddy simulation in combination with a real-gas description of thermodynamics and transport properties. A stationary acoustic field is obtained by modulating the normal acoustic velocity at the lateral boundaries of the computational domain. This study specifically focuses on the interaction of the jet with the acoustic field to investigate how the round transcritical jet changes its shape and mixes with the surrounding fluid. Different modulation amplitudes and frequencies are used to sweep a range of conditions. When the acoustic field is established in the domain, the jet length is notably reduced and the jet is flattened in the spanwise direction. Two regimes of oscillation are identified: for low Strouhal numbers a large amplitude motion is observed, while for higher Strouhal numbers the jet oscillates with a small amplitude around the injector axis. The minimum length is obtained for a Strouhal number of 0.3 and the jet length increases with increasing Strouhal numbers after reaching this minimum value. The mechanism of spanwise deformation is shown to be linked with dynamical effects resulting from reduction of the pressure in the transverse direction in relation with increased velocities on the two sides of the jet. A propagative wave is then introduced in the domain leading to similar effects on the jet, except that a bending is also observed in the acoustic propagation direction. A kinematic model, combining hydrodynamic and acoustic contributions, is derived in a second stage to represent the motion of the jet centerline. This model captures details of the numerical simulations quite well. These various results can serve to interpret observations made on more complex flow configurations such as coaxial jets or jet flames formed by coaxial injectors.

Frequency tunable surface magneto elastic waves

We use the transient grating technique to generate narrow-band, widely tunable, in-plane surface magnetoelastic waves in a nickel film. We monitor both the structural deformation of the acoustic wave and the accompanying magnetic precession and witness their intimate coupling in the time domain. Strikingly, when an in plane magnetic field is applied parallel to the acoustic propagation direction, we witness its resonant coupling to the ferromagnetic resonance.

Failure of the Ingard–Myers boundary condition for a lined duct: An experimental investigation

This paper deals with experimental investigation of the lined wall boundary condition in flow duct applications such as aircraft engine systems or automobile mufflers. A first experiment, based on a microphone array located in the liner test section, is carried out in order to extract the axial wavenumbers with the help of an "high-accurate" singular value decomposition Prony-like algorithm. The experimental axial wavenumbers are then used to provide the lined wall impedance for both downstream and upstream acoustic propagation by means of a straightforward impedance education method involving the classical Ingard-Myers boundary condition. The results show that the Ingard-Myers boundary condition fails to predict with accuracy the acoustic behavior in a lined duct with flow. An effective lined wall impedance, valid whatever the direction of acoustic propagation, can be suitably found from experimental axial wavenumbers and a modified version of the Ingard-Myers condition with the form inspired from a previous theoretical study [Aurégan et al., J. Acoust. Soc. Am. 109, 59-64 (2001)]. In a second experiment, the scattering matrix of the liner test section is measured and is then compared to the predicted scattering matrix using the multimodal approach and the lined wall impedances previously deduced. A large discrepancy is observed between the measured and the predicted scattering coefficients that confirms the poor accuracy provided from the Ingard-Myers boundary condition widely used in lined duct applications.

Impact on acoustic propagation by internal waves and tides in the region of shelf and slope.

Internal waves (IWS) and tidal activities were investigated in the South China Sea (SCS) and the northeastern seas of Taiwan. These oceanic processes cause large fluctuations and impact on underwater acoustic propagation. These effects include two-dimensional (2-D) and three-dimensional (3-D) effects. The 2-D and 3-D effects are related to the angle between the directions of sound propagation and IW front. When the IW front are from 20 to 90 deg with respect to the acoustic propagation direction, acoustic mode coupling is the dominant factor which could be sufficiently predicted by the N×2D simulations. Acoustic energy is exchanged between modes and is re-distributed among the water columns. However, when the angles between the wave front and the acoustic wave propagation are 0–20 deg, the horizontal refraction effect dominates over mode coupling and the fully 3-D calculation is needed. The acoustic energy would be refracted, as a consequence, resulting in energy focusing and defocusing. These effects are clearly seen by the series of data collected in the SCS and the region of Northern East of Taiwan. Computer modeling results are used to manifest experiment data results in this research. [This work is supported by National Science Council of Taiwan.]

