Sound Speed In Water Research Articles

The limits of using closed-cell foam as a pressure release condition when dealing with underwater acoustics

Closed-cell foams are frequently used in both the research and industry communities to provide a pressure release condition for underwater sound. In practice, closed-cell foams can effectively imitate a pressure release condition even if the thickness of the foam is as small as 1/60th the wavelength of the sound in water, however these conditions start to break down as the sound speed in water is reduced. The presence of even a small volume fraction of bubbles can cause the sound speed to drop in such a way that the water can couple with the closed-cell foam and prevent the interface from providing a pressure release condition. Measurements made in an acoustic resonator are used to show how the idealized system breaks down. [This work was supported by the Office of Naval Research.]

Tuned elastic shells with matched acoustic impedance and sound speed in water

Tuning of an elastic shell to match the acoustic properties of the surrounding fluid (water) is done with an internal mechanism consisting of a concentered central mass supported by an axisymmetric distribution of elastic stiffeners. The effective impedance and index are both matched at low frequencies (ka < 1) by adjusting the mass and stiffness of the internal oscillator for a given shell thickness. The subsonic flexural modes of the shell, which are excited by the point attachments of the stiffeners, are ultimately suppressed in the low frequency range with a sufficiently large number of stiffeners. As a result, the scattering cross-section of the tuned shell-stiffener-mass system is negligible compared to the empty shell at frequencies below the resonance of the internal oscillator. An optimal shell thickness exists which maximizes the frequency range of water-like behavior. Furthermore, tuning the effective properties of each such shell in an array is used to achieve acoustic transparency and lensing. Inclusion of damping decreases the magnitude of the internal oscillator resonance without significantly affecting the scattered field. Several planar simulations are presented demonstrating the applications.

Shock Hugoniot Equations of State for Binary Ideal (Toluene/Fluorobenzene) and Nonideal (Ethanol/Water) Liquid Mixtures

Laser shock Hugoniot data were obtained using ultrafast dynamic ellipsometry (UDE) for both nonideal (ethanol/water solutions with mole percent χ(ethanol) = 0%, 3.4%, 5.4%, 7.5%, 9.7%, 11%, 18%, 33%, 56%, 100%) and ideal liquid mixtures (toluene/fluorobenzene solutions with mole percent χ(toluene) = 0%, 26.0%, 49.1%, 74.9%, 100%). The shock and particle velocities obtained from the UDE data were compared to the universal liquid Hugoniot (ULH) and to literature shock (plate impact) data where available. It was found that the water UDE data fit to a ULH-form equation suggests an intercept of 1.32 km/s, lower than the literature ambient sound speed in water of 1.495 km/s (Mijakovic et al. J. Mol. Liq. 2011, 164, 66-73). Similarly, the ethanol UDE data fit to a ULH-form equation suggests an intercept of 1.45 km/s, which lies above the literature ambient sound speed in ethanol of 1.14 km/s. Both the literature plate impact and UDE Hugoniot data lie below the ULH for water. Likewise, the literature plate impact and UDE Hugoniot data lie above the ULH for ethanol. The UDE Hugoniot data for the mixtures of water and ethanol cross the predictions of the ULH near the same concentration where the sound speed reaches a maximum. In contrast, the UDE data from the ideal liquids and their mixtures are well behaved and agree with ULH predictions across the concentration range. The deviations of the nonideal ethanol/water data from the ULH suggest that complex hydrogen bonding networks in ethanol/water mixtures alter the compressibility of the mixture.

Exploration of third-order nonlinear acoustics for projection of narrow-beam lower-frequency underwater beams

Projection systems are considered where two or three frequencies (e.g., f1, f2, and/or f3) are simultaneously projected into water in a parallel fashion. High near-field amplitudes produce beams of frequencies equal to any linear combination of f1, f2, and f3, with integer coefficients n1, n2, and n3 (possibly zero or negative). Interest here is in the case where the magnitudes of the coefficients sum to three, associated with a third-order nonlinearity. The question addressed is that of how large the amplitude of the far-field signal will be. The considered causes of the nonlinearities are (1) the convective derivative term in the total time derivative of the fluid velocity, and (2) the higher coefficients in the expansion of the fluid density in terms of the deviation of the pressure from its ambient value. These coefficients are derived from data reported by Holton et al. [J. Acoust. Soc. Am. (1968)] on the sound speed in water. A perturbation technique is explored starting with the basic nonlinear equations of compressible time-dependent fluid dynamics, where at each step one has a simultaneous set of coupled linear and homogeneous equations with the source terms dependent on the solutions of the analogous equations corresponding to the previously considered orders.

