**4.1.2 Excitation by mechanical shock: estimation of the frequency range**

The mechanical excitation in question is ensured via the core of an electromagnet. As an indication, figures 20a and 20b show the temporal and frequency responses of the sensor.

Fig. 20a. Temporal response of a mechanical shock

Fig. 20b. Spectral response associated with the shock

Low Frequency Acoustic Devices for Viscoelastic Complex Media Characterization 235

Fig. 21. Evolution of the power of the acoustic signal received during the fouling test and

fouling in the plate heat exchanger under very specific operating conditions.

with the right choice and positioning of the sensors.

and which are optimised with the application in mind.

Three different cases were thus studied:

In conclusion, this work concerned the monitoring of fouling using acoustics. By adopting a multi-stage experimental protocol we have been able to show that the variation in the acoustic signal can be used to predict variations in the pressure drop as well as the state of

Finally, this study illustrates an example of a non-intrusive acoustic technique for the local monitoring in real time of the fouling of plate heat exchangers. The results show that it is possible to follow the relative kinetics of the state of fouling in each zone of the exchanger

This chapter has proposed a synopsis of all the work that has led to the development of novel low frequency sensors. By using structural resonance modes excited by a transducer, these sensors present the advantage of having small sized sources with regard to the acoustic wavelength generated. These sensors are omni-directional but can nevertheless present significant contact areas with the medium to be characterised. This is the case for sensors developed for the characterisation of gels. The close contact of the elements set in resonance with the medium enables phenomena linked to changes in state to be monitored easily. Various applications have led us to develop sensors with very different geometries

Indeed, for each need expressed, the approach consisted in optimising not only the geometry of the sensors but also their optimum position according to the problem posed.

• identical near-field coupled sensors, through the medium to be characterised. They were used for monitoring the evolution of the ultrasonic values to characterise a sol-gel transition or the cohesion kinetics of a medium. For certain applications, the sensors are immersed in the medium. This direct immersion is essential for characterising fragile

cleaning

**5. Conclusion** 

media.

The curves show the temporal response and the frequency range of the sensor following a stress induced by a mechanical shock of short duration. The experiments carried out on the overall system (sensor & exchanger) in real configuration show that the temporal response is maximum 4 ms and its frequency response is around a central frequency of approximately 4 kHz.
