Acoustic Field Intensity Research Articles

Mapping of microwave-induced phonons by μ-Brillouin spectroscopy: hypersons in ZnO on silicon

High performance Brillouin microscopy has been used as a versatile method in order to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a ZnO film deposited on silicon. Filtering properties and acoustic field distribution emitted by inter-digital transducers as well as propagation losses are investigated by μ-Brillouin spectroscopy. It turns out that the acoustic field intensity decreases dramatically outside the immediate excitation area situated below the inter-digital finger structure.

Characterization of high intensity focused ultrasound transducers using acoustic streaming

A new approach for characterizing high intensity focused ultrasound (HIFU) transducers is presented. The technique is based upon the acoustic streaming field generated by absorption of the HIFU beam in a liquid medium. The streaming field is quantified using digital particle image velocimetry, and a numerical algorithm is employed to compute the acoustic intensity field giving rise to the observed streaming field. The method as presented here is applicable to moderate intensity regimes, above the intensities which may be damaging to conventional hydrophones, but below the levels where nonlinear propagation effects are appreciable. Intensity fields and acoustic powers predicted using the streaming method were found to agree within 10% with measurements obtained using hydrophones and radiation force balances. Besides acoustic intensity fields, the streaming technique may be used to determine other important HIFU parameters, such as beam tilt angle or absorption of the propagation medium.

Method of analyzing and displaying blood volume using myocardial blood volume map

A contrast echo image of the myocardium and a cardiac chamber obtained by use of ultrasound diagnostic equipment is analyzed so as to determine the volume of blood within the myocardium. The contrast echo image is divided into a plurality of calculation regions, each being considered to have a uniform acoustic field intensity and covering a portion of a myocardium image region and a corresponding portion of a cardiac chamber image region. For each calculation region, a difference in intensity between the myocardium image region and the cardiac chamber image region is obtained, and the myocardium image region is colored in accordance with the difference.

Near-field acoustic intensity measurements using an accelerometer-based underwater intensity vector sensor

An accelerometer-based underwater acoustic intensity vector sensor is used to measure the acoustic nearfield of a single spherical source, and a pair of sources that vibrate in or out of phase with each other. The intensity sensor consists of co-located pressure and inertial sensors within a neutrally buoyant probe body. The design of this probe has been published previously. The measurements were performed in a large tank at a frequency of 5 kHz for two sources of different sizes, corresponding to ka values of 0.7 and 1.2 respectively, where k is acoustic wavenumber and a is the source radius. By way of validation, the acoustic intensity field from two closely spaced, interacting spherical radiators is predicted using the exact theory of the translational addition theorem for spherical wave functions. The predictions using this theory compare favorably well with the measured intensity field. Beam pattern and calibration data obtained for the intensity sensor suggest that underwater acoustic intensity generated by simple and complex sources can be measured to an accuracy of ±1 dB provided that ka is less than approximately 0.2.

The acoustic intensity and directivity of the modern concert harp was investigated

The acoustic intensity and directivity of the modern concert harp was investigated in two distinct scenarios. Previous studies [Bell and Firth, Acustica (1989); Gautier and Dauchez, Appl. Acoust. (2004)] have relied on mechanical excitation of the strings. Their findings identify two main sources for acoustic radiation, the soundboard and the soundbox holes, the second of which proved to be the dominant source in three frequency ranges. To gain better knowledge of the acoustics of the harp under natural playing conditions, new measurements were conducted in which harp players excited the instrument. One particular goal was to demonstrate the effect of the player on the sound radiation. The absorption and diffraction introduced by the performer diminished the relative importance of the radiation from the soundbox holes. A detailed description of the acoustic intensity field with and without the player is given. In addition, the acoustic profiles of a variety of extended techniques used in contemporary music were investigated. A complete timbral description of these techniques demonstrates the breadth of character the modern harp can produce.

Impact of structural support on vector sensor acoustic performance

Vector sensors hold great promise for the exploitation of undersea acoustics by providing the ability to measure the acoustic intensity. Current sensor developments can measure the acoustic acceleration in three dimensions as well as the pressure, which when combined through a cross‐correlation produces the acoustic intensity field surrounding the sensor. Mathematical studies indicate that a gain of 4.6 dB is achievable by beamforming the sensor’s three cardioid outputs with the pressure sensor’s omnidirectional response. In these studies, the mechanics of physically retaining the sensor in position is not considered, but is necessary to allow the sensor to freely sense particle velocity/acceleration and not shadow the sensor to the surrounding pressure field. A set of PVC frames was constructed to hold an individual sensor, which was then combined to form an array of sensors for use in a long‐term shallow water acoustic field intensity study. These frames are modeled to determine the level of forward and backscatter caused by the flooded PVC pipes and the possible impact to the theoretical patterns. The physical structures are then calibrated in a controlled measurement facility to determine if the accelerometers move freely in all three dimensions to generate the cardioid patterns necessary to achieve the gains predicted by intensity models over similar pressure sensor configurations.

