China
Africa
Explore chapters and articles related to this topic
Fundamentals and Basic Terminology
Published in David A. Bies, Colin H. Hansen, Carl Q. Howard, Engineering Noise Control, 2018
David A. Bies, Colin H. Hansen, Carl Q. Howard
The specific acoustic impedance is the ratio of acoustic pressure to associated particle velocity. It is important in describing the propagation of sound in free space and is continuous at junctions between media. In a medium of infinite extent, it is equal to the characteristic impedance, ρc, of the medium. The specific acoustic impedance is also important in describing the reflection and transmission of sound at an absorptive lining in a duct or on the wall or ceiling of a room and in describing reflection of sound in the ground plane. It will find use in Chapters 5 and 8. In Chapter 5, the characteristic impedance of the ground is denoted Zm and is defined as the ratio of acoustic pressure, p, to particle velocity, u, in the ground.
Diagnostic Ultrasound
Published in Michael Ljungberg, Handbook of Nuclear Medicine and Molecular Imaging for Physicists, 2022
The sound speed is thus a characteristic of the material in question, defined by the material’s elasticity and density. Now, what happens at a reflection is that the wave propagating in a medium encounters a different medium with other acoustic properties. What determines the magnitude of the reflection is the difference in acoustic impedance between the two media. The acoustic impedance can be understood as the resistance for the volume elements in the model to move from their equilibrium position. It can be shown that acoustic impedance equals the sound speed times the density of a medium. Hence, this is also a material-specific parameter that also depends on density and elasticity, but in a different way than sound speed does.
Ultrasound Imaging
Published in Kayvan Najarian, Robert Splinter, Biomedical Signal and Image Processing, 2016
Kayvan Najarian, Robert Splinter
Reflection results from a combination of changes in acoustic impedance, usually in the order of the size of the wavelength. Often the reflection on microscopic scale will fall under the principle of scattering in ultrasound imaging. The ratio of reflected to refracted sound waves is dependent on the acoustic properties of both media at either side of the interface. The acoustic impedance of the tissues often characterizes these properties. In order to quantitatively analyze this phenomenon, assume that an incident acoustic wave front hits the interface of the two media with an angle θi (with respect to the normal to the interface). In addition, assume the two media to have different acoustic impedances ZA1 and ZA2. Further, show the incident pressure as Pi, the reflected pressure as Pr, and the transmitted pressure as Pt. Then, the pressure reflection coefficient, r, and the pressure transmission coefficient, t, are defined as follows: r=PrPi=ZA2cosθ1−ZA1cosθ2ZA2cosθ1+ZA1cosθ2
Enhanced reactivity by energy trapping in shocked materials: reactive metamaterials for controllable output
Published in Combustion Theory and Modelling, 2022
Donald Scott Stewart, Kibaek Lee, Alberto M. Hernández
Acoustic impedance contrasts between component materials controls the reflection and transmission properties of incident pressure waves. But there are significant differences. Energetic materials with pre-defined (and similar) engineering crystalline or laminate structures, typically are subjected to strong shock impacts with loading pressures in the range of 10's to 100's of Kilobars. Whereas phononic metamaterials experience small deformations and are modelled by linear elasticity theory. The lead shock wave preconditioning in a shock-initiated metamaterial controls the acoustic radiative interactions between the composite constituents. The post-shock region of shock-initiated metamaterial will have particles (voids) that actively deform, in the prescence material streaming and mixing. Reactive metamaterials undergo a chemical reaction that occur in the volume of the matrix material, volume of the particles, at interfaces between the matrix and the particles, and any combination.
Partial Discharge Detection and Localization in Power Transformers based on Acoustic Emission: Theory, Methods, and Recent Trends
Published in IETE Technical Review, 2021
Viral B. Rathod, Ganesh B. Kumbhar, Bhavesh R. Bhalja
The acoustic impedance (Z) of any medium is the product of density (ρ in kg/m3) of the material and the propagation velocity (c in m/s) of the acoustic wave (Z = ρc). It is also called as a characteristic impedance of a medium. When an AE wave propagates through the medium of different acoustic impedance, the reflection and refraction of acoustic waves take place. Due to this reduction in energy of transmitted wave will take place. If there is a large difference in acoustic impedance of two medium through which the AE wave propagates, only some part of the incident AE wave will transmit from one medium to another medium. The relation between the incident AE wave, the reflected AE wave, and the transmitted AE wave is shown in Figure 7 for a medium of two different acoustic impedance [5,8–10].
Developing a Speckle Pattern to Evaluate Adhesive Joints Using Digital Image Correlation
Published in Research in Nondestructive Evaluation, 2020
Seyed Fouad Karimian, Tsuchin Philip Chu
Embedding a speckle pattern into the adhesive bond is the significant part of this research. Similar to optical DIC, where a speckle pattern is applied to a material’s surface to monitor whole-field strain, the speckle pattern in the bondline allows DIC technique to analyze whole-field strain of the adhesive itself. DIC technique analyzes the adhesive by comparing speckle images during deformation. Optical DIC is only able to view surface strain rather than monitoring strain within the adhesive. Using ultrasonic imaging allows us to view the strain in the adhesive by following an embedded speckle pattern [21–23]. To create speckle images of the adhesive for the DIC technique to compare, a material with a higher acoustic impedance than the adhesive must be applied into the adhesive for ultrasonic imaging. The difference in acoustic impedance of material is due to the material density ρ and acoustic velocity υ [24]. The equation for acoustic impedance (Z) is: