Charles M. Loeffler

Elastic wave contributions in high‐resolution acoustic images of fluid‐filled, finite cylindrical shells in water

Recent studies of plane wave scattering by finite, evacuated cylindrical shells at oblique incidence describe monostatic echo contributions from surface elastic waves. These waves follow helical paths along the shell’s surface and are guided back in the source direction after reflection from the cylinder truncation. The present research examines similar effects viewed in the time signature of a high‐resolution acoustic imaging system. The experimental technique utilizes a narrow beam consisting of a short tone burst to probe a cylindrical shell along its axis of symmetry. The echo returns contain information which yields insight into local scattering processes at the impedance discontinuities of the finite scatterer. The cylindrical shell has a thickness‐to‐radius ratio h/a=5.5%. It is immersed in water and is subject to both exterior and interior fluid loading. Prominent artifacts are shown to coincide with near axial coincidence of the s0 and a0 Lamb waves. The echo contributions are extremely sensitive...

Relationship between material parameters and target strength of fluid-filled spherical shells in water: calculations and observations

Thin fluid-filled spherical shells have been used as passive sonar targets for many years. They possess a large target strength which is highly dependent on the sound-speed mismatch between the fluid contained within the shell and the exterior fluid surrounding the shell. In the past, to obtain the appropriate mismatch, the interior fluid mixture contained chlorofluorocarbons (CFCs). Due to a recent production ban on CFCs, it is necessary to choose alternative fluids. The present research analyzes the backscattering target strength of a fluid-filled spherical shell as a function of several material parameters as a guide to choosing alternative fluids and shell materials. Calculations over a broad range of material values display the target strength dependence on the interior fluid parameters as well as the parameters defining the metallic shell. The range of material values presented here is far larger than any previous study addressing the focusing effects of fluid-filled spherical shells. The results should aid in determining liquid fillers and shell materials which yield the maximum possible backscattered returns. Also, several experiments were conducted with stainless steel shells containing a hydrochlorofluorocarbon (HCFC), dichlorofluoroethane. The results are compared with results found from calculations as well as from other experiments involving shells containing a previously used CFC mixture.

Backscattering of obliquely incident plane waves by a composite cylindrical shell constructed of isotropic and transversely isotropic layers.

Acoustic scattering from a transversely isotropic cylindrical shell excited by an obliquely incident plane wave is examined. The shell is comprised of N layers which may be described by isotropic or transversely isotropic material parameters. The present research solves the boundary value problem for a transversely isotropic, infinite cylindrical layer within the framework of exact 3‐D elasticity theory. The layers which comprise the shell are connected via a ‘‘propagator matrix’’ which relates the interior and exterior boundary conditions. The backscattering form function is then constructed for several commonly used composite materials which display transverse isotropy. The results for anisotropic shells are compared to results found for isotropic shells with similar parameters. Here, attention will be given to the similarities (and dissimilarities) of the scattering mechanisms which are the chief contributors to the backscattering form function as the degree of transverse anisotropy is increased. Analy...

Three‐dimensional auditory display of passive sonar data

Submarine sonar operators typically use visual and monaural audio outputs to locate obstacles, ships, and animals in the surrounding ocean. Recently, the effectiveness of using 3‐D, rather than monaural, sonar audio outputs was investigated. A multi‐channel signal processing system has been developed that converts sonar signals collected by elements of a hydrophone array into binaural signals for the left and right ears, which provide accurate 3‐D aural imaging of an underwater acoustical environment. An operator who listens to the binaural signals via stereo headphones will perceive that he is immersed in the underwater acoustical environment. The system incorporates two banks of FIR filters (one for the left ear and one for the right ear) to process the sonar data. The FIR filters simultaneously beamform the hydrophone data and filter the data with head‐related transfer functions in order to create binaural signals that provide 3‐D sound. [Work supported by Applied Research Laboratories: U. T. Internal ...

Sound scattering by a fluid‐filled cylindrical shell in water

Previous calculations and observations of backscattering by cylindrical shells in water usually involve shells which are either empty or subject to extremely light interior loading conditions such as air. Typical echo signatures display distinct contributions described by specular reflection and guided waves launched along the shell structure. The present research analyzes the backscattering effects as a consequence of filling the interior cavity with a higher impedance fluid such as water. Energy transferred into the cavity couples into radiation mechanisms which drastically increase the fine structure in the backscattering form function. Dispersion curves derived from full 3‐D elasticity theory via the Watson transform methodology display a complicated mode structure which is a combination of the guided waves found on a cylindrical shell in vacuum and the normal modes of a fluid‐filled cylindrical cavity satisfying rigid boundary conditions. The radiation damping of the associated curves yields insight ...