Statistics of the Parseval sum, match filter, and instantaneous single‐frequency transmissions in an ocean waveguide

During the ONR Main Acoustic Clutter Experiment of 2003 on the New Jersey Continental Shelf, short duration broadband pulses with 50‐ to 150‐Hz bandwidths were transmitted at 50‐s intervals from a moored source array in the 300‐ to 1500‐Hz range. The signals were measured by a towed horizontal receiving array at varying ranges from the source. The broadband data are processed in a variety of ways including the use of Parseval sum and match‐filter, and as an instantaneous single frequency transmission. The ping‐to‐ping fluctuations in the measured acoustic data are a result of changes in the waveguide modal intereference structure due to both motion of the array and presence of time‐dependent random internal waves. Our analysis shows that the Parseval sum and match‐filter outputs for the direct arrival have significantly smaller standard deviations compared to the instantaneous single frequency transmission. We account for these observations by modeling the broadband acoustic field intensity in both a static waveguide and with Monte Carlo simulations in a fluctuating waveguide, as well as with the use of statistical distributions for the broadband acoustic field. We present an approach for empirically quantifying the range‐dependent degradation in the match‐filter output in a multimodal waveguide.

Vector intensity field scattered by a rigid prolate spheroid

The short-wavelength, steady-state vector intensity field scattered by a rigid prolate spheroid (10:1 fineness ratio) is investigated analytically. The intensity field is the product of the scalar acoustic pressure and the acoustic particle velocity fields which are computed from the Helmholtz equation in prolate spheroidal coordinates. Particular emphasis is placed on results in the forward-scattered direction. It is found that the equivalent plane wave intensity varies by less than ±0.5dB (relative to the incident acoustic intensity) in the far-forward-scattered direction with the selected parameters. This illustrates that the forward-scattered pressure is masked by and interferes with the incident wave. The reactive intensity in the forward-scattered direction is found to be −25dB relative to the incident active intensity, and the computed phase difference between pressure and particle velocity is 4° at ranges approaching 10 spheroid lengths at a reduced frequency of h=41.7; in the absence of the spheroid, the phase difference due to the incident plane wave alone is exactly 0°. The study demonstrates that unique information, extending beyond direction-of-arrival estimation, can be derived from the total acoustic intensity field and may allow inferences to be made regarding the presence or absence of the spheroid.

Stochastic differential equation analysis for ocean acoustic energy scattering by internal waves

Stochastic coupled mode theory is used to study acoustical scattering of a narrow-angle, low-frequency acoustical beam. Random index of refraction perturbation fields are described using the Garrett-Munk internal wave spectrum. The stochastic differential equations for the first- and second-order statistical moments of the acoustical field are derived using the Markov approach similar to Dozier, Tappert, and Chernov theories. The main objective of the research is to account for cross-mode correlations and to show that the method can be used for a calculation of the detailed structure of the mean acoustic field intensity distribution. For this purpose a narrow finite-frequency beam was taken as an example, and the intensity of the scattering field was calculated as a function of frequency and sound source depth. The coefficients of the matrix differential equation were obtained analytically, and an efficient numerical computation was obtained. Comparisons to Monte Carlo simulation show that the method can be used for detailed statistical sound field analysis in long-range propagation.

Acoustic vector fields in underwater waveguides

The ability to compute the sound pressure level as well as the vectors associated with the acoustic particle motion has existed for some time. However, propagation studies and ambient noise investigations have typically focused only upon the sound pressure levels that would be observed by an omnidirectional hydrophone or array of hydrophones. Recent interest in geophones and accelerometers for use as vector and dyadic sensors should encourage the investigation and analysis of the underlying vector fields contributing to the acoustic intensity and energy density fields. The frequency domain properties of the acoustic vector field generated by monopole sources having frequencies <1kHz in a simple iso-velocity waveguide are presented in order to build a fundamental understanding of the related quantities. Subsequently, similar field quantities computed for more realistic environments such as downward refracting profiles and deep-water profiles supporting convergence zone propagation will be discussed. Regions and phenomena associated with perturbations in the energy flux density will be highlighted.

