Acoustic Velocity Distribution Research Articles

Localization of acoustical sources rotating in the cylindrical duct using a sparse nonuniform microphone array

The Doppler effect caused by the rotation of turbine fans brings significant challenges to the identification of fan noise sources inside the nacelle. Current methods of rotating source localization inside the cylindrical duct are executed with tough requirements for the microphone array mounted on the duct wall, which includes a fair enough number of microphones and equal-space distribution of each microphone in the azimuthal direction. A methodology based on nonuniform measurements and duct spin modes superposition (NMDMS) is proposed to identify rotating sources with high spatial resolution and few side lobes even near cut-on frequencies, which requires much fewer microphones and no uniform distribution of each microphone in the circumference. The sparsity in the azimuthal domain of the duct field generated by multiple rotating sources is verified theoretically, after which the azimuthal mode in each duct cross-section is reconstructed by the inhomogeneous measurements through the orthogonal matching pursuit (OMP) method. Followed by the identification of mode amplitude in the rotation frame, the sound pressure distribution as well as the axial acoustic velocity distribution on the source plane is reconstructed through duct modes summation. Numerical simulations and experiments are implemented to validate the method. Localization results indicate that rotating sources identification using the proposed method could not only save more than half the microphones without requirements of equal-space distribution but obtain good accuracy of localization and remarkably fewer side lobes.

A damage localization technique using wave front shapes in composite laminates without knowing the velocity profile

Composite laminates are widely used in various fields, but their structures are prone to cracks and damage. Due to the difference in angles of the instantaneous direction of the wave front propagation and the direction of the energy flow in an anisotropic material, the use of Lamb waves for damage localization in composite laminates is a challenging task. Establishing the wave front shape equation can overcome the difficulty of damage localization caused by anisotropy, but this usually requires a priori knowledge of the acoustic velocity distribution of the laminates, which is not convenient for efficient damage localization. In this paper, a damage localization method based on wave front shapes for composite laminates without any knowledge of the velocity profile is presented. Numerical simulation and experimental results show that the proposed method works. This method shows good damage localization accuracy and has broad application prospects in non-destructive testing for plate structures with strong anisotropy.

Comparative research on 3D visualization of underwater acoustic fields using near real-time database

In order to realize the near real-time prediction of the ocean acoustic field, this paper conducted a comparative study of HYCOM data and NMDC data for the application of hydrographic datasets, and drew pseudo-color maps of ocean surface temperature distribution and sound velocity distribution characteristics in the area of 4°N~26°N and 105°E~1236°E respectively. Moreover, the 3D visual distribution of sound velocity in the ocean acoustic field in a 1° grid region of 114°E~115°E, 18°N~19°N is also conducted. Comparisons of the results with the two datasets shown that the acoustic velocity distribution characteristics generated by these two datasets remain consistent overall, and the data generated by HYCOM has more detailed features than the NMDC, however, NMDC data has smaller size and advantageous data type of acoustic velocity, which reduces the requirements for computer hardware and offers a faster processing speed.

Image Human Thorax Using Ultrasound Traveltime Tomography with Supervised Descent Method

The change of acoustic velocity in the human thorax reflects the functional status of the respiratory system. Imaging the thorax’s acoustic velocity distribution can be used to monitor the respiratory system. In this paper, the feasibility of imaging the human thorax using ultrasound traveltime tomography with a supervised descent method (SDM) is studied. The forward modeling is computed using the shortest path ray tracing (SPR) method. The training model is composed of homogeneous acoustic velocity background and a high-velocity rectangular block moving in the domain of interest (DoI). The average descent direction is learned from the training set. Numerical experiments are conducted to verify the method’s feasibility. Normal thorax model experiment proves that SDM traveltime tomography can efficiently reconstruct thorax acoustic velocity distribution. Numerical experiments based on synthetic thorax model of pleural effusion and pneumothorax show that SDM traveltime tomography has good generalization ability and can detect the change of acoustic velocity in human thorax. This method might be helpful for the diagnosis and evaluation of respiratory diseases.

Manipulation of acoustic wavefront by gradient metasurface based on Helmholtz Resonators

We designed a gradient acoustic metasurface to manipulate acoustic wavefront freely. The broad bandwidth and high efficiency transmission are achieved by the acoustic metasurface which is constructed with a series of unit cells to provide desired discrete acoustic velocity distribution. Each unit cell is composed of a decorated metal plate with four periodically arrayed Helmholtz resonators (HRs) and a single slit. The design employs a gradient velocity to redirect refracted wave and the impedance matching between the metasurface and the background medium can be realized by adjusting the slit width of unit cell. The theoretical and numerical results show that some excellent wavefront manipulations are demonstrated by anomalous refraction, non-diffracting Bessel beam, sub-wavelength flat focusing, and effective tunable acoustic negative refraction. Our designed structure may offer potential applications for the imaging system, beam steering and acoustic lens.

