Louis R. Dragonette

Acoustic reflection from elastic spheres. I. Steady‐state signals

Measurements are made of the acoustic reflected pressure from aluminum and tungsten‐carbide spheres for ka values between 5 and 20. Only the farfield monostatic case is considered. Experimental values are compared to a harmonic‐wave solution. Excellent agreement is observed when the wave speeds in the sphere material are adjusted by an amount smaller than the known uncertainty in the wave‐speed values in the solids. The position of rapid changes in the reflection solution with ka is found to be very sensitive to shear‐wave speed and insensitive to longitudinal‐wave speed. The sensitivity to ambient temperature of the position of a selected minimum in the reflection solution is examined computationally for an aluminum sphere. A shift in the value of ka at which a minimum is calculated is also observed experimentally. Long pulses are used to approximate steady‐state conditions and agreement with the steady‐state theory shows that this approximation is adequate.

Time–frequency analysis of acoustic scattering from elastic objects

Some characteristics of an insonified object are generally present in the signal carried by the returning scattering wave. As an echo is the result of the wave interaction with a material structure, such a response is distinctive for a body of given shape and composition. Traditionally, to express the information content in the echo signature, either the frequency response (transfer/form function) or the time signature (impulse response) is employed; however, for a detailed study of the scatterer’s structure, a joint time and frequency analysis is performed. The aim of this analysis is to develop a simple processing algorithm for extracting the prominent features, which can then be used to determine the physical parameters of the object. The approach is based on the modified version of the Wigner distribution function (WDF) and utilizes an image‐processing technique to depict the outstanding highlights of the scatterer’s response in a two‐dimensional time and frequency display. The physical parameters of ...

Calibration technique for acoustic scattering measurements

Tungsten carbide spheres are used as calibration targets in laboratory acoustic scattering measurements. Though the steady‐state response of any metal sphere in water greatly differs from a rigid body return, over almost the entire frequency spectrum, the rigid body and elastic returns can be separated in a short pulse, broadband experiment. This rigid body echo can then be used as a reference to normalize the scattering returns from targets of interest.

Observation of Waves Radiated from Circular Cylinders Caused by an Incident Pulse

Schlieren visualization and hydrophone measurements are used to observe the radiated wavefronts which result when an acoustic pulse is incident on a metal cylinder in water. The range of size parameter ka from 138 to 1419 is considered. The wavefront positions are traced by the refraction, internal reflection, and radiation of shear and compressional waves. In the case of solid cylinders, many wavefronts display an apparent circumferential property derived from the incidence of energy from the normal to the appropriate critical angle. Identification of one of these wavefronts as resulting from previously identified “Rayleigh‐type” wave propagation and a single incident angle is denied, although the circumferential property is verified. A previously identified faster circumferential wave is attributed to a composite wavefront resulting from direct compressional transmission and an increasing number of its internal reflections. Other wavefronts depending on mode conversions are also identified. A mechanism ...

Acoustic reflection from elastic spheres and rigid spheres and spheroids. II. Transient analysis

Curves relating the reflected acoustic pressures to frequency for a rigid sphere and spheroid and for elastic spheres of aluminum, brass, and tungsten carbide in water are obtained. Experimental measurements using single short acoustic pulseforms are compared with theory. Excellent agreement is obtained for the limited ranges of ka over which the experiments were done. Only the case of monostatic reflection is considered.

Broadband acoustic scattering measurements of underwater unexploded ordnance (UXO)

In order to evaluate the potential for detection and identification of underwater unexploded ordnance (UXO) by exploiting their structural acoustic response, we carried out broadband monostatic scattering measurements over a full 360 degrees on UXOs (two mortar rounds, an artillery shell, and a rocket warhead) and false targets (a cinder block and a large rock). The measurement band, 1-140 kHz, includes a low frequency structural acoustics region in which the wavelengths are comparable to or larger than the target characteristic dimensions. In general, there are aspects that provide relatively high target strength levels ( approximately -10 to -15 dB), and from our experience the targets should be detectable in this structural acoustics band in most acoustic environments. The rigid body scattering was also calculated for one UXO in order to highlight the measured scattering features involving elastic responses. The broadband scattering data should be able to support feature-based separation of UXO versus false targets and identification of various classes of UXO as well.

Measurement of Rayleigh phase velocity and estimates of shear speed by schlieren visualization

A schlieren technique is used to give a direct accurate measurement of Rayleigh phase velocity on various materials. Plane solid interfaces underwater are insonified by finite acoustic beams. Sound incident at the Rayleigh angle produces a null strip in the radiated field which uniquely identifies the Rayleigh angle. This null strip is caused by the mutual cancellation of equal amplitude specular and Rayleigh radiations which are 180° out of phase. Rayleigh phase velocity is then calculated from the measured Rayleigh angle by a simple equation. The measured Rayleigh velocities are used to give estimates of shear velocity, and these estimates are compared with direct shear velocity measurements. The attenuation of the Rayleigh wave in the low‐MHz region is due to radiation into the water and is found to be directly proportional to frequency and inversely proportional to material density as predicted by theory.

T‐matrix implementation of forward scattering from rigid structures

The T‐matrix method is applied to compute forward scattering from rigid structures. The method requires evaluation of matrix elements Qmmlk whose imaginary parts with l<k are difficult to evaluate for long, slender, nonellipsoidal structures because the evaluation involves integrations of large oscillatory functions. A new approach for the evaluation of these elements is shown for finite cylinders where the antisymmetric part of the matrix is written in a form involving integrals that are easier to compute. Forward scattering results are shown for spheroids and cylinders with spheroidal endcaps where the length of the cylindrical section is shown to have little effect on forward scattering. The magnitude of the forward scattering at the high‐frequency limit is seen to be proportional to the cross‐sectional area, which is in agreement with the high‐frequency Kirchhoff approximation.

Monostatic reflection of a plane wave from an absorbing sphere

The monostatic reflection from a lucite sphere in water is measured and compared with the exact classical scattering theory. Experimental results do not agree with theory which neglects absorption, in direct contrast to the excellent agreement found when metal spheres are used as targets. The theory is modified to include the effects of absorption of shear and compressional waves in lucite, and agreement between experiment and the modified theory is demonstrated.Subject Classification: 20.30, 20.15.

Calibration method for acoustic scattering measurements using a spherical target

A method for calibrating acoustic backscattering instrumentation utilizing spherical body as a standard target. A spherical body made of high specific acoustic impedance material, such as tungsten carbide, is positioned a given distance from a source/receiver transducer which is energized to produce a short acoustic pulse directed toward the sphere. Acoustic signals reflected from the sphere are detected by the transducer and processed in the time domain to separate the rigid portion of the return from the elastic portions. The rigid portion is corrected for the transducer to sphere distance, the reflectivity of the sphere, and for the radius of the sphere. The resultant corrected signal represents the incident acoustic pulse produced by the transducer.