Ultrasonic acoustic impedance

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]. 

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. 

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

The impedance is practically important because it determines the proportion of an ultrasonic wave which is reflected from a boundary between materials. When a plane ultrasonic wave is incident on a plane interface between two materials of different acoustic impedance it is partly reflected and partly transmitted (Figure 3). The ratios of the amplitudes of the transmitted (At) and reflected (Ar) waves to that of the incident wave (Aj) are called the transmission (T) and reflection coefficients (R), respectively. 

The greater the difference in acoustic impedance between the two materials the greater the fraction of ultrasound reflected. This has important consequences for the design and interpretation of ultrasonic experiments. For example, to optimize the transmission of ultrasound from one material to another it is necessary to chose two materials with similar acoustic impedance. To optimize the reflection coefficient materials with very different acoustic impedance should be used. The acoustic impedance of a material is often determined by measuring the fraction of ultrasound reflected from its surface. 

Solids usually have larger ultrasonic velocities and acoustic impedance, than liquids, which have larger values than gasses. Air has a very low acoustic impedance compared to liquids or solids which means that it is difficult to transmit ultrasound from air into a condensed material. This can be a problem when ultrasound is used to test dry materials, e.g., biscuits or egg shells. A small gap of air between an ultrasonic transducer and the sample to be tested can prevent ultrasound from being transmitted into the material. For this reason coupling materials (often aqueous or oil based) can be placed between the transducer and sample to eliminate the effects of the air gap, or alternatively soft-tip ultrasonic transducers can be used. 

Measurement cell. The measurement cell should be made of a material which does not react with the sample. The cell walls should be of an appropriate thickness and acoustic impedance so that any reverberations in the cell walls do not interfere with the signal from the sample. The internal walls of the cell should be smooth and parallel so that scattering or oblique reflection of the ultrasonic wave do not cause errors in the velocity and attenuation measurements. Ultrasonic measurements are particularly sensitive to temperature and so it is important to either use a thermostated measurement cell, or to measure the temperature and make a suitable correction. 

For efficient transfer of power from the generator to the medium, usually water, the two must be acoustically matched. The discontinuity can be smoothed by fixing a 2/4 thick layer of material having an acoustic impedance intermediate between that of the radiating surface material and water, and polymers having impedances of about 3.5 Mrayl are readily available. The velocity of sound in them is approximately 2500 ms-1 so that the thickness required at 50 kHz is about 12 mm. In practice the transducer is often bonded to an ultrasonic cleaning tank and then the tank and water become a complicating part of the transducer. 

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. 

At a boundary between two media having different acoustic impedance a radiation force F, or radiation pressure, applies when an acoustic wave reaches that interface. This should not to be confused with sound pressure. The magnitude of the force generated by radiation pressure depends on the intensity of the sonic or ultrasonic field, and on the shape of the field. When it impinges on a body (e.g. a target) immersed in a liquid it will also depend on the size and the shape of the material from which that body is formed. When the ultrasonic wave is completely absorbed the surface is submitted to a steady force [21],... 

ECHO is based on the principle of differential acoustic impedance (or tissue density) and the laws of reflection and refraction. Sound waves directed across tissues from a transducer will reflect back sound waves of different frequencies. The ability of the ultrasonic beam to penetrate chest wall structures is inversely proportional to the frequency of the signal. With transthoracic ECHO, frequencies of 2.0 to 5.0 MHz are used commonly in adults, and frequencies of 3.5 to 10.0 MHz are used in children. Serial determinations in a given patient using the same conditions and ECHO images (windows) provide the best form of internal control to allow comparisons of test results. In clinical trials, echocardiograms are read and interpreted independently by two or three clinicians to provide a means of control. 

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... 

Next, at measuring pomt B where the edge of the Teflon sheet is located, the time-domain waveform shows only reflection from the Teflon sheet. The disappearance of the reflection from the bottom surface of the CFRP sample is assumed to be due to the existence of a low-density region in the neighborhood of the Teflon sheet edge, which reflects almost all the ultrasonic waves because of its lower acoustic impedance. 

Ultrasonic methods There exist a variety of methods which are used widely in the aircraft industry. The technique measures changes in acoustic impedance caused by defects in a bonded assembly when an ultrasonic transducer is liquid-coupled to it. The high frequency sound waves are simply scattered by the presence of porosity or voids in the bondline. Interpretation of the data can be difficult, although substrate thickness is not generally a limitation. 

Hard, glassy, brittle thermoplastics such as polystyrene (PS) and polymethylmethacrylate (PMMA) have low attenuations, of order 6-10 dB/cm at 10 MHz, and in the case of PS, a low acoustic impedance. Ductile polymers such as polycarbonate (PC), many polyolefins and impact-modified thermoplastics generally have high absorption coefficients, in the range 20-40 dB/cm. The same molecular structures and mobiUty, which contribute to ductihty, may also contribute to absorption of ultrasonic energy. Not surprisingly, rubbers and, by extension, any polymer above its... 

Gerlach R, Kraus O, Fricke J, Eccardt PC, Kroemer N, Magori V (1992) Modified siUca aerogels as acoustic impedance matching layers in ultrasonic devices. J Non-Cryst.SoUds 145 227-232... 

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