Acoustic intensity measurement of the sound field radiated by a concert harp

An experimental study of the acoustic radiation of a concert harp is presented. Using a stationary mechanical excitation placed at one string location, the acoustic radiation of the complete instrument is investigated. Two main acoustic radiation sources are identified: the soundboard and the soundbox holes. Acoustic intensity vectors are measured in two planes located in the near field of these two sources, making use of a 3D intensity probe. A detailed description of the acoustic intensity field radiated by these two sources is given. Calculations of acoustic power radiated by the soundboard surface and by the soundbox holes allow us to quantify the relative importance of these two sources. In three frequency ranges, it is shown that the soundbox holes radiate more than the soundboard. In the first range, it is suggested that the acoustic radiation of the instrument can be described by two point sources of sound implying that in this range the harp acts as an equivalent elastic Helmholtz resonator.

MEAN AND VARIANCE OF THE FORWARD FIELDS PROPAGATED THROUGH 3D RANDOM INTERNAL WAVES WITH RAYLEIGHT-BORN SCATTERING

The mean and variance of the acoustic field forward propagated through an ocean waveguide containing random internal waves is modelled using a normal mode formulation [1] that takes into account the 3-D interaction of the acoustic field with medium inhomogeneities. The effects of dispersion, attenuation and redistribution of modal energy due to random multiple scattering in the forward azimuth are treated analytically. The inhomogeneous medium's scatter function density is modelled using the Rayleigh-Born approximation to Green's theorem to account for random fluctuations in density and compressibility caused by the internal waves. The generalized waveguide extinction theorem [2] is applied to determine the attenuation due to scattering from the internal wave inhomogeneities. Simulations for typical continental shelf environments quantitatively show how coherent and incoherent acoustic field intensities vary with range, depth and internal wave field properties

The chemical percolation devolatilization model applied to the devolatilization of coal in high intensity acoustic fields : Veras, C. A. G. et al. Journal of the Brazilian Chemical Society, 2002, 13, (3), 358–367

Nicofluprole is a novel insecticide of the phenylpyrazole class conferring selective antagonistic activity on insect GABA receptors. After repeated daily dietary administration to Wistar rats for 28/90 days, Nicofluprole induced increases in thyroid (and liver) weight, associated with histopathology changes. Nicofluprole did not inhibit thyroid peroxydase nor sodium/iodide symporter, two key players in the biosynthesis of thyroid hormones, indicating the absence of a direct thyroid effect. The results seen in rats suggested a mode of action of Nicofluprole driven by the molecular initiating event of CAR/PXR nuclear receptor activation in livers, with key events of increases in liver weight and hypertrophy, decreasing circulatory thyroid hormones, a compensatory increase in TSH release and follicular cell hypertrophy. To explore the relevance of these changes to humans, well established in vitro rat and human sandwich-cultured hepatocytes were exposed to Nicofluprole up to 7 days. A concentration-dependent CYP3A induction (PXR-activation), an increase in T4-glucuronoconjugation accompanied by UGT1A/2B inductions was observed in rat but not in human hepatocytes. The inductions seen with Nicofluprole in rat (in vivo and in vitro in hepatocytes) that were absent in human hepatocytes represent another example of species-selectivity of nuclear CAR/PXR receptor activators. Importantly, the different pattern observed in rat and human models demonstrate that Nicofluprole-related thyroid effects observed in the rat are with no human relevance.

A novel cavitation probe design and some preliminary measurements of its application to megasonic cleaning

Cavitation is an effective physical mechanism for concentrating mechanical energy. Accordingly, it has wide applications in such diverse fields as sonochemistry, in which chemical reactions are initiated or accelerated, or in the electronic component cleaning industry in which particles (and other materials) are removed from surfaces. However, devices designed to act as cavitation monitors have had little success, partly because their intrusiveness often affects the cavitation field itself. Presented here is a brief description of a unique cavitation monitor that utilizes the phenomenon of sonoluminescence as an indirect quantifier of cavitation. It appears to work efficiently over a broad range of acoustic field intensities and its application to megasonic cleaning has provided interesting and valuable insights into this technology.

The Chemical Percolation Devolatilization Model Applied to the Devolatilization of Coal in High Intensity Acoustic Fields

The chemical percolation devolatilization model (CPD) was extended for the prediction of drying and devolatilization of coal particles in high intensity acoustic fields found in Rijke tube reactors. The acoustic oscillations enhance the heat and mass transfer processes in the fuel bed as well as in the freeboard, above the grate. The results from simulations in a Rijke tube combustor have shown an increase in the rate of water evaporation and thermal degradation of the particles. The devolatilization model, based on chemical percolation, applied in pulsating regime allowed the dynamic prediction on the yields of CO, CO2, CH4, H2O, other light gases as well as tar which are important on ignition and stabilization of flames. The model predicted the quantity and form of nitrogen containing species generated during devolatilization, for which knowledge is strategically indispensable for reducing pollutant emissions (NOx) in flames under acoustic excitation .

