Ultrasonic continuous wave

Continuous wave methods are the most accurate means of making ultrasonic measurements. Even so, they are used less frequently than pulse methods because measurements are more time consuming and laborious to carry out, are more difficult to automate, and the measurement cell requires a high degree of precision engineering. These techniques therefore tend to be used in specialized research laboratories where accurate measurements are important. Continuous wave ultrasound is utilized in a variety of different techniques, but the most commonly used is the interferometer [10,11]. 

The signal generator applies a continuous sine wave of suitable frequency and amplitude to the transducer. The transducer generates an ultrasonic sine wave which propagates into the sample and is reflected back and forth between the reflector plate and transducer. Standing waves are set up in the sample, and the amplitude of the signal received by the transducer... 

Radiation pressure is a steady constant pressure for continuous wave signals. For acoustic pulses, however, this pressure varies periodically at the pulsing frequency. Radiation pressure thus provides a mechanism for producing foree at frequencies other than the normal ultrasonic frequencies and potentially in the audible zone. For small particles and non-absorbing interfaces, radiation pressure has a direction and amplitude which depend on the elastic properties of the material in question. This extra force may also result in particle movement. 

There are two groups of ultrasonic resonators, fixed path-length and fixed wavelength. In a fixed pathlength resonator, a continuous wave containing a single ultra sonic frequency is transmitted across the measurement cell. The... 

Augustsson P, Aberg LB, Sward-Nilsson AMK, Laurell T (2009) Buffer medium exchange in continuous cell and particle streams using ultrasonic standing wave focusing. Microchimica Acta 164 269-277... 

FIGURE 9. Basic ultrasonic waveforms, (a) Continuous wave (b) pulse. 

Recently, Petersson et al. [12] have developed a Lab-on-a-Chip device for particle separation based on this ultrasonic principle. In order to demonstrate the separation of particles with different physical properties in a continuous laminar flow, they have prepared a microfluidic device with a 750 xm wide and 250 p,m deep channel. As a test material, a mixture of milk and blood was used. The ultrasound half standing wave at 2 MHz was produced by a piezoelectric ceramic, which was glued directly on the bottom of the microfluidic device. Milk was used as an initial test medium (Fig. 9a and 9b) to confirm that the hpid particles within the milkfat were attracted toward the pressure anti-nodes of the half standing wave, which exist along the microfluidic channel wall. In addition, when they infused a mixture of milk and blood, they found efficient separation of blood cells from the mixture (Fig. 9c). This demonstration showed that a mixture of heterogeneous conponents can be effectively separated by utilizing ultrasonic standing waves, combined with the laminar flow in microfluidic channels. The separation efficiency of erythrocytes from the mixture was about 70%, while more than 80% of the milkfat lipid particles were... 

Flow-through systems are used in a continuous mode of operation in which a mix of fluid and particles is pumped through the device and the ultrasonic standing wave field concentrates the particles into a nodal plane. In this way a barrier-free filter is created. 

One measures this rate by observing the increase in attenuation of the ultrasonic shear wave as the area of contact grows. Growth continues until the substrate is covered with the polygonal units rather than flattened spheres. 

Another variation of the normal beam ultrasonic technique is illustrated in Fig. 6 for a so-called resonance method. In this case, continuous wave ultrasonic energy travels from the ultrasonic transducer through the entire adhesive bond structure. When encountering a delamination, notice that the wave resonance set up is of a different wavelength than one where ultrasonic energy can nicely traverse the entire adhesive bond structure. There are many resonance techniques and instrumentation possibilities on the market that work very well for cohesive bonding problems and in some cases are even acceptable for adhesive defect situations. Standing waves are established and the frequency observed is equal to the wave velocity over twice the thickness, in this case either twice d or twice D. 

Fundamentals Pulsed Methods Continuous Wave Methods Musical Instruments Transducer Arrays Medical Ultrasonics Nondestructive Evaluation Geophysical Exploration Marine Sonar and Sonar In Animals Processing Technology Sonochemistry... 

The Nd YAG laser was directed through the clear substrate onto the ultrasonic horn via an arrangement of three mirrors. The laser was also passed through a focusing lens to narrow the beam. This lens was placed three and one-eighth inches from the mirror nearest to the ultrasonic horn. The laser power reaching the horn was measured to be twenty-six Watts, and the spot size was six mm. The laser was used at a current of 35 Amp and continuous wave (CW) mode. 

The Doppler meter may be used wherever small particulate solids, bubbles or droplets are dispersed in the fluid and are moving at essentially the same velocity as the fluid stream which is to be metered. A continuous ultrasonic wave is transmitted, again at an acute angle to the wall of the duct, and the shift in frequency between the transmitted and scattered waves is measured. This method of measurement of flowrate is frequently used for slurries and dispersions which present considerable difficulties when other methods are used. 

Doppler ultrasonic flowmeters depend upon the reflection of a continuous ultrasonic wave (frequency 0.5-10 MHz) from particulate matter (scatterers) contained in the fluid. Hence they may be used to monitor the rate of flow of dirty liquids. The transducer involved can act both as transmitter and receiver and is generally of the clamp-on type (Fig. 6.4). If the scatterers can be assumed to be moving at the velocity of the liquid, then the volumetric rate of flow Q is related to the Doppler frequency shift AtoD by ... 

In practice ultrasound is usually propagated through materials in the form of pulses rather than continuous sinusoidal waves. Pulses contain a spectrum of frequencies, and so if they are used to test materials that have frequency dependent properties the measured velocity and attenuation coefficient will be average values. This problem can be overcome by using Fourier Transform analysis of pulses to determine the frequency dependence of the ultrasonic properties. 

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