Ultrasound wave motion

Ultrasonic testing (UT) uses mechanical waves to test objects for their quality. The ultrasound wave is generated by a mechanical vibration from a transducer that converts an electrical signal into mechanical motion and vice versa. By inputting a short pulse of electrical energy or a tone burst to the transducer, a pressure pulse will be... 

Ultrasound passes through an elastic medium as a longitudinal wave, which is a series of alternating compressions and rarefactions. This means that hquid is displaced parallel to the direction of motion of the wave. Ultrasound comprises sound waves typically in the range of 20 kHz to approximately 500 MHz. The frequency (/) and the acoustic amplitude (PA,max) are the most important properties to characterize the pressure wave. The variation of the acoustic pressure (Pa) of an ultrasound wave as a function of time (t) at a fixed frequency is described by Eq. (2)... 

Cavitation is the formation of gaseous cavities in a medium upon ultrasound exposure. The primary cause of cavitation is ultrasound-induced pressure variation in the medium. Cavitation involves either the rapid growth and collapse of a bubble (inertial cavitation) or the slow oscillatory motion of a bubble in an ultrasound field (stable cavitation). Collapse of cavitation bubbles releases a shock wave that can cause structural alteration in the surrounding tissue [13]. Tissues contain air pockets trapped in the fibrous structures that act as nuclei for cavitation upon ultrasound exposure. The cavitational effects vary inversely with ultrasound frequency and directly with ultrasound intensity. Cavitation might be important when low-frequency ultrasound is used, when gassy fluids are exposed, or when small gas-filled spaces are exposed. 

The term sonochemistry indicates the use of sound waves to generate chemical and physical effects which can be harnessed in multiple applications (Fig. 1). Although such effects can be obtained at a wide range of frequencies, the word sonochemical is invariably linked to ultrasound, i.e. sound we cannot hear (typically above 20 kHz). Natural phenomena are good sources of both ultrasonic (e.g. animal communication or navigation) and infrasonic waves (such as earthquakes and tidal motion). Ultrasonics is currently of interest to lay people because of medical imaging, metal cleaning, industrial and dental drills and non-destructive material characterisation. 

The term ultrasound describes sound waves with a frequency range from 16kHz up to several megahertz. Vibrational motions are transmitted by oscillating devices into a fluid and cause pressure waves. This varying sound pressure is superimposed on the static pressure of the liquid. Fluids are generally capable... 

Most applications in ultrasound deal with rather small particle velocities v. This can be described by the acoustic Mach number M = v/cl. A 20-kHz high-intensity step horn with peak amplitude of 100 pm creates a Mach number of about 0.01. For low Mach numbers, the acoustic approximation of incompressible liquids is applicable. In certain cases, for example focused sound fields, much higher Mach numbers are present and lead to distortion of the sinusoidal wave form. Density and sound pressure no longer satisfy a simple linear equation with a fixed speed of sound c. A pressure-dependent sound speed is observed (material nonlinearity). Convective terms in the equation of motion cannot be neglected at higher Mach numbers and lead to a more complicated situation (convective nonlinearity) where the real speed of the wave front consists of two parts, the speed... 

Doppler Ultrasound. Doppler ultrasound has been widely used to provide quaUtative measurements of the average flow velocity in large to medium-size vessels if the vessel diameter is known. These include the extracranial circulation and peripheral limb vessels. It is also used in an assessment of mapped occlusive disease of the lower extremities. The frequency used for Doppler ultrasound is typically between 1 and 15 MHz. The basis of this method is the Doppler shift, which is the observed difference in ftequency between sound waves that are transmitted from simple piezoelectric transducers and those that are received back when both transmitter and receiver are in relative motion. The average frequency shift of the Doppler spectrum is proportional to the average particulate velocity over the cross-sectional area of the sample. When us to measure blood flow, the transducers are stationary and motion is imparted by the flowing blood cells. In this event, red cell velocity V is described by the relationship... 

The origin of sonochemical effects in liquids is acoustic cavitation. Ultrasound is transmitted through a medium via pressure waves by inducing vibrational motions of molecules, which alternately compress and stretch the molecular structure of the medium due to a time-varying pressure. Molecules start to oscillate around their mean position, and provided that the strength of the acoustic field is sufficiently intense, cavities are created in liquids. This will happen if the negative pressure exceeds the local tensile strength of the liquid. 

Several theories aimed at explaining the phenomena have been proposed, each of which is founded on completely different concepts. Sripaipan et al. [21] proposed a nematic layer with free ends, in which the interaction between the longitudinal oscillations (induced by the motion of the free ends of the layer in compression) and the traverse oscillations establishes steady flow of the liquid and, as a result, rotation of the molecules. However, these authors used incorrect dispersion relations and their calculations are not consistent with observed layer compression patterns. Nagai and coworkers [26,27] hypothesized that with normal incidence of an ultrasound beam on the layer the rotation of molecules is attributable to radiation fluxes. Radiation fluxes are the steady acoustic flows caused by radiation forces in a traveling acoustic wave, the only provision being that the width of the ultrasound beam is smaller then the dimensions of the cell. In reality, radiation fluxes can only occur near the boundaries of the beam and produce a compression effect that is smaller than the one that is actually ob-... 

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