Journal of Theoretical and Computational Acoustics

The Ivory-billed Woodpecker (Campephilus principalis) is an elusive bird that has repeatedly been feared extinct only to be rediscovered during the past hundred years. The most recent rediscovery, which took place in Arkansas, was announced in an article that was featured on the cover of Science in 2005. Despite published reports of sightings in Florida and Louisiana in the years that followed, the issue became controversial when nobody managed to obtain a clear photo and critics attacked relatively weak video evidence that was presented in the original article. The issue began to develop into a science scandal when the critics used specious arguments to delay the publication of the strongest evidence, which consists of three videos that were obtained during encounters with birds that were identified in the field as Ivory-billed Woodpeckers and that show field marks, body proportions, flights, and other behaviors that are consistent with that species but no other species of the region. The scandal culminated in a decision by the U.S. Fish & Wildlife Service to declare the species extinct in 2021, which was made without addressing the strongest evidence. An update is given here on recent developments. The decision to declare the species extinct was based on a five-year review of evidence for persistence. During an interview, it came to light that the person who performed the review was unaware of basic facts about the strongest evidence. Evidence that was obtained at another site in Louisiana [Latta et al., “Multiple lines of evidence suggest the persistence of the Ivory-billed Woodpecker (Campephilus principalis) in Louisiana,” Ecology and Evolution (2023)] is discussed and compared with the strongest evidence. Videos obtained with a drone on a sunny day often show white markings that do not correspond to actual field marks. It is demonstrated that a video that was purported to show an Ivory-billed Woodpecker on the basis of apparent white markings is apparently a Pileated Woodpecker (Dryocopus pileatus). The bird in that video appears to have black trailing edges on the dorsal surfaces of the wings, which are consistent with the Pileated Woodpecker, not the Ivory-billed Woodpecker. The wingbeat frequency and an upward swooping landing also seem to be consistent with the Pileated Woodpecker but are not consistent with historical accounts of the Ivory-billed Woodpecker or flights appearing in videos that contain the strongest evidence.

With the increasing importance of vehicle acoustic comfort, the topics of precise absorber simulation and characterization approaches are coming into forefront. The behavior of widely used porous mass-spring systems, consisting of foams coupled with heavy layers, can be strongly influenced by numerous factors. Among them are boundary conditions between trim materials and exciting structures. This paper focuses on investigating trim-structure interface phenomena with the help of modern measurement techniques. Using acquired experimental results, numerical studies have been conducted to account for observed phenomena. In particular, parameter optimization algorithms have been implemented for intrinsic material parameter characterization purposes. Moreover, sensitivity analysis investigations have been performed to substantiate the developed approaches.

The Helmholtz equation is a reliable model for acoustics in inviscid fluids. Real fluids, however, experience viscous and thermal dissipation that impact the sound propagation dynamics. The viscothermal losses primarily arise in the boundary region between the fluid and solid, the acoustic boundary layers. To preserve model accuracy for structures housing acoustic cavities of comparable size to the boundary layer thickness, meticulous consideration of these losses is essential. Recent research efforts aim to integrate viscothermal effects into acoustic boundary element methods (BEM). While the reduced discretization of BEM is advantageous over finite element methods, it results in fully populated system matrices whose conditioning deteriorates when extended with additional degrees of freedom to account for viscothermal dissipation. Solving such a linear system of equations becomes prohibitively expensive for large-scale applications, as only direct solvers can be used. This work proposes a revised formulation for the viscothermal BEM employing the Schur complement and a change of basis for the boundary coupling. We demonstrate that static condensation significantly improves the conditioning of the coupled problem. When paired with an iterative solution scheme, the approach lowers the algorithmic complexity and thus reduces the computational costs in terms of runtime and storage requirements. The results demonstrate the favorable performance of the new method, indicating its usability for applications of practical relevance in thermoviscous acoustics.