Inversion of seabed acoustic parameters in shallow water using the warping transform

In this paper, a method is described for inverting geoacoustic parameters of the seabed from short range field data recorded by single hydrophone. The original data in time domain are processed by a warping operator at first, and then, the dispersion curve and the mode amplitude ratios are extracted separately from the warped data. The velocity and the density in the bottom are inverted from the dispersion curve, and the attenuation from the mode amplitude ratios, respectively. The performance of the method is examined using simulated data and then experimental data from the North Sea of China. The source used in the experiment was a small explosive charge that provided good signal to noise ratio over the frequency band from 200 Hz to 1 kHz. The depth of the water was about 30 m, and the water sound speed was nearly constant with depth. The seabed geoacoustic parameters are inverted from the data received at different ranges from 2 to 14 km. The results from the different ranges are consistent with a simple half space model of the bottom. The seabed velocity is about 1600 m/s.

An acoustic resonance measurement cell for liquid property determinations up to 250 °C

This paper reports on the development of a compact, rugged, and portable measurement cell design for the determination of liquid sound speed at temperatures up to 250 °C and pressures up to 3000 psi. Although a significant amount of work exists in the literature on the characterization of fluids, primarily pure water, over a wide range of pressures and temperatures, the availability of experimentally determined sound speed in water between 100 °C and 250 °C is very limited. The need to measure sound speed in liquids up to 250 °C is of both fundamental interest, as in the case of basic equations of state, and applied interest, such as for characterizing geothermal or petroleum downhole environments. The measurement cell reported here represents an advancement in the established room temperature swept frequency acoustic interferometry measurement for liquid sound speed determinations. The paper details the selection of materials suitable for high temperature operation and the construction of the measurement apparatus. Representative sound speeds as a function of temperature and pressure are presented and are shown to be in very good agreement with an internationally accepted standard for water sound speed.

Acoustic scattering from a finite plate: Generation of guided Lamb wavesS0,A0andA

In the domain of renewable energies, marine current turbines constitute one of the possibilities of producing electrical energy. Naked-eye inspection, or with the aid of video monitoring systems of these machines to ensure their perfect working order, can be difficult in a turbid environment. Acoustic methods are conceivable. The study focuses on the blades of these machines, by considering rectangular plates. The propagation of Lamb waves in a plate is studied by analyzing experimental time signals obtained from acoustic scattering. These signals are analyzed employing the ray theory. In vacuum, the flexural wave is the A(0) Lamb wave, whilst in water this wave splits in a bifurcation: the A wave with a phase velocity always smaller than the sound speed in water, and the A(0) wave with a phase velocity always higher than the sound speed in water. In the central bandpass of the transducers used in the experiments, mainly the A and S(0) waves exist. However, signals observed in the third harmonic bandpass of the transducers are also analyzed. In order to complement these results, resonance frequencies of the plate studied are calculated taking into account the boundary conditions and compared with the resonance frequencies of the experimental spectra.

Classroom measurements of sound speed in fresh/saline water

A tabletop apparatus permitting demonstrations and hands-on student exercises in the measurement of sound speed in water as a function of both temperature and salinity, at atmospheric pressure, is described. By measuring sound speed using a differential technique with a precisely measured path length difference Δx, errors due to uncertainty in original path length are minimized. The apparatus can be used to measure sound speed in freshwater baths of varying temperature and room temperature baths of varying salinities to provide the student a clear picture of sound speed variation along two axes.

