Fast Field Program Research Articles

Prediction Method of Underwater Acoustic Transmission Loss Based on Deep Belief Net Neural Network

The prediction of underwater acoustic transmission loss in the sea plays a key role in generating situational awareness in complex naval battles and assisting underwater operations. However, the traditional classical underwater acoustic transmission loss models do not consider the regional hydrological elements, and the performance of underwater acoustic transmission loss prediction under complex environmental conditions in a wide range of sea areas is limited. In order to solve this problem, we propose a deep learning-based underwater acoustic transmission loss prediction method. First, we studied the application domains of typical underwater acoustic transmission loss models (ray model, normal model, fast field program model, parabolic equation model), analyzed the constraint rules of its characteristic parameters, and constructed a dataset according to the rules. Then, according to the characteristics of the dataset, we built a DBN (deep belief net) neural network model and used DBN to train and learn the dataset. Through the DBN method, the adaptation and calculation of the underwater acoustic transmission loss model under different regional hydrological elements were carried out in a simulation environment. Finally, the new method was verified with the measured transmission loss data of acoustic sea trials in a certain sea area. The results show that the RMSE error between the underwater acoustic transmission loss calculated by the new method and the measured data was less than 6.5 dB, the accuracy was higher than that of the traditional method, and the prediction speed was faster, the result was more accurate, and had a wide range of adaptability in complex seas.

Acoustic wave three-dimensional ground reflection and bouncing

This paper deals with the development of a noise propagation tool for the prediction of perceived noise due to aircraft/rotorcraft flying in a non-uniform atmosphere over a non-flat ground. The far-field noise is predicted through a direct ray tracing method, once the near-field aeroacoustic solution, used to define the aircraft/rotorcraft equivalent noise source, is known from a near-field aeroacoustic solver. The atmospheric effects are taken into account in the ray tracing algorithm through standard formulations. A new algorithm capable of taking into account the realistic orography and terrain properties handles the interaction between sound rays and ground. The ground orography is automatically generated by exploiting the open-source Application Programming Interface Open-Elevation, whereas the soil composition is defined using the CORINE Land Cover inventory. The new algorithm, applying the ground absorption on each ray independently and moving the waves interference evaluation to a post-processing space–time interpolation process, allows the inclusion of the contributions of multiple-reflecting rays in the evaluation of the receiver noise. The numerical investigation is aimed at validating the proposed tool against a Fast Field Program solver, as well as at assessing the effects of the realistic description of the terrain on the predictions of perceived noise.

Long-range atmospheric infrasound propagation from subsurface sources.

In seismology and ocean acoustics, the interface with the atmosphere is typically represented as a free surface. Similarly, these interfaces are considered as a rigid surface for infrasound propagation. This implies that seismic or acoustic waves are not transmitted into the atmosphere from subsurface sources, and vice versa. Nevertheless, infrasound generated by subsurface sources has been observed. In this work, seismo-acoustic modeling of infrasound propagation from underwater and underground sources will be presented. The fast field program (FFP) is used to model the seismo-acoustic coupling between the solid earth, the ocean, and the atmosphere under the variation of source and media parameters. The FFP model allows for a detailed analysis of the seismo-acoustic coupling mechanisms in frequency-wavenumber space. A thorough analysis of the coupling mechanisms reveals that evanescent wave coupling and leaky surface waves are the main energy contributors to long-range infrasound propagation. Moreover, it is found that source depth affects the relative amplitude of the tropospheric and stratospheric phases, which allows for source depth estimation in the future.

Levin's collocation method for predicting sound fields in the presence of sound speed gradients

Sound propagation in a stratified medium has been studied for decades. The Ray tracing method, which is one of the most popular methods, calculates the ray path and propagating angle of a sound ray. However, the ray method is not accurate under many circumstances compared with the wave-based numerical schemes. The Fast field program (FFP), an efficient method for numerical evaluation of the Fourier integral, is accurate but requires significant computational resources for high frequency sound fields because of the highly oscillatory functions found in the integrand. In this paper, a Levin’s collocation method is introduced and used in the evaluation of the sound fields above a locally reacting ground with the presence of the sound speed gradient. The method is far more efficient than most other available numerical techniques such as the FFP and Parabolic Equation method.