Interior acoustic problems require accurately representing the boundary conditions of all acoustically interacting surfaces to achieve precise acoustic predictions. The complex-valued boundary admittance fully characterizes these properties. Yet, conventional approaches to determine boundary admittances, such as the impedance tube, have limitations which do not accurately represent real-world conditions. This motivates in situ methods, where the acoustic boundary admittance is estimated in the actual mounting condition based on sound pressure measurements at certain observation points within the domain. In contrast to existing deterministic methods, a Bayesian approach is employed in this work, which provides probability distributions for the boundary admittances rather than point estimates. This offers valuable insights into the uncertainty associated with the estimation, proving beneficial for applications where a comprehensive understanding of uncertainty is desired. A finite element model is utilized to generate sound pressure data and serves as the forward model during the inference process. This makes it particularly suited for applications that involve pre-existing geometrical models, such as digital twin applications and model updating. The proposed method is applied to a two-dimensional car cabin model, demonstrating the framework’s efficacy in accurately inferring the acoustic boundary admittance using just ten observation points.

Common multi-frequency solution strategies for acoustic systems are either associated with high computational costs or resort to model reduction strategies, which potentially induce considerable approximation errors. In that context, this work proposes a fast multi-frequency solver for acoustic Boundary Element Method (BEM) analyses that combines conventional preconditioners with an accelerated recycling strategy, achieving in that way a fast and accurate iterative solution. Specifically, by approximating both the system and the related preconditioner by individual affine expressions, an upfront Galerkin projection on a global deflation basis is facilitated, therefore enabling the deployment of the accelerated recycling scheme. The latter acts by accelerating the construction of the related deflation projectors, therefore leading to a significantly faster recycling procedure. Finally, the efficiency of the global deflation basis is guaranteed through an Automatic Krylov subspaces Recycling for Deflation (AKR-D) [D. Panagiotopoulos, W. Desmet and E. Deckers, An accelerated subspaces recycling strategy for the deflation of parametric linear systems based on model order reduction, Comput. Methods Appl. Mech. Eng. 403 (2023) 115765] algorithm and thus, the cumulative number of iterations required by an iterative solver within a frequency sweep is drastically decreased. The proposed multi-frequency solver is tested on two industrially relevant examples of an interior and an exterior scattering problem, benchmarked against the traditional solution strategies.

While acoustic analysis with the finite element method is commonly done under the assumption that the source location is stationary, this is not always a valid assumption. This paper derives a method to simulate moving sound sources using an unfitted mesh approach, where the source boundary is described by cutting through elements, known as the cut finite element method (cutFEM). By using such an approach the mesh stays static during simulation, even for large source movements, thus avoiding expensive re-meshing or mesh morphing steps. To be able to describe a wide variety of acoustic sources, both sources with a large and thin thickness are considered. Several time integration schemes are assessed and a method to determine the stability of the resulting time-varying system of equations is derived. The paper concludes by demonstrating the derived methods with two examples, where it is shown that the unfitted mesh approach is a viable way to simulate moving sources.

In this study, we focus on the prediction of the pressure field scattered from an immersed cylindrical shell partially coated by a soft rubber, impacted by an acoustic plane wave. As the coating covers only a partial portion along the circumference of the shell, the considered system is not axisymmetric. As a result, a spectral (Fourier) resolution of the mathematical problem would induce the coupling of the different circumferential orders, which can lead to prohibitive computing times. To circumvent this issue, the reverse Condensed Transfer Function (rCTF) method has recently been developed to decouple vibroacoustic subsystems initially coupled along lines or surfaces. From an analytical model of the fully coated shell impacted by the acoustic plane wave and a finite element (FE) model of the missing coating material, the rCTF approach predicts the vibroacoustic behavior of the coated shell with a voided section instead of the removed part. This voided section can then be filled by a FE model of the water domain replacing the removed coating material, using the direct CTF approach. The principle of the rCTF approach, some numerical validations, and results for the scattering from the partially coated shell are presented in this paper.