Neural time and movement time in choice of whistle or pulse burst responses to different auditory stimuli by dolphins

Echolocating dolphins emit trains of clicks and receive echoes from ocean targets. They often emit each successive ranging click about 20 ms after arrival of the target echo. In echolocation, decisions must be made about the target--fish or fowl, predator or food. In the first test of dolphin auditory decision speed, three bottlenose dolphins (Tursiops truncatus) chose whistle or pulse burst responses to different auditory stimuli randomly presented without warning in rapid succession under computer control. The animals were trained to hold pressure catheters in the nasal cavity so that pressure increases required for sound production could be used to split response time (RT) into neural time and movement time. Mean RT in the youngest and fastest dolphin ranged from 175 to 213 ms when responding to tones and from 213 to 275 ms responding to pulse trains. The fastest neural times and movement times were around 60 ms. The results suggest that echolocating dolphins tune to a rhythm so that succeeding pulses in a train are produced about 20 ms over target round-trip travel time. The dolphin nervous system has evolved for rapid processing of acoustic stimuli to accommodate for the more rapid sound speed in water compared to air.

Enhancement and suppression of the response of a water-filled spherical shell resonator due to the interaction of modes.

In this paper we report on the suppression and enhancement of modal amplitudes in a water-filled, spherical shell resonator as a function of sound speed in water. The resonator, which has a 1-in.-thick wall and a 9.5-in. diameter, was driven by a transducer bolted to the external wall. The drive frequency was swept over specific resonant modes whose frequencies have different dependencies of the sound speed in the water. Theoretical analysis of the resonant modes revealed that the suppression or enhancement of the response occurs for specific values of the sound speed when the initially distinct resonant frequencies of two modes meet at a crossover point. Additional theoretical modeling shows that the suppression or enhancement is caused by the interaction between the two modes when their resonant frequencies intersect.

Seagrass leaves in 3-D: Using computed tomography and low-frequency acoustics to investigate the material properties of seagrass tissue

As a preliminary step toward development of a predictive model of sound speed in water containing seagrass, the acoustic and material properties of leaf tissue for Thalassia testudinum, Syringodium filiforme and Halodule wrightii were assessed using a one-dimensional acoustic resonator and computed tomography imagery. The high-resolution three-dimensional imagery provided valuable information about leaf morphology and produced precise estimates for plant tissue volume and the volume of air contained within the aerenchyma of the plant tissue, which in turn yielded measurements of the plant tissue density ( Thalassia = 1032.34 ± 115.42 kg·m − 3 ; Syringodium = 664.17 ± 10.38 kg·m − 3 ; Halodule = 964.96 ± 21.72 kg·m − 3 ). The acoustic resonator technique was used to investigate the species-specific and biomass-dependent differences in the propagation of sound through seawater containing seagrass leaves in the 0.2 kHz to 10 kHz frequency band. The three species each caused a unique decrease in sound speed with an increase in biomass. The values for leaf air volume, tissue volume and tissue density were then combined with the results of the acoustic experiments and an effective medium acoustic model to determine the acoustic compressibility of the leaves. Further implementation of two effective medium models failed to yield measurements of leaf tissue compressibility. Failure of these models indicates that tissue elasticity and structure must be considered for a complete understanding of acoustic propagation in seagrass meadows.

Flexural Waves in Fluid-Filled Tubes Subject to Axial Impact

We experimentally studied the propagation of coupled fluid stress waves and tube flexural waves generated through projectile impact along the axis of a water-filled tube. We tested mild steel tubes, 38–40 mm inner diameter and wall thicknesses of 0.8 mm, 6.4 mm, and 12.7 mm. A steel impactor was accelerated using an air cannon and struck a polycarbonate buffer placed on top of the water surface within the tube. Elastic flexural waves were observed for impact speeds of 5–10 m/s and plastic waves appeared for impact speeds approaching 20 m/s for a 0.8 mm thickness tube. We observed primary wave speeds of 1100 m/s in a 0.8 mm thickness tube, increasing to the water sound speed with 6.4 mm and 12.7 mm thickness tubes. Comparison of our measurements in the 0.8 mm thickness tube with Skalak’s water hammer theory indicates reasonable agreement between the predicted and measured peak strains as a function of the impact buffer speed (1956, “An Extension to the Theory of Water Hammer,” Trans. ASME, 78, pp. 105–116). For thick-walled tubes, the correlation between the experimentally determined peak pressures and strains reveals the importance of corrections for the through-wall stress distribution.

