Frequency ultrasound waves

The word ultrasound has become common knowledge due to the widespread use of ultrasound scanning equipments in medical applications. Ultrasound refers to sound waves having frequencies higher than those to which the human ear can respond (p, > 16 KHz) (Hz = Hertz = cycles per second). High frequency ultrasound waves are used in medical equipments. The ultrasound frequencies of interest for chemical reactions (about 20-100 KHz) are much lower than those used for medical applications, but the power used is higher. 

The previous subsection described single-experiment perturbations by J-jumps or P-jumps. By contrast, sound and ultrasound may be used to induce small periodic perturbations of an equilibrium system that are equivalent to periodic pressure and temperature changes. A temperature amplitude 0.002 K and a pressure amplitude 5 P ss 30 mbar are typical in experiments with high-frequency ultrasound. Fignre B2.5.4 illustrates the situation for different rates of chemical relaxation with the angular frequency of the sound wave... 

The easily accessible frequency range of sound and ultrasound waves confines the range of applicability of... 

Servant G, Laborde JL, Hita A, Caltagirone JP, Gerard A (2003) On the interaction between ultrasound waves and bubble clouds in mono and dual-frequency sonoreactors. Ultrason Sonochem 10 347-355... 

The pioneering work on the chemical applications of ultrasound was conducted in the 1920 s by Richards and Loomis in their classic survey of the effects of high frequency sound waves on a variety of solutions, solids and pure liquidsQ). Ultrasonic waves are usually defined as those sound waves with a frequency of 20 kHz or higher. The human ear is most sensitive to frequencies in the 1-5 kHz range with upper and lower limits of 0.3 and 20 kHz, respectively. A brief but useful general treatment of the theory and applications of ultrasound has been given by Cracknel 1(2). 

Essentially all imaging from medical ultrasound to non-destructive testing relies upon the same pulse-echo type of approach but with considerably refined electronic hardware. The refinements enable the equipment not only to detect reflections of the sound wave from the hard, metallic surface of a submarine in water but also much more subtle changes in the media through which sound passes (e. g. those between different tissue structures in the body). It is high frequency ultrasound (in the range 2 to 10 MHz) which is used primarily in this type of application because by using these... 

The first area involves low amplitude (higher frequency) sound and is concerned with the physical effect of the medium on the wave and is commonly referred to as low power or high frequency ultrasound . Typically, low amplitude waves are used for analytical purposes to measure the velocity and absorption coefficient of the wave in a medium in the 2 to 10 MHz range. Information from such measurements can used in medical imaging, chemical analysis and the study of relaxation phenomena and this will be dealt with later. 

The potential of sonochemistry was identified over sixty years ago in a wide ranging paper entitled The Physical and Biological Effects of High Frequency Sound-Waves of Great Intensity [13]. Over the few years which followed this paper a great deal of pioneering work in sonochemistry was carried out and, as a result of this, two reviews on the applications of ultrasound in polymer and chemical processes were published... 

Ultrasound is used to obtain information about the properties of a material by measuring the interaction between a high frequency sound wave and the material through which it propagates. This interaction depends on the frequency and nature of the ultrasonic wave, as well as the composition and microstructure of the material. The parameters most commonly measured in an ultrasonic experiment are the velocity at which the wave travels and the extent by which it is attenuated. To understand how these parameters are related to the properties of foods it is useful to consider the propagation of ultrasonic waves in materials in general. 

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. 

An ultrasound device emits high-frequency sound waves that can pass through a material, he absorbed, or reflect off the surface of a material. Waves are reflected at the border between tissues with different densities, such as an organ and a tumor. The larger the difference in density, the greater the reflection. 

The presence of undissolved gas and of cavitation bubbles affects the transparency and refractive index of a liquid. Thus when a sonicated liquid is irradiated with light, X-rays, y-rays, or even high-frequency ultrasound, the attenuation and (or) refraction of the wave can be used to detect both the cavitation threshold and bubble density, and their variation with time. This is possible even within a very short period of the order of one ultrasonic cycle [138,139]. 

Ultrasound waves are mechanical vibrations (frequency 20 kHz-10 GHz) produced by a piezoelectric device. These waves can be established in a liquid sample and produce cavitation. Very high temperatures are associated with the locations where cavitation occurs, so the effect can be exploited to assist sample preparation [143]. 

Ultrasound imaging is a non-invasive, portable and relatively inexpensive imaging modality, which is used extensively in the clinic. An ultrasound transducer (also called scanhead) sends short pulses of a high-frequency sound wave (1-10 MHz) into the body. At interfaces between two types of tissue, the wave will be refracted and part of the sound wave is reflected back due to Snells law. How much is reflected depends on the densities of the respective tissues, and thus the speed of the sound wave within the different tissues. In addition, parts of the sound wave are also backscattered from small structures at tissue boundaries or within the tissue. High-frequency sound waves propagate weU through soft tissue and fluids, but they are more or less stopped by air or bone. In clinical practice, this limitation is referred to as an acoustic window . The transducer not only sends the wave into the body but also receives part of the reflected and/or backscattered wave, also named echo . In clinical practice, ultrasound is used in a... 

For the spatial resolution of an ultrasound system, three different dimensions must be considered the axial, the lateral, and the elevation dimension. The axial resolution along the axis of the transducer is defined as the closest separation of two echoes that can be resolved and improves at higher frequencies. However, the penetration depth of ultrasound waves decreases with increasing frequencies. Therefore, lower frequencies (1-3 MHz) are used for studies of deep-lying structures. 

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