Journal of Computational Acoustics

This work studies the acoustic imaging problem with compressed sensing (CS) and microphone arrays. The CS algorithm with Basis Pursuit (BP) algorithm has shown satisfying results in acoustic imaging, the maps of which are characterized by super-resolution. However, the performance of the CS algorithm with the BP algorithm is limited to Restricted Isometry Property (RIP), and the algorithm has a long CPU-time. We propose a new CS algorithm with Orthogonal Matching Pursuit (OMP) algorithm for acoustic imaging. The performance of the OMP algorithm with regard to RIP is examined through numerical simulation in this work. The simulation results and CPU-time for OMP algorithm are compared with those of the BP algorithm and the conventional beamformer (CBF). When the RIP does not hold, satisfying results can still be obtained by the OMP algorithm, and the CPU-time for OMP algorithm is far less than BP algorithm. In order to validate the feasibility of the OMP algorithm in acoustic imaging, an experiment is also conducted in a semi-anechoic room. Two mobile phones are served as sound sources. We investigate the mobile phones sources and compare the experimental results with those of BP algorithm and CBF method. The OMP algorithm can locate the main sources at low frequencies, while the CBF method can just give a rough indication and fails for low frequencies due to the width of its main lobe. Due to many reconstructed sources outside of the expected source positions existing on the map, the BP algorithm fails to locate the main sources at low frequencies.

A numerical model for a clamp-on transit-time ultrasonic flowmeter (TTUF) under multi-phase flow conditions is presented. The method solves equations of linear elasticity for isotropic heterogeneous materials with background flow where acoustic media are modeled by setting shear modulus to zero. Spatial derivatives are calculated by a Fourier collocation method allowing the use of the fast Fourier transform (FFT) and time derivatives are approximated by a finite difference (FD) scheme. This approach is sometimes referred to as a pseudospectral time-domain method. Perfectly matched layers (PML) are used to avoid wave-wrapping and staggered grids are implemented to improve stability and efficiency. The method is verified against exact analytical solutions and the effect of the time-staggering and associated lowest number of points per minimum wavelengths value is discussed. The method is then employed to model a complete TTUF measurement setup to simulate the effect of a flow profile on the flowmeter accuracy and a study of an impact of inclusions in flowing media on received signals is carried out.

In this work, a diffusion equation model (DEM) is applied to a room acoustics case for in-depth sound field analysis. Background of the theory, the governing and boundary equations specifically applicable to this study are presented. A three-dimensional geometric model of a monumental worship space is composed. The DEM is solved over this model in a finite element framework to obtain sound energy densities. The sound field within the monument is numerically assessed; spatial sound energy distributions and flow vector analysis are conducted through the time-dependent DEM solutions.

Choosing the right number and type of elements in modern commercial finite element tools is a challenging task. It requires a broad knowledge about the theory behind or much experience by the user. Benchmark tests are a common method to prove the element performance against analytical solutions. However, these tests often analyze the performance only for single elements. When investigating the complete mesh of an arbitrary structure, the comparison of the element’s performance is quite challenging due to the lack of closed or fully converged solutions. The purpose of this paper is to show a high-precision comparison of eigenfrequencies of a real structure between experimental and numerical results in the context of an element performance check with respect to a converged solution. Additionally, the authors identify the practically relevant accuracy of simulation and experiment. Finally, the influence of accuracy with respect to the number of elements per standing structural bending wave is shown.

This paper presents a review of basic concepts of the boundary element method (BEM) for solving 3D half-space problems in a homogeneous medium and in frequency domain. The usual BEM for exterior problems can be extended easily for half-space problems only if the infinite plane is either rigid or soft, since the necessary tailored Green’s function is available. The difficulties arise when the infinite plane has finite impedance. Numerous expressions for the Green’s function have been found which need to be computed numerically. The practical implementation of some of these formulas shows that their application depends on the type of impedance of the plane. In this work, several formulas in frequency domain are discussed. Some of them have been implemented in a BEM formulation and results of their application in specific numerical examples are summarized. As a complement, two formulas of the Green’s function in time domain are presented. These formulas have been computed numerically and after the application of the Fourier Transformation compared with the frequency domain formulas and with a FEM calculation.

In recent years, boundary element method (BEM) and finite element method (FEM) implementations of acoustics in fluids with viscous and thermal losses have been developed. They are based on the linearized Navier–Stokes equations with no flow. In this paper, such models with acoustic losses are applied to an acoustic metamaterial. Metamaterials are structures formed by smaller, usually periodic, units showing remarkable physical properties when observed as a whole. Acoustic losses are relevant in metamaterials in the millimeter scale. In addition, their geometry is intricate and challenging for numerical implementation. The results are compared with existing measurements.

Acoustic radiation modes (ARMs) and normal modes (NMs) are calculated at the surface of a fluid-filled domain around a solid structure and inside the domain, respectively. In order to compute the exterior acoustic problem and modes, both the finite element method (FEM) and the infinite element method (IFEM) are applied. More accurate results can be obtained by using finer meshes in the FEM or higher-order radial interpolation polynomials in the IFEM, which causes additional degrees of freedom (DOF). As such, more computational cost is required. For this reason, knowledge about convergence behavior of the modes for different mesh cases is desirable, and is the aim of this paper. It is shown that the acoustic impedance matrix for the calculation of the radiation modes can be also constructed from the system matrices of finite and infinite elements instead of boundary element matrices, as is usually done. Grouping behavior of the eigenvalues of the radiation modes can be observed. Finally, both kinds of modes in exterior acoustics are compared in the example of the cross-section of a recorder in air. When the number of DOF is increased by using higher-order radial interpolation polynomials, different eigenvalue convergences can be observed for interpolation polynomials of even and odd order.

The wide concern on absolute calibration of microphones at high pressure levels prompts the development of the pistonphone technique. However, as the sound pressure level goes higher, the linear hypothesis, which is applicable for the small amplitude sound wave, will no longer be valid. The nonlinear characteristics of the finite amplitude sound wave will produce high order harmonic components and also some other complex frequency components, which eventually result in the distortion of the sound pressure in the pistonphone, and should be quantitatively calculated to assess the accuracy of absolute sound pressure calibration of microphones at high sound pressure levels using pistonphones. In this paper, the linearized wave equations were built based on the perturbation method and the Euler system. Then, the distributed parameter expressions for the finite amplitude sound pressure both neglecting and considering the intermodulation characteristic have been explicitly derived. Nonlinear distortion characteristics of the sound pressure in the pistonphone have been studied, and the effects of the intermodulation on the sound pressure distortion have been evaluated. Computations reveal that the nonlinear distortion characteristics of the sound pressure produced by pistonphones should be quantitatively considered when the sound pressure level reaches 148dB, and the intermodulation characteristic can be neglected when the sound pressure level is lower than 174dB.