Underwater acoustic intensity measurements using a pressure/particle-acceleration intensity vector sensor

The acoustic field of simple sources is investigated by measuring the simultaneous, colocated acoustic pressure and particle acceleration with a neutrally buoyant underwater acoustic intensity vector sensor. This sensor consists of a piezoceramic hollow-cylinder pressure transducer, and a pair of miniature accelerometers mounted inside of the cylinder. A free-field calibration is performed under the ice of a flooded quarry, while in a laboratory test tank, the 2-D acoustic intensity field is mapped for various acoustic source configurations. These include a single spherical source and a pair of spherical sources that vibrate in or out of phase with each other. A sinusoidal signal of various frequencies between 2 and 20 kHz is gated in all of these experiments in order to suppress the effects of environmental reflections. The receiving beam pattern of the intensity sensor is also measured and reported. An analytical model for the interaction of two closely spaced spherical radiators is developed, and predictions from it compare well with the measured intensity field. Some limitations in the experiments, and future work are discussed. [Work supported by ONR, Code 321SS.]

Sound-Assisted Fluidized Bed Combustion of Fine Particles

The sound-assisted fluidized bed combustion of powders of tyre derived fuel (TDF) and of a biomass (Robinia Pseudoacacia) has been studied to assess the benefits deriving from the application of an acoustic field on the performance of the combustor. Experiments consisted of steady combustion, either ordinary or sound-assisted, of TDF and Robinia fine particles. High intensity acoustic fields of 120Hz reduce carbon elutriation rates by a factor of about 1.5, increasing the fixed carbon conversion efficiency by 5-8% and 2-3% for TDF and Robinia respectively. Sound frequencies either lower or higher than 120Hz reduce the effectiveness of sound, approaching the performances of ordinary fluidized bed combustors. Results have been analyzed assuming that fine particles are present in the bed either freely moving in the interstices of the bed (free fines) or attached onto coarse inert bed particles (attached fines). Accordingly the inventory of carbon under steady state conditions in the bed depends on the competition between the following processes: elutriation of free fines, adhesion of fines onto bed material, attrition of fine-coarse aggregates and combustion of fine particles. In this framework, acoustic fields have the property of enhancing the formation of fine-coarse aggregates.

Proving an algorithm for calculation of acoustic field intensity

We propose, and prove mathematically, an algorithm for calculation of the acoustic field intensity of a point monochromatic source in an inhomogeneous medium. This algorithm is based on the notion of ray-beam concentration introduced here. Comparative calculations using the proposed and previously known algorithms are performed.

Studying drop cluster and array behavior using an acousto-electric levitator

An acousto-electric levitator is developed for studying the formation and behavior of drop clusters and arrays. The device enables us to form suspended 2-D drop clusters and arrays by using electric and acoustic fields to levitate and manipulate drops. The spacing and the size of drops can be easily controlled by varying electric and/or acoustic field intensities. Experiments have been done on single- and multi-component drops, such as octane and dodecane, which are primary materials in natural fuels. Studies are focused on the evaporation rate of each drop due to the interaction in the array. The acoustic frequency has been increased from 28 kHz to more than 150 kHz so that generation and control of smaller drops, associated with spray combustion, are possible. The formation process of clusters and arrays, and the force balance among drops are analyzed. A theoretical calculation based on the diffusion-controlled model is compared with these experimental results. This high-frequency acoustic levitator is also used to study solid particle clusters whose size is around 10 μm by using a mixture of solid particles and liquid, and then letting the liquid evaporate. [Work supported by NASA through JPL Grant No. 958722 and NASA Grant No. NAG3-2147.]

The study of evaporation of multicomponent drops using the acoustoelectric levitator

The evaporation of single drops and drop arrays of multicomponent liquids is investigated using a novel acoustoelectric levitator. By simultaneous levitation and charging of the drops, 2D drop arrays with the drop diameters varying from 0.2 to 1 mm can be formed. Also by controlling both the acoustic and electric field intensities, arrays with different drop sizes and spaces between drops can be created. This allows us to study the interference effects among drops in an evaporating dense spray. Mixtures of different liquids such as alkanes (heptane, dodecane, and hexadecane) are studied. The evaporation rates of those mixtures with different molar ratios of components and different numbers of drops in an array are measured. The drop size and spacing are carefully controlled and varied. A video camera is used to record the drop size change with time, and the diameters of drops are calculated from the pictures. The results are compared with a theoretical analysis based on the diffusion-controlled evaporation model. [Work supported by NASA through JPL Grant No. 958722.]