Acoustic coupling

Figure 3.7 demonstrates the resolution that can be achieved using water-coupled acoustic microscopy. It represents a remarkable achievement in the development of the instrument. Never before had it been possible to obtain an acoustic image through water with such high resolution. Nevertheless,... 

Noise emissions from gas-turbine engines, resulting from unsteady flow interactions with rigid and moving surfaces (compressor and diffuser noise) and from combustion (core noise), must also be reduced without sacrificing performance. Studies addressing simultaneous pollutant and noise emission reductions are rare. The current study may lead to a better understanding of the coupled acoustic and pollutant behavior of advanced combustor concepts. 

The same group has also presented an alternative way to couple acoustic energy into a microfluidic chip using a flat transducer and a refractive element placed on top of the chip. This setup is used in combination with dielectrophoretic (DEP) forces with an outlook to combined manipulation of bioparticles or individual cells using the two forces simultaneously. 

Kaduchak G, Sinha DN, Lizon D (2002) Novel cylindrical, air-coupled acoustic levitation/concenlration devices. Rev Sci Instrum 73 1332-1336... 

Moving away from the dispersion relation for infinite layers, the transmissivity or reflectivity of an A-repeat ADBR can be calculated using the theory of Mizuno and Tamura (Mizuno and Tamura 1992) which extracts the net transmission from transfer matrix theory for waves passing from a mirror substrate to a detector. For a fuller model that can include the details of transducer layers and coupling fluids used to couple acoustic waves into an ADBR, the matrix model of Mitsas and Siapkas has been used (Mitsas and Siapkas 1995). At normal incidence for acoustic waves and when... 

It must efficiently couple acoustic energy into and out of the tissue, i.e. its acoustic impedance must be close to that of tissue (l.SMrayl). 

Within this range we can modify the volume fraction without losses in the coupling factor and thereby can adjust the acoustic impedance to our demands. 

The properties of the piezocomposite material mentioned above offer special benefits when the transducer is coupled to a material of low acoustic impedance. This especially applies to probes having plastic delay lines or wedges and to immersion and medical probes. These probes with piezocomposite elements can be designed to have not only a high sensitivity but also at the same time an excellent resolution and, in addition, the effort required for the probe s mechanical damping can be reduced. 

The Champ-Sons model is a most effieient tool allowing quantitative predictions of the field radiated by arbitrary transducers and possibly complex interfaces. It allows one to easily define the complete set of transducer characteristics (shape of the piezoelectric element, planar or focused lens, contact or immersion, single or multi-element), the excitation pulse (possibly an experimentally measured signal), to define the characteristics of the testing configuration (geometry of the piece, transducer position relatively to the piece, characteristics of both the coupling medium and the piece), and finally to define the calculation to run (field-points position, acoustical quantity considered). 

Up to now it was demonstrated, that the probe design enables a fast positioning and that the acoustical parameters of the probe ensure a reliable crack detection implying even a coupling control. Now, the final customer s requirement was the inspection of the blades without demounting them from the engine. 

ISONIC - Complete Ultrasonic Weld Inspection Documentation by Continuous Recording of Manual Probe Manipulations, Acoustic Coupling and Echoes. 

After setting up the test system, the inspeetor can fully concentrate on scanning the test zones. In the case of coupling failure, an acoustic alarm horn will sound and a visual alarm message will appear on the PC display. 

For special applications such as air coupled testing a special programmable transmitter board was developed. This transmitter generates rectangular and burst signals, which increase the acoustical power in an optimized frequency range, and provides a superior signal-to-noise ratio. 

This corresponds to the physician s stethoscope case mentioned above, and has been realized [208] by bringing one leg of a resonatmg 33 kHz quartz tiinmg fork close to the surface of a sample, which is being rastered in the x-y plane. As the fork-leg nears the sample, the fork s resonant frequency and therefore its amplitude is changed by interaction with the surface. Since the behaviour of the system appears to be dependent on the gas pressure, it may be assumed that the coupling is due to hydrodynamic mteractions within the fork-air-sample gap. Since the fork tip-sample distance is approximately 200 pm -1.120), tire teclmique is sensitive to the near-field component of the scattered acoustic signal. 1 pm lateral and 10 mn vertical resolutions have been obtained by the SNAM. 

The chemical effects of ultrasound do not arise from a direct interaction with molecular species. Ultrasound spans the frequencies of roughly 15 kH2 to 1 GH2. With sound velocities in Hquids typically about 1500 m/s, acoustic wavelengths range from roughly 10 to lO " cm. These are not molecular dimensions. Consequently, no direct coupling of the acoustic field with chemical species on a molecular level can account for sonochemistry or sonoluminescence. 

Acoustic Coupling When the shell-side fluid is a low-density gas, acoustic resonance or coupling develops when the standing waves in the shell are in phase with vortex shedding from the tubes. The standing waves are perpendicular to the axis of the tubes and to the direction of cross-flow. Damage to the tubes is rare. However, the noise can be extremely painful. 

Thus far we have discussed the direct mechanism of dissipation, when the reaction coordinate is coupled directly to the continuous spectrum of the bath degrees of freedom. For chemical reactions this situation is rather rare, since low-frequency acoustic phonon modes have much larger wavelengths than the size of the reaction complex, and so they cannot cause a considerable relative displacement of the reactants. The direct mechanism may play an essential role in long-distance electron transfer in dielectric media, when the reorganization energy is created by displacement of equilibrium positions of low-frequency polarization phonons. Another cause of friction may be anharmonicity of solids which leads to multiphonon processes. In particular, the Raman processes may provide small energy losses. 

The normal modes for solid Ceo can be clearly subdivided into two main categories intramolecular and intermolecular modes, because of the weak coupling between molecules. The former vibrations are often simply called molecular modes, since their frequencies and eigenvectors closely resemble those of an isolated molecule. The latter are also called lattice modes or phonons, and can be further subdivided into librational, acoustic and optic modes. The frequencies for the intermolecular modes are low, reflecting, the... 

There are numerous other inspection techniques that have been developed in the last couple of decades such as holographic interferometry, acoustical holography, acoustic emission, thermal emission scanning, etc. They all have been developed to address shortcomings of more popular inspection techniques but for the most part remain niche techniques. 

The use of electromagnetic acoustic transducers (EMAT) obviates the coupling problems already referred to, and has been applied successfully to the inspection of boiler tubes. Ultrasonic time of flight diffraction (TOFD), developed by the Harwell Laboratory" , is utilised to fingerprint flaws (cracks) in process plant. Subsequent examination at, say, six month intervals can indicate any growth or extension of the crack. It is claimed that changes in crack height of 0-5 mm or less be estimated. 

In this mode, acoustic-pressure oscillations are similar to those established in a closed organ pipe. The resulting pressure oscillations then couple with the pressure-sensitive combustion processes to further excite the oscillating pressure and thus produce the high-pressure amplitudes. 

The most important parameter in the analysis of pressure-coupled combustion instability is the acoustic admittance Y, which is the ratio of the amplitude of the acoustic velocity V to the amplitude of the acoustic pressure amplitude of the acoustic velocity V to the amplitude of the acoustic pressure P ... 

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