Sound intensity

Perception of sound by the human ear is nor related to. sound power hut to sound intensity, defined as... 

A weighting scale, dBA The unit of sound intensity expressed as a logarithmic scale, related to a reference level of 10 W m"-. The A weighting scale is the most commonly used scale, as it reduces the response of sound meters to very high and low frequencies and emphasize those within the range audible by the human ear. 

Sound intensity The sound power distributed over unit area, in units of W m -. 

The attenuation of sound intensity / follows Eq. (4-29), which is analogous to Beer s law. 

Sound intensity is a measure of energy, and its units are watts per meter. Intensity is proportional to the square of pressure ... 

Sound intensity level is defined in a similar manner to sound pressure level. In this case the equation is ... 

It can thus be seen that it is important not only to express the unit but also to state sound pressure level, sound intensity level or sound power level. 

The role of cavitation in ultrasound degradation has been confirmed repeatably in most experiments where cavitation was prevented, either by applying an external hydrostatic pressure, by degassing the solution, by reducing the sound intensity or the temperature, polymer chain scission was also largely suppressed [117]. 

For cavitation to occur in a liquid, it has to overcome the natural cohesive forces present in the liquid. Any increase in these forces will tend to increase the threshold pressure and hence the energy required to generate cavitation. In highly viscous liquids, severe attenuation of the sound intensity occurs and the active cavitating zone gets reduced substantially. Moholkar et al. [56] have confirmed this fact with experiments with different liquids and reported that for highly viscous liquids, cavitational effects are not observed. 

Table 3-7 Sound Intensity Levels for a Variety of Common Activities1...

Thus, as the sound intensity increases, higher temperatures and pressures exist within the bubble interior, which then enhance the overall decomposition rate of the PCBs. In addition, the formation of organic free - radicals is also enhanced at higher sound intensities. These observations are consistent with previous reports of enhanced decomposition kinetics at higher acoustic intensities. 

Fig. 1 First - derivative ESR spectrum indicating the accumulation of PBN-phenyl after 10 minutes of sonicating an Ar-saturated solution of 2-PCB (4.6 iM) at 20 kHz and varying sound intensities.

One of the most important characteristics necessary to completely identify a wave is its intensity, where the intensity is a measure of the sound energy the wave produces. For a sound wave in air, the mass (m) of air moving with an average velocity (v) will have associated with it a kinetic energy of (mv )/2 (joules). In the strictest sense the intensity is the amount of energy carried per second per unit area by the wave. Since the units of energy are joules (J) and a joule per second is a watt (W), then the usual unit of sound intensity (especially in sonochemistry) will be W cm. As we will see later (Eq. 2.13), the maximum intensity (I) of the sound wave is proportional to the square of the amplitude of vibration of the wave (P ). This will have important repercussions in our study of chemical systems. 

According to the above expressions the value of a/f is a constant for a given liquid at a given temperature. Any increase in sound frequency,/ must result in a compensatory increase in a and thus a more rapid attenuation of the sound intensity with distance (Eq. 2.14). This has important consequences. Consider for example the passage of sound through water at room temperature. According to Fox and Rock [10] the value of a// for a wide variety of frequencies in water is 21.5 x 10 cm. Using this value the absorption coefficients at 21.5 kHz and 127.0 kHz can be deduced to be... 

Eq. 2.14) at which the sound intensities are reduced to half of their original values to be 35 km and 1 km respectively. Calculations such as these demonstrate clearly... 

Transient cavitation bubbles are voids, or vapour filled bubbles, believed to be produced using sound intensities in excess of 10 W cm. They exist for one, or at most a few acoustic cycles, expanding to a radius of at least twice their initial size, (Figs. 2.16 and 2.20), before collapsing violently on compression often disintegrating into smaller bubbles. (These smaller bubbles may act as nuclei for further bubbles, or if of sufficiently small radius (R) they can simply dissolve into the bulk of the solution under the action of the very large forces due to surface tension, 2a/R. During the lifetime of the transient bubble it is assumed that there is no time for any mass flow, by diffusion of gas, into or out of the bubble, whereas evaporation and condensation of liquid is assumed to take place freely. If there is no gas to cushion the implosion... 

In Chapter 2 we explained why there existed a cavitation threshold i. e. a limit of sound intensity below which cavitation could not be produced in a liquid. We suggested that only when the applied acoustic amplitude (P ) of the ultrasonic wave was sufficiently large to overcome the cohesive forces within the liquid could the liquid be tom apart and produce cavitation bubbles. If degradation is due to cavitation then it is expected that degradation will only occur when the cavitation threshold is exceeded. This is confirmed by Weissler who investigated the degradation of hydroxycellulose and observed that the start of degradation coincided with the onset of cavitation (Fig. 5.21). 

When ultrasound is used as energy carrier, a sound intensity in the range from 5-10 W cm-2 is employed. This energy is sufficient to heat the material up to or even above its melting point. As a result, the diffusion velocity of the free radicals in turn increased. In addition, in the fluid phase of the matrix, sonochemical reactions are possible, based on cavitation. Such cavitation is associated with... 

The loss of compression in the damaged ear is a possible cause of the phenomenon of loudness recruitment. Loudness is the perceptual correlate of sound intensity. Loudness recruitment is defined as the unusually rapid growth of loudness with an increase in sound intensity [Moore et al., 1985], and often accompanies sensorineural hearing impairment... 

The hypnotic state is a psychological construct or, if induction has been successful, an experiential reality to the hypnotized person. It is not defined by external measurements. There are no obvious behavioral manifestations that clearly indicate hypnosis has occurred and no known physiological changes that invariably accompany hypnosis. The hypnotic procedure, on the other had, the words that they hypnotist says aloud, is highly amenable to physical measurement. An investigator can film the hypnotic procedure, tape-record the hypnotist s voice, measure the sound intensity of the hypnotist s voice, and accumulate a variety of precise, reproducible physical... 

Decible The unit used for the measurement of the intensity of sound on a logarithmic scale, based on measurements of sound intensity in watts per square meter and related to a reference. For instance, 10 watts/m2 is the intensity of the quietest sound perceptible to human ear. 

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