Acoustics Wave Propagation

Autor: Eric Brandão. Revisão Técnica: William D'Andrea Fonseca. "Este livro aborda os princípios para a modelagem e caracterização da propagação do som em ambientes, bem como os fundamentos para o desenvolvimento de projetos de recintos com uma qualidade acústica adequada. A obra apresenta conceitos básicos sobre o som e processamento de sinais, propagação sonora em ambientes (em baixas, médias e altas frequências), cálculo e medição de parâmetros objetivos, como tempo de reverberação, claridade, espacialidade e inteligibilidade da fala etc. Além disso, aborda a modelagem, a caracterização e o projeto de dispositivos utilizados no tratamento acústico de ambientes (absorvedores e difusores). O capítulo final trata de diretrizes para o projeto de vários tipos de ambientes, como estúdios, salas de concerto, teatros, salas de aula, cinemas etc. A obra é destinada a professores, alunos e profissionais que atuam no projeto acústico de ambientes, como os engenheiros acústicos, engenheiros civis, arquitetos, físicos e técnicos de áudio; também serve de base para pessoas que almejam um bom entendimento sobre o tema da acústica de salas. O livro conta com sólido desenvolvimento matemático e diversos exemplos e discussões, contribuindo para solidificar os conceitos apresentados." Preview/amostra em: https://www.blucher.com.br/livro/detalhes/acustica-de-salas-1163

Pipeline blockage can result in the reduction of system carrying capacity, energy and resource wastage, and potential increase in the probability of occurrence of environmental and health problems. The primary purpose of this paper is to demonstrate a methodology to simulate acoustic wave propagation (AWP) in partially-blocked fluid-filled pipes with flow under different flow velocities. This was achieved by using computational fluid dynamics (CFD) software in conjunction with an external user-defined function (UDF) file which introduces compressibility effect into the CFD solution. The use of finite element analysis (FEA) to simulate AWP in fluid-filled pipelines with blockage has been previously reported without flow. In this paper, both CFD and FEA techniques were used to simulate the AWP in air-filled pipelines without mean flow. However, it is shown that only the CFD technique could be used to simulate the AWP in air-filled pipelines with mean flow. The secondary purpose of the paper is to use the simulated acoustic waveforms to illustrate a blockage identification procedure for detection, localisation and sizing of blockages in fluid-filled pipes using Time of Flight (ToF) methodology. It is shown that the ToF accurately detects and locates blockages in blocked pipelines. The errors in the localisation of blockages were less than 4%. Furthermore, the effects of blockage size and the mean flow velocity on the AWP characteristics are established. It is shown that both the size of the blockage in a pipe and the mean flow velocity affect the pressure amplitude of the transmitted and reflected waveforms.

Leakage of oil and gas pipe systems, water pipes and other pipe networks can cause serious environmental, health and economic problems. In order to minimise the damages brought to the environment, human health and the economic issues, rapid non-destructive detection of pipeline leakage is imperative. In recent works, number of non-destructive testing (NDT) methods was used in detecting this defect in pipeline systems such ultrasonic, magnetic particle inspection, pressure transient and acoustic wave methods. In this study, the acoustic wave method and a modal frequency technique are used to detect leakage in pipeline system. Finite element analysis (FEA) was employed to simulate acoustic wave propagation in fluid-filled pipes with leakage. Furthermore, experimental testing was conducted to validate some of the numerical results. The experiment performed consisted of the measurement of acoustic wave propagation in a straight fluid-filled pipe. The FEA analysis of fluidfilled pipe can be used to simulate the acoustic wave propagation and acoustic wave reflectometry of a fluid-filled pipe with leakage of different using the ACAX element in order for accurate predictions. Also, the measured signal of acoustic wave propagation in pipeline from the experiment can be decomposed and de-noised to identify and locate leakages of different sizes.

The problem we study cane be defined as follows: In three-dimensional space, we consider two homogeneous media - "air" and "water" - separated by a plane interface. There is a point source of sound in "air" at height h above the interface. Its emission is given by a function F(t) of time. We are interested in the behavior of pressure in water, at time t, depth z, and distance r from a vertical line going through the source.
This problem has been discussed innumerable times, both for monochromatic and transient sources [1-6]. The feature of interest to us is the existence, first recognized in the monochromatic regime, of contributions to the total solution called lateral waves. They tend to be concentrated near the interface, and have properties of propagation and attenuation different from waves in homogeneous media. They decrease exponentially with depth, and their attenuation length is frequency-dependent. Consequently, their penetration depth decreases with frequency. For a recent discussion [7], it should be mentioned that this attenuation does not correspond to absorption of energy, a lateral wave is merely a contribution to an elastic propagation.
The classical methods of resolution, while well adapted to the case of monochromatic sources, are less suited to the description of transients. This is due to the fact that waves of different frequencies both follow different paths and undergo different attenuations. In such time-and-frequency dependent situations, it is natural to apply wavelet transform techniques.
We shall first, briefly describe the time behavior of the WT  of the acoustic field at a fixed point under water.  (This is not the same as attempting to use wavelet techniques to solve partial differential equations of the propagation problem). It is straightforward to write down an expression for the propagator for our problem. This expression is much smoother than the propagator itself. and allows selective reconstitutions with arbitrary precision. A natural decomposition into three contributions appears corresponding to branch points of the integrand. In a second part,  we use the different contributions to obtain a formula for reconstruction of the time-dependent source.