Propagation Of Acoustic Energy Research Articles

Nonreciprocal control and cooling of phonon modes in an optomechanical system.

Mechanical resonators are important components of devices that range from gravitational wave detectors to cellular telephones. They serve as high-performance transducers, sensors and filters by offering low dissipation, tunable coupling to diverse physical systems, and compatibility with a wide range of frequencies, materials and fabrication processes. Systems of mechanical resonators typically obey reciprocity, which ensures that the phonon transmission coefficient between any two resonators is independent of the direction of transmission1,2. Reciprocity must be broken to realize devices (such as isolators and circulators) that provide one-way propagation of acoustic energy between resonators. Such devices are crucial for protecting active elements, mitigating noise and operating full-duplex transceivers. Until now, nonreciprocal phononic devices3-11 have not simultaneously combined the features necessary for robust operation: strong nonreciprocity, in situ tunability, compact integration and continuous operation. Furthermore, they have been applied only to coherent signals (rather than fluctuations or noise), and have been realized exclusively in travelling-wave systems (rather than resonators). Here we describe a scheme that uses the standard cavity-optomechanical interaction to produce robust nonreciprocal coupling between phononic resonators. This scheme provides about 30 decibels of isolation in continuous operation and can be tuned in situ simply via the phases of the drive tones applied to the cavity. In addition, by directly monitoring the dynamics of the resonators we show that this nonreciprocity can control thermal fluctuations, and that this control represents a way to cool phononic resonators.

Regression analysis of the results of planned computer experiments in machine mechanics

The possibility is considered on the basis of the planned computational experiment at the stage of mathematical modeling to efficiently construct convolutions of the received information. This allows us to build a direct functional relationship between the parameters and the quality criteria of the dynamic system. formulas for calculating the coefficients for some types of regression dependencies are obtained, pre-calculated estimates of the variances from below and from above are given both for the coefficients and for the values of the regression function (i.e., the error forecast of the quality criterion). The results of research on the mathematical model of acoustic emission of loom elements are presented; a mathematical model for the propagation of acoustic energy by the design of the machine is constructed; Such assessments made it possible to develop recommendations for reducing machine noise.

Offshore impact pile driving as a source of opportunity for geoacoustic investigations

Noise generated by offshore impact pile driving can radiate into and propagate through the air, water, and sediment. Most of the recent studies have focused on predicting acoustic pressure field in water to assess environmental impacts. In this study, we focus on the propagation of acoustic energy along the interface between water and ocean bottom. We modeled the interface wave propagation using the commercial Finite Element (FE) code, Abaqus 6.11. A field test is planned for this summer using a scaled model impact pile driving off the dock in the Bay Campus University of Rhode Island (URI). Interface data (particle velocities at the water-sediment interface) will be collected using the Shear Measurement System. Our efforts will focus on identifying the arrivals corresponding to various wave types using data-model comparison. In addition to the interface waves, we will also model the Mach wave front arrival angle. We will explore the possibility of utilizing the arrival times and angles corresponding to these wave types for setting up an inverse problem. This test will also verify the possibility calculating compressional wave speed using a single three-axis geophone by measuring arrival angle of Mach wave front at the interface generated by impact pile driving. [Work sponsored by the Link Foundation Ocean Engineering and Instrumentation PhD Fellowship program.]

High frequency sound propagation in a network of interconnecting streets

We propose a new model for the propagation of acoustic energy from a time-harmonic point source through a network of interconnecting streets in the high frequency regime, in which the wavelength is small compared to typical macro-lengthscales such as street widths/lengths and building heights. Our model, which is based on geometrical acoustics (ray theory), represents the acoustic power flow from the source along any pathway through the network as the integral of a power density over the launch angle of a ray emanating from the source, and takes into account the key phenomena involved in the propagation, namely energy loss by wall absorption, energy redistribution at junctions, and, in 3D, energy loss to the atmosphere. The model predicts strongly anisotropic decay away from the source, with the power flow decaying exponentially in the number of junctions from the source, except along the axial directions of the network, where the decay is algebraic.

A Study of Far Field Directivity Pattern of Bio-Inspired EMFit Emitters

Bats use echolocation to navigate their environment and to localize and identify prey. A biomimetic sensor uses propagation of acoustic energy at higher frequencies to extract information from the environment. EMFit is a new material with very good properties for air. In our attempt to optimize the actual sonar head used by self-governed robots we used this material to construct ultrasound transducers capable of working in a similar way as bats' emitter. We performed measurements, calculated and analyzed the far field directivity pattern for two types of transducers: linear array and hemi-cylindrical array. Experimental studies of the directivity pattern in far field for these two types of EMFit transducers highlighted the capabilities of these two emitters.

