Correspondence between sound propagation in discrete and continuous random media with application to forest acoustics

Although sound propagation in a forest is important in several applications, there are currently no rigorous yet computationally tractable prediction methods. Due to the complexity of sound scattering in a forest, it is natural to formulate the problem stochastically. In this paper, it is demonstrated that the equations for the statistical moments of the sound field propagating in a forest have the same form as those for sound propagation in a turbulent atmosphere if the scattering properties of the two media are expressed in terms of the differential scattering and total cross sections. Using the existing theories for sound propagation in a turbulent atmosphere, this analogy enables the derivation of several results for predicting forest acoustics. In particular, the second-moment parabolic equation is formulated for the spatial correlation function of the sound field propagating above an impedance ground in a forest with micrometeorology. Effective numerical techniques for solving this equation have been developed in atmospheric acoustics. In another example, formulas are obtained that describe the effect of a forest on the interference between the direct and ground-reflected waves. The formulated correspondence between wave propagation in discrete and continuous random media can also be used in other fields of physics

Recent Progress in Acoustic Travel-Time Tomography of the Atmospheric Surface Layer

Acoustic tomography of the atmospheric surface layer (ASL) is based on measurements of the travel times of sound propagation between sources and receivers which constitute a tomography array. Then, the temperature and wind velocity fields inside the tomographic volume or area are reconstructed using different inverse algorithms. Improved knowledge of these fields is important in many practical applications. Tomography has certain advantages in comparison with currently used instrumentation for measurement of the temperature and wind velocity. In this paper, a short historical overview of acoustic tomography of the atmosphere is presented. The main emphasis is on recent progress in acoustic tomography of the ASL. The tomography arrays that have been used so far are discussed. Inverse algorithms for reconstruction of the temperature and wind velocity fields from the travel times are reviewed. Some results in numerical simulations of acoustic tomography of the ASL and reconstruction of the turbulence fields in tomography experiments are presented and discussed. Zusammenfassung Die akustische Tomographie der atmosph¨arischen Bodenschicht basiert auf Messungen der Laufzeit von Schallwellen zwischen Sendern und Empf¨angern, welche ein tomographisches Messfeld bilden. Anschließend werden dann die Temperatur- und Windgeschwindigkeitsfelder innerhalb eines tomographischen Volumens oder einer Fl¨ache rekonstruiert, wobei verschiedene inverse Algorithmen angewendet werden k¨onnen. Eine verbesserte Kenntnis dieser meteorologischen Felder ist f ¨ur viele praktische Anwendungen bedeutsam. Tomographische Verfahren haben bestimmte Vorteile gegen¨uber den momentan genutzten Messverfahren f ¨ur die Temperatur und Windgeschwindigkeit. In dieser Ver ¨offentlichung wird ein kurzer ¨Uberblick zur Entwicklung der akustischen Tomographie der Atmosph¨are pr¨asentiert. Der Schwerpunkt der Arbeit liegt auf der Darstellung des aktuellen Fortschritts in der akustischen Tomographie der atmosph¨arischen Bodenschicht. Die bisher genutzten tomographischen Messfelder werden vorgestellt. Inverse Algorithmen f ¨ur die Rekonstruktion von Temperatur- und Windgeschwindigkeitsfeldern aus akustischen Laufzeiten werden bewertet. Einige Resultate der numerischen Simulation der akustischen Tomographie der Bodenschicht und der Rekonstruktion von turbulenten Feldern meteorologischer Gr ¨oßen in tomographischen Experimenten werden dargestellt und diskutiert

Staggered-grid finite-difference acoustic modeling with the Time-Domain Atmospheric Acoustic Propagation Suite (TDAAPS).

This document is intended to serve as a users guide for the time-domain atmospheric acoustic propagation suite (TDAAPS) program developed as part of the Department of Defense High-Performance Modernization Office (HPCMP) Common High-Performance Computing Scalable Software Initiative (CHSSI). TDAAPS performs staggered-grid finite-difference modeling of the acoustic velocity-pressure system with the incorporation of spatially inhomogeneous winds. Wherever practical the control structure of the codes are written in C++ using an object oriented design. Sections of code where a large number of calculations are required are written in C or F77 in order to enable better compiler optimization of these sections. The TDAAPS program conforms to a UNIX style calling interface. Most of the actions of the codes are controlled by adding flags to the invoking command line. This document presents a large number of examples and provides new users with the necessary background to perform acoustic modeling with TDAAPS

Equations for finite-difference, time-domain simulation of sound propagation in moving media with arbitrary Mach numbers

Finite-difference time-domain (FDTD) techniques for sound propagation have become increasingly popular. In moving media, such as the atmosphere, starting equations for FDTD calculations are often limited to low Mach numbers, which may result in significant errors. In this article, two coupled equations for the sound pressure and acoustic particle velocity are derived from the linearized fluid dynamic equations. These coupled equations are valid for arbitrary Mach numbers, in the high-frequency approximation, and can be used in FDTD calculations and other methods for sound propagation in moving media. For low Mach numbers, the equations derived are valid for arbitrary frequencies and are consistent with equations from the literature

Coupled mode transport theory for sound transmission through an ocean with random sound speed perturbations: Coherence in deep water environments

The article of record as published may be found at http://dx.doi.org/10.1121/1.4818779Second moments of mode amplitudes at fixed frequency as a function of separations in mode number, time, and horizontal distance are investigated using mode-based transport equations and Monte Carlo simulation. These second moments are used to study full-field acoustic coherence, including depth separations. Calculations for low-order modes between 50 and 250 Hz are presented using a deep-water Philippine Sea environment. Comparisons between Monte Carlo simulations and transport theory for time and depth coherence at frequencies of 75 and 250 Hz and for ranges up to 500 km show good agreement. The theory is used to examine the accuracy of the adiabatic and quadratic lag approximations, and the range and frequency scaling of coherence. It is found that while temporal coherence has a dominant adiabatic component, horizontal and vertical coherence have more equal contributions from coupling and adiabatic effects. In addition, the quadratic lag approximation is shown to be most accurate at higher frequencies and longer ranges. Last the range and frequency scalings are found to be sensitive to the functional form of the exponential decay of coherence with lag, but temporal and horizontal coherence show scalings that fall quite close to the well-known inverse frequency and inverse square root range laws.Office of Naval Research Ocean Acoustics Program Code (322)Office of Naval Research Ocean Acoustics Program Code (322