Wind turbine low frequency and infrasound propagation and sound pressure level calculations at dwellings.

This study was developed to estimate wind turbine low frequency and infrasound levels at 1238 dwellings in Health Canada's Community Noise and Health Study. In field measurements, spectral peaks were identifiable for distances up to 10 km away from wind turbines at frequencies from 0.5 to 70 Hz. These measurements, combined with onsite meteorology, were in agreement with calculations using Parabolic Equation (PE) and Fast Field Program (FFP). Since onsite meteorology was not available for the Health Canada study, PE and FFP calculations used Harmonoise weather classes and field measurements of wind turbine infrasound to estimate yearly averaged sound pressure levels. For comparison, infrasound propagation was also estimated using ISO 9613-2 (1996) calculations for 63 Hz. In the Health Canada study, to a distance of 4.5 km, long term average FFP calculations were highly correlated with the ISO based calculations. This suggests that ISO 9613-2 (1996) could be an effective screening method. Both measurements and FFP calculations showed that beyond 1 km, ISO based calculations could underestimate sound pressure levels. FFP calculations would be recommended for large distances, when there are large numbers of wind turbines, or when investigating specific meteorological classes.

A Legendre-Galerkin spectral method for constructing atmospheric acoustic normal modes.

A Legendre-Galerkin spectral method is applied to the construction of atmospheric acoustic normal modes above level ground, represented by a complex impedance. A search in the complex plane for modal eigenvalues is replaced by a complex symmetric matrix eigenvalue problem. The Legendre-Galerkin spectral method projects the acoustic normal modes onto an orthogonal basis of Legendre polynomials. The matrix eigenvalue problem can be solved by readily available, public domain, software. Prior knowledge of the location of a set of nearby real eigenvalues is unnecessary since the complex symmetric matrix formulation embodies an approximation of all of the physical constraints of the problem. The normal modes are used to compute the acoustic field, due to a harmonic point source in the atmosphere, including a group of discrete modes radiating into the upper atmosphere, usually associated with the continuous spectrum. The validity of the acoustic field calculation is tested in a comparison with the fast field program and interpreted, with aid of the normal modes, in a downward-refracting atmosphere.

Seismo-Acoustic numerical simulation of North Korea nuclear tests

A seismo-acoustic event is an event in which seismic energy transfers to acoustic energy in the oceans and/or atmosphere and vice versa. Although measurements confirms the coupling between the mediums, a numerical investigation of these events may provide us with a deeper understanding of the phenomena. In this presentation, a finite element, elasto-acoustic Fast Field Program (FFP) is presented and is applied to the recent 2013 and January 6th, 2016 North Korea underground nuclear tests. The aim of this study is to model the elastic and acoustic wave fields of these events and use this information to estimate the source depths. The modeled depths will be compared to those from seismo-acoustical observations.

Accurate fast field formulation for a uniformly moving source traveling above an impedance plane

The Lorentz transform, which can be applied to a uniformly moving source, effectively converts the time-domain wave equation to an equivalent problem in frequency-domain due to a stationary source. Standard fast field program (FFP) implementations apply the property of conjugate symmetry in the wavenumber domain for improved efficiency. Recent literature [D. Dragna et al., AIAA 52, 1928–1939 (2014)] suggests the use of a Dopplerized frequency-dependent impedance model to account for the effects of source motion. Consequently, the FFP kernel function is no longer identical for the positive and negative wavenumbers. Additional complications are introduced by the necessity to compute the positive and negative horizontal separation distances in the Lorentz frame to obtain a complete time history of sound pressures for the source approaching and receding from the receiver. Further development of the FFP algorithm in the Lorentz frame is explored such that a frequency-dependent impedance model is developed. Both moving line and point monopole sources are considered in the current investigation. Results are validated against direct numerical integration schemes and with the published time-domain solutions. Applications for an aircraft operating in cruise condition is also examined in the present study. [Work Sponsored by the Federal Aviation Administration.]