Internal wave observations and transmission losses in the Santa Barbara Channel

Measurements from three vertical line arrays of temperature probes, deployed in a triangular configuration in the Santa Barbara Channel in August 2002, permitted determination of speed and direction of tidally generated internal solitary wave events (ISWEs). Two arrays were deployed 2 km apart along a 60-m isobath, with the third normal to the isobath. Sound propagation measurements were also conducted along the isobath. Highly correlated ISWEs were evident on all three arrays every 12 h. Measurements of time delays suggest that these waves emanated from a single site between two of the Channel Islands. Because the ISWE propagation direction was nearly normal to the acoustic propagation direction, acoustic mode coupling due to ISWEs is expected to be minimal. Time-series temperature measurements reveal that average near-surface sound speeds were systematically different when ISWEs were present or absent, and changed abruptly during transitions. Numerical simulations with a normal mode propagation code (C−SNAP) indicate that frequency-dependent differences in transmission loss on near-bottom hydrophones may be as large as 7 dB in narrow frequency bands (5%–10%) between 0.5 and 2 kHz when an ISWE is present versus when it is absent. [Work supported by ONR.]

Influence of shallow water currents on the performance of a broadband time-reversing array

In recent shallow water experiments, the temporal and spatial focusing properties of time-reversing arrays (TRAs) were shown to be robust in a reciprocal medium and useful for underwater applications. The presence of oceanic currents in coastal environments leads to nonreciprocal acoustic propagation. In this case, time-reversal invariance is modified because the propagation speed inhomogeneity depends on the direction of acoustic propagation. Therefore, similarly to phase coherent reciprocal transmissions, a TRA will be influenced by the current-induced effects but not by the scalar contributions due to temperature or salinity. TRA performance, in the presence of steady currents, is investigated both theoretically using a simple first order normal mode formulation and numerically using a parabolic equation code for moving media, GCPEM [D. Mikhin, J. Acoust. Soc. Am. 105, 1362 (1999)]. In a multipath shallow ocean environment, the retrofocus field is shifted relative to its location in a nonmoving medium. This shift depends on the current speed and the range-depth dependency of the ocean current profile because each acoustic mode is influenced differently. The possibility of using TRAs for monitoring coastal currents will be discussed.

In vivo modeling of the HM3 lithotripter

A computer model for the propagation of shock waves in the HM3 lithotripter has recently been developed using the KZK equation [Averkiou and Cleveland, J. Acoust. Soc. Am., submitted]. The model has been extended to account for propagation through inhomogeneous tissue. The tissue was modeled as a layered medium with properties varying in the direction of acoustic propagation. Tissue absorption was modeled as consisting of four relaxation processes which provided an excellent fit to empirical absorption characteristics. Results were compared to waveforms measured in pigs [Cleveland et al., Ultrasound Med. Biol. 24, 293–306 (1998)]. Measured and calculated waveforms were in good agreement, including the rise time of the shock. However, at the focus measured, peak pressure was less than calculated numerically. It is supposed that small-scale arbitrary inhomogeneities, that the model does not account for, produced aberration effects that lead to the reduction in measured amplitude. The model predicted large negative pressures in the perirenal fat where cavitation events have been detected. [Work supported by NIH PO1-DK43881.]

Acousto-optic superlattice modulator using a fiber Bragg grating

A modulator is reported in which an extensional acoustic wave is launched along a fiber Bragg grating. The acousto-optic superlattice effect causes an enhancement in reflectivity within a narrow spectral region on both sides of the Bragg wavelength. For a fixed acoustic propagation direction, the Doppler shift can be either positive or negative, depending on whether the wavelength of the incident light lies above or below the Bragg condition. The device can function as a Bragg cell and a tunable filter.

