Impedance, acoustic characteristic

Reflection of acoustic waves incident on a planar interface between two isotropic media is most easily considered in terms of impedances. Acoustic characteristic impedance is defined as minus the ratio of traction to particle displacement velocity,... 

The sonographic technique is based on reflection) it works by measuring and portraying differences of impedance. Differences in acoustic characteristics of structures and tissues are the basis of sonographic differentiation. 

Acoustic impedance of these materials is around 1,000-2,000x10 kg/(m s), more than ten times the value of ideal acoustic impedance. That is, the current performance of ultrasonic transducers with an acoustic matching layer is restricted by the limitations of the acoustic characteristics of its acoustic matching layer. Silica aerogel has an extremely low density as a solid. We focused on this fact and researched the potential regarding the acoustic matching layer of ultrasonic transducers, as we describe in more detail in the next section. 

A resonance in the layered stracture occurs when echoes between two boundaries travel back and forth due to differences in acoustic impedances at the boundaries. For multi-layer structures a number of resonances can be observed depending on their geometry and condition. For each particular defect-free structure and given transducer we obtain a characteristic resonance pattern, an ultrasonic signature, which can be used as a reference. 

CH2—CI2—) —(—CF2— CFH—) (39). Ceramic crystals have a higher piezoelectric efficiency. Their high acoustic impedance compared to body tissues necessitates impedance matching layers between the piezoelectric and the tissue. These layers are similar in function to the antireflective coatings on a lens. Polymer piezoelectric materials possess a more favorable impedance relative to body tissues but have poorer performance characteristics. Newer transducer materials are piezoelectric composites containing ceramic crystals embedded in a polymer matrix (see Composite materials, polymer-MATRIX Piezoelectrics). 

For the convenience of the reader, the values of the sonic characteristics of representative and frequently encountered materials are listed in Table 1. Energy which is not transferred is reflected. Maximum transfer takes place when the acoustic impedance of all materials are equal When the angle of incidence is normal to the. interface, the fraction of the reflected incident energy is found as follows ... 

Impedance and reflection. Another important characteristic of a material is its specific acoustic impedance (Z). The specific acoustic impedance is defined as the ratio of the acoustic excess pressure (P) and the particle velocity (U) ... 

The other important consideration concerns the transmission of ultrasound (and other forms of energy) from one medium to another and the importance of impedance matching . When wave energy is transferred from one medium to another then a part is transmitted and the rest reflected. The ratio of reflected to transmitted energies depends on the characteristic impedances of the two media and the transmission is total if these are matched. In the case of acoustic waves the specific impedance (Z) of a medium is given by the product of the density p and the velocity of sound v. that is... 

For plane waves propigating in an isotropic homogeneous medium, three acoustic properties are important the speed of sound, the attenuation coefficient (to be discussed), and the characteristic impedance of the media. This impedance z is defined as the ratio of the acoustic pressure to the particle velocity associated with the wave motion in the material. For simple free-field plane waves, tliis is simply the product of the sound speed and density p. 

It is of interest to note that the lengthy and complex calculation Debye made was published in the same (first) edition of the Journal of Chemical Physics as an article by Bernal and Fowler, who first suggested several seminal concepts about the structure of water that are now commonly accepted in solution theory. The velocity amplitude is measured in cm s". It is the ratio of the pressure of the ultrasonic wave to the characteristic acoustic impedance of the media. 

Like the ultrasonic velocity and attenuation coefficient, the acoustic impedance is a fundamental physical characteristic which depends on the composition and microstructure of the material concerned. Measurements of acoustic impedance can therefore be used to obtain valuable information about the properties of materials. 

Fig. 17. Wireline log characteristics, smoothed acoustic impedance curve and 3D seismic response over the Upper Angel Formation at Angel-2. Note that the major dolomite-cemented zones (black bars) are identifiable on the basis of neutron, density, resistivity and sonic log profiles. The zones appear as discrete layers at this location, with a cumulative thickness of 164 m, and are not fully cemented but contain some residual porosity. The dolomite-cemented zones occur both above and below the gas-water contact (GWC). The smoothed acoustic impedance curve shows that the zones produce a visible seismic response which is mappable. For an example of a line through the 3D seismic volume see Ryan-Grigor Schulz-Rojahn (1995 their Fig. 10a,b).

Characteristic) acoustic impedance of quartz Resistance ratio... 

The variable k is called the scattering coefficient, (from wave scattering theory) and the equations are called scattering equations. They express the behavior of the wave equation at the boimdary between acoustic tube segments of different characteristic impedances, where part of the incoming wave is... 

Figure 32 shows the schematic structure of the tested CFRP sample. The CFRP sample had a multilayered structure of CFRP cloth. The carbon fibers in one layer of cloth were aligned unidirectionally and the adjacent layers of cloth were set cross-directionally. The thickness of CFRP cloth was 70 pm, and the examined CFRP sample was composed of 72 layers of doth (about 5 mm thickness in all). In order to model the separation of the CFRP layers (defect), a Teflon sheet (80 pm in thickness) was sandwiched between the 54th and the 55th layers of CFRP cloth, at a level 3.8 mm below the CFRP top surface. The sound velocity and the acoustic impedance of the CFRP sample are about 3 X 10 m/s and 5 x 10 Pa s/m, respectively. Water was used as the coupling medium because of its low ultrasonic attenuation characteristics, and the CFRP... 

When the acoustic waves move into the boundary of the propagation media, acoustic impedance determines basic characteristics such as reflection or transmission. Referring to Figure 33.1, if Zp is the acoustic impedance of the piezoelectric element, is the acoustic impedance of air, Z is the acoustic impedance of the acoustic matching layer, and d is the thickness of the acoustic matching layer, acoustic impedance matching conditions can be represented by (33.2), with thickness being represented by (33.3) [3]. 

Simultaneous in-phase and out-of-phase reflections within a particular locale greatly increase the possibility of phase cancellation, which is accomplished if both high- and low-impedance particle types are mixed together in a matrix material, or if two or more particle types with merely different acoustic impedance characteristics are combined. When two or more particle types are mixed into a matrix material base in the correct proportions, a synergistic effect... 

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