# Browsing by Subject "Underwater acoustics"

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Item The acoustic ecology of submerged macrophytes(2011-12) Wilson, Christopher James, 1985-; Wilson, Preston S.; Dunton, Kenneth H.; Shank, G. Christopher; McClelland, James W.; Fuiman, Lee A.Show more Underwater acoustics has recently emerged as a viable tool for assessing ecosystem health and exploring the estuarine soundscape. Recent acoustic surveys have mapped distributions of both seagrass meadows and kelp forests, and scientists are currently developing remote sensing capabilities to improve ecological assessments of these communities. Furthermore, researchers are beginning to focus on the propagation and ecological significance of bioacoustic signals within estuarine landscapes. The research presented here includes a thorough examination of the interaction of acoustic energy and macrophyte tissue as it pertains to habitat assessment and ecosystem function. Modeling experiments investigated the interaction of acoustic energy and submerged macrophyte tissue. Both seagrasses and kelp exhibited a similar acoustic response by increasing the acoustic compressibility of a seawater medium. The increase in acoustic compressibility was driven by free-gas volumes contained within the macrophyte tissue. Interestingly, the tissue served to limit the acoustic compressibility of the gas volume below the magnitude predicted by effective medium models. Separate inquiries of high-frequency sound propagation and the seagrass canopy revealed a significant temporal component to acoustic transmission. Specifically, sound transmission throughout a seagrass canopy was altered by the formation of free gas bubbles and the pressurization of aerenchyma channels, which was mediated by photosynthesis. The photosynthetic controls on sound propagation were species-specific, and patterns of acoustic transmission provided a reasonable proxy for gross primary production in Syringodium filiforme plants. Finally, the interaction of sound energy and submerged macrophytes appears to have important ecological implications. This research suggests that seagrass meadows scatter high-frequency sound energy and provide an acoustic refuge to fish from marine mammal predators. This refuge is highly seasonal, specific to different seagrass species and dependent on the abundance of above-ground biomass. Seagrasses also may influence the transmission of low-frequency sounds used by soniferous fish. Propagation characteristics of low-frequency sounds are highly dependent on frequency and result in differential transmission distances among individual fish species. It is clear from this body of work that submerged macrophytes are an important feature of the underwater soundscape. Future research should continue to exploit this feature for remote sensing purposes and examine its ecological significance.Show more Item An evaluation of competing geoacoustic models and their applicability to sandy ocean sediments(2017-12-01) Bonomo, Anthony Lucas; Hamilton, Mark F.; Isakson, Marcia J.; Wilson, Preston S; Ghattas, Omar; Kallivokas, Loukas FShow more This dissertation studies five models that make up a cross section of the geoacoustic models that have been used to study sandy sediments: a simple fluid model, the effective density fluid model (EDFM) of Williams, the viscous grain-shearing (VGS[lambda]) model of Buckingham, the Biot-Stoll model, and the corrected and reparameterized extended Biot (CREB) model of Chotiros. The first objective is to use numerical experiments and model/data comparisons to determine the usefulness and assess the physical validity of these five models. The second objective is to ascertain the current state of knowledge of sandy sediments and describe what truths can be learned from model/data comparisons. To complete these objectives, the models' predictions of geoacoustic quantities such as wave speeds, attenuations, and bottom loss are compared with published measurements and to each other through Bayesian inference and computational studies. It is determined that while each model has its uses, no one model fully captures the wave physics of sandy sediments.Show more Item Characterization of underwater acoustic metamaterials inspired by transformation acoustics(2021-08-16) Cushing, Colby Walker; Haberman, Michael R. (Michael Richard), 1977-; Wilson, Preston S.; Norris, Andrew N; Cummer, Steven A; Hamilton, Mark FShow more Underwater acoustic metamaterials (AMMs) use engineered subwavelength structures to produce novel effective material properties to control water-borne acoustic wave propagation. Transformation Acoustics (TA) is a mathematical construct that is used to specify the material properties that are necessary to achieve exotic manipulation of acoustic fields, and as such, AMMs can be the means through which physical realization of these devices can occur. An acoustic cloak is one possible device that can be designed using TA, but it is one that has garnered the most attention in academic studies due to the multi-faceted