Anna L. Pate

Wave‐number domain separation of the incident and scattered sound field in Cartesian and cylindrical coordinates

Two‐surface Cartesian coordinate system and cylindrical coordinate system measurement techniques are applied to obtain the scattered sound fields of a general shape. This decomposition method is based on the principle that any waveform can be decomposed using a two‐dimensional spatial Fourier transform into wave components that propagate in a known manner. The Cartesian method was developed by Tamara [J. Acoust. Soc. Am. 88, 2259–2264 (1990)] to measure the reflection coefficients of waves incident at oblique angles onto planar surfaces, so our paper focuses on the optimization of this method when applied to scattering investigations. A two‐surface technique to separate the incident and scattered field is also developed in cylindrical coordinates. The separation process is carried out in the wave‐number domain in a manner similar to what has been developed for Cartesian coordinates. However, because the incident and scattered fields are not directly separable in cylindrical coordinates, knowing general pr...

Sensitivity of the wave‐number domain field separation methods for scattering

The two‐surface field separation technique for obtaining the sound field scattered from an object has been shown to be feasible for both the Cartesian and cylindrical coordinate systems [Cheng et al., J. Acout. Soc. Am. 97, 2293–2303 (1995)]. Investigations of the sensitivity of implementing these separation techniques to the measurement parameters and their errors will be reported. The results of this sensitivity investigation are helpful in deciding the accuracy of the far‐field target strength results and designing experimental procedures and equipment that ensure accurate results. The separation and the spatial processing parameters that are considered include the distance between measurement points in each measurement surface, the aperture size of each measurement surface, and the distance between measurement surfaces. Recommendations for these parameters are made. Next the sensitivity to errors in the experimental geometry is explored. For the Cartesian coordinate system skew and curvature of the me...

Ultrasonic Transducer Calibration for Reciprocity-Based Measurement Models

A general formulation regarding the relationship between scatterer properties and experimental signals was presented in a companion paper[1]. This formulation was developed for the case of acoustic wave scattering in an immersion measurement. In order to perform the theoretical calculations, the formulation requires knowledge of the radiation and reception characteristics of the probes involved and the combined properties of the pulser and the receiver (system efficiency). Thus, the objective of this paper is to discuss methods for obtaining the characteristics of measurement systems as required in the above-mentioned formulation. As an example, the case of acoustic wave scattering from a sphere of finite size is examined. Experimental verifications of results are included.

A study on source localization in plates.

Structural intensity and a force distribution function are used to localize the sources on a plate based on the knowledge of the plate surface velocity. The plate surface velocity is measured by means of a laser vibrometer which, unlike accelerometer, does not change the plate loading and mass. Two structural intensity formulations are used. The first formulation is based on simple flexural motion that only includes the out‐of‐plane motion and its special derivations. The second formulation includes all three motion components of the plate displacement. Midlin’s equations of plate vibration, which take into account the rotatory inertia and shear effect on flexural motion, are used to calculate the plate in‐plane motions and force distribution function. The in‐plane motions are then used to calculate the structural intensity. Some numerical difficulties are overcome by calculating structural intensity in k space. Results show that both structural intensity and the force distribution function can clearly sh...

Reciprocity-Based Measurement Models for Ultrasonic NDE

Quantitative NDE objectives such as the evaluation of flaw signals and the assessment of inspection capabilities generate the need for models of ultrasonic scattering measurements capable of including all aspects of the measurement process: radiation and reception characteristics of probes, propagation of finite beams, scattering of nonplane-wave fields, and the like. Once such models are developed and validated, that is, proven able to predict the received voltages obtained in actual ultrasonic experiments, they can be used to gain a thorough understanding of the NDE inspection process. Specifically, these models will be useful in investigations of the factors that significantly affect ultrasonic inspections, and they will form the basis for the proper interpretation of flaw signals[1].