TEMPORAL CHARACTERISTICS OF ACOUSTIC RAY PROPAGATION THROUGH "INFINITE" AND "BOX MODEL" TURBULENCE

In this work, we investigate the temporal characteristics of acoustic ray propagation through simulated, weakly turbulent temperature fields. In a first set of experiments, we generate ensembles of random scalar fields from randomly oriented Fourier temperature modes. Then, by integrating the ray trace equations, we estimate the distribution of arrival times for rays propagating a distance R through them. We demonstrate that these arrival time distributions are Gaussian for both axial and 3-D propagation and are primarily determined by the lower wave numbers of the 1-D fluctuation spectrum. In a second set of experiments, we generate random fields comprised of Fourier modes prescribed on a lattice, as in "box model" turbulence. In these simulations, we find that acoustic travel times are significantly affected both by the periodicity of the fields and by the direction of acoustic propagation with respect to the orientation of the box. Both effects can ultimately be attributable to an inadequate representation of the low wave number region of the 1-D spectrum. We suggest that these artifacts of simulated periodic fields may preclude their use for acoustic propagation studies.

Internal‐tide inversions from a 1000‐km Pacific acoustic propagation experiment in the Pacific

The Slice89 experiment sent sound pulses through the Pacific over a 1000‐km path. Different identified wavefronts showed clear M2 internal‐tidal signals in their arrival‐time series. we have examined internal‐tide models composed of the first three vertical modes in order to fit the Slice89 data. The horizontal directions of the three modes were allowed to be different. The horizontal structure of the models varied from plane waves to waves that are localized along the direction of acoustic propagation. It has been found that the best‐fit internal‐tide model within the parametrization, and the resulting fit is reasonable. These results along with those of other experiments in the same geographical area have been compared.

Estimating backscattering strength for a correlation log

A correlation log is a type of sonar system used for measuring the velocity of a surface vessel or of a submersible vehicle. The operation is similar to that of a conventional echo sounder, and the direction of acoustic propagation is vertically downwards. Since a correlation log can be used for either bottom tracking or water tracking, it is of interest to relate the transmitted power level to the echo levels from both bottom and volume scatterers, which is a means of predicting the log's performance. This is done by modeling the transmission channel in terms of the sonar equations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Equivalent-circuit model for interdigital transducers with varying electrode widths

A recently developed bulk-wave equivalent-circuit model that allows the electric fields in a transducer to be arbitrary functions of distance along the direction of acoustic propagation has been adapted for surface-acoustic-wave transducers. Experimental results are presented that confirm the validity of this model.

Observation of optical activity in Brillouin scattering experiments

Analytical expressions have been developed for calculating the diffraction geometry for Brillouin scattering experiments in which the applied acoustic beam and the optical wave vectors lie in a plane containing the optic axis of uniaxial crystals. A computer program solves for the diffraction angles for the general cases where the choice of crystal, crystal face, direction of acoustic propagation, plane of optical incidence, and amount of optical activity are all arbitrary. It is shown that for acoustic propagation along the X axis in quartz, the optical activity produces easily detectable changes in the angles of the incident and diffracted light beams in the XZ plane when polarization change occurs on diffraction. Moreover for this case, the optical activity produces a cutoff acoustic frequency below which Brouillin scattering with polarization rotation does not occur. Experimental measurements verify the predicted angular changes to within the accuracy permitted by the finite acoustic beam width.

Magnetoacoustic Studies of the Fermi Surface of Aluminium

AbstractMagnetoacoustic measurements are made on very pure aluminium single crystals (R293 °K/R4.2 °K up to 26 500) in the frequency range 70 to 150 MHz. The directions of acoustic propagation are 〈100〉, 〈110〉, 〈111〉, 〈112〉, and 16° from 〈120〉 towards the nearest 〈100〉. In all cases a large number of oscillations are observed. The deduced Fermi surface dimensions for the second zone are in general agreement with Harrison's free‐electron model. The data obtained with the direction of propagation along the three principal crystallographic directions agree with earlier investigations by KAMM and BOHM, and BEZUGLYI et al. Third zone dimensions are obtained which are in agreement with Ashcroft's calculations. The conditions for magnetoacoustic oscillations are discussed. Large‐angle intersection between the plane of the electron orbit and the contour at the point of interaction with the Fermi surface seem to be a fundamental requirement for pronounced oscillations. With wave vector along 〈110〉 orbits are reported which are probably due to magnetic breakthrough at a field strength of 370 Gauss.