complexity of designing, fabricating, and experimentally validating performance. For an object to be indistinguishable from the background medium, an acoustic cloak requires either anisotropic effective dynamic density or anisotropic effective dynamic stiffness, but further requires these properties to be spatially dependent. Devices predicted by TA, such as cloaks, can be expensive and difficult to fabricate due to the geometric complexities of the substructures required to realize the required material properties. In some cases, state-of-the-art additive manufacturing methods are the only means of fabricating such devices. Devices predicted by TA are also difficult to characterize because of their frequency dependent dynamic behavior. Further, since the AMMs of interest in this work operate primarily at low frequencies, freefield methods are not practical. Therefore, this work investigated the characterization of select underwater AMMs predicted by TA using new and existing experimental apparatuses and techniques while evaluating the applicability of different fabrication processes to physically realize testable samples. Three materials were investigated in this work: (i) a layered AMM comprised of aluminum, air-filled, honeycomb panels coupled to one another by compliant rubber rods demonstrating anisotropic effective dynamic density, (ii) a hexagonal elastic lattice comprised of bimode unit cells that exhibits a focusing beam pattern in an underwater acoustic field by exploiting spatially varying density and stiffness, and (iii) an additively manufactured, titanium, diamond-shaped, elastic pentamode lattice that demonstrates anisotropic sound speeds due to anisotropic effective dynamic stiffness. Significant effort was expended on experimental validation of the AMM's acoustic properties, but this work also explored the relationships between homogenization theory, numerical simulation, and fabrication methods, in order achieve more robust understanding of the underwater acoustic AMMs studied here.Show more Item Design and implementation of an underwater acoustic transponder(2011-05) Perrine, Kenneth Avery; Evans, Brian L. (Brian Lawrence), 1965-; Hall, Neal A.Show more A transponder for underwater acoustic data communications is prototyped. The mobile transponder emits a data sequence whenever it detects a ping from a base station. The data sequence includes GPS coordinates and UTC time sent over a conservative and brief 12 kbps turbo-coded BPSK link, and a 6 kB JPEG image sent over an ambitious 67 kbps turbo-coded 16-QAM link. The range of the transponder from the base station can also be accurately derived. Several challenges exist in decoding the underwater signals at the base station receiver, including Doppler distortion and multipath. While experimental results show that the ranges for decoding the 16-QAM signals with a single hydrophone are limited to less than 25 m, the BPSK signals prove to be much more robust, decoding at ranges of up to 625 m. Experiments with delays and transducer tether length indicate methods for improving reliability in the presence of reverberation and thermocline. This transponder uses mostly off-the-shelf parts and is anticipated to be improved when paired with advanced sonar array devices.Show more Item Eigenray sonar simulation in two-layer media(2016-05) Clark, Jonathan Michael; Ling, Hao; Story, MarkShow more In this work, an eigenray finding method for simulating refractive multipath phenomena in an unbounded underwater environment is presented. To simplify the problem, the environment consists of a horizontally-homogeneous two-layer medium in which isogradient sound speed profiles are used in each layer. Eigenrays are found through an iterative Newton-Raphson method on the Snell invariant, and the travel time is found from the computed Snell invariant. A derivation for the maximum number of ray paths that need to be considered to account for all refractive multipaths is included. Given a point source/point target pair, the method will output a launch angle and travel time for the eigenray connecting the source and target. The time accuracy of the eigenray finding method is confirmed using a forward ray-trace. From the travel time of eigenrays, time series data is formed for each receive element. Refractive multipath phenomena are shown using the time series data of a target near the layer boundary. An FFT beamformer is used to beamform the simulated time series data which reproduces an image of the acoustic targets, which shows a time resolution sufficient to support the coherent addition of signals in beamforming.Show more Item In situ measurements of compressional and shear wave speed and compressional wave attenuation using the acoustic coring system(2023-12) Garcia, Dante Daniel; Wilson, Preston S.; Ballard, Megan S.Show more In situ measurements of geoacoustic properties provide direct characterization of the seabed at near ambient conditions. The Acoustic Coring System (ACS) is a gravity corer equipped with acoustic probes that obtains in situ compressional wave (30–200 kHz) and shear wave (400--1200 Hz) measurements as the corer penetrates the seabed. During the April 2022 R/V Endeavor coring survey, the ACS was deployed at 36 locations within the New England Mud Patch (NEMP) and New England Shelf Break areas. Data from these experiments were analyzed to characterize the depth-dependent structure of the seabed geoacoustic properties as well as their spatial variability. The in situ measurements are interpreted in the context of stratigraphic layering measured by a seismic survey. Depth-dependent profiles of compressional wave speed from a subset of these deployments in the NEMP are compared to profiles previously collected at nearby locations in 2016. In situ compressional wave records from both areas are compared with ex situ sediment core measurements, including data collected from core loggers and laboratory analyses. Additionally, novel in situ shear wave speed measurements from the NEMP and surrounding areas are introduced.Show more Item An investigation of the combustive sound source(2010-08) McNeese, Andrew Reed; Knobles, David P.; Wilson, Preston S.Show more This thesis describes the development and testing of the Combustive Sound Source (CSS), which is a broadband underwater sound source. The CSS is being developed as a clean, safe, and cost effective replacement to underwater explosive charges, which exhibit an inherent danger to marine life and researchers using the charges. The basic operation of the CSS is as follows. A combustible mixture of gas is held below the surface of the water in a combustion chamber and ignited with an electric spark. A combustion wave propagates through the mixture and converts the fuel and oxidizer into a bubble of combustion products, which expands due to an increase in temperature, and then ultimately collapses to a smaller volume than before ignition, producing a high intensity, low frequency acoustic signal. The thesis begins by discussing the background, history, and purpose of developing the CSS. It continues by describing the current apparatus and the essential components and convenient features added to the latest mechanical design. The general operation is discussed along with a description of an experiment conducted to determine the acoustic output and robustness of the current CSS. The results of this experiment are presented in terms of the effect of volume, ignition depth, oxidizing gas, combustion chamber size, and repeatability of acoustic signatures. Discussion of apparatus robustness is presented to suggest improvements for future CSS designs.Show more Item Laboratory measurements of sound speed and attenuation of water-saturated granular sediments(2012-05) Argo, Theodore F., 1982-; Wilson, Preston S.; Chotiros, Nicholas P.; Ezekoye, Odfodike A.; Hamilton, Mark F.; Swinney, Harry L.Show more The propagation of acoustic waves through water-saturated granular sediments has been widely studied, yet existing propagation models can not adequately predict the speed and attenuation of sound across the range of frequencies of interest in underwater acoustics, especially in loosely packed sediments that have been recently disturbed by storms or wave action. Advances in modeling are currently dependent on experimental validation of various components of existing models. To begin to address these deficiencies, three well-controlled laboratory experiments were performed in gravity-settled glass beads and reconstituted sand sediments. Sound speed and attenuation measurements in the 0.5 kHz to 10 kHz range are scarce in the literature, so a resonator method was used to investigate a reconstituted sand sediment in this range. The literature contains laboratory and in situ measurements of sound speed and attenuation at higher frequencies, but existing models can not predict both the speed of sound and attenuation simultaneously in some sediments. A time-of-flight technique was used to determine the speed of sound and attenuation in monodisperse water-saturated glass beads, binary glass bead mixtures, and reconstituted sediment samples in the frequency range 200 kHz to 900 kHz to investigate the effect of sediment inhomogeneity. The effect of porosity, independent of changes in other sediment physical properties, has not been demonstrated in the experimental literature. Therefore, a fluidized bed technique was used to independently vary the porosity of monodisperse glass bead samples from 0.37 to 0.43 and a Fourier phase technique was used to determine the speed and attenuation of sound. Collecting these results together, measured sound speeds showed positive dispersion below 50 kHz while negative dispersion was observed above 200 kHz for some samples. Attenuation measurements showed an approximately f⁰̇⁵ dependence in the low frequency regime and an approximately f³̇⁵ dependence for large-grained samples in the high frequency regime. The laboratory experiments presented in this work demonstrate that both sound speed and attenuation in idealized loosely packed water-saturated sediments can not be simultaneously predicted by existing