Local Acoustic Pressure Research Articles

Acousto-optical metasurfaces for high-resolution acoustic imaging systems

This work introduces a paradigm for acoustic imaging in which a metasurface converts the acoustic waves scattered by remote scenes into coherent light focused into images by conventional optical cameras. The metasurface is composed of acousto-optical unit cells that sense the local acoustic pressure and use the resulting signal to modulate the amplitude of the electric field produced by a laser on an optical aperture. We derive the general design requirements for the image reconstruction in the optical domain and validate the concept through acoustic field measurements of the ultrasound scattered from an object submerged in water followed by numerical simulations predicting how this field is processed by the metasurface and camera. We show that this approach has two main advantages compared to traditional acoustic imaging systems. First, the acoustic-to-optic wavelength down-conversion leads to effective acoustical apertures very large compared to the physical size. Second, the unit cells are not synchronized electronically and thus the complexity of the metasurface increases only linearly with the number of unit cells, which is a significantly slower increase compared to conventional synchronized arrays. This work shows that these advantages lead to compact acoustic cameras providing image resolutions higher than possible with conventional acoustic imaging methods.

Flame-acoustic response measurements in a high-pressure, 42-injector, cryogenic rocket thrust chamber

This work presents measurements of acoustically driven flame dynamics in a 42-element, cryogenic oxygen-hydrogen rocket thrust chamber under supercritical injection conditions. The experiment shows self-excited combustion instabilities for certain operating conditions, and this work describes the nature of the flame dynamics driving the acoustic field, as far as it can be ascertained from state-of-the-art optical measurements. Optical access has been realized in the combustion chamber with both fibre-optical probes and a viewing window. The probes collect point-like measurements of filtered OH* radiation. Their signals were used to calculate the gain and phase of intensity oscillations with respect to acoustic pressure for both stable and unstable operating conditions. Through the window, synchronized high-speed imaging of the flame in filtered OH* and blue radiation wavelengths was collected. The 2D flame response was related to the local acoustic pressure to investigate the distributed intensity and phase relationships. The flame response from OH* measurements is in agreement with the theory of Rayleigh. For stable conditions the oscillations of combustion and pressure were out of phase, whereas for an excited chamber 1T mode the oscillations were closely in phase. The integrated Rayleigh index from blue imaging was not consistent with the OH* results. The reason lies in the depth of field captured by this type of imaging, and must be used in a complementary fashion together with OH* imaging. The flame response values and 2D visualization presented in this work are expected to be of value for the validation of numerical modelling of combustion instabilities.

Raman Spectroscopy Diagnostics of the Local Time Profile of an Ultrasound Beam in Water

It has been demonstrated for the first time that pulsed laser Raman spectroscopy can be used for diagnostics of a local acoustic pressure profile with a peak pressure drop of 50 MPa and a carrier frequency of 2.0 MHz in the focus of an ultrasound beam propagating in water. A 527-nm 10-ns laser pulse has been focused into the waist of the ultrasound beam at an angle of 90°. Backscattered photons have been recorded in a gated spectrum analyzer. It has been found that the Raman spectra at the times corresponding to the maximum and minimum acoustic pressures are significantly different. This feature has been used for point-to-point reconstruction of the acoustic pressure profile; for this purpose, the delay between the ultrasound and laser pulses is consequently increased with a step of 50 ns. It has been shown that, within the measurement error, the resulting changes in the position of the center of the stretching OH vibration band of water molecules in the Raman spectrum reproduce the acoustic pressure profile directly measured using a PVDF hydrophone at the laser sensing point. The results obtained can be used to develop a new method for remote diagnostics of the time profile of acoustic pressure and monitoring the local dynamic of the compression-tension processes in water up to critical pressures corresponding to the cavitation rupture, when the use of the hydrophone can lead to its damage.

Blood-brain barrier disruption in humans using an implantable ultrasound device: quantification with MR images and correlation with local acoustic pressure.