Enhanced Propagation of Aviation Noise in Complex Environments: A Hybrid Approach

Accurate prediction of sound levels around airports and below flight paths can help faithfully represent the impact of aviation noise on communities. However, for large scale assessments, as are often performed by the U.S. Federal Aviation Administrationis environmental models, the accuracy of a model must be weighed against its efficiency. The hybrid propagation model (HPM) is capable of predicting propagation through complex environments. It is a composite of three methods—the generalized terrain parabolic equation (GTPE), fast field program (FFP), and straight ray models—each utilized in a different region of elevation angles from the source. If propagation conditions do not warrant use of the full model, one of the component models with faster runtime can be chosen as a surrogate. Analyses of cases using different source heights and including uneven terrain, refractive atmosphere, and ground type transitions, indicate when a detailed propagation model is needed, or when a simpler model is sufficient.

The effective sound speed approximation and its implications for en-route propagation

The effective sound speed approximation is widely used in underwater and outdoor sound propagation using common models such as ray tracing, the parabolic equation, and wavenumber integration methods such as the fast field program. It is also used in popular specialized propagation methods such as NORD2000 and the Hybrid Propagation Model (HPM). Long ago when the effective sound speed approximation was first introduced, its shortcomings were understood. But over the years, a common knowledge of those shortcomings has waned. The purpose of this talk is to remind everyone that for certain situations the effective sound speed approximation is not appropriate. One of those instances is for the propagation of sound from aircraft cruising at en-route altitudes when wind is present. This is one situation where the effective sound speed approximation can lead to substantially incorrect sound level predictions on the ground. [Work supported by the FAA. The opinions, conclusions, and recommendations in this material are those of the authors and do not necessarily reflect the views of FAA Center of Excellence sponsoring organizations.]

Source motion modeling for high-speed aircraft noise propagation

A continuous source motion model has been developed to represent a cruising aircraft traveling at high altitudes. The numerical implementation is based on the Lorentz transform (LT) and a Fast Field Program (FFP) formulation, which is used to compute the sound fields due to a monopole point source traversing in a horizontally stratified atmosphere parallel to a ground surface. To reduce the computational expenses, a one-dimensional LT-FFP is performed to obtain the pressure time-history for overhead flight conditions. The continuous source motion model is compared against a heuristic model which applies a Doppler shift to the stationary source sound field. A linear sound speed profile was selected to simplify the ray model implementation. A parametric study involving the source Mach number and source emission frequency has been performed in a variety of environmental conditions. The differences in the predicted maximum sound pressure levels between the two models can be as large as 9 dB under certain cond...

Short range sound propagation in shallow water and geo-acoustic parameters inversion

The distribution of short range sound field is complicated in shallow water because layered structures of seabed and multiple reflection from surface have significantly effect on field, propagation model is established by using fast field program. Matched-field processing experiments are carried out respectively during two typical hydrographic seasons in north Yellow Sea. The seabed parameters are inverted based on Bayesian theory.The Results show that inverted parameters of two experimental data had a well consistency and multiple layered structure of seabed affected sound field at short range.