models within the uncertainties of the model input parameters, but the independent effect of porosity on sound speed can be predicted.Show more Item Modeling three-dimensional acoustic propagation in underwater waveguides using the longitudinally invariant finite element method(2014-08) Goldsberry, Benjamin Michael; Hamilton, Mark F.; Isakson, Marcia J.Show more Three-dimensional acoustic propagation in shallow water waveguides is studied using the longitudinally invariant finite element method. This technique is appropriate for environments with lateral variations that occur in only one dimension. In this method, a transform is applied to the three-dimensional Helmholtz equation to remove the range-independent dimension. The finite element method is employed to solve the transformed Helmholtz equation for each out-of-plane wavenumber. Finally, the inverse transform is used to transform the pressure field back to three-dimensional spatial coordinates. Due to the oscillatory nature of the inverse transform, two integration techniques are developed. The first is a Riemann sum combined with a wavenumber sampling method that efficiently captures the essential components of the integrand. The other is a modified adaptive Clenshaw-Curtis quadrature. Three-dimensional transmission loss is computed for a Pekeris waveguide, underwater wedge, and Gaussian canyon. For each waveguide, the two integration schemes are compared in terms of accuracy and efficiency.Show more Item Numerical prediction of underwater noise induced by non-cavitating/cavitating propellers(2022-05-11) Kim, Seungnam; Kinnas, Spyros A.; Liljestrand, Howard M.; Sepehrnoori, Kamy; Hamilton, Mark F.; Kim, Moo-Hyun; Wilson, Preston S.Show more In this work, a systematic procedure of analyzing marine propeller performance and its underwater acoustics is delineated. The main focus is laid on the numerical prediction of hull pressure fluctuations and far-field acoustics induced by marine propellers via frequency- or time-domain approaches. As the first step, a hydrodynamic boundary element method (BEM) is implemented to analyze the comprehensive marine propeller performance in open water and ship-behind conditions. An elaborate numerical scheme modeling developed tip vortex cavitation is proposed for cavitating propellers. In the frequency domain approach, a potential-based panel method (or pressure-BEM) solving for the diffraction potential on the ship hull is coupled with the hydrodynamic BEM to predict the propeller-induced hull pressure fluctuations. Infinite speed of sound is assumed in the near-field, neglecting the time travel of acoustic waves to reduce the Helmholtz equation to the Laplace equation, which can be solved by a standard BEM solver. Far-field underwater noise is investigated in the time domain using the Ffowcs Williams-Hawkings (FW-H) formulation. The model propeller is mounted on the ship with an infinite plane assumed at the design draft to consider the pressure release or solid wall boundary conditions. Time-dependent surface quantities from the hydrodynamic analysis are taken as the known noise sources to the FW-H equation given in the form proposed by Farassat. The complex hull geometry and its disturbance to the medium are considered by solving the boundary integral equations. A half-space domain is modeled by modifying standard full-space Green’s function to consider the image model beyond the infinite plane. The propeller-induced noise, its reflections from the hull, and their sums as the total noise will be investigated in full-space or half-space domains. Numerical predictions, including the time history of acoustic pressures and noise spectra, under various operating conditions are compared with experimental measurements, observations, and results from finite volume methods to validate the feasibility and accuracy of the present methodShow more Item On the propagation of longitudinal stress waves in finite solid elastic horns(1966) Martin, Gordon Eugene, 1925-; Hixson, Elmer L.Show more This dissertation consists of an investigation of the propagation of longitudinal stress waves in finite solid elastic horns or tapered bars. A large number of publications during the last several decades have reported improved theories for other structures such as rods of uniform cross section. However, there appears to have been no publication wherein the effects of lateral inertia and/or shear have been included in a theory of longitudinal waves in horns. These effects are included in the results presented here. A new wave equation and appropriate boundary conditions are obtained, and the special case of the conical horn is discussed in detail including theoretical and experimental results. The historical progress in the study of fluid as well as solid horns is surveyed. A previously-unpublished duality principle is derived which shows that the results from fluid horn theory have a one-to-one correspondence with the one-dimensional theory of solid horns. The major advances in the theory of longitudinal waves in rods of uniform cross section