One of the goals in this study was to set up a semiautomatic method to estimate blood-brain barrier disruption obtained in patients with glioblastoma by using an implantable, unfocused, ultrasound device. Another goal was to correlate the probability of significant ultrasound-induced signal enhancement (SUISE) with local acoustic pressure in the brain. Gd-enhanced MR images acquired before and after ultrasound treatments were analyzed prospectively. The image sets were segmented, normalized, and coregistered to evaluate contrast enhancement. The volume of SUISE was calculated with voxels labeled as gray or white matter, in a cylindrical region of interest, and with enhancement above a given threshold. To validate the method, the resulting volumes of SUISE were compared to qualitative grades previously assigned by 3 clinicians for 40 ultrasound treatments in 15 patients. A parametric study was performed to optimize the algorithm prediction of the qualitative grades. The 3D acoustic field in the brain was estimated from measurements in water combined with simulations accounting for ultrasound attenuation in brain and overlaid on each MR image to correlate local acoustic pressure with the probability of SUISE (defined as enhancement > 10%). The algorithm predicted grade 2 or 3 and grade 3 openings with areas under the receiver operating characteristic curve of 0.831 and 0.995, respectively. The probability of SUISE was correlated with local acoustic pressure (R2 = 0.98) and was 3.33 times higher for gray matter than for white matter. An algorithm for evaluating blood-brain barrier disruption was validated and can be used for future clinical trials to further understand and quantify this technique in humans.Clinical trial registration no.: NCT02253212 (clinicaltrials.gov).

Acoustic Characterization of the CLINIcell for Ultrasound Contrast Agent Studies.

Ultrasound contrast agents consist of gas-filled coated microbubbles that oscillate upon ultrasound insonification. Their characteristic oscillatory response provides contrast enhancement for imaging and has the potential to locally enhance drug delivery. Since microbubble response depends on the local acoustic pressure, an ultrasound compatible chamber is needed to study their behavior and the underlying drug delivery pathways. In this study, we determined the amplitude of the acoustic pressure in the CLINIcell, an optically transparent chamber suitable for cell culture. The pressure field was characterized based on microbubble response recorded using the Brandaris 128 ultrahigh-speed camera and an iterative processing method. The results were compared to a control experiment performed in an OptiCell, which is conventionally used in microbubble studies. Microbubbles in the CLINIcell responded in a controlled manner, comparable to those in the OptiCell. For frequencies from 1 to 4 MHz, the mean pressure amplitude was -5.4 dB with respect to the externally applied field. The predictable ultrasound pressure demonstrates the potential of the CLINIcell as an optical, ultrasound, and cell culture compatible device to study microbubble oscillation behavior and ultrasound-mediated drug delivery.

A Method for Sizing Noise Protective Unit Design Features

The purpose of this paper is to develop a sizing method to decide the best design features of a noise protective unit (NPU). The method of “inverse matrix” was used to assess the local acoustic pressure drop when a wave goes through the multi-layer shell of the NPU. Solutions of the wave equations were used in each of the layers taking into account: 1) the incidence angle of the sound waves and the layer loss factor; 2) the input boundary conditions establishing the equality of the normal components of particle velocity and acoustic pressure at medium borders; 3) the system of algebraic complex equations and corresponding matrix equations. The telemetric information obtained during a real launch was processed by means of specially developed software. A novel method of sizing the best design features for a noise protective unit makes it possible to reduce vibroacoustic loads on spacecraft to acceptable levels. The comparison of the data calculated using this method and the experimental data showed their good agreement. The devised method appears to have considerable promise when developing new launch vehicles able to protect spacecraft from noise and vibration. At critical frequencies the levels of acoustic pressure decreased by half to two thirds.

Imaging local acoustic pressure in microchannels

A method for determining the spatially resolved acoustic field inside a water-filled microchannel is presented. The acoustic field, both amplitude and phase, is determined by measuring the change of the index of refraction of the water due to local pressure using stroboscopic illumination. Pressure distributions are measured for the fundamental pressure resonance in the water and two higher harmonic modes. By combining measurement at a range of excitation frequencies, a frequency map of modes is made, from which the spectral line width and Q-factor of individual resonances can be obtained.