The impact of ocean sound speed variability on the uncertainty of geoacoustic parameter estimates

This paper investigates the influence of water column variability on the estimates of geoacoustic model parameters obtained from matched field inversions. The acoustic data were collected on the New Jersey continental shelf during shallow water experiments in August 2006. The oceanographic variability was evident when the data were recorded. To quantify the uncertainties of the geoacoustic parameter estimates in this environment, Bayesian matched field geoacoustic inversion was applied to multi-tonal continuous wave data. The spatially and temporally varying water column sound speed is parametrized in terms of empirical orthogonal functions and included in the inversion. Its impact on the geometric and geoacoustic parameter estimates is then analyzed by the inter-parameter correlations. Two different approaches were used to obtain information about the variation of the water sound speed. One used only the profiles collected along the experimental track during the experiment, and the other also included observations collected over a larger area. The geoacoustic estimates from both the large and small sample sets are consistent. However, due to the diversity of the oceanic sound speed, more empirical orthogonal functions are needed in the inversion when more sound speed profile samples are used.

Measurement of sound transmission through mud at Dodge Pond, Connecticut.

Questions important to the sonic detection of buried ordinance are whether the sound dispersion and attenuation of muddy bottoms can be predicted and verified. Wood and Weston [Acustica (1964)] measured compressional speeds in harbor mud 3% less than that of water with attenuation considerably less than those of sandy/silty sediments. A recent theoretical treatment [Pierce and Carey, POMA, 7001 (2009)], making use of the Mallock–Wood equation and of a card‐house theory of the structure of mud, estimates the slow sound speed to depend on porosity as 1−(0.35)(1−φ). Present measurements at frequencies between 1 and 10 kHz with a buried array in the depositional mud at the bottom of Dodge Pond, which contains considerable gas microbubbles, yield speeds of the order of 60% of the sound speed in water. The initial measurements on the disturbed sediment were found to be strongly influenced by scattering from larger bubbles whereas the results after a period of 10 months showed the effect of a smaller size distribution of bubbles. Estimates based on the Dodge Pond measurements and on the card‐house theory of the propagation characteristics and of the effect of micro‐bubbles are discussed. [Sponsored by SERDP‐NSWC‐PCD.]

Marginal probability distributions for seabed parameter values from simultaneous processing of vertical and horizontal apertures.

Information about the physical parameter values for the seabed in a littoral region can be extracted from acoustic measurements in the water column. While acoustic data from both horizontal line arrays (HLAs) and vertical line arrays (VLAs) have been processed successfully to infer seabed properties, the simultaneous signal processing of acoustic data recorded on both a vertical and a horizontal aperture for seabed properties is a new research area. Relative to only a bottom‐mounted horizontal aperture, an L‐array (bottom mounted HLA and a VLA) provides access to the sensitivity of the depth‐dependence of the acoustic field to seabed parameters. However, the utility of the additional information can be negated by uncertainty in the assumed water sound speed profile. Acoustic data collected on two L‐arrays deployed on the New Jersey continental shelf are examined for the additional information about the seabed that can be obtained relative to the case for a single aperture. The information from simultaneou...

Card‐house theory of mud sediments containing kaolinite and its acoustical implications.

A major component of harbor mud is the clay mineral kaolinite, Al2Si2O5(OH)4. Isomorphous substitution, Al atoms occasionally replacing Si atoms, causes a net positive charge of roughly one electronic charge per 400 Si atoms. Kaolinite platelets have diameters and thicknesses of the order of 1 μm and 20 nm. The platelet’s positive charge induces a thin negative charge layer of suspended negative ions on both sides, layer thickness dependent on temperature, creating a continuous sheet of transversely aligned longitudinal electrical quadrupoles. Platelets repel each other weakly when placed face to face. The natural configuration is where the only connection between platelets are with the edge of one touching the center of the face of another. The resulting configuration resembles a house of cards. Because the platelets are thin, the porosity is close to unity, and the sound speed is slightly less than the sound speed in water. This type of mud weakly resists shear because any shearing leads to an increase in electrostatic energy within the card house. Theories are presented to answer why bubbles in mud tend to have a flattened shape and to explain the small but nonzero attenuation of sound in mud. [Work sponsored by Strategic Environmental Research and Development Program.]

