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

Ocean sound channel ray path perturbations from internal-wave shear and strain

Author Posting. © Acoustical Society of America, 2005. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 118 (2005): 2899-2903, doi:10.1121/1.2062127.The relative importance of internal-wave strain and internal-wave shear on perturbation of acoustic ray trajectories in the ocean is analyzed. Previous estimates based on the Garrett-Munk internal-wave spectral model are updated using data from recent field studies of internal waves. Estimates of the ratio of the rms shear effect to the rms strain effect based on data from the upper kilometer of ocean are as high as 0.25–0.4, exceeding the estimates of 0.08–0.17 stemming from the model. Increased strength of three phenomena that have shear to strain ratios higher than the internal-wave average can cause this effect. These are near-inertial waves, internal tides, and vortical modes.This work was funded by grants from the U.S. Office of Naval Research

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

Infrasound generation by tornadic supercell storms

Acoustic wave generation by turbulence in the stratified, moist atmosphere is studied. It is shown that in the saturated moist air turbulence in addition to the Lighthill's quadrupole and the dipole sources of sound related to stratification and temperature fluctuations, there exist monopole sources related to heat and mass production during the condensation of moisture. We determine acoustic power of these monopole sources. Performed analysis shows that the monopole radiation is dominant for typical parameters of strong convective storms. Obtained results are in good qualitative agreement with the main observed characteristics of infrasound radiation by strong convective storms such as total acoustic power and characteristic frequency

Personalized Restoration via Dual-Pivot Tuning

Generative diffusion models can serve as a prior which ensures that solutions of image restoration systems adhere to the manifold of natural images. However, for restoring facial images, a personalized prior is necessary to accurately represent and reconstruct unique facial features of a given individual. In this paper, we propose a simple, yet effective, method for personalized restoration, called Dual-Pivot Tuning - a two-stage approach that personalize a blind restoration system while maintaining the integrity of the general prior and the distinct role of each component. Our key observation is that for optimal personalization, the generative model should be tuned around a fixed text pivot, while the guiding network should be tuned in a generic (non-personalized) manner, using the personalized generative model as a fixed ``pivot". This approach ensures that personalization does not interfere with the restoration process, resulting in a natural appearance with high fidelity to the person's identity and the attributes of the degraded image. We evaluated our approach both qualitatively and quantitatively through extensive experiments with images of widely recognized individuals, comparing it against relevant baselines. Surprisingly, we found that our personalized prior not only achieves higher fidelity to identity with respect to the person's identity, but also outperforms state-of-the-art generic priors in terms of general image quality. Project webpage: https://personalized-restoration.github.i

Sound propagation from a ridge wind turbine across a valley

Sound propagation outdoors can be strongly affected by ground topography. The existence of hills and valleys between a source and receiver can lead to the shielding or focusing of sound waves. Such effects can result in significant variations in received sound levels. In addition, wind speed and air temperature gradients in the atmospheric boundary layer also play an important role. All of the foregoing factors can become especially important for the case of wind turbines located on a ridge overlooking a valley. Ridges are often selected for wind turbines in order to increase their energy capture potential through the wind speed-up effects often experienced in such locations. In this paper, a hybrid calculation method is presented to model such a case, relying on an analytical solution for sound diffraction around an impedance cylinder and the conformal mapping (CM) Green's function parabolic equation (GFPE) technique. The various aspects of the model have been successfully validated against alternative prediction methods. Example calculations with this hybrid analytical-CM-GFPE model show the complex sound pressure level distribution across the valley and the effect of valley ground type. The proposed method has the potential to include the effect of refraction through the inclusion of complex wind and temperature fields, although this aspect has been highly simplified in the current simulations. This article is part of the themed issue 'Wind energy in complex terrains'

An iterative three-dimensional parabolic equation solver for propagation above irregular boundaries

This paper describes the development of an iterative three-dimensional parabolic equation solver that takes into account the effects of irregular boundaries and refraction from a layered atmosphere. A terrain-following coordinate transformation, based on the well-known Beilis-Tappert mapping, is applied to the narrow-angle parabolic equation in an inhomogeneous media. The main advantage of this approach, which has been used in two dimensions in the past, is the simplification of the impedance boundary condition at the earth surface. The transformed initial-boundary value problem is discretized using the Crank-Nicholson marching scheme in the propagating direction and second-order finite-differences in the transversal plane. The proposed method relies on an efficient iterative fixed-point solver which involves the inversion of tridiagonal matrices only. The accuracy of the method is evaluated through a comparison with boundary element simulations in a homogeneous atmosphere above a Gaussian hill. Results show that transversal scattering occur in the shadow zone of the obstacle where the 2D parabolic equation underestimates the pressure amplitude. The model is particularly suited for the simulation of infrasound in a three-dimensional environment with realistic topographie

Interstellar Scintillation of PSR J0437-4715

We studied the turbulence spectrum of the local interstellar plasma in the direction of PSR J0437-4715, on the basis of our observations and those reported earlier by others. We combine these data to form a structure function for the variations of phase along the line of sight to the pulsar. For observations that did not report them, we infer modulation indices from a theoretical model. We find that all of the observations fit a power-law spectrum of turbulence with index n=3.46+/-0.20. We suggest that differences among reported values for scintillation bandwidth and timescale for this pulsar arise from differences in observing parameters. We suggest that refractive effects dominate for this line of sight, with refraction angle about twice the diffraction angle at 330 MHz observing frequency. We suggest that the scattering of this pulsar lies in a layer of enhanced turbulence, about 10 pc from the Sun. We propose that the flux variations of the extragalactic source PKS 0405-385 arise in the same scattering layer.Comment: 9 pages, 3 figures, 2 table