Acoustic general principle

This observation illustrates a general principle the total influence of any structure behind a boundary can be combined and represented as the acoustic input impedance of the boundary. Hence the input impedance at the interface between media 1 and 2 is defined by a rearrangement of Equation 5 as ... 

One of the most sensitive methods to detect volatile compounds released by the plants is that of laser photoacoustic spectroscopy (LPAS), which permits the identification of many molecules signalling plant defence mechanisms (e.g. see Harren and Reuss (1997)). The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption the general principles of photoacoustic spectroscopy were briefly outlined in Section 5.3. 

The general principle of acoustic measurements for layer characterization is to follow the ultrasonic eedio reflected on the top of a layer and the one reflected on the bottom of the layer. One can write ... 

As we do not know the acoustic velocity in particle deposit, the general principle of acoustic method for thickness measurement has to be modified. It was proposed to first measure the signal on the clean membrane, at t = 0, then the deposit echo on a fouled membrane (Figure 11.15). 

Figure 4 Acoustic beam characterization set-up for the immersion SW array probe a/ Angle beam characterization principle b/ General view of the automatic bench...

Up to now, sensors using this parameter have not been taken into consideration, as they are generally not selective. Water for instance is traceable in air because its velocity of sound is significantly higher. The VOS of carbon dioxide is just around 1/3 the VOS of air. In a mixture of air, water and C02 none of the compounds can be quantified. As the VOS of carbon monoxide is similar to that of air, CO cannot be quantifiedby this method either. Hence, sensors based on acoustic principles cannot be taken into consideration neither for the measurement of single species in a flue gas flow nor for the identification of fuel gases. 

Measurements of liquid density are closely related to quantity and liquid-level measurements since both are often required simultaneously to establish the mass contents of a tank, and the same physical principle may often be used for either measurement, since liquid-level detectors sense the steep density gradient at the liquid-vapor interface. Thus, the methods of density determination include the following techniques direct weighing, differential pressure, capacitance, optical, acoustic, and nuclear radiation attenuation. In general, the various liquid level principles apply to density measurement techniques as well. 

Since ultrasound in general follows the principles delineated in acoustics, its development naturally reflects the developments in acoustics. For the purpose of this review, only some of the major developments in acoustics and ultrasonics will be highlighted. For in-depth review of this area, readers are referred to a website developed by Dr. Joseph Woo,12 which contains some of the best information on the history of the study of acoustics and biomedical application of ultrasound. 

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