Reliability analysis of in-situ stress measurement using circumferential velocity anisotropy

In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is based on the distribution of acoustic velocity in different directions around rock cores. The heterogeneity of core samples, such as fractures and gravel contained, can also lead to wave velocity anisotropy. Therefore, the corresponding reliability evaluation method is established to exclude some other anisotropy factors caused by non-tectonic stresses. In this paper, the reliability of testing results is evaluated from three aspects, i.e. phase difference, anisotropy index and waveform, to remove the factors caused by non-tectonic stresses.

An Approach for Implementation of Underwater Acoustic Communication Channel using 2-D TLM Modeling and Cross-Correlation Function

In underwater acoustic communication, acoustic signals from transducers or hydrophones are used. And the underwater acoustic communication channels are very complicated, because of vertical distribution of acoustic velocity according depths, and reflections from boundaries like as surface or bottom. For the implementation of the underwater acoustic communication channel, the image method or ray tracing method have been used. In this paper, we introduce a new approach for implementation of underwater acoustic communication channel using the simulation of the Transmission Line Matrix Modeling and cross-correlations from the input and output signals.

Geostatistical Simulation of Acoustic Log Data for Seismic Depth Conversion

Seismic reflection methods measure the time a seismic wave takes to travel through the ground, from the user defined source to a series of signal monitoring sensors known as geophones. The measured times need to be depth converted to allow for integration with other geological data. In order to convert from time to depth, an estimate of the rock volume velocity field must be made. The velocity field estimate can be made by assignment of velocity estimates to a geological model independent of the seismic processing. This article presents the results of using the acoustic geophysical log data extrapolated via sequential Gaussian simulation to derive the velocity field. The uncertainties associated with the velocity estimates were significant and provided the means to assess confidence limits for the actual depth determination. The technique is assessed by application to a major coal deposit, approximately 2.1 m thick and 210 m deep. Considering only the uncertainty associated with estimating the velocity field, half of the confidence interval values showed approximately 1 m of uncertainty in depth. The application of sequential Gaussian simulation to model the 3D distribution of acoustic velocity can be extended to other geophysical log parameters or derived estimates.

Precise Positioning of a Sea Bottom Transponder (4)

In the previous papers, it is shown that not only the position of a bottom transponder but also the underwater acoustic velocity distribution can be determined, if the position of surface transponders and ranges between the surface and bottom transponders are known. Also, a theoretical possibility was shown that the position of the bottom transponder can be obtained in the order of centimeters by using equipments available now. They were extended to three-dimensions in the third paper. More realistic results were obtained there. In the present paper, the theory is extended to include the effects of current. The basic nonlinear equations are obtained by a variational principle. Some two-dimensional numerical calculations are conducted, and the convergence of the iteration calculations is verified. The numerical results show that the effects of current can be obtained and can't be neglected for precise positioning.

Three Dimensional Calculation

According to the first report, not only the position of a sea bottom transponder but also the underwater acoustic velocity can be determined, if the position of sea surface transponders and the distances between the surface and bottom transponders are known. In the second report, a theoretical possibility of obtaining the position of the sea bottom transponder with accuracy of centimeter order is shown. For the purpose, estimation of the underwater acoustic velocity distribution as a part of solution is extremely important. Effects of errors in the positions of the surface transponders and the distances between the surface and bottom transponders are also obtained in detail. In the previous papers, two dimensional calculations are conducted.In the present paper, more realistic simulations are conducted by introducing three dimensional calculations. And a new idea called MIL (Method of Incremental Layers) are developed to stabilize numerical calculations.

Low Prandtl number gas mixtures as a working fluid in a thermoacoustic refrigerator

Prandtl number (Pr) is the dimensionless ratio of kinematic viscosity to thermal diffusivity, and is about 0.7 for most ideal gases. This value can be lowered significantly by mixing two gas species having molecular weights that are very different, resulting in a minimum Pr of 0.22 for He-Xe mixtures. This can be used to minimize the nuisance effect of viscous shear losses for a thermoacoustic refrigerator as well as for other types of heat engines. The principle of thermoacoustic heat transport will be briefly discussed [J. Wheatley, T. Hofler, G. W. Swift, and A. Migliori, J. Acoust. Soc. Am. 74, 153–170 (1983)]. However, changing the viscosity of the working fluid also changes the details of the acoustic velocity distribution, thereby modifying the basic thermoacoustic heat transport mechanism. Measurements indicate that this effect may be more important than the simple reduction of viscons shear losses. [Work supported by the Office of Naval Research and the Office of Naval Technology.]

