Acoustical measurements

In line with discussions included in previous sections, ultrasonic experiments carried out on fresh water by different investigators indicate that the stabilization of gas microbubbles, acting as gas nuclei for ultrasonic cavitation, is always attributable to the presence of surface-active substances in the water (ref. 15-17,25). As a starting point, one should consider that laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). Sirotyuk (ref. 25) estimates that the content of surface-active substances in ordinary distilled water amounts to 10 7 mole/liter, and in tap water it is 10 6 mole/liter or higher. These values indicate the appreciable content of such substances in both cases (ref. 122), although they differ by roughly an order of magnitude in absolute value. It is essentially impossible to completely remove 

There is other acoustical evidence to support the belief of Sirotyuk and other investigators that stable microbubbles serve as cavitation nuclei in fresh water. As noted by Sirotyuk (ref. 25), numerous experiments have disclosed that the cavitation threshold of water is increased by degassing of the liquid or by the 

From such microbubble-dissolution measurements, Bemd (ref. 16,17) outlined a physical model to explain much of the dynamic behavior of film-stabilized microbubbles.- One problematic aspect of this dynamic behavior involved the question of how a gas nucleus can be surrounded by a relatively impermeable film and yet subsequently act to produce cavitation when a gas/water interface is needed to initiate cavitation. Bernd (ref. 16) explains that if the stabilized gas microbubble enters a low-pressure area, the gas within the microbubble will attempt to expand. The surfactant film may also elastically attempt to expand. The surfactant film will then be expanded until essentially the surface tension of the water alone acts to contract the microbubble, since the protective shell no longer acts. The film has either been ruptured upon expansion, or it has expanded until it is ineffectual. Thus the microbubble (i.e., gas nucleus) should be capable of expanding to form a cavitation void or acquire additional gas in the form of water vapor or from surrounding dissolved gas. In addition, Bernd points out that it is reasonable to expect a gas microbubble to acquire such an effective 

In addition to acoustical methods, which take advantage of the fact that gas nuclei (i.e., stable microbubbles) are elastic bodies and thus absorb sound energy (ref. 4,5,9,25,26,31,32,50), another class of methods for detecting these gas microbubbles that has been employed repeatedly is based on their optical behavior. Specifically, most of these optical methods involve detection of these long-lived microbubbles in water from the light scattered by them (ref. 5,26,59,60,127). 

More recent light-scattering studies (ref. 26,59,60) of microbubble populations in fresh water, using laser-light sources, have yielded very similar results. For example, Keller s laser-scattered-light technique (ref. 26) provided precise measurements of the size and number of freestream gas nuclei (i.e., long-lived microbubbles) in a cavitation tunnel from microbubble spectra 

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Acoustic emission tests Acoustic insulation Acoustic measurements Acoustic microscopy Acoustic spectroscopy Acoustic waves... 

C. M. Harris, ed.. Handbook of Acoustical Measurement and Noise Control, 3rd ed., McGraw-HiU Book Co., Inc., New York, 1991. 

Acoustic Measurements. Measurement of the propagation of ultrasonic acoustic waves has been found useful for determining the viscoelastic properties of thin films of adhesives. In this method, the specimen is clamped between transmitting and receiving transducers. The change in pulse shape between successive reverberation of the pulse is dependent on the viscoelastic properties of the transmitting material. Modulus values can be calculated (267,268). 

This aspect is not included here, but is related to optical flow diagnostics. It is based again on the principle of the optical Doppler effect. Multifunctional equipment is available for noncontact measurements of flow-induced vibration on surfaces of structural elements, for acoustic measurements, and for calibration of accelerometers and vibration transducers. 

After injection starts, but before the granulator reaches normal operating conditions, the airflow drops suddenly for approximately 10-15min. The drop in the airflow can easily be seen in the score plot for the acoustic measurements. The score plot of the process data recorded simultaneously is displayed in Figure 9.16 and does not detect the change. 

L.Baranek, "Acoustic Measurements," Wiley,NY(I949) 6)L.L.Baranek, "Acoustics," McGraw-Hill, NY(1954)... 

One acoustical measure of the direct to reverberant ratio is called the clarity index, and is defined as ... 

There are a large number of subjective attributes of reverberation which have been discussed in the literature. Most of these are monaural attributes directly correlated with acoustical measures that can be derived from the EDR. Consequently, it is convenient to think of the EDR as representative of all the monaural objective measures of a room impulse response. Presumably, the fine details of this shape are irrelevant, particularly in the late response, but no systematic study has been done to determine the resolution required to perceptually reproduce a reverberant response from its EDR. 

Separate from his earlier-described acoustical measurements, Medwin (ref. 31) conducted a direct photographic determination of microbubble populations in coastal ocean waters. His results indicated the presence of several million microbubbles per cubic meter (i.e., >103/liter) within the diameter range of 20 to 100 pm, both at the surface and at a depth of 10 feet in Monterey Marina (ref. 31,33). However, as a result of his technique of illumination, Medwin suggested that his count probably included many non-bubbles (ref. 31). 

H. Medwin, In situ acoustic measurements of bubble populations in coastal ocean waters, J. Geophys. Res. 75 (1970) 599-611. 

When considering water circulation, quantitative and qualitative generalizations and a comparison of the the results of the studies of the Black Sea currents performed using modern observation techniques (autonomous buoys stations, shipborne acoustic measurements, drifter and altimetric satellite observations) with different types of hydrodynamic models were presented. The horizontal and vertical structure of the general water circulation in the Black Sea is shown, together with its seasonal and interannual variabilities. The principal mechanisms for the current formation under external hydro- and thermodynamical forcing are discussed. 

Acoustic measurements can also be used as a structural probe, since the acoustic properties, especially sound absorption, are related to many structural factors, such as transition temperatures, morphology, cross-link density, etc. Finally, they can be used as a source of engineering data, especially in the building and construction field for the absorption of unwanted sound, the construction of acoustically transparent windows, underwater acoustics, etc. 

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

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