Acoustical impedances

The basics of the method are simple. Reflections occur at all layers in the subsurface where an appreciable change in acoustic impedance is seen by the propagating wave. This acoustic impedance is the product of the sonic velocity and density of the formation. There are actually different wave types that propagate in solid rock, but the first arrival (i.e. fastest ray path) is normally the compressional or P wave. The two attributes that are measured are... 

Of course the typical seismic trace has many hundreds of reflections in it, all the way down from the surface to the deepest times measured. These days, engineers and geologists prefer to see the seismic in terms of the acoustic impedance rather than reflection data and this can be obtained by inversion from the seismic volume. Aseismic volume is made up of hundreds of thousands of traces. 

D is basically a succession of 2D or 3D surveys repeated at intervals of time during which it is expected that some production effect has occurred, of sufficient magnitude to effect the acoustic impedance contrast seen by the propagating waves. For example, this oould be changes in the water or gas saturation, or changes in pressure. 

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. 

There have been numerous efforts to inspect specimens by ultrasonic reflectivity (or pulse-echo) measurements. In these inspections ultrasonic reflectivity is often used to observe changes in the acoustical impedance, and from this observation to localize defects in the specimen. However, the term defect is related to any discontinuity within the specimen and, consequently, more information is needed than only ultrasonic reflectivity to define the discontinuity as a defect. This information may be provided by three-dimensional ultrasonic reflection tomography and a priori knowledge about the specimen (e.g., the specimen fabrication process, its design, the intended purpose and the material). A more comprehensive review of defect characterization and related nondestructive evaluation (NDE) methods is provided elsewhere [1]. 

Piezocomposite ceramic has a higher efficiency than pure ceramic so that a higher signal amplitude is obtained. Moreover, the required acoustic impedance can be adjusted aceording to the ciency being reduced. 

Within this range we can modify the volume fraction without losses in the coupling factor and thereby can adjust the acoustic impedance to our demands. 

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

K) is the Fourier transform of the logarithmic fluctuation of acoustics impedance. 

Here, Zi, Z2 and Zz are acoustic impedance of each materials as shown in Table 3. The order of Zi, Z2 and Z3 becomes Zi Z3 for the material on ultrasonic wave s incidence side. 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

Flere, Zi and Zi are the acoustic impedances of the material on a ultrasonic wave incidence side and confrontaion with the incidence side. 

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). 

Acoustic impedance The shock impedance in the limit of an infinitesimal disturbance. Independent of pressure. 

Figure 8.2. Target configuration for dynamic tensile fracture experiments on rock. PMMA buffers and windows were not used on those rocks with an acoustic impedance comparable to PMMA. Velocity for these tests was measured at the rock-free surface.

The acoustic impedance Z is defined as the ratio of a variation of acoustic pressure to the induced velocity of matter ... 

The only catch in the use of equation 6.4 is that the acoustic impedance must be known. There are a number of cases where a compromise has to be made with accuracy due to incomplete or restricted knowledge of the material constants of the coating material ... 

Improving the accuracy of a quartz crystal microbalance with automatic determination of acoustic impedance ratio. 

An interpretation of the fact that, for some explosives at least, the detonation velocity does not continue to rise with rise in density, but goes thru a maximum and detonation finally fails when the density exceeds a critical value is reptd by Dunkle (Ref 5) and Price Refs 9 10). Roth (Ref 4), on the basis of results reported in Refs 1, 2 St 3, suggests the existence of a property he calls Widerstand ("resistance or "impedance ) of value equal to the product of loading density and detonation velocity, analogous to acoustic impedance and shock impedance (See abstract of Roth s paper at the end of this item)... 

Detonation, Shock Impedance and Acoustic Impedance in. Acoustic impedance is the ratio between sound pressure and particle velocity. A sound wave, on reaching a boundary between two media, has part of its energy reflected at the boundary and part transmitted into the 2nd medium. The relationships depend on the values of the acoustic impedance in the two media. Swenson (Ref 2) showed that ... 

In view of the formidable technical difficulties, the results that have been achieved are all the more impressive. Figure 3.3(a) (see colour plate section) is an acoustic image of a bipolar transistor on a silicon integrated circuit taken at 4.2 GHz. There is a very severe acoustic impedance mismatch between helium and a material such as silicon. Even for normally incident waves 99 per cent of the power is reflected straight off the surface, and for waves incident at an angle greater than 3° all the power is reflected. For a lens of N.A. = 0.5,... 

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