Sonoluminescence in varying acceleration environments

In order to prepare for experiments in variable-g environments, some of the multiple effects caused by the time-varying acceleration of acoustic resonators used in bubble levitation experiments are considered. The coupled effects of the induced changes in ambient pressure (due to a changing hydrostatic head) and bubble position (due to a change in buoyant body force) were modeled. Changing the ambient pressure, while holding the acoustic pressure amplitude constant, causes changes in the radial bubble response and diffusive equilibrium requirements. Changing the bubble levitation position causes both the local acoustic pressure amplitude and gradient to change, which will again impact bubble response. If the bubble remains in a stable diffusive equilibrium, both of these effects will force the bubble’s equilibrium radius to change. By using an empirical relation for emitted sonoluminescence intensity versus bubble response, the variation of emitted light intensity as a function of the changing ambient acceleration can be predicted. The predicted results are shown to agree quantitatively with existing experimental data from other research groups. An experiment was designed and built to fly onboard a KC-135 aircraft that has been reinforced for parabolic flight maneuvers to simulate micro and hyper gravity acceleration. Thesis advisor: R. Glynn Holt Copies of this thesis may be obtained by contacting the advisor, Glynn Holt, Dept. of Aerospace and Mechanical Engineering, Boston University, 110 Cummington St., Boston, MA 02215. E-mail address: rgholt@bu.edu

Potential errors in contrast-agent-mediated estimates of regional myocardial perfusion arising from myocardial anisotropy

Recently developed imaging modalities based on the nonlinear response of contrast agents are dependent upon the local acoustic pressure. Thus, regional variations of acoustic pressure arising from myocardial anisotropy can result in nonuniform ‘‘apparent perfusion’’ in the absence of a perfusion defect. Measurements of regional variation of myocardial attenuation in the septum and lateral walls of excised sheep hearts are reported. Studies were carried out on hearts obtained immediately after slaughter from 8 healthy adult sheep. These measurements of attenuation were carried out in orientations analogous to those encountered in the apical four-chamber view, which is the preferred view for clinical studies of myocardial perfusion using contrast-agent-enhanced echocardiography. The video signal analysis method introduced by the University of Wisconsin group was used to analyze data obtained with a commercially available (ATL) clinical scanner using a linear array (L 7-4) operating in fundamental mode. Tissue mimicking phantoms were imaged under identical conditions. Normalizing sample data by reference data from the phantoms yields quantitative estimates of slope of attenuation. Results of these experimental measurements document significant transmural variations in attenuation that could lead to apparent perfusion anomalies in contrast-agent-mediated estimates of regional myocardial perfusion. [Work supported by NIHHL40302.]

Bubble dynamics and SBSL in a variable acceleration environment

In order to prepare for pending experiments in variable g environments, some of the multiple effects caused by the time‐varying acceleration of acoustic resonators used in bubble levitation experiments were considered. The coupled effects of the induced changes in ambient pressure (due to a changing hydrostatic head) and bubble position (due to a change in the buoyant body force) were modeled. Changing the ambient pressure, while holding the acoustic pressure amplitude constant, causes changes in the radial bubble response and diffusive equilibrium requirements. Changing the bubble levitation position causes both the local acoustic pressure amplitude and gradient to change, which will again impact bubble response. If the bubble is required to remain in a stable diffusive equilibrium, both of these effects will force the bubble equilibrium radius to change. Finally, by using an empirical relation for emitted SL intensity versus bubble response, the variation of emitted light intensity as a function of the changing ambient acceleration is obtained. Where possible, these results are compared to experimental data. [Work supported by NASA.]

The effects of ambient acceleration on bubble levitation and SBSL

In order to prepare for pending experiments in variable-g environments some of the multiple effects caused by the time-varying acceleration of acoustic resonators used in bubble levitation experiments are considered. The coupled effects of the induced changes in ambient pressure (due to a changing hydrostatic head) and bubble position (due to a change in the buoyant body force) are modeled. Changing the ambient pressure, while holding the acoustic pressure amplitude constant, causes changes in the radial bubble response and diffusive equilibrium requirements. Changing the bubble levitation position causes both the local acoustic pressure amplitude and gradient to change, which will again impact bubble response. If the bubble must remain in stable diffusive equilibrium, both of these effects will force the bubble equilibrium radius to change. Finally, by using an empirical relation for emitted SL intensity versus bubble response, the variation of emitted light intensity as a function of the changing ambient acceleration is obtained. [Work supported by NASA.]

Sonochemical reactor optimization using computational acoustics techniques

The designer of a sonochemical reactor is confronted by a complex problem characterized by a large number of design parameters: reactor shape and dimensions, number, positions and power of ultrasound sources, frequency, solvant, temperature, etc.? A wide parametric study cannot be conducted experimentally due to time and cost constraints and a numerical model seems the only viable approach. Such a model must reflect the inherent complexity of the system and at the same time be simple enough to provide relevant results in a reasonable amount of time. This paper proposes an iterative modeling methodology based on the following steps: (1) a finite-element model of the fluid in the reactor, (2) a relationship between local acoustic pressure and gas fraction released by cavitation, and (3) equivalent acoustic properties for the cavitating medium. After a few iterations, this model leads to two main results: (1) the pressure field in the reactor and (2) the cavitation zone and the distribution of gas fraction in this cavitation zone. One can further postprocess the pressure field to predict the streaming forces on the fluid and therefore predict the flow pattern using classical CFD techniques.