Measurement of Acoustic Velocity and Acoustic Attenuation Constant Distribution in Soft Tissue by Ultrasonic CT

An experimental data acquisition system for ultrasonic CT is developed. The system displays cross-sectional images representing acoustic velocity distribution and acoustic attenuation constant distribution within a subject. Experiments using a sponge phantom and animal tissues are performed. The attenuation images show high values on the borders of objects because of the wave characteristics such as reflection and refraction of ultrasound.

Directional Acoustic-Velocity Distribution in a Petroleum Reservoir

Abstract Experimental conditions, theoretical considerations, and results of adirectional acoustic-velocity survey in the plane of formation of an oilreservoir in Pennsylvania are given. More than 60 blasting-cap acoustic sourceswere discharged opposite the Bradford Third sand in 10 different wells locatedwithin a 1,200- by 1,350-ft rectangular array of 41 wells. Each source wasmonitored simultaneously by 11 different hydrophones strategically locatedwithin the 41-well array. Both acoustic sources and sensors were located at theformation elevation, about 2,000 ft below the surface, so that the in-situvelocities determined were in the plane of the sandstone formation. Well-to-well acoustic velocities were calculated and are presented in tabularand graphical form. The velocity data were further analyzed statistically toyield over-all reservoir velocity trends. Results of the study show that thereservoir is acoustically heterogeneous, with local variations in P-wavevelocity of as much as 20 percent. The over-all P-wave velocity trend for thereservoir was greater in the east-west direction than in the north-southdirection by about 700 ft/sec. The mean acoustic velocity for all wells in alldirections was 15,621 ft/ sec. The acoustic-velocity distribution is also correlated to directionalpermeability data obtained by oriented core analyses. Introduction The directional variation of geophysical properties affecting wave propagationhas long been known. A brief examination of the history and evolutionaryprocesses of sedimentary rock should leave no doubt about the nature of theformations. However, little attention has been given to directional propertiesin the evaluation and recovery of natural resources. Evaluation of thedirectional acoustic (seismic) velocity distribution in reservoirs is perhapsthe most promising means of evaluating directional reservoir properties. Themagnitude of directional variation can be predicted to a degree by consideringthe lithology and the geological features of an area. However, there are alwaysuncertainties in the study of wave propagation through the earth; numerousvariables influencing wave-propagation velocities have been determined. This research was initiated originally to aid in acoustically mapping thegrowth of hydraulically induced fractures in time and space, and to assist indelineating existing fracture systems. The reservoir investigated had been thesubject of other research programs, and considerable information was available.Using the available information, however, large directional variations inacoustic velocities over small areas would not necessarily have beenanticipated. As will be shown, the velocity variations were substantial andwere correlatable, to a degree, with other geophysical and reservoirproperties. Since the late 1940's and early 1950's, a tremendous amount of work has beendone in active and passive seismic monitoring and in the general area of wavepropagation through geological materials. Most of the work falls into one offour general categories:active seismic exploration,passive earthquakeor nuclear undergroundexplosion monitoring,active oil and gas welllogging, androck noise and slope-stability studies. Obviously, eachcategory directly or indirectly involves the directional nature of propagationvelocities. The first two categories, and usually the fourth, are associatedwith very low frequency monitoring systems, such as geophones usually operatingin the 0- to 50-hz range. Frequencies used in Category 3 are usually in the20-khz range since the waves typically are transmitted only 3 ft verticallyfrom a transmitter to a receiver.

Corrections of ray theory

In previous reports it was shown that the failure of ray theory (for a case of a ray field with the amplitude function and its first derivative continuous) can be corrected by distributing sources throughout the volume having a density proportional to the Laplacian of the amplitude function. The theory still holds with the restriction relaxed. Symbolic functions may appear on surfaces, implying a source or dipole distribution on them. This has been applied to correct for diffraction on a shadow zone boundary (for which a different proof has been reported), at a caustic, and at a discontinuity of acoustic velocity distribution (e.g., corners). Some examples are discussed.