Improved interference rejection in code‐division multiaccess underwater acoustic communications: The near‐far problem.

Underwater acoustic communication is known for the signal latency due to the slow sound speed in water. This creates a severe scheduling problem for signal transmission in an underwater acoustic network involving mobile platforms or sensors, particularly when the ranges between the individual nodes are varying with time. Multiple‐access communication methods such as code‐division multiple‐access (CDMA) that are widely used in radio frequency communications (mobile cellular network) have been considered. The problem is that the code orthogonality between the different users is destroyed by the presence of multipath arrivals in an underwater acoustic channel and severe Doppler shift in the signal. As a result, the ability to reject interference that is well known for CDMA signal is severely degraded resulting in a poor bit error rate. One approach is to recover the signal orthogonality by equalizing the multipath arrivals. However, performance of the equalization algorithm is itself degraded in the presence of interference. A different approach using the hyperspace cancellation by coordinate zeroing method is adapted in this paper for interference rejection without requiring channel equalization. Good performance is obtained using at‐sea data. [This work is supported by the US Office of Naval Research.)

Mid-frequency acoustic propagation in shallow water on the New Jersey shelf: Mean intensity

Mid-frequency (1-10 kHz) sound propagation was measured at ranges 1-9 km in shallow water in order to investigate intensity statistics. Warm water near the bottom results in a sound speed minimum. Environmental measurements include sediment sound speed and water sound speed and density from a towed conductivity-temperature-depth chain. Ambient internal waves contribute to acoustic fluctuations. A simple model involving modes with random phases predicts the mean transmission loss to within a few dB. Quantitative ray theory fails due to near axial focusing. Fluctuations of the intensity field are dominated by water column variability.

Measuring the equivalent beam angle of echo sounders with split-beam transducers

When trying to measure the numerical density of marine animal populations with scientific echo sounders, several technical system parameters are determined through a standard-target calibration. The acoustic scattering properties of standard targets are known a priori. In a typical standard-target calibration the system gain and the transducer beam pattern are estimated. The system gain parameter is used both when measuring volume backscattering strength and when measuring target strength. The calibrated beam pattern is currently used only for target strength estimation. However, measurements of mean volume backscattering strength also require knowledge of the effective sampling volume of the acoustic beam, namely the equivalent beam angle. This parameter is typically not determined during an ordinary echo sounder calibration. A method for measuring the equivalent beam angle and the transducer angle sensitivity of a specific split-beam transducer is described, and results from exemplary calibrations are presented. Changes in the calibration parameters as a function of environmental parameters, especially the sound speed in water, are discussed.

Correcting bathymetry measurements for water sound speed effects using inversion theory

We present a method to accurately estimate the bathymetry and water sound speed in shallow waters using overlapping swaths obtained from a Multi-Beam Echo Sounder (MBES). The method is designed to correct the errors in the bathymetry manifested mainly in the outer beams of the MBES. The errors are caused by deviations in sound speed, which occur, for example, in estuaries where fresh river water mixes with seawater. Simulations show that we are able to simultaneously estimate the mean sound speed and the bathymetry. This is accomplished by minimizing the differences between the MBES measurements in the overlap region using a Levenberg-Marquardt optimization routine. We also present an application of the method to real data obtained at the fairway to the harbour of Rotterdam, The Netherlands. Analysis shows that the inverted sound speed agrees well with the measured sound speed at the transducer. However, the uncertainties are larger when the track-to-track distance is ̃2 times the depth. Therefore, we also discuss the optimal distance between sailed tracks. The method appears to be a promising tool for the accurate mapping of sea floors in shallow-water areas with a complex sound-speed profile.