**3.5 Case of highly absorbent matter**

226 Acoustic Waves – From Microdevices to Helioseismology

Heated to a constant temperature of 35 °C, the experimental mould was instrumented with a transmitter and two ultrasonic receivers spread out so as to integrate the signals transmitted through two paths presenting enough interfaces between the grains. This thus reduced

measurement dispersion linked to the random number of interfaces (Figure 12).

Thermometer

Scales

Fig. 12. On the left: The experimental measuring device with a horizontal cross-section of the mould at sensor level and on the right: omni-directional, optimised sensor in elongation

Figure 13 shows that the amplitude of the ultrasonic signal is a parameter that is sensitive to

**Cohesive** 

**Time** 

Whey ejected

Fig. 11. Estimation of the propagation zone of the main signal beam

Transmitter

variations in the properties of the medium under investigation.

**Uncontrolled rapid evacuation of the whey** 

> **Constant ratio between deformation and stress (***incompressible medium***)**

Fig. 13. Typical curve reflecting the cohesion phenomenon as seen by the variation in the

Receiver1 Receiver2

Controlled pressure

mode at 246 kHz

ultrasonic amplitude

The characterisation of media using ultrasounds is often limited by the heterogeneous nature of the matrix which can, in the case of cosmetic, pharmaceutical and agro-food products, be viscoelastic and heterogeneous (foam or emulsion for example). Wave attenuation in such media is mainly due to viscous absorption and scattering from heterogeneities. The higher the frequency the greater the attenuation, hence the necessity to analyse the media using low frequencies in order to characterise the evolving matter.

The search for a compromise between the analysis frequency and the volume of the medium to be characterised led us to propose specific sensor geometries associated with specific excitation conditions.

In order to manage this constraint, a very low frequency acoustic technique was adapted so as to communicate sufficient energy to a particularly absorbent sample. This was achieved by mechanical excitation caused by a shock. An electrical image of this excitation is obtained using a second identical sensor used as the synchronisation reference (Figure 14).

Fig. 14. Schematic diagram
