Seite - 201 - in Advanced Chemical Kinetics

material, thefaster theultrasonicwavewillpropagatewithin it. It is important tokeepinmind that theultrasonicvelocitychanges significantlywith temperature [26–29]. Mathematically, the speedof sound is computeddividing thedistance travelledby thepulse by the timespent to travel it (timeof flight), asdisclosed inEq. (5). υ¼2 �Δs t (5) Here, Δs is the distance separating the ultrasonic surface and the reflecting interface (the travelling distance is twice this value) and t is the time required for the ultrasonic pulse to transpose thatdistanceandreturnto the transducer.Thisprocesscanberepeatedmanytimes, dependingon theattenuationand thedistance fromthe transducerand the reflecting surface. After each subsequent reflection, the pulse amplitude will decrease, as a consequence of attenuation. The multiple reflections will remain until the sound energy is completely absorbed in theprocess.Figure2exhibits thatmulti-reflectionbehaviour. Whileplanning the experimental set-up for thepulse/echomethod, onemust be aware about theabsorptionof the liquidunder investigation,aswellas thedistancebetweenthetransducer and the reflecting surface. The pulse frequency plays a key role, as ultrasonic attenuation is exponentially proportional to the frequency. In general, water is used as reference once its behaviourboth forattenuationandultrasonicvelocityareverywellknown[26–29]. Figure2. Ultrasonicpulseandreflections. Ultrasound as a Metrological Tool for Monitoring Transesterification Kinetics http://dx.doi.org/10.5772/intechopen.70501 201