Deducing ground structure using seismic pulses originating from an outdoor explosive source

Near‐surface layering of ground soil can influence propagation of acoustic and seismic pulses originating from above‐surface sources. Simultaneous recording of acoustic air pressure and seismic radial and vertical particle velocities resulting from a small, above ground explosion is used to obtain information about soil structure near the surface. Assuming nonlinear effects to be small at the ranges of interest here, a numerical model called Fast Field Program for Layered Air Ground Systems (FFLAGS), developed originally for continuous sound sources above a porous elastic ground is used to model a porous elastic layered ground system. Suitable optimisation methods were used to predict a set of best fit parameters for the near‐surface ground structure. It is shown that the model can explain multiple seismic arrivals and give a reasonable prediction of wave speeds and layer depths while the pressure pulse can predict permeability of the surface soil.Near‐surface layering of ground soil can influence propagation of acoustic and seismic pulses originating from above‐surface sources. Simultaneous recording of acoustic air pressure and seismic radial and vertical particle velocities resulting from a small, above ground explosion is used to obtain information about soil structure near the surface. Assuming nonlinear effects to be small at the ranges of interest here, a numerical model called Fast Field Program for Layered Air Ground Systems (FFLAGS), developed originally for continuous sound sources above a porous elastic ground is used to model a porous elastic layered ground system. Suitable optimisation methods were used to predict a set of best fit parameters for the near‐surface ground structure. It is shown that the model can explain multiple seismic arrivals and give a reasonable prediction of wave speeds and layer depths while the pressure pulse can predict permeability of the surface soil.

Prediction of seismic pulses from an outdoor explosive source

Near-surface layering of ground soil can influence propagation of seismic pulses originating from above-surface sources. Furthermore, in a cold climate snow and frozen ground can add to this layering effect. A numerical model called Fast Field Program for Layered Air Ground Systems (FFLAGS), developed originally for continuous sound sources above a porous elastic ground, is used to predict radial and vertical seismic signals from an above ground explosive charge recorded by a geophone. Simultaneous acoustic air pressure recording of the same charge is used to make a prediction of the source pulse shape. This procedure produces an effective linear source. In other words, nonlinear effects are assumed to be small at the ranges of interest here. Subsequently, suitable optimization methods were used to predict a set of best fit parameters for the near-surface ground structure. [Work supported in part by US Army through its European Research Office.]

Predicting acoustic and seismic pulses from outdoor explosions

To support the formulation of a simple model that can be used for assessment and prediction of ground vibration levels due to above-ground explosions, a fast field program for layered air ground systems (FFLAGS), developed originally for continuous sound sources, has been extended to enable predictions of propagation from impulsive sound sources in a refracting atmosphere above layered porous and elastic ground. Using a mixture of measured and best-fit parameters, the pulse version of the code (PFFLAGS) has been found to give predictions of the air pressure spectrum above ground and the vertical component of solid particle velocity near the ground surface that compare tolerably well with published data for the spectra and waveforms of acoustic and seismic pulses at short range (60 m) in grass- and snow-covered ground and at longer range (3 km) in forested terrain. The predicted seismic pulses in the presence of the snow cover explain a phenomenon frequently observed for shallow snow covers whereby a high-frequency ground vibration is modulated by a lower-frequency layer resonance [Work supported in part by SERDP SEED Project SI-1410 and Contract Number W90C2K9887EN010001 through the European Research Office of the U.S. Army.]

Equations for finite-difference, time-domain simulation of sound propagation in moving inhomogeneous media and numerical implementation

Finite-difference, time-domain (FDTD) calculations are typically performed with partial differential equations that are first order in time. Equation sets appropriate for FDTD calculations in a moving inhomogeneous medium (with an emphasis on the atmosphere) are derived and discussed in this paper. Two candidate equation sets, both derived from linearized equations of fluid dynamics, are proposed. The first, which contains three coupled equations for the sound pressure, vector acoustic velocity, and acoustic density, is obtained without any approximations. The second, which contains two coupled equations for the sound pressure and vector acoustic velocity, is derived by ignoring terms proportional to the divergence of the medium velocity and the gradient of the ambient pressure. It is shown that the second set has the same or a wider range of applicability than equations for the sound pressure that have been previously used for analytical and numerical studies of sound propagation in a moving atmosphere. Practical FDTD implementation of the second set of equations is discussed. Results show good agreement with theoretical predictions of the sound pressure due to a point monochromatic source in a uniform, high Mach number flow and with Fast Field Program calculations of sound propagation in a stratified moving atmosphere.