Fabrication of low‐sound‐speed particulate composites for acoustic applications

Low‐sound‐speed elastic materials are of great interest in a wide range of applications. The present research addresses the fabrication of a low‐sound‐speed elastic material in the form of a particulate composite. A polymeric matrix material is combined with highly compressible particu‐lates (40‐μm diameter). At frequencies well below the monopole resonance of the particles, dilatational sound speeds as low as 400 m/s have been measured in experiments. The attenuation characteristics of the material are highly dependent upon frequency and may display transmissive or absorptive properties. The measured sound speeds and attenuations compare well with theoretical predictions for composites with relatively low volume fractions of particulates. Applications of these materials will also be discussed. [Work sponsored by the ARL:UT Independent Research and Development Program.]

Frequency‐dependent classification cues resulting from localized interactions on a target surface

Much work has been done to extract information about a scatterer by analyzing the spectrum of its echo return. Typical applications rely on phase matched circumnavigations of a surface guided wave (a ‘‘resonance’’ of the structure). The present research incorporates high‐frequency scattering mechanisms which interact in a small localized area located about the specular points of a curved surface. The scattering contributions from such mechanisms promptly follow the onset of the specular echo in the time signature and are independent of the backside of the scatterer. The mechanisms presented here include the effects of thickness resonances and ‘‘negative’’ group velocity surface guided waves. It is shown that by temporally isolating the region about the specular return in the backscattered time signature, it is possible to gain information related to the thickness and/or material properties of a scatterer. This is demonstrated both computationally and in experiments involving cylindrical shells and finite ...

Line extraction from sonar images of elastic objects

Acoustic imaging of elastic targets often results in linear flashes in the image. Delineating these target features could be used in subsequent target detection and recognition schemes; however, the difficulty in this task is not to be underestimated due to the presence of background reverberation or ambient noise in the sonar image. A powerful line‐fitting algorithm which locates the linear flashes in sonar images of elastic objects is presented. The algorithm initially groups the image pixels into neighboring clusters by using the parameters of an intensity‐weighted least‐mean‐squared line fit locally computed over the entire image. The algorithm uses the information of these groups in order to extract the dominant lines from the image. It is noteworthy to realize that compared to the majority of line and edge detection schemes, the algorithm actually parametrizes for each line extracted its length, orientation, location, and intensity. By culling these lines based on their parameters, particular linear...

Appearance of elastic wave contributions in high resolution acoustic images

Studies involving plane wave scattering by finite cylindrical shells at oblique incidence display monostatic echo contributions from surface elastic waves. These waves follow helical paths along the shell’s surface and are guided back in the source direction after reflection from the cylinder truncation [X.‐L. Bao, 1461–1465 (1993)]. The present research examines such effects viewed in the time record of a high resolution sonar system. The target consists of a finite cylindrical shell with a radius to thickness ratio of 5%. The shell is immersed in water and is subject to both exterior and interior fluid loading. It is ensonified with short tone bursts of narrow beam width over a range of incidence angles 90≥φ≥50, where φ is measured relative to the cylinder axis. The backscattered echo signature exhibits prominent artifacts as a result of scattering processes involving the s0 and a0 Lamb waves interacting with the cylinder truncation. The echo contributions are extremely sensitive to the angle of inciden...

Scattering of a narrow acoustic beam by a truncated fluid‐filled cylindrical shell

Previously a method of describing spherical acoustic waves in cylindrical coordinates was applied to the problem of point source scattering by an elastic infinite fluid‐filled cylindrical shell [S. Dodd and C. Loeffler, J. Acoust. Soc. Am. 97, 3284 (A) (1995)]. This method is applied to numerically model monostatic oblique incidence scattering from a truncated cylinder by a narrow‐beam high‐frequency imaging sonar. The narrow‐beam solution results from integrating the point source solution over the spatial extent of a line source and line receiver in the frequency domain. The cylinder truncation is treated by the method of images, and assumes that the reflection coefficient at the truncation is unity. The scattering form functions, calculated using this method, are applied as filters to a narrow bandwidth, high ka pulse to find the time domain scattering response. The time domain pulses are further processed and displayed in the form of a sonar image. These images compare favorably to experimentally obtai...