Application of a nonlinear layered model to hyperthermia

The use of ultrasound as a heating modality in cancer therapy gives rise to the need for delivering prescribed exposures of high‐intensity ultrasound to deep tissue volumes. It is known that the attenuation coefficient of nonlinear media depends upon the local acoustic pressure amplitude for a given fundamental frequency. It is also known that such anomalous effects occur at biomedical frequencies and intensities [e.g., E. L. Carstensen et al., Acustica 51, 116–123 (1982) and F. Dunn et al., Br. J. Cancer 45, 55–58 (1982)]. In this presentation a series algorithm [M. E. Haran and B. D. Cook, J. Acoust. Soc. Am. 73, 774–779 (1983)] is applied to study the propagation of plane finite amplitude waves through multiple layered media. It is shown that in some cases harmonic distortion and the resulting anomalous attenuation are sufficient to reduce the heat generation to a large degree. It is hoped that this elementary model will aid in the development of more effective acoustic sources and exposure protocols.

Pulsating combustion of coal in a Rijke type combustor

This paper describes results obtained in an investigation of the feasibility of burning coal in a pulsating mode of combustion in a combustor whose design was based upon the Rijke tube principles. The combustor consisted of a vertical tube opened at both ends with a coal burning bed located in the middle of its lower half. In this configuration, heat supplied by the combustion process spontaneously excites the fundamental, longitudinal acoustic mode of the combustor. Once excited, the interaction between the combustion process and the local acoustic pressure and velocity oscillations helps to maintain the oscillations and it also results in a more efficient and intense combustion process. This study demonstrated that in contrast to the difficulties and limitations experienced by previously developed coal burning pulsating combustors, the combustor constructed for this investigation can burn unpulverized coal stably and continuously under either self aspirating or forced flow modes of operation. In the latter case, either fuel rich or fuel lean operation is possible with optimum operation occurring near stoichiometric conditions, suggesting that systems utilizing a coal burning, Rijke type pulsating combustor should possess high thermal efficiens. The fact that the combustor can operate under fuel rich conditions indicates that it can possibly be used as a coal gasifier. Finally, the paper describes CO, CO2, and particulate productions by the device and its uses the CO and CO2 concentrations to determine combustion efficiencies which ranged between 89 and 98.5 percent for air/fuel ratios between 1.03 and 1.22, respectively.

Do Locally Reacting Acoustic Liners Always Behave as They Should?

BY definition, a liner is locally reacting (or point reacting) when the local response of the liner is dependent only upon the local acoustic pressure and not upon the structure of the acoustic field in the duct. This implies that no sound propagation may take place in the liner parallel to the liner surface. Helmholtz resonator-type acoustic liner materials are normally assumed to be locally reacting. However, it is shown that different behavior arises when neighboring cells are interconnected (e.g., by water drain holes) or when the cell cross dimensions are not small enough with respect to acoustic wavelength. The assumption of local reaction is then violated and the liner intended to be locally reacting will not behave as it should, that is, not as expected. In these cases the twomicrophone technique of in-situ impedance measurement will provide erroneous data. Further, it is shown that when using the two-microphone technique on really locally reacting liners the location of the surface microphone has to be chosen carefully in order to provide accurate data.

Response of Anisotropic Cylindrical Shells due to Local Acoustic Pressure

The dynamic equation of motion for anisotropic shells is first formulated with the effects of internal fluid, internal pressure, and viscous damping taken into account. For practical purposes, the system is reduced to a Donnell-type equation with the radial displacement as the single dependent variable. The response of the shell due to local acoustic pressure of discrete frequency is then analyzed by using the normal-mode solution. The calculation is facilitated by first plotting the free vibration spectra of the cylinder. Based on the spectra, the resonant modes for a given forcing frequency are isolated and the response can be accurately calculated by considering only the components of the pressure identical to those modes. Since the Fourier-series expansion for a local load usually involves thousands of modes, the process proposed here greatly simplifies the response calculation. Numerical results obtained herein are presented in charts.