Ultrasound attenuation, absorption

Interaction between ultrasound and tissue can be characterized by a number of parameters like speed of propagation, attenuation, absorption coefficients etc. As ultrasound velocity is characteristic for a particular tissue the possibility is opened for ultrasound velocity tomography. The principles hereof and implications for medicine have been described by Greenleaf et al. (1975). In ultrasound velocity tomography transmitters and receivers are positioned around the object under study. Times of flight of ultrasonic pulses from transmitter to receiver are measured accurately. By rotating the transmitters and receivers in a plane around... 

Homogeneous liquids do not scatter ultrasound because they do not contain any discontinuities. Attenuation in these systems is solely due to absorption caused by thermodynamic relaxation processes. In a pure homogeneous liquid, which is not highly attenuating ( .e. a adiabatic compressibility and the density by the equation... 

It was observed that ultrasound stimulation (ultrasonication) also accelerated the crystallization of the more stable polymorphs of CB (111). A fundamental study of the effects of ultrasonication on the polymorphic crystallization of PPP and LLL showed that several factors, such as pressure effect, shear flow, cavitation, and thermal energy caused by absorption of attenuated ultrasound wave, may play concurrent effects of ultrasonication. As a result, there are optimal conditions for temperature and duration of ultrasonication to increase the rate of crystallization and the occurrence of the more stable polymorphs (20). This effect was also observed in CB (111). 

C. Ultrasound (200 kHz, 300 W) was stimulated to a 250-mL sample of CB at 32.3°C during cooling before crystallization. Form II occurred without ultrasonication, whereas Form V was observed when ultrasonication was done for 3 seconds. Further ultrasonication for 9 seconds formed a mixture of Form II and Form V, and only Form II was observed by the ultrasonication for 15 seconds. It is assumed that there are conflicting effects by ultrasonication promotion of nucleation by pressure effect and retardation of nucleation by thermal energy caused by absorption of attenuated ultrasound wave. The former effect may prevail at the ultrasonication for 3 seconds. The temperature rise, however, of the sample caused by absorption of attenuated ultrasound wave was 2°C for 9 seconds and 3.9°C for 15 seconds, and the latter effect may result in the case of cooling from above the melting point of CB. 

It has been proposed that the absorption of ultrasound energy depends on the thermo-elastic relaxation of ice crystals in food and is affected by ice crystal orientation and size, impurities present in the ice crystals, as well as temperature (Kissam et al., 1981). Miles and Shore (1978) and Shore et al. (1986) found that ultrasound was more attenuated in frozen meats than in unfrozen tissues and that the attenuation increased markedly with temperature, reaching the maximum near the initial freezing points of the foods before decreasing rapidly at higher temperatures, thus making ultrasound particularly suitable for the controlled thawing of foods (Miles et al., 1999 Torley and Bhandari, 2007). 

Active ultrasound uses a source of sound radiation, which is appHed to a process sample, with a detector placed such that modification to the signal can be detected and related to changes in the sample. Signal attenuation, velocity measurements and wavelength selective absorption provide the means of probing the sample. This approach promises to provide both chemical and physical information but as yet has not been used extensively. A number of on-line polymer-related studies have been reported in which polymer flow behaviour, viscosity, blend characterisation, and foaming-process monitoring have been examined [19]. 

The observation that a system in chemical equilibrium can absorb ultrasounds dates back to the 1930s. During this period, Bazulin attributed the absorption of ultrasounds by acetic acid to the existence of hydrogen bonds. When an acoustic wave is made to cross a medium, alternate regions of compression and depletion of the particles that comprise it are produced. Eventually, this acoustic wave is attenuated within the space covered, in the same way that an electromagnetic wave is attenuated, because no medium is completely transparent. If species in chemical equilibrium that absorb ultrasound at the frequency used are added to this medium, and if the relaxation for the equilibrium situation is slower than that in the ultrasonic frequency, the regions of compression and depletion will not occur with the same phase or amplitude as the pure solvent. By comparison of the two situations it is possible to obtain the velocity of relaxation of the system in equilibrium. The... 

In the previous section we demonstrated the use of ultrasonic velocity measurements to characterise creaming, and indirectly to characterise flocculation. However, there is more information to be obtained from an emulsion using ultrasonic spectroscopy. This involves measurement of phase velocity and attenuation of ultrasound as a function of frequency after propagation through the emulsion. There are a number of mechanisms by which ultrasound is attenuated by the emulsion, resulting in characteristic ultrasonic properties. Figure 4.15 shows the prineipal mechanisms of absorption. 

---