Frequency and angle spreading due to rough interface scattering

A narrow band passive sonar system sensing propagation of acoustic energy reflected from the ocean bottom in shallow water incurs detection losses due to frequency and angle limitations of the processor and receiving array. These losses are expressible as ratios of integrals of a suitably defined bottom scattering function. This talk presents a modeling approach which uses propagation calculations and a tangent plane approximation to the Helmholtz scattering integral at planes within a layered bottom sediment to analyze sensitivities of the spreading problem and to predict system losses. Model results for a few hypothetical but representative sonar systems will be presented.

Effects of solitons on acoustic energy flow in three dimensions

The impact of a train of nonlinear solitons on the propagation of acoustic energy in shallow water is examined. The soliton perturbations are based on an analytic formulation that produces a train of five soliton waves. Each wave front is parallel and has infinite extent in the horizontal direction. The acoustic field is modeled using a three-dimensional (3D) split-step Fourier parabolic equation (SSF/PE) approach defined in Cartesian coordinates. The standard PE approximation is employed in both depth and cross-range directions. Both pressure and particle velocity fields are computed in a self-consistent manner, allowing a full description of the 3D acoustic intensity field which describes the flow of energy in the presence of the solitons. Individual, low-order modes are extracted from the propagating field so that the impact on specific modes may be examined. The analysis is performed at various frequencies and for various source-receiver geometries relative to the soliton train. Emphasis is placed on the focusing and defocusing of acoustic energy between the various soliton waves. The impact of such soliton perturbations on signal variability and bearing resolution at the receiver will be quantified. [Work supported by ONR 321OA.]

Hydroacoustic blockage prediction and measurement at Diego Garcia using the Adiabatic Mode Parabolic Equation Model

Underwater explosion monitoring with sparse sensors at long ranges relies on the efficient propagation of acoustic energy in the sound fixing and ranging (SOFAR) channel. When sound traveling in this channel encounters an island or seamount, it will either diffract, scatter, or be converted into seismic energy. Signals observed on the opposite side of these obstructions have been affected by some combination of these processes, and models of global detection and localization depend on knowing these effects. We present a study using the Adiabatic Mode Parabolic Equation (AMPE) model to predict these processes in three dimensions at the Chagos Archipelago. Predictions at 5, 10, and 20 Hz are compared with measurements of approximately 300 T-wave signals from six years of earthquakes on either side of the Chagos Archipelago. These have been recorded at the hydrophone arrays around Diego Garcia. The result of this 360-degree analysis, and the agreement with observed data, demonstrate the utility of the model in understanding the physical effects of these obstructions.

On the usefulness of waterborne measurement of particle velocity in geoacoustic inversion

Recent advances in sensor design have led to the development of receiving systems whose elements are vector sensors, i.e., sensors that simultaneously measure the acoustic pressure and fluid motion due to the propagation of acoustic energy at the sensor location. As such, arrays of vector sensors can provide more information about the sound field than arrays made of traditional hydrophones, and thus are attractive for various applications including the inversion for environmental properties. The fundamental question addressed by this paper is: does the use of acoustic vector data versus pressure-only data improve the results of an inversion scheme based on waterborne observations and matched field processing? To our knowledge no experimental data are available yet to allow a detailed comparison of performance between standard (pressure-only) arrays and vector sensor arrays for environmental inversions. For a preliminary study we will simulate the inversion of vector sensor data in the South Elba environment for which pressure-only inversion results and ground truth data are available following the Yellow Shark'94 and Blue Planet'07 experiments. Broadband signals received on fully-populated and sparse arrays will be considered.

On the propagation of acoustic energy in the vicinity of a bubble chain

In this paper, experimental data on the propagation of acoustic energy in the vicinity of a vertical chain of discrete air bubbles are presented. The acoustic energy was created naturally during the formation of each bubble at the bottom of the chain. Previous work has reported that the root-mean-squared pressure distribution is highly anisotropic in the vicinity of a bubble chain. A new experimental set-up has been developed to obtain ‘snapshots’ of the instantaneous acoustic pressure field using a triggering technique with two hydrophones. This methodology allowed coordinated measurement of the acoustic signal in the near and far field and the data were used to construct the instantaneous spatial distribution of acoustic energy around the bubble chains. The results show that the phase speed in the direction of the bubble chain has values substantially lower than the speed of sound in pure water. Bubble chains of different configurations were investigated and it was found that this speed of propagation is reduced for chains consisting of larger and more closely spaced bubbles.

Sound channeling through bubbly streams

The presence of even small amount of bubbles can have a strong effect on the transmission and reduction of the sound waves through the medium in which they occur. The study presented here examines experimentally and numerically the propagation of acoustic energy in the vicinity of streams of discrete air bubbles. The acoustic energy was created naturally during the detachment of bubbles at the bottom of the stream. Previous work [A. Nikolovska, J. Acoust. Soc. Am. 115, 2473 (2004)] has reported that the root-mean-squared pressure distribution is highly anisotropic in the bubbles vicinity. A new experimental technique has been developed to obtain time frames of the instantaneous acoustic pressure field using two hydrophones. This technique allowed coordinated measurement of the acoustic energy distribution in the near and far field. The results show that the phase speed in the direction of the bubble stream has values considerably lower than the speed of sound in pure water. Different bubble stream configurations were investigated and it was found that this speed of propagation reduces and the quality factor of the sound pulses increases as the air volume fraction in the bubble streams is increased. [Work supported by RCOM.]

Winter PRIMER Ocean-Acoustic Solitary Wave Modeling Studies

In this paper, we present results from a joint oceanographic-acoustic study of solitary waves and their effects during the 1997 winter PRIMER4 experiment on the shelfbreak south of Cape Cod, MA. The study addresses the acoustic effects induced by solitary waves and associated oceanographic phenomena. Solitary wave generation and propagation simulations are produced by the Lamb model [J. Geophys. Res., vol. 99, pp. 848-864, 1994]. The model is nonhydrostatic and is formulated in 2.5 dimensions using terrain following coordinates. Acoustic field calculations are performed with a parabolic equation acoustic model along the path of solitary wave train propagation. The oceanographic model is initialized from density profiles derived from conductivity-temperature-depth (CTD) casts using analytical functions. The model is forced with a prescribed semidiurnal tidal velocity. An ocean background current is introduced. Simulations based on parameters derived from measurements show the following: 1) internal solitary waves of elevation propagate onto the shelfbreak region; 2) opposing ocean currents enhance the formation of solitary waves at the shelfbreak; 3) deepening of the winter mixed layer results in less penetration of the solitary waves on to the shelf; 4) density structure, mixed-layer depth, tidal forcing, and ocean currents control the formation of solitary waves of elevation at the shelfbreak; 5) energy conversion, from semidiurnal barotropic to semidiurnal barcoclinic tides and to internal solitary waves, occurs; 6) amplitudes and periods of modeled solitary waves are in the range of thermistor chain measurements; and 7) lower mixed-layer densities increase the phase speed of simulated solitary waves. Acoustic field calculations are coupled to the propagation of the solitary wave packets through the sound-speed changes that are derived from the oceanographic simulations. Acoustic model predictions show signal intensity fluctuations similar to the anomalous loses in acoustic energy observed in the Yellow Sea data taken by Zhou [J. Acoust. Soc. Amer., vol. 90, pp. 2042-2054, 1991]. In some cases, the presence of solitary waves on the shelf enhances the propagation of acoustic energy onto the shelf. This was observed for acoustic simulations where the acoustic source was located beyond the shelfbreak and at a depth greater than the shelf depth.

Micro-machined ultrasonic transducer (MUT) substrate that limits the lateral propagation of acoustic energy

A micro-machined ultrasonic transducer (MUT) substrate that reduces or eliminates the lateral propagation of acoustic energy includes holes, commonly referred to as vias, formed in the substrate and proximate to a MUT element. The vias in the MUT substrate reduce or eliminate the propagation of acoustic energy traveling laterally in the MUT substrate. The vias can be doped to provide an electrical connection between the MUT element and circuitry present on the surface of an integrated circuit substrate over which the MUT substrate is attached.

Inversion of sediment parameters in a shallow-water range-dependent environment

The propagation of acoustic energy in a range dependent environment will be analyzed. The broadband acoustic signals from explosive sources collected during the Shelf Break PRIMER Experiment will be used for this study. Time–frequency behavior of these signals will be analyzed using wavelets. Upslope propagation along varying bathymetry (from 800 m at source location to 90 m at the receiver) and sound speed is considered. A simple model for the sediment compressional speeds will be extracted using a hybrid inversion scheme. This hybrid scheme uses a genetic algorithm and Levenberg–Marquardt nonlinear least-squares method. Inversion is based on group speed dispersion. Theoretical group speeds will be calculated based on range-dependent propagation. [Work supported by ONR.]

Multiscale estimation of bottom properties

In shallow water areas, the ocean bottom plays an important role in the propagation of acoustic energy. It is therefore important to be able to determine the bottom acoustic properties with adequate resolution and accuracy. Further, if the bottom model is to be capable of predicting the field over a range of frequencies, the model has to be extracted from broadband data. The possibility of obtaining the bottom properties at different scales of resolution is explored. Scaling functions derived from wavelets are used as the basis functions for estimating the bottom properties. Modal inverse method and the multifrequency data acquired during the Hudson Canyon experiment are used for this study.

Subcritical penetration into a viscoelastic solid due to interface roughness: Laboratory experiment and perturbation theory model

The propagation of acoustic energy into sub-bottom sediments at grazing angles below the compressional critical angle has been well documented and various mechanisms to explain this propagation have been proposed. Due to the complexity of the in-situ environment the propagated energy might not be the result of a single mechanism, but a sum of a combination of several mechanisms. In order to better consider the validity of one of the more prominent mechanisms, rough surface scattering, a simplified laboratory environment has been constructed in which to make measurements of penetration due to interface roughness. In this environment, the sediment has been replaced by a homogenous viscoelastic solid. To preserve the uniformity of the solid, both the transmitter and the receiver are located in the water column, where their radiation patterns are well characterized. Use of an automated positioning and measurement system and a large interface area ensures that the series of measurements are both repeatable and statistically meaningful. A perturbation theory model has been constructed for comparison with the measurements. In the experiment its accuracy is evaluated as the surface roughness is increased through the limit of perturbation theory. [Work supported by ONR.]

Understanding acoustic propagation in shallow water via animations.

Modeling the propagation of acoustic energy in shallow water is difficult due to the complexity of the environment. Roughness at the air–sea interface and bottom, inhomogeneities in the water column and sediment, and discrete scatterers affect the propagation. Although many modeling techniques are available to study this problem, arguably none is general enough to provide robust and accurate results over a large region when several complicating physical features are present. Instead, simplifying assumptions are often made, and the effect of such assumptions on the model’s validity is uncertain. Here, propagation in shallow water is studied via animations of the results obtained using the finite‐difference time‐domain method (a powerful, but computationally expensive, modeling technique with few inherent assumptions). Such animations provide insight into the relative importance of certain physical features that is not easily obtained from ‘‘still’’ data. Two different types of animations are discussed. One employs a ‘‘side‐by‐side’’ comparison in which time‐domain propagation is shown in both the presence and absence of a certain physical feature. In the other, time is held constant and a physical feature is varied from frame to frame. Creation of animations using freely available software is also discussed. [Work supported by ONR.]

Finite difference simulations of propagation in a shallow water environment

The propagation of acoustic energy in a shallow water environment is studied using a two-dimensional, staggered-grid finite-difference model. Such a model allows one to view the propagation of energy in the time domain throughout the region of interest. Since fields are available ‘‘everywhere,’’ the model can be used to guide the development of robust schemes for the detection of scatterers in the water column or buried in the sediment. In addition, it can be used to benchmark approximate models for acoustic propagation and scattering in shallow water. This paper considers the effect of different physical features on the possible identification of scatterers. Such scattering mechanisms as top and bottom roughness and inhomogeneities in the water column are considered. Monte Carlo simulations are used to gauge the relative effects of these features. Frequencies on the order of 7.5 kHz are considered for depths up to 30 m. The implementation used here includes several enhancements over traditional finite-difference models. For example, typical finite difference models approximate continuously varying material interfaces by an interface that appears like a staircase—the material properties change only at discrete location. For this research a conformal technique is used to model more accurately the continuous interface. [Work supported by ONR.]

Direct numerical calculation of acoustics: solution evaluation through energy analysis

The propagation of acoustic energy from a sound source to the far field is a fundamental problem of acoustics. In this paper the use of computational fluid dynamics (CFD) to directly calculate the acoustic field is investigated. The two-dimensional, compressible, inviscid flow about an accelerating circular cylinder is used as a model problem. The time evolution of the energy transfer from the cylinder surface to the fluid, as the cylinder is moved from rest to some non-negligible velocity, is shown. Energy is the quantity of interest in the calculations since various components of energy have physical meaning. By examining the temporal and spatial characteristics of the numerical solution, a distinction can be made between the propagating acoustic energy, the convecting energy associated with the entropy change in the fluid, and the energy following the body. In the calculations, entropy generation is due to a combination of physical mechanisms and numerical error. In the case of propagating acoustic waves, entropy generation seems to be a measure of numerical damping associated with the discrete flow solver.

Acoustic tomography in the Straits of Florida

Variability of the Florida Current has been monitored via acoustic tomography. A reciprocal tomography experiment was conducted in the eastern half of the Florida Straits during mid‐October and November, 1983. A triangular array of transceivers, with leg separations of approximately 45 km, was deployed at 27°N. The presence of a surface mixed layer in the region allowed for the ducted propagation of acoustic energy in the surface layer. A deeper layer was sampled by an unresolved group of refracted, bottom‐reflected ray arrivals. Incorporating the complete set of arrivals, we are able to obtain depth‐dependent estimates of the temperature field, current velocity, and relative vorticity. The oceanography of the region has been shown to be dominated by the lateral shifting of the surface and subsurface core of the Florida Current. The influx of westward flowing water through the Northwest Providence Channel at 26°N also appears as a large‐scale signal in the eastern Florida Straits. Low‐frequency fluctuations of temperature, current velocity, and vorticity occur at periods ranging from several days to nearly 2 weeks, and are intimately related to meandering of the Florida Current system.