are reviewed also. The engineering method for the derivation of improved theories of wave propagation is applied to horns. Namely, assumed forms for the components of particle displacement are used to form the Lagrangian and incorporate it into Hamilton's Principle. The solution of the resultant integral equation leads to an (Euler) wave equation and the corresponding boundary conditions. It is shown that the horn theory satisfies the reciprocity conditions for a physical system only if the appropriate boundary conditions are used. The conical horn is discussed as a special case of a horn with lateral inertia effects only. It is shown that the solution of the wave equation is a linear combination of the two Legendre functions divided by the axial coordinate. The theory of the finite conical horn with arbitrary end conditions is derived. The new theory is compared with the one-dimensional theory with the conclusion that small but significant lateral effects are present if the horn is long and thin. Comparison of theoretical results with experimental data published three decades ago indicated that the previous experimental results were not sufficiently accurate to test the theory. Therefore, an experimental study is reported wherein two identical horns are cemented with a thin ferroelectric disc between them to provide excitation. Measured values of critical frequencies of resonances are compared with results from theory incorporating the effects of the cement joint and the disc as a linear piezoelectric device. A separate experiment for the evaluation of the dynamic compliance of cement joints is described. The comparison of theoretical and experimental frequencies of resonances of conical horns shows that the relative error is reasonably small (.01 - 1.%). Possible causes for the error due to experimental aspects are given. It is concluded further that the theory should be extended with higher order representations of particle displacement used to include more correctly the effects of lateral inertia and shear.Show more Item The impact of salinity diffusion, poroelasticity, and organic carbon in sediment acoustics(2019-05) Venegas, Gabriel Ricardo; Wilson, Preston S.; Hamilton, Mark R; Haberman, Michael R; Salamone, Salvatore; Lee, Kevin M; Ballard, Megan SShow more To optimize the use of sound in waters on the continental shelf for naval, commercial, and environmental monitoring applications, the acoustic properties of the ocean bottom must be well understood. The effects of 1) pore water salinity variability on acoustic reflectivity, 2) poroelasticity on geoacoustic inference, and 3) organic carbon on sediment properties were formerly-considered insignificant in sediment acoustics, but due to advancements in other areas of underwater acoustics systems and modeling, have now become significant. Three separate but related studies were conducted to begin to quantify these effects. 1) A high-frequency acoustic reflection experiment was performed on a water-clay interface, while varying the salinity of the water. Results demonstrated significant changes in reflectivity at high incident angles, as well as a transient effect explained by a new coupled salt diffusion/reflection model. Using the model, the effective diffusion coefficient of salt in clay was inferred from the experiment, and reflectivity was then simulated at lower frequencies and longer time-scales. From this modeling effort, at a given time-scale of fluctuation, a characteristic frequency was identified, below which the reflectivity should not be assumed temporally invariant. 2) A model geoacoustic inference procedure was performed on a layered waveguide consisting of water and water-saturated glass beads contained within a glass tube. The resonance frequencies of the system were measured and compared with simulations of the experiment. Within each simulation, various sediment acoustics models were used. The only model that allowed for self-consistency between the inference and an independent set of high-frequency sound speed measurements, was a model that accounted for poroelastic effects. 3) A sediment constituent that has great value to the planet and is ubiquitous in natural marine sediment, organic carbon, has been ignored in sediment acoustics models. To begin to explore this relationship, sediment cores were extracted from a T. testudinum seagrass meadow in the Lower Laguna Madre, Texas, USA. A strong correlation between organic carbon and the primary-wave modulus was identified using a custom-built automated broad-band core and resonance logger and an elemental analyzer. The sediment properties attained from the cores were compared, and a theory explaining the correlations was developed. The acoustic sensitivity to organic carbon in a seagrass meadow has demonstrated promise toward developing an acoustic tool to more rapidly quantify marine organic carbon stores, which is needed in climate science. However, a larger-scale study is required to determine its applicability across a broader range of seagrass meadows and sediment typesShow more