Surf-generated noise signatures: a comparison of plunging and spilling breakers.

Range-time-frequency distributions of surf-generated noise were measured within the surf zone during the SandyDuck'97 experiment at Duck, NC. A 24-phone, 138-m, bottom-mounted, linear array located along a line perpendicular to the shore at a depth of 1 to 3 m recorded the surf-generated noise. Concurrent video measurements of the location, size, and time-evolution of the individual breaking waves directly above the array were made from a nearby 43-m tower. Source level spectra are obtained by using a modified fast field program to account for water column and geoacoustic propagation from the distributed source region to an individual hydrophone. The length, location, and orientation of the leading edge of breakers are tracked in time from rectified video images. It is observed that the source levels from spilling breakers are lower (approximately 5-10 dB) than those produced by plunging breakers that occurred during the same time period. Plunging breakers generated time-frequency signatures with a sharp onset while spilling breakers' signatures had a gradual low-frequency precursor. Range-time signatures of plunging breakers indicate a burst of acoustic energy while spilling breakers' signatures depict sound being generated over a longer time period with the source region moving with the breaking surface wave.

A hybrid BIE/FFP scheme for predicting barrier efficiency outdoors

To assess the acoustical performance of noise screens in the presence of an arbitrary sound speed profile, a numerical scheme based on a combination of the boundary integral and fast field program methods is developed. The Green’s function required in the boundary integral is evaluated by a fast field formulation. The procedure is validated by comparing its predictions with other numerical results for simple cases and with measurements for indoor model experiments simulating downward refracting conditions. It is predicted that the performance of a noise screen placed 50 m from the source is reduced considerably by moderate downwind conditions.

A new boundary-element method for predicting outdoor sound propagation and application to the case of a sound barrier in the presence of downward refraction

A new boundary-element method for predicting outdoor sound propagation over uneven ground is presented. This allows the sound field around complex boundaries (various absorptive properties and shapes) and in the presence of refraction to be calculated accurately. The total sound pressure is expressed as the sum of the incident pressure and the pressure scattered by the obstacles in the propagation medium, involving layer potentials. The Green’s function used in this formulation takes meteorological and ground effects into account and relies on recent models for propagation in inhomogeneous media, such as normal modes, residue series, the parabolic equation, or the fast-field program. In this paper, this new method, called Meteo-BEM, is derived, based on both boundary-element methods (BEM) in a quiescent medium, and propagation models in inhomogeneous media. The hypothetical case of a rigid, thin noise barrier on a flat ground, under a known sound-speed gradient condition, is studied. Comparison of numerical simulations with experimental results shows that this new method is a powerful tool for outdoor sound propagation prediction, which gives rise to many applications and developments.

A benchmark fast-field program model for infrasound propagation

Three decades ago Pierce [e.g., Pierce, Posey, and Iliff, J. Geophys. Res. 76, 5025 (1971)] developed an exact theory for acoustic wave propagation in a temperature- and wind-stratified atmosphere. The theory includes both the acousto-gravity wave component (i.e., the Lamb wave) and the effect of a vector wind that can vary with height in both magnitude and direction. In this paper, a fast-field program (FFP) is discussed which numerically integrates Pierce’s differential equations, thus eliminating the need to approximate the solutions with a normal-mode expansion. For a stratified atmosphere, the FFP solution can serve as a benchmark for testing parabolic equation (PE) solutions that include the Lamb wave and a vector wind. Pierce’s theory is outlined and a general numerical approach is given for solving the fundamental equations of the theory. The numerical solutions are compared with known solutions obtained by other methods, such as the parabolic equation and analytic models. [Work supported by the U.S. Army Space and Missile Defense Command.]