Mode localization phenomenon analysis on infinite ribbed plates

An analysis of the mode localization phenomenon is demonstrated for disordered structural systems of infinite ribbed plates in air and in water. While for infinite ordered ribbed plates there exist frequency passbands for which vibrations propagate extensively throughout the structure, the introduction of disorder disables the propagation of vibrations to arbitrarily large distances and confines vibrations within a region close to the excitation force. Based upon the mathematical model built in a paper to be published, inertance transfer functions and vibrational responses due to line force excitation are implemented to investigate the passband and stopband characteristics and the localization phenomenon of multiple‐rib plates in air and in water. Two facts are confirmed. First, water provides an energy path that reduces the decoupling effect between the bays of ribbed plates observed in the presence of air and consequently, diminishes the localization phenomenon. Second, passbands can merge at certain fr...

Characterization of Ultrasonic Immersion Transducers

In any ultrasonic NDE experiment, the distributed field properties of the transducer involved represent an important aspect of the overall measurement process. The normal velocity profile across the active face of the transducer is typically used to characterize these properties. In quantitative NDE applications, a simple parametric form is usually assumed for this profile, such as a rigid piston with either the nominal or so-called active probe diameter taken as the parameter value. It has been shown in related studies that such an approach does not characterize all UT transducers accurately in all measurement situations (particularly nearfield versus farfield locations). Thus a new method for individual transducer characterization is presented herein that is based on reconstructions of the unknown probe velocity profile from measurements of the radiated field.

Investigation of scattering with acoustic holography.

The objective of this paper is to investigate sound scattering of a rigid sphere using the acoustic holography method. A piston source, which is vibrating sinusoidally, illuminates a sphere. The superimposed incident and scattered fields are measured in the near field of the sphere using acoustic holography. These two fields are decomposed in the wave‐vector domain and, therefore, the scattered field is extracted. Numerical simulations are performed and the effect of various parameters is investigated. Specifically, the distance between two holography planes, the sampling rate, and the aperture size are investigated in the field separation technique. In addition, experimental studies were conducted inside an anechoic chamber with a baffled loudspeaker as a source and a cast‐iron sphere as a scatterer. The experiments demonstrate the feasibility of the field separation technique based on two‐plane acoustic holography. [Work supported by David Taylor Research Center.]

Vibration and acoustic response of a ribbed plate excited by a line or point force.

An analytic formulation for the vibrational and acoustic responses caused by application of a point or a line force to the rib of a single‐ribbed, infinite plate with negligible fluid loading has been investigated previously by the same authors [‘‘Vibrational and Acoustic Response of a Ribbed Infinite Plate Excited by a Force Applied to the Rib,’’ Proc. 2nd Int. Congress on Recent Developments in Air and Structure‐Borne Sound and Vibration, March 1992]. An expression for the same responses of ribbed plate system due to similar excitation away from the rib was formulated. Numerical solutions were obtained by using the same technique as before. The results again show that the structural intensity in the ribbed plate is an excellent tool for force and rib localization. Acoustic radiation patterns between the previous case of force on the rib and the present case of force away from the rib are quite different as shown in 3‐D pattern results. Moreover, the interaction between force and rib plays an important r...

Acoustic imaging of ribbed plates

In recent years, a number of measurement techniques have been developed for structural intensity in various systems such as beams and plates. These techniques are based on motion measurements by either contact sensors (e.g., accelerometers) or noncontact methods (e.g., near‐field acoustic holography). It has been shown that it is possible to calculate the two‐dimensional structural intensity vector in plates on the basis of acoustic imaging. However, the method is very sensitive to errors occurring in the image of the reconstructedsurface motion. Thus the goal of this paper is to investigate the accuracy of the surface velocity when calculated from two different acoustic imaging methods: near‐field acoustic holography and a new acoustic imaging method based on surface motion expansion in a set of orthonormal basis functions. The imaging is performed by using a singular value decomposition method along with additional constraints to calculate appropriate coefficients in this expansion. Comparisons between imaging of thin homogeneous plates and ribbed plates will be discussed. [Work supported by ONR.]