Ultrasound applied frequency

The credibility of the hot spot theory is reinforced by its ability to account for the effects of extrinsic variables on the sonochemical process. Nevertheless, the frequency of ultrasound applied is surprisingly irrelevant to the course of the reaction. Cleaning baths produce a range of frequencies which often vary from day to day, or even during the course of a reaction, and yet this has no discemable effect on the sonochemistry observed. 

The parameters that best characterize an ultrasonic field are the wave frequency and the ultrasonic energy applied, with the latter being expressed as power, intensity, or acoustic pressure. The frequency does not represent a critical point because the variation of the applied frequency is usually less than 5-10% of the nominal frequency provided by the manufacturer. In contrast, the actual applied acoustic energy could be very different from the electrical consumption, and its direct measurement is difficult. In the literature, it is possible to identify a wide variety of methods for that purpose, but the results provided may be difficult to compare one with another. The majority of these methods are based on the measurement of physical or chemical changes produced by ultrasound, and a classification was proposed by Berlan and Mason (1996) ... 

Ultrasonic methods can also be applied to velocity measurements based on measurement of the Doppler shift in the frequency of an ultrasonic wave scattered from a moving particle. The angle between the velocity vector and the direction of ultrasound propagation must be known, which practically limits the appHcation of the technique to the measurement of unidirectional flows. However, this Hmitation may be overcome again by the use of an array of transducers [11]. 

Ultrasound can be used to supplement or replace the microwave radiation. Ultrasound effects a high frequency mechanical vibration that warms the interior of the exposed object. In this case, the sample can exposed, and heated, as a whole. However, because ultrasonic waves can be so readily focused, it is also possible to apply them in bundled form so that they act on certain selected regions of the blank, for instance by sweeping along a raster. 

Acoustic cavitation (AC), formation of pulsating cavities in a fluid, occurs when a powerful ultrasound is applied to a non-viscous fluid. The cavities are formed when the variable acoustic pressure in the rarefaction phase exceeds the cohesive strength of the fluid. Under acoustic treatment (AT), cavities grow to resonance dimensions conditioned by frequency, amplitude of oscillations, stiffness properties and external conditions, and start to pulsate synchronously (self-consistently) with acoustic pressure in the medium. The cavities undergo significant strains (compared to their dimensions) and their size decreases under compression up to collapsing. This nonlinear behavior determines the active, destructional character of the cavities near which significant shear velocities, local pressure and temperature bursts occur in the fluid. Cavitation determines the specific character of acoustic treatment of the fluid and effects upon objects resident in the fluid, as well as all consequences of these effects. 

Figure 7.6 Clinical prototype for treatment with low-frequency ultrasound. 12 W of 55 kHz ultrasound is applied to a skin area of 0.8 cm 2 until the impedance is below 10 KX2. The hand grip serves as the return electrode for the impedance measurement. The coupling media and ultrasonic horn are within the handpiece housing. Reprinted with permission from Ref. 9. Copyright 2004 Mary Ann